NPL Report No CMMT(B)56 - adhesivestoolkit.com

NPL Report No CMMT(B)56

NITS Adhesives Project 1

Report No 6 April 1996

TEST METHODS FOR DETERMINING SHEAR PROPERTY DATA FOR ADHESIVES SUITABLE FOR DESIGN.

Part 1: Notched-beam shear (Iosipescu) and notched-plate shear (Arcan) methods for bulk and joint test specimens.

Part B FULL REPORT

By B C Duncan and G D Dean

Centre for Materials Measurement and Technology, National Physical Laboratory,

Queens Road, Teddington, Middlesex, UK, TW11 0LW


© Crown Copyright 1996 Reproduced with permission of the Controller of HMSO

National Physical Laboratory Teddington, Middlesex, UK, TW11 0LW

Extracts form this report may be reproduced provided that the source is acknowledged.

Approved on behalf of Managing Director, NPL by Dr M K Hossain, Director,

Centre for Materials Measurement and Technology.


TEST METHODS FOR DETERMINING SHEAR PROPERTY DATA FOR ADHESIVES SUITABLE FOR DESIGN.

Part 1: Notched-beam shear (Iosipescu) and notched-plate shear (Arcan) methods for bulk and joint test specimens.

By: B C Duncan and G D Dean

MTS Adhesives Project 1 Report No 6 Part B FULL REPORT April 1996

ABSTRACT

The stress/strain behaviors to failure of adhesives under shear stress are needed for

designing adhesively bonded structures. There are several test methods for measuring these

properties. The current report describes V-notch shear methods for testing bulk and joint

specimens.

Since the loading arrangement for bulk specimens in the Iosipescu test is unsuitable for

compliant specimens, the study has concentrated on the Arcan test. A shear extensometer

was designed and constructed to measure shear strains in the Arcan test. Two extensometers

were used to measure the strain on each face of the specimen simultaneously and the

average of these strains is taken as the shear strain. Modulus measurements made using the

Arcan test, with bulk and joint specimens, are typically reliable to within better than ± 20 %..

There is scope for increasing the accuracy through minor improvements to the apparatus.

Arcan bulk specimen tests enable shear stress/strain measurements to be made with

adequate accuracy at strains below 20 % strain although some less ductile adhesives may fail

prematurely because of local tensile stress concentrations. The measurement of shear strains

in bulk specimens above 20 % are susceptible to errors arising from buckling deformations

of the specimen and performance limitations of the extensometer however accuracy can be I

improved by using the crosshead displacement. These problems associated with the

measurement of large shear strains in bulk specimens are not experienced in joint specimen

tests as the shear displacements are small. The joint test appears to be more reliable for

measuring stress/ strain curves to failure for compliant adhesives.

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CONTENTS

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...1

1.

2. 2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.2

3. 3.1 3.1.1 3.1.2 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.3 3.3.1 3.3.2 3.3.3

4. 4.1 4.1.1 4.2 4.2.1 4.2.2 4.2.3 4.3

5. 5.1 5.2 5.2.1 5.2.2 5.2.3 5.2.4 5.2.5 5.3 5.3.1 5.3.2 5.4

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...4

NOTCHED SPECIMEN SHEAR TEST METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...5 LITERATURE REVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...5 The Iosipescu test method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...5 The Arcan test method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...7 Adhesive joint tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..7 Shear strain measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 SHEAR MEASUREMENTS REPORTED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...8

EXPERIMENTAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...9 APPARATUS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...9 Iosipescu test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...9 Arcan test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...9 SHEAR STRAIN MEASUREMENT METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...10 Strain Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...10 Description of the extensometer for the Arcan shear test . . . . . . . . . . . . . . . . . . . . . . . . . 10 Uncertainties in shear strain measurements made with the extensometer . . . . . . . . . . . . 12 Displacement Transducers.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...13 Data recording equipment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...13 CALIBRATION OF SHEAR EXTENSOMETERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...13 Tesa displacement transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...13 Arcan extensometer lever ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...13 Arcan extensometer gauge length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...14

TEST SPECIMENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...15 BULK SPECIMENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...15 Finite element analysis of bulk Arcan specimen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 JOINT SPECIMENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...16 Preparation of joint specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...16 Finite element analysis of the Arcan joint specimen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Joint test requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...18 MATERIALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...18

EXPERIMENTAL MODULUS MEASUREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...20 TESTS ON BULK IOSIPESCU AND ARCAN SPECIMENS USING STRAIN GAUGES . . 21 BULK SPECIMEN MODULI MEASURED WITH THE ARCAN SHEAR EXTENSOMETER 22 Stress/strain curves obtained during modulus measurements . . . . . . . . . . . . . . . . . . . . . 22 Measured modulus values for stiff materials . . . . . . . . . . . . . . . . . . . . . . . . . . . ...23 Measured modulus values for compliant materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Discriminating between specimens with different properties . . . . . . . . . . . . . . . . . . . . . . 24 Influence of specimen thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...25 THE MEASUREMENT OF MODULUS OF ARCAN JOINT SPECIMENS . . . . . . . . . . . . . 25 Stress/strain curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...25 Measured Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...25 SUITABILITY OF TEST METHODS FOR MODULUS MEASUREMENT . . . . . . . . . . . . . 26

continued . . . . . . . . . . . . .

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6. 6.1 6.1.1 6.1.2 6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.2.5 6.3 6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.5 6.5.1 6.5.2 6.5.3

7. 7.1 7.2 7.2.1 7.2.2

8.

9.


MEASUREMENT OF STRESS-TRAIN TO FAILURE USING THE ARCAN TEST...... 27 STIFF BULK SPECIMENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...27 Tests using the Arcan shear extensometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...27 Failure tests using strain gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...28 COMPLIANT BULK SPECIMENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...28 Problems with strain determination at large deformations . . . . . . . . . . . . . . . . . . . . . . . . 29 Out of plane distortions of the test specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...29 A method for extrapolating large strains from the crosshead movement . . . . . . . . . . . . . 30 Stress/strain behaviour of compliant specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Failure modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..31 STRAIN RATE EFFECTS IN BULK SPECIMENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 JOINT SPECIMENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...33 Features of joint tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...33 Stiff Adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...34 Compliant adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..35 Failure modes in joint specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...36 SUITABILITY OF THE TEST METHOD FOR STRESS/STRAIN TO FAILURE . . . . . . . . . 37 Strain measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...37 Failure modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...37 Reliability of stress-strain curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...38

RECOMMENDATIONS ON THE USE OF NOTCHED-SPECIMEN SHEAR TESTS . . . . . 39 IMPROVEMENTS TO TEST METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...39 USE OF THE TEST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...40 Modulus Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40 Stress-strain to failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...40

ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...41

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...41

TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...44

APPENDIX I ASSESSMENT OF UNCERTAINTIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...56

APPENDIX II CORRECTION FOR DISPLACEMENT OF THE ADHERENDS . . . . . . . . . . . . . . 62

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...64

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TEST METHODS FOR DETERMINING SUITABLE FOR DESIGN.

Part 1: Notched-beam shear and joint test specimens.

MTS Adhesives Project 1

SHEAR PROPERTY DATA FOR ADHESIVES

(Iosipescu) and notched-plate shear (Arcan) methods for bulk

Report No 6B April 1996

By: B C Duncan and G D Dean

1. INTRODUCTION

In order to employ finite element methods for design with adhesives, data are needed on their stress/strain behaviour to failure under well defined states of stress. An important part of this data requirement is satisfied by a measurement of the stress/strain curve under a shear stress. For this purpose, a number of test methods exist that employ test specimens in the form of a bonded joint. For the determination of strain in these tests, it is very difficult to measure the very small displacements in the adhesive with accuracy and reliability. Where bulk specimens of the adhesive can be prepared, there is scope for achieving greater accuracy through the use of a larger gauge length in the specimen for the determination of strain.

In Project 1 of the MTS Programme on adhesives, a number of bulk and joint specimen tests have been developed or improved for the measurement of shear property data for a variety of adhesive materials. Four types of adhesives were selected that exhibit different characteristics in their shear behaviour. One objective of this work was to specify how the accuracy and reliability of data produced by each method can be optimised through features such as the design the apparatus and extensometers, the choice of specimen geometry and the use of correction procedures for sources of error.

In this report, two related test methods are described that can be used for testing either bulk or joint specimens of adhesives. These are the notched-beam shear or Iosipescu method and the notched-plate or Arcan method. In two separate reports [1,2], torsion methods, suitable for both bulk and joint specimens, and the thick adherend shear method are described.

A further objective of this work was to compare results from these test methods obtained on specimens prepared from the same source of adhesive. A variety of different types of adhesive have been studied. It should then be possible to make certain recommendations regarding the most suitable test method for a particular adhesive type. This work should also shed further light on the issue as to whether the properties of bulk specimens of a particular adhesive are representative of the material in the thin layer in an adhesive joint. The comparison of results from the bulk and joint specimen tests is described in a further report [3].

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2. NOTCHED SPECIMEN SHEAR TEST METHODS

The two V-notched shear tests (V-notched beam and V-notched plate) described in this report rely on the same principles, differing only in the method of applying load and the specimen geometry (rectangular beam and ‘square’ plate as shown in Figures 1 and 2). The gauge section of the test piece is the region between the two notches, the rest of the test piece can be considered tabs for applying loads. When a load is applied to either specimen, as illustrated in Figure 1 (parallel to the axis between the V-notches), the stress state in the gauge section is pure shear and uniform except in the regions near the notch roots. The shear stress is ‘to a good approximation equal to the applied load divided by the cross- sectional area of the specimen between the notches (product of the notch root separation and the specimen thickness). The shear strain can be determined in the centre of the gauge section by measuring displacements in the direction of the applied load, in the central region of the gauge section. Test fixtures, described in Section 3, can be made to carry out both shear tests in standard tensile test machines.

The notched-beam test (referred to in this report as the Iosipescu test) and the notched-plate test (referred to as the Arcan test) were developed to test the shear properties of bulk test pieces. Joint test pieces can be manufactured by bonding together two metal adherends along the central axis between the V-notches.

The purpose of the present report is to describe the features of notched specimen test methods in the measurement of the shear properties of polymeric adhesives. The report concentrates on the notched-plate method, using both bulk and joint specimens, which appears to offer advantages over the notched-beam method for lower modulus materials. The performance and utility of the tests are discussed in detail with reference to measurements made on four typical engineering adhesives. Section 2 discusses reports in the literature on notched-specimen shear tests. The experimental apparatus used in the current work, particularly the Arcan test and shear extensometer, are described in Section 3. Section 4 concentrates on the test specimens and materials. Shear modulus measurements made with the notched-specimen shear tests are covered in Section 5 and stress/strain curves to failure are discussed in Section 6. Section 7 describes recommended uses of the tests and potential improvements.

2.1 LITERATURE REVIEW

2.1.1 The Iosipescu test method

V-notched shear tests, the subject of the current report, were first introduced by Iosipescu to study the failure strength of metals and welds in shear [4]. The test specimen adopted by Iosipescu was a short, wide beam with two symmetrical notches cut in the centre of each of the long edges. The specimen is loaded at four points on the specimen edge as shown in Figure 1. Iosipescu reported photoelastic measurements which seemed to confirm that the stress distribution between the notches of the specimen was pure shear and uniform over much of the gauge section [4]. Shear tests using this specimen geometry and loading method will be referred to as Iosipescu tests.

This test seemed to offer a number of advantages, viz:

● compact, easily manufactured specimens

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● uniform shear stress applied to the sample ● shear stress determined simply from applied load and specimen dimensions ● both modulus and strength measurements can be made ● loading fixture can be mounted in a tensile test machine ● the reduced area promotes shear failure in the test section.

The Iosipescu test was adapted for shear tests on other materials particularly composites. It was adopted as a standard method for testing composites in shear by ASTM, becoming method ASTM D30.04 [5]. Many materials tests conducted using the Iosipescu test have been reported in the literature [6-10]. These indicate that the test gives reasonably reliable results in comparison to other test methods. However, some problems have been reported for this test in the following areas:

Deterrnination of shear strain

● The small gauge length makes accurate strain measurements difficult. ● Shear strains on opposite sides of the specimen can differ considerably.

Stress distributions

● Whilst the shear stress distribution is pure shear in most of the region between the notch roots, there are tensile stress concentrations near the notch roots that can lead to early failure [7].

● Stress concentrations are a function of the loading positions, the notch angle, the radius of the notch roots, the notch root depth and, for composites, the material anisotropy [7]. The failure criteria determined through this test will be influenced by the precise specimen dimensions.

● Transverse tensile stresses exist outside of the test section and can cause premature tensile failure outside of the gauge section [7].

● The shear stress in the central section is not accurately equal to the average stress applied to the sample (stress = load/cross section area) and a small correction to the measured data is needed [8].

Difficulties in applying load

● As the load is applied at points on the edge of the specimen compressive failure of the specimen may occur at these locations.

● Loading from the edge may also cause bending of the specimen about its long axis. This would cause different levels of strain on the opposite faces, a phenomenon which has been reported by several groups [7,9]. Adhesives are generally more compliant than the composites for which ASTM D30.04 is designed and it was thought that the rubbery adhesives, such as the polyurethane, would be extremely difficult to test using this method due to bending of the specimen.

● As the specimen is loaded along the edges, a fairly thick specimen is required to minimise these testing problems. ASTM D30.04 recommends a minimum thickness of 3-4 mm. The requirement for thick specimens preparation of bulk specimens of adhesives.

may be difficult to fulfil in the

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.


2.1.2 The Arcan test method

As the Iosipescu test method requires relatively thick test pieces, it was decided to concentrate on another notched specimen test method. The notched plate test was developed by Arcan [11] and is referred to in this work as the Arcan test. The Arcan test also uses two symmetrical notches cut at ± 45° to define the test section. However, as shown in Figure 2, the test piece adopted resembles a ‘butterfly’ rather than a beam. The specimen is gripped in a loading frame and load is applied via the faces thus avoiding the potential instability of edge loading. Thinner test specimens can be used. This test has been used in studies of composites and adhesives [11-15].

The original specimen suggested [11, 12] was large and circular in shape with a notched centre. The outer ring could be gripped in a tensile testing machine with the region between the notches experiencing a shear deformation. The specimen could be rotated to produce combined tensile and shear stresses. Photoelastic studies, in the non-rotated situation, showed that the stress in the region between the notches was, as with the Iosipescu test, predominantly uniform shear [11, 12]. Other studies showed that the shear stress in the centre of the notch section is uniform but there are stress concentrations near the notch roots.

A compact, butterfly shaped test specimen, such as shown in Figure 2, has been adopted by some researchers to simplify specimen manufacture [13, 14]. This has the same geometry as the gauge section of the original Arcan specimen. Clamps as shown in Figure 1 take the place of the outer region of the specimen and are used to mount the specimen in a tensile test machine. Photoelastic and finite element studies have shown that the stress distribution in the compact specimen is the same as in the original Arcan specimen [12]. Figure 2 shows the test specimen adopted for this work.

Although the stress distribution in the gauge section is predominantly uniform shear, stress concentrations and tensile /compressive stresses will be present. The exact distribution is dependent on specimen geometry, particularly around the notch roots. Finite element investigations were performed for the Arcan test for the work reported here and these are discussed in Section 4. The Arcan test has most of the advantages of the Iosipescu test but avoids the difficulties inherent in applying loads via the edge of the test specimen. Therefore, the Arcan test would seem to be better suited for testing compliant materials such as adhesives. It also shares some of the disadvantages of the Iosipescu method as well as introducing other potential problems such as the test piece slipping in the jaws of the grip or the specimen twisting/bending when loads are applied.

2.1.3 Adhesive joint tests I —

Both the Iosipescu and the Arcan test have been suggested as being suitable for studying the shear properties of adhesives [15-17]. Wycherley et al [16,17] used the Iosipescu test as a basis for an adhesive joint test. Two shaped adherends were bonded together with the bondline forming the test section between the notch roots. Shear strain was measured using a strain gauge based shearometer designed and built by the authors. Several stress-strain curves to failure were presented for a flexible adhesive which were reasonably consistent. However, no data on the low strain moduli were given. Weissberg and Arcan [15] proposed that the Arcan test could be used to characterise adhesive joints. Although their work concentrated on fracture of cracked specimens they did demonstrate that the Arcan test could be used to determined stress strain to failure.

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2.1.4 Shear strain measurement

One difficulty in performing V-notch shear tests is the difficulty in measuring shear strains. Strain gauges rosettes with elements mounted at ± 45° are generally used for measuring the strain of bulk specimens in the Iosipescu [5,7,9] and Arcan [14] tests. Strain gauging specimens for testing can be expensive both in terms of the cost of the gauges and the time required to apply the gauge to the specimen.

Previous tensile tests on bulk specimens of relatively stiff epoxy adhesives (TE251 and AV119 with Young’s modulus = 3000 MPa) showed that strain gauges have an appreciable stiffening effect [18]. This stiffening will be magnified with more compliant materials. An additional problem with strain gauges is that the maximum strain that they can withstand is less than 10% which is much less than the breaking strain of many flexible adhesives. Tests have shown that strain gauged epoxy specimens tend to fail prematurely in tension [19]. Adhesive joint specimens with thin bondlines are obviously not amenable to strain gauging. Alternative means of measuring shear strain were explored, and a shear extensometer for the Arcan test capable of measuring both bulk and joint specimens was constructed as part of this study and is described in Section 3.

2.2 SHEAR MEASUREMENTS REPORTED

Data reported in the literature from a number of organisations [6,7,8,14] were further analysed to determine the levels of accuracy of the Iosipescu and Arcan test methods. The repeatability for composite materials with high shear moduli (= 4-6 GPa) reported by Weinberg [6] and Broughton et al [8], shown in Tables 1 and 2 respectively, is acceptable, the 95 % confidence band for the modulus is generally less than 20 % of the mean value whilst that for the shear strength is of the order of 10% or less. Broughton et al reported that for two of the materials, torsion data were available and agreed, to within 10%, with the Iosipescu data.

Wilson’s data [7], shown in Table 3, indicate that the inter-laboratory reproducibility is not as good as this, the 95 % confidence band for the modulus being around 20-30 %. whilst the reproducibility y in shear strength ranges from 5 to 30 %. Weinberg’s data [6] on polymer specimens indicate that the repeatability in modulus values for low modulus polymers can be much worse than for composites, showing 95 % confidence bands of 50 % or higher. The data on shear strengths were less scattered.

Voloshin and Arcan [14] reported moduli for a fibre reinforced material using the Arcan test, the data are summarised in Table 4. The moduli measured agreed well with the theoretically expected value of 3.48 GPa. The 95 % confidence bands for-the measurements are around 10% which is comparable with the data from the Iosipescu tests.

The tests reported using. V-notch shear tests have concentrated on ‘stiff’ composite materials with shear moduli greater than 2 GPa. The uncertainties in the moduli and shear strengths reported are of the order of 10 to 20 % of the mean values. None of the reports attribute these uncertainties to materials variability and the precision of these methods appears to be much poorer than is generally available from standard tensile tests. The only data reported using Iosipescu measurements for materials with similar stiffness to adhesives show even poorer precision. These experiments indicate that there may be problems achieving accurate data for adhesives using the Iosipescu test.

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3. EXPERIMENTAL

3.1 APPARATUS

3.1.1 Iosipescu test

. .

The Iosipescu test was carried out according to the test procedure of ASTM D30.04 [5]. The loading arrangement of this testis shown in Figure 3. This was installed in an Instron 4500 universal testing machine. The applied load was measured by a load cell located in the crosshead of the test machine. Load was applied via the edges of the specimen and a shear stress is established in the central region of the specimen between the notches. Strain measurements were made using ± 45° strain gauges which were bonded on each side of the specimen, at the centre of the area between the notches (Figure 4). The two gauges were wired together as a half bridge to average the strain signal from each side of the specimen. Data for individual faces were not obtained. The load and strain data were recorded using a PC running Instron’s Series IX mechanical test software which calculated the shear modulus of the material.

The modulus of each specimen was determined several times from slopes of stress against strain between 0.05 and 0.25 % strain. The strains were kept small (< 0.5 %) at all times so that applied deformations were completely reversible. Once repeatable moduli data, consistent with the expected values calculated from tensile tests, had been measured each specimen was loaded to failure. From the resulting stress-strain curves yield and break points could be identified.

3.1.2 Arcan test

The Arcan test was performed using a specially manufactured loading jig which could be mounted in a tensile testing machine. This jig is shown in Figure 5. The specimen is clamped between two sets of grips which have interior faces that have been impregnated with diamond dust to improve the grip on the specimen. Adaptors connect the grips to the testing machine pull rods.

The specimen is clamped into the loading jig using a rigid frame to align the specimen and grips (Figure 6). The rear halves of the upper and lower grips are fitted, over the locating posts, onto the frame. The specimen is placed on the grips, with the holes aligned with the bolt holes in the grips. The front halves of the grips are then fitted and the entire assembly is fastened using four bolts (B1) which fit through the four holes in the specimen and corresponding holes in the grips. Each bolt is tightened to the same tension (25 mNm-l) using a torque driver. A small rectangular support frame is bolted to the loading jig, using 4 long thin bolts (B2), to support the jig when it is placed in the testing machine.

The jig is then lifted off the locating posts, the adaptors are fitted using pins (Figure 5) through the holes used to secure the jig in the clamping frame. The jig is mounted in the testing machine by connecting the adaptors to the pull rods. When the jig is in place the support frame is removed and two ‘tuning fork’ spacers are used to centre the test jig in the adaptors.

Moduli measurements were performed by pulling the loading jig in tension causing shear strains and stresses in the specimen in the region between the notches. Tests were stopped

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at small strains (around 0.5 % for brittle specimens and 17. for ductile specimens) for which the deformation was completely reversible. Test speeds were chosen to maintain consistency in measured strain rates between different tests and materials. Typically strain rates of 1 % strain per minute or 4 % strain per minute were chosen.

The load experienced by the test piece was measured using a load cell connected to the top pull rod. In the tests reported here, two load cells with measurement ranges O -1000 N and 0-5000 N were used. The 1000 N load cell was used when measuring modulus, particularly of compliant specimens (eg the polyurethane or the acrylic), where the loads were small and high sensitivity was required. The 5000 N load cell was used when measuring stiffer specimens where loads were greater and sensitivity not so important and when performing shear tests to failure. The shear stress was calculated from the applied load divided by the cross-section area of the specimen between the notch roots (the product of the distance between the notch roots and the specimen thickness).

Strain was measured in the region between the notches using initially strain gauges (as in the Iosipescu test) but for the majority of tests a specially designed extensometer was employed (described in section 3.2.2). The strain from both faces was recorded and averaged to obtain the shear strain. Both bulk and joint adhesive specimens can be measured using the method outlined above. However, for joint specimens the measured strains may need correction for the deformation of the adherends.

3.2 SHEAR STRAIN MEASUREMENT METHODS

3.2.1 Strain Gauges

The strain gauges used in the shear test measurements are more fully described in an earlier report [1]. Briefly, strain gauges for measuring shear properties are available as 90° biaxial rosettes consisting of two strain elements mounted at 90° to each other [18,20]. They are mounted on the centre of the v-notched test piece at ± 45° to the direction of loading as indicated in Figure 4. The shear strain is deterrnined from the sum of the two tensile strains generated at* 45° to the line joining the notch roots (ie in the directions of the strain gauge elements).

3.2.2 Description of the extensometer for the Arcan shear test

Shear extensometers have been described in the literature [17,21,22,] for the thick adherend shear test and the Iosipescu test and used with mixed effectiveness. The relative displacements of points/knife edge contacts on either side of the centre of the shear deformation of the test piece are measured in the direction of the applied load and used to determined the shear strain.

A shear extensometer for the Arcan test was designed as part of this work. This also determines the relative displacement of points on either side of the gauge section. However, the transducers and the support for the extensometer are remote from the specimen which minimises any potential effect of the extensometer on the testpiece behaviour. The Arcan shear extensometer uses sensitive displacement transducers rather than strain gauges to monitor displacements.

The Arcan shear extensometer constructed for this work is shown in Figures 7, 8 and 9.

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. .


Figure 9 shows the extensometer attached to the lower Arcan grips, one of the upper grips has been omitted for clarity. Each extensometer measures the vertical displacement of two points (a and b) on either side of the centre line of the specimen. The horizontal separation of the points ab is small, 3 mm or less, so that strain is measured in the region of uniform shear stress. The difference in measured displacements (d, - d~, experienced by the two points is the shear displacement (d). The shear strain (E) measured by each extensometer is calculated from the shear displacement divided by the extensometer gauge length 1.

d (da-db) e=—=

1 1 (1)

The extensometer gauge length 1 is defined for a bulk specimen test as the separation of the needles on the surface of the specimen (generally about 3 mm) and for a joint specimen test as the bondline thickness (around 0.5 mm). Shear displacements in joint specimen tests are much smaller than in bulk specimen tests and the accuracy of the strain measurements will be less.

The extensometer shown in Figures 7, 8 and 9 consists of a pair of levers and displacement transducers with associated signal processing (section 3.2.3). The levers move independently. Contact between the extensometer and the points (a and b) on the specimen being measured is made by the needle points (N) of the extensometer levers as shown in Figure 8. The two needles are close together on the specimen to restrict measurements to the region of uniform shear stress (Figure 9).

Each of the levers is pivoted using a precision bearing (P) and slack in the pivot should be minimal. The thumbscrew (T) through the lever on the opposite side of the pivot contacts the head of a displacement transducer. The hinge (H) and the spring (S) allow the levers to move back and forward independently. The spring provides the force between the needle point and the specimen. This can be adjusted by varying the height of the screw. If the contact force is sufficient then the needle will not slip on the surface of the specimen. The hinge and spring allow forward movement of the lever to maintain contact as the deflection of the lever increases.

When the specimen is deformed by applying a shear stress, the vertical distances between the measurement points and the lower grip, on which the extensometer is mounted, increase. The needle contacts follow the movement and the extensometer levers pivot through a small arc. The displacement transducer measures the vertical deflection of the opposite end of the lever. For each lever, this measured displacement (d~) is related to the true displacement of the specimen (d,) by the lever ratio for the particular lever used (k). .

d, = kin (2)

The lever ratio (k) is the ratio of the distances between the centre of the pivot and the needle point and transducer contact (see Section 3.3.2 and Appendix I), this is generally adjusted to be one.

The pins at the base of the extensometer locate it on the lower grip of the loading jig (Figure 7). The extensometer is secured in place using a screw fitting. The cam is used for engaging the extensometer. When the cam is back the needle points do not contact the specimen. The extensometer is fitted to the test jig with the cam in the back position. When the cam is

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moved to the forward position the needles are brought into contact with specimen. The securing pin fits through both levers, holding them in position. This is removed after the needles are positioned on the specimen to allow free rotation about the pivot.

The thumbscrews are the contact between the extensometer and the displacement transducers. These can be adjusted to set the extensometer outputs. Counterbalance springs between the transducer end of the levers and the base of the extensometer compensate for the upwards force on the levers from the springs in the displacement transducers.

Two extensometers are used in the Arcan test to measure the strain on opposite faces of the test specimen. The reliability of the test is much improved if the average of the strains is used as the shear strain. In this arrangement four levers, each with its own lever ratio, and four displacement transducers are required.

3.2.3 Uncertainties in shear strain measurements made with the extensometer

There are several sources of uncertainty in the measurement of shear strains using the current design of extensometer. In bulk specimen tests, at large strains the specimen deforms to a severe extent and there are many problems when measuring strains. The problems of measuring strains at large deformations are discussed in section 6.2.

The lever arm of the extensometer describes an arc while measuring displacement. The levers will move forward as the displacement increases but the displacement transducers are fixed. Therefore, the point of contact will move across the head of the displacement transducer as the displacements are applied. The heads of the displacement transducers are rounded and it is likely that moving the point of contact on the displacement transducer head will introduce erroneous displacement readings. The errors could be reduced if the heads of the displacement transducers were flat. When the lever is at the zero position and moves only through very small arcs (such as measuring a few % strain) these erroneous displacements will be negligible. However, when the arc is large, measuring large strains, divergence from the true displacement may become significant. It is best to work within small arc movements and reset the extensometer by pulling the cam back, securing the levers using the pin and re-engaging the extensometer if the displacements measured become too large.

The extensometer is sensitive to out of plane movements of the specimen which can arise from small twists of the specimen. These will lead to inaccurate shear displacement measurements. To try and compensate for this, two extensometers are used to determine the shear strain. Each extensometer measures the strain on one face of the specimen. These strains are averaged. In this way it is anticipated that bending or twisting of the specimen will be predominantly corrected for during the test. –

Uncertainties in the measurement of strain arise primarily from uncertainties in the extensometer lever ratios (section 3.3.2), displacement transducer resolution, and the extensometer gauge length (section 3.3.3) Uncertainty analyses for the extensometer in both bulk and joint tests are shown in Appendix I. Uncertainties in the strain measurements are estimated at approximately ± 6 % for the bulk specimen test and ± 7.5% for the joint specimen test. Uncertainties in moduli will be similar. Other uncertainties not readily quantifiable include slippage of the specimen in the grips, twisting of the specimen or in- plane bending.

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3.2.4 Displacement Transducers

The displacement transducers used with the Arcan extensometer were manufactured by Tess Metrology and each unit is comprised of two linear action induction displacement probes with a travel of ± 1 mm which were connected to a two channel Tesamodul length measurement unit which measures the outputs of the probes. The displacement transducer unit could be used on two sensitivities 0-200 pm with a resolution of 0.1 pm and 0 -2000 pm with a resolution of 1 pm and these were used for low strain modulus measurements and high strain tests to failure respectively. The displacement could be read from the display

.— on the amplifier unit and was also available as a DC voltage signal which could be recorded using a datalogging PC. The output of the displacement transducer could be set as a single channel (A or B), the sum of the channels (A+B) or the difference between the channels (A-B or B-A). It was not possible get the output from both channels (A and B) simultaneously,

. . and so, the difference output (A-B) was measured during tests. This is a limitation when assessing the test method, a better system would allow the A and B channel of each extensometer (ie 4 channels in total) to be read simultaneously.

3.2.5 Data recording equipment

The test data for the Arcan test was collected using Instron’s Multi Axis data acquisition software (MAX) running on a PC. The software could record signals from up to 8 channels simultaneously at collection rates of 5 points per second. Input channels were configured for each range of each measurement device used (load cell and extensometers). This was done by setting known input signals to the data acquisition system and calculating scaling factors to convert the voltages read into engineering units (eg N, mm, strain etc). The data recorded was analysed using Microsoft Excel v4.0 spreadsheets.

3.3 CALIBRATION OF SHEAR EXTENSOMETERS

3.3.1 Tess displacement transducers

The Tess displacement transducers used with the Arcan extensometer were calibrated against a set of inspection grade A gauge blocks. The transducer was clamped into a rigid stand, gauge blocks of different thickness were placed between the transducer head and the stand base and the displacement reading was noted. The slope of reading against displacement for each transducer was within 0.5 % of 1.000 V per mm on both sensitivities.

3.3.2 Arcan extensometer lever ratio

The lever ratio of each of the Arcan extensometer levers was-determined, using a travelling microscope accurate to 0.01 mm, by measuring the lengths of the two halves of the lever. Figure 8 shows the points on the extensometer used to define the lever lengths.

length 1. the point of the needle (N) and the centre of the pivot (P), and; length 2. the centre of the pivot (P) and the centre of the thumbscrew forming

the displacement transducer contact (T).

The position of the needle in the lever was adjusted to give a lever ratio (1/2) close to 1.00. — The lever ratios of both levers on each extensometer were set to be equal to within at least

0.5 % of each other. Uncertainties in the measurement of strain due to uncertainties in the

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lever ratio are estimated at approximately 1.5 % (Appendix I).

3.3.3 Arcan extensometer gauge length

In bulk specimen tests the extensometer gauge length is the horizontal separation of the needle contacts on the specimen. It is not possible to determine this when mounted on a specimen so it was measured for both extensometers before each test. This was done with the aid of a blanking cap which could be attached to the base of each extensometer which gives a flat surface at the same distance as a specimen. Adhesive tape was placed on the face of the blanking cap. The extensometer needles were brought forward onto the blanking cap using the cam and the holding pin was removed. The extensometer needles make marks on the tape corresponding to the positions that they would take up on a specimen. The distance between the marks on the tape was measured using a travelling microscope accurate to 0.01 mm. It was assumed that carefully mounting the extensometer with the holding pin in place would prevent the needle points changing separation. Analysis of point separation measurements made during tests showed that the extensometer gauge length is usually repeatable to around ± 0.15 mm (around ± 5% with a 3 mm separation). Altering the design of the extensometer to improve the repeatability of the needle separations would increase the reliability of the test data.

In joint tests the extensometer gauge length is taken as the bondline thickness in the centre of the joint specimen. The uncertainty in this distance is the uncertainty in the measurement of the bondline thickness. This is typically around ± 5 % which is comparable to the uncertainty in the extensometer gauge length in the bulk specimen test.

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4. TEST SPECIMENS

4.1 BULK SPECIMENS

All the bulk adhesive test specimens were cut from sheets of adhesive prepared by methods developed earlier in the project [23]. Iosipescu specimens were manufactured to the standard dimensions specified in ASTM test method D30.04 [5], all specimens were between 4.0 and 4.5 mm thick. The test specimen is shown in Figure 1. Strain gauges were fixed to the specimen between the notches on both faces.

The bulk Arcan testpiece is shown in Figure 2. The notch radius of 1.5 mm was selected with the aid of finite element calculations to minimise corrections to the shear stress in the central region, see section 4.1.1. Test specimens were between 2 mm and 4 mm thick.

Shear strains were measured in the centre of the specimens. No strain measurement devices were placed within 2 mm of the notch roots. The gauge length of the extensometer is the separation of the needles. Shear stress is defined as the applied load divided by the cross- sectional area of the specimen between the notch roots.

The four bolt holes are to locate the bolts for clamping the specimen. The specimen is gripped by the faces with the bolts merely supplying the pressure to the jaws. The location of the four bolt holes through the specimen is critical, wrongly located holes can make loading the specimen impossible. In some cases, where the misalignment is small, the specimen can be loaded into the test machine with difficulty but there is the possibility y that the specimen is prestressed and this could lead to premature failure.

4.1.1 Finite element analysis of bulk Arcan specimen

Finite element calculations were performed by TWI using ABAQUS v5.3 FEA software. This section summarises the analysis performed. The shear stress distribution for a bulk testpiece with the dimensions indicated in Figure 2 having 1 mm radius notch roots is shown in Figure 10. The shear stress is uniform in the centre of the specimen between the notch roots (A and B). Stresses near the notch roots are slightly greater than in the centre of the specimen. Stress levels in the 2 mm notch radius testpiece are slightly greater along the vertical centre axis (line AB) of the specimen. This is shown in Figure 11 which shows shear stress plotted against distance along AB for 1 mm, 1.5 mm and 2 mm radius specimens.

The shear stress is zero at each notch root, peaks near each root and decreases slightly to a broad minimum at the centre of the specimen. The decrease is less (and the peak less pronounced) for larger notch radii. The shear stress at the centre of the specimen is slightly greater than the nominal mean shear stress of 100 MPa calculated from the load divided by cross-section area (length AB x thickness of specimen). The increased shear stress is dependent on the root radius - 7.5% at 2 mm, 4.5% at 1.5 mm and 1% at 1 mm.

The stress slowly decreases in the direction perpendicular to the central vertical axis. This is illustrated in Figure 12 which shows shear stress plotted along the central horizontal axis (CD). As any strain measuring device will occupy a finite area, a correction is necessary to give the mean stress within the area in which the strain is measured. The 1.5 mm radius notch root was chosen for the test specimens used in this research as the shear stress correction for the region measured by an extensometer (with a 3 mm gauge length) or strain

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gauge (around 3 mm wide) mounted in the centre of an Arcan specimen, taking into account the stress distribution about the centre line, is negligible.

The principal stress distribution (combination of shear and tensile stresses) in the test piece was also calculated. Figure 13 shows that the stress distribution in the centre of the specimen is uniform and comparable with the shear stress alone shown in Figure 11. However, close to the notch roots the principal stress increases significantly. Figure 14 shows the principal stress calculated along the notch edge for a 1 mm notch radius specimen. There is a peak in stress (ca 250% of the nominal shear stress) about 2.5 mm from the notch root. This peak stress is due to tensile stress concentrations and, while having negligible effects during low strain modulus determinations, is liable to lead to failure of the specimen at this point.

At low strains the shear stress distribution in the centre of the specimen is uniform and by choosing a 1.5 rnm notch root radius the need to apply corrections to the measured shear stress is minimised. However, at high strains the stress concentrations near the notch root become important and the test is likely to be terminated by tensile failure criteria.

4.2 JOINT SPECIMENS

4.2.1 Preparation of joint specimens

Arcan joint specimens were manufactured from two symmetrical metal adherends, each forming half of the arcan specimen as shown in Figures 15 and 16. Each adherend was 19.75 ± 0.1 mm wide to give a nominal bondline thickness of 0.5 mm. Adherends were prepared with bond lengths of 5 mm or 10 mm. All adherends were around 6 mm thick. Most joint specimens were made using steel adherends to minimise deformation of the adherends in the test although a few were prepared with aluminium adherends to improve the gripping of the specimen. The bonded faces of the adherends were grit blasted and solvent washed prior to bonding (Figure 15).

Initially, specimens were manufactured using a specially constructed jig with PTFE spacers which are bolted into the jig to define the bond thickness. One adherend was mounted in the jig and clamped firmly in place against the spacers. Adhesive was applied to the face. The second adherend was then fitted into the jig and pressed against the spacers, squeezing out any excess adhesive, and clamped into place using shims to prevent any movement. The joints were cured in the jigs and released when cure was complete.

This method of joint preparation proved somewhat unreliable in practice. There were problems in aligning the PTFE spacers which lead to many instances of non-uniform (wedge shaped) bondlines. These misaligned specimens were difficult to mount in the Arcan loading jig as the bolt holes did not correspond to the locations in the loading jigs. Misaligned specimens bonded using rigid adhesives (either epoxy) would either break when clamped firmly into the grips or fail during low strain modulus measurements.

To avoid these alignment problems, an alternative method of joint preparation was devised. The jig shown in Figure 16 was constructed and consists of a metal base with 4 bolt holes, corresponding to the location of the clamping bolts on the Arcan loading stage, was constructed. One adherend is fixed in place, using two tapered bolts to align the specimen. Adhesive is applied to the surface to be bonded. The second adherend is pushed into

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. . . .

. NPL Report No CMMT(B)56

.-..

. .

—

position and fixed in place using another two tapered bolts. After the specimen is cured, the bolts are removed to release the specimen. The alignment of specimens prepared in this way is acceptable and a high proportion of specimens produced thus can be tested.

After the joint was cured (and post-cured where applicable) excess adhesive was cleaned off the joint specimen face by mechanical grinding. To achieve a better grip during testing, the faces of the adherends were roughened by shot blasting with special care being taken to mask the bondline to avoid damage. Excess adhesive at the ends of the bondline (at the notch roots) was removed using a round needle file to create a concave notch root. Finite element calculations suggested that this reduces stress concentrations and is the most appropriate specimen geometry (Section 4.2.2).

4.2.2 Finite element analysis of the Arcan joint specimen

Finite element calculations to determine stress distributions in the adhesive joint specimens were performed using ABAQUS v5.3 by TWI. The results are more fully discussed by Dickerson [24]. The overall geometry of the testis shown in Figure 15. The finite element mesh and regions analysed are shown in Figure 17. Calculations were performed for flat, concave and convex end fillets with bondlines of 5 or 10 mm. Most of the analysis was performed assuming flat end spew fillets.

Figure 18 shows shear stress distributions calculated along the length of the bond at the centre and at both the edges of the bondline, for flat end fillets. This indicates that there is a uniform stress distribution in the centre of the specimen but at the very ends of the bondline, at the adhesive-adherend interface, there are local shear stress concentrations. The size of the stress concentration predicted is dependent on the mesh geometry chosen, Figure 19. The shear stress in the centre of the bond is slightly greater than at the interface with the adherends. Figure 20 shows that the size of the stress concentration region, assuming flat fillets, is the same for 5 mm and 10 mm long bonds.

The shear stress concentration is dependent on the geometry of the adhesive fillet at the end of the bond. Figure 21 suggests that a concave fillet minimises the shear stress concentrations.

Analysis of the X-X tensile (direct) stresses, as shown in Figure 22, predicts tensile and compressive peel stress concentrations occurring near the ends of the bonded region at the interface between adhesive and adherend. Tensile peel stresses further than 0.3 mm from the end of the bond are negligible. Figure 23 shows that the shape of the adhesive spew fillet geometry is important in determining the peel stress distributions. The peak peel stress is much lower in the case of the concave spew fillet than in the case of the flat or convex fillets. Figure 24 shows that there is no difference between the stress concentrations for 5 mm and 10 mm long bonds.

The FEA calculations indicate that the shape of the adhesive fillets at the ends of the adhesive bond should be concave to minimise stress concentrations and reduce premature failure of the specimen. The end peel stress will give rise to high peel forces between the adhesive and adherends and may be larger than the shear stress. It is likely that the joint may fail through a peel mechanism due to the high X-X tensile forces pulling the joint apart at the ends rather than through failure in the adhesive.

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4.2.3 Joint test requirements

The dimensions of the joint specimen need to be measured accurately. The distance between the notch roots was measured using a travelling microscope. Measurements were made along the bondline on either face of the specimen and the results averaged to give the bondline length. Where the two sides differed by more than 0.1 mm, the excess adhesive was re-filed to equalise the distances. The thickness of the joint was measured at the bondline using a micrometer accurate to 0.01 mm. The bondline thickness (nominally 0.5 mm) was measured at 3 places along the bondline (the centre and either end) on each face of the specimen (6 measurements in total) using a traveling microscope. The average thickness of the bondline on each side was used as the nominal gauge length for the extensometer. The uncertainty in the measured bondline thickness was typically 0.02-0.03 pm which for a typical 0.5 mm thick bondline corresponds to an uncertainty of approximately ± 5 % in the gauge length.

The shear strain in a joint specimen is determined from the displacement of the two bond edges in the direction of the applied deformation. To a first approximation this is the shear displacement measured by the extensometer divided by the bondline thickness. The shear

—

stress is defined as the applied load divided by the cross-sectional area of the adhesive bond (the product of bond length and adherend thickness).

As the extensometer needles make contact in the metal adherends, a finite distance from the edge of the bondline, they will measure deformations of the adherends between the points in addition to the deformation of the adhesive. The majority of the joint specimens were constructed using steel which has a shear modulus of 27 GPa which is much greater than that of any of the adhesives which have maximum shear moduli around 1 GPa. The deformation of the adherend should be much smaller than that of the adhesive bondline. A method for correcting measured strains for the deformation of the adherends assuming a simple elastic model is shown in Appendix II. For the case of an epoxy specimen the true stain is typically 5-7 % lower than the measured strain and the modulus is correspondingly higher. The corrections for the two compliant adhesives were negligible (less than 1 %) and data for these are quoted without correction. .-

4.3 MATERIALS

The materials used in this work are shown in Table 5. These consisted of 4 structural adhesives and 2 general purpose plastics. Materials were identified by a 3 character code, all sheets of material manufactured were numbered and individual specimens cut from the sheet identified by a final letter. Thus two specimens made from the same material having the same number in their identifier originate from the same-sheet. This is important when comparing specimens cut from the same sheet.

The adhesives used in this study were supplied, with the exception of the polyurethane, in the same form as in the earlier work [23]. The properties should thus be comparable, within materials variation, with the tensile measurements reported earlier [19].

The polyurethane adhesive 3M 3532 was not available in the form used previously (1 litre tins of each part which were poured into a piston driven dispensing and mixing machine) and the only available packs (4 oz tubes) were unsuitable for the preparation of bulk specimens. The manufacturer (3M) was able to supply four 400 ml twinpack cartridges of

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-.

. -.’

. . . .

.

. . .

an equivalent adhesive Scotchweld DP 609, an alternative name for the 3532. However, batch numbers were not visible on the tubes and it is not known whether the material supplied represents a single batch. Each cartridge was individually identified with a three letter code (HPC, HPD, HPE and HPF) so that specimens manufactured from individual cartridges could be identified. The polyurethane could be mixed and dispensed in the same way as the 2-part Epoxy TE251 and bulk specimens were manufactured using the procedure developed for the TE251 [23].

The data for the polyurethane measured in this work are not directly comparable to the previous tensile measurements, even allowing for the different packs supplied, as the storage and test conditions are different. The tensile tests reported previously were performed at a temperature of 22°C on specimens stored under 507. relative humidity. The shear and tensile tests reported here were performed on specimens stored under 0 % relative humidity and the tests were performed at 23 °C, conditions which could be maintained more reliably. It is known [19] that the mechanical properties of the polyurethane are very sensitive to moisture content, temperature and test rate.

Test specimens of the 2-part epoxy Evode TE251 were made from a single batch of material. The l-part epoxy Ciba AV119 was supplied in three batches but tensile tests showed that variation between the batches was insignificant. Although early tensile and shear tests used specimens made from the epoxy adhesives and stored at 50 % relative humidity, the moduli measured using the Arcan shear extensometer should be comparable as the effect of adsorbed water vapour on the modulus of these materials at room temperature is small (less than 5 %) in comparison with the variation in materials properties.

The acrylic Permabond F241 was supplied in three batches. The variations between tensile specimens prepared from the same batch of material were large and it was not possible to ascertain whether there is a significant batch to batch variation. Tensile measurements show large differences in the Young’s moduli of sheets manufactured at different times (ranging from 400 to 1000 MPa). The moduli of specimens manufactured with the same thickness at the same time were relatively consistent. The modulus seemed to depend on the specimen thickness although no definite conclusions could be drawn [19]. The acrylic F241 is the has the shortest cure time of the 4 adhesives studied at 5-10 minutes whereas the other adhesives take between 1 hour and 24 hours to cure. The heat generated by the exothermic polymerisation reaction is large. Therefore, the acrylic F241 will be the most difficult adhesive to cure reproducibly and large variations in material properties can be expected. The acrylic adhesive is most likely to show variations in properties with specimen thickness.

The two plastics (acetal and polypropylene) were supplied by RS components. A single sheet of each was received from which 4 mm thick shear and tens-de specimens were cut.

The ranges of Young’s moduli (E) and Poissons ratios (v) for these materials, determined using tensile tests [18,19] on bulk specimens, are shown in Table 5. For small strains the shear modulus (G) can be calculated from the tensile modulus and Poisson’s ratio:

G= E (3) 2(1 +U)

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Shear moduli for each material expected from the tensile test data are shown in Table 5. These can be compared with the data obtained from the shear measurements for evaluation of the test methods. As the properties of individual sheets of adhesives can vary, better comparison of the test methods can be made using different test pieces cut from the same sheets and stored under the same conditions.

5. EXPERIMENTAL MODULUS MEASUREMENTS

The experimental study concentrated on the Arcan test and the shear extensometer. Some illustrative Iosipescu and the Arcan test measurements, using bulk specimens of the epoxy adhesives, were made using strain gauges. The expected limitations of the bulk Iosipescu test and strain gauges for testing adhesives have been discussed earlier. In principle joint specimen tests could be performed using the Iosipescu method but there was no shear extensometer available for this loading stage.

Each modulus test consisted of a number of measurements performed by cycling the specimen between zero strain and a chosen low strain (typically 0.5 % to 1 %) with the mounting of the specimen and extensometer undisturbed between each measurement. When the mounting of the specimen or extensometer was altered a new test was considered to have started. Each individual measurement was recorded and the modulus calculated. The modulus for each test was determined from the mean of the measurements made during the test. The uncertainty (at the 95 % confidence level) was calculated from the standard deviation of the measurements.

The moduli for the epoxy and acrylic adhesives and the two engineering plastics were calculated from the slope of stress against mean-strain readings at low strains. Strain rates around 1 % strain per minute were used. Applied strains were limited to less than 1 % to avoid irrecoverable deformations. The tensile moduli were calculated from the stress/strain curve between 0.05 % and 0.25 % strain as specified in ISO 527-1 [25]. The straightest region of the stress/mean-strain curve was chosen to calculate shear modulus. The region between 0.05 % and 0.25 % strain was the preferred region but in many cases extreme curvature or discontinuities in the measured curves meant that the modulus had to be calculated in higher strain regions.

The polyurethane adhesive is a strongly viscoelastic material and the stress/strain plot, at low strain, has a high degree of curvature. Therefore, rather than calculate moduli from the slope of the stress/strain curve the secant modulus was calculated from the slope of a line joining the origin to the stress/strain curve at 1 % strain (ie modulus = stress divided by strain at 1 % strain). This analysis was used for both tensile and shear measurements. The modulus of polyurethane was determined at a strain rate of 4 % per minute.

The materials are divided into two classes for the purposes of discussions. The l-part epoxy AV119, the 2-part epoxy TE251 and the acetal have shear moduli around 1000 MPa, and are .

referred to as ‘stiff' materials. The acrylic F241, polyurethane and polypropylene have shear moduli between 50 Mpa and 500 MPa, and are classed as ‘compliant’ materials.

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5.1 TESTS ON BULK IOSIPESCU AND ARCAN SPECIMENS USING STRAIN GAUGES

Measurements of the shear properties of bulk adhesive specimens using strain gauges were made only on the stiffest adhesives, the two epoxies (TE251 and AV119). These shear specimens had been stored at 50 % relative humidity prior to testing and their moduli could be up to 5 % less than would be the case had the specimens been stored dry. The shear moduli measured for Iosipescu test specimens are shown in Table 6. The shear moduli measured using strain gauged Arcan specimens are shown in Table 7. Typical stress strain curves measured using Arcan test specimens ofAV119 and TE251 are shown in Figures 25 and 26 respectively.

Previously [18] it has been shown that strain gauges have a stiffening effect on bulk adhesive specimens, increasing the measured modulus by up to 20%. Test data from the strain gauge types known to have the largest stiffening effects have been omitted and the data presented has not been corrected for the stiffening by the strain gauges (assumed to be less than 5%). The modulus values determined from the two tests show good repeatability, with the data obtained from tests on a single specimen agreeing very well. The standard deviations in the measured moduli of the strain gauged specimens are low. The shear moduli measured from both test methods agree well for both of the materials and are within the ranges expected from the tensile test data shown in Table 5.

The stress-strain plots shown in Figures 25 and 26 are typical of those obtained by modulus measurements at low strain. Each figure shows the stress-strain curves determined from each face of the specimen together with the stress-mean strain curve. The plot of stress against mean strain is smooth and linear to the origin. The data from the individual sides is not as linear and the individual strains can differ by up to ± 20 % of the mean strain (at 0.5% strain). At higher strains the magnitude of the differences between the strains stops increasing, the stress-strain plots for the individual faces become parallel and the relative differences between the faces are less. Similar differences between the strains determined on different faces are also encountered in tensile tests. It is usually possible to reduce the differences to less than ± 5 % of the measured strain, by remounting the specimen.

Experiments conducted using different orientations of the loading jig in the testing machine showed that each extensometer was able to produce strain measurements greater or less than the mean strain. Moduli calculated from the mean-strain data were independent of the orientation. The difference between the strains experienced by the individual faces is therefore not due to differences in the performances of the extensometers but maybe caused by in-plane bending of the specimen. This effect should be compensated for by averaging the strains. For an individual specimen, the modulus determined from the mean strain differs little between tests where there are large differences between the faces and tests where there are only small differences between the faces.

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5.2 BULK SPECIMEN MODULUS MEASUREMENTS MADE USING THE ARCAN SHEAR EXTENSOMETER

Moduli measurements made using bulk specimen tests and the Arcan extensometer are summarised for each material in Tables 8 to 12. All the data were determined from the mean of the strains measured by the extensometers. The data and uncertainties quoted represent the mean and 95 % confidence interval calculated from the standard deviation for the measurements of modulus made during each test. Where there is more than one value quoted for a specimen, the data represent measurements made in different tests where the specimen has been removed from the test jig and re-mounted between batches of measurements. The shear moduli calculated from tensile tests on specimens cut from the same sheets as shear specimens are also included in these tables for comparison. As some of the adhesives (eg the acrylic) show large variations in tensile moduli between different sheets, comparisons are best made between shear and tensile tests performed on specimens cut from the same sheet.

5.2.1 Stress/strain curves obtained during modulus measurements

Although the stress against mean-strain curves were generally straight lines, there was often poor agreement between the strains measured on each face of the specimen. Figure 27 shows well behaved stress-strain curves determined during moduli measurements using the epoxy adhesive AV119. Such well behaved curves were not always obtained, Figures 27 to 30 show typical stress strain curves for all four of the adhesives.

Figure 28 shows a modulus measurement on a specimen of the TE251 epoxy adhesive. The slopes of the stress-strain curves for the individual sides of the specimen differ considerably, The moduli calculated from these curves are shown on the plot, there are significant differences between the two extensometers. As the measured curves are not linear at low strain (the strain reading on one side goes negative), moduli were calculated from the curves at higher strains (0.2 to 0.5 %).

A modulus measurement for the acrylic F241 is shown in Figure 29. The discontinuity around 0.2% strain is a typical feature of Arcan tests on low stiffness test pieces. Beyond this discontinuity, the stress-mean strain curve is strait and the individual extensometer readings are reasonably parallel. The moduli recorded on the plot were calculated between 0.3 and 0.6 % strain.

A similar measurement on polyurethane is shown in Figure 30 and, again, there is a discontinuity in the stress-strain curve, This material is highly viscoelastic and the stress- strain graph is very curved. Following the analysis of the tensile tests [19], the modulus is defined as the secant (ie stress divided by strain) at 1% strain, these are recorded on the figure. The discontinuities in the stress-strain curve are repeatable for a series of measurements in a single test on a specimen but, as Figure 31 shows, not reproducible between different tests on a single specimen. The discontinuity has a significant effect on the modulus determined in a test and variations in the location of the discontinuity will lead to uncertainties in the modulus determined.

A plot of strain against time measured in the test in Figure 30 is shown in Figure 32. The measured stress vs time is shown in Figure 33. Although the crosshead speed is uniform, the rate of strain applied to the specimen is not constant. At some points during the test

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—

(most often at the very start), very little strain or stress is applied as slack in the loading assembly is taken up. Once all the slack has been taken up, the strain rate settles to a constant value and no further discontinuities occur. However, at low strain the strain rate is not constant which causes problems in testing rate-sensitive materials such as the polyurethane. The strain rate at the same crosshead speed can not be repeated in different tests, reducing the reproducibility of the data. To obtain some degree of comparability between the test data for polyurethane specimens, the cross head speed was set to give a strain rate of approximately 4 % per minute at the 1 % strain level.

The discontinuities in the stress/strain curves, believed to be due to slack in the loading assembly, are a particular problem with the low modulus materials. There is scope for improving the accuracy of the Arcan test by employing a stiffer loading assembly with less slack.

5.2.2 Measured modulus values for stiff materials

Moduli measurements shown in Tables 8-10 for the stiff materials (the epoxies and acetal) made with the Arcan extensometer show a significant degree of scatter. Whilst the repeatability of modulus values obtained from measurements during a test is generally good, the range of modulus values for individual test specimens from different tests can vary by up to 20 % of the measured values. For example Table 8 shows measured values for the modulus of the Acetal specimen 002C of 930 MPa and 1160 MPa. Generally, different tests on a specimen will give values within ± 10 %.

The shear moduli determined for stiff specimens are usually within 20% of the values expected from tensile measurements using specimens cut from the same sheets. The average of the mean moduli for each of the materials determined from the tensile tests and bulk Arcan tests agree within the measurement uncertainties calculated from the standard deviations. The uncertainty in the Arcan value is around 2-3 times the uncertainty in the tensile value. The average Arcan test modulus for each material is less than the average of the shear moduli calculated from tensile tests by around 3-5 %.

The uncertainties in the moduli calculated from the standard deviations in the measured values are greater than the uncertainties in the measurement apparatus quantified in Appendix I. These differences are most likely due to effects such as specimen twist which are not easily quantifiable and thus excluded from the uncertainty estimation. The magnitudes of the uncertainties are comparable with the test data reported in the literature which were discussed in Section 2.2 and smaller than those measured by the Iosipescu test for materials with similar moduli (Table 1).

5.2.3 Measured modulus values for compliant materials

The data for the compliant materials (polypropylene, polyurethane and acrylic) show higher scatter than the stiff materials. The polyurethane (Table 11) and the acrylic (Table 12) have significantly variable material properties, shown by both tensile and Arcan tests, which may be due to specimens being from different batches of material or experiencing different curing conditions. The variation in moduli measured for single Arcan specimens is high and test measurements for a single specimen differ by up to up to 30%. Restricting the calculation of mean modulus to specimens manufactured from the same material batch as the joint specimens (acrylic specimens M02-376, 377, 379 and 381, polyurethane specimens HPD001

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and HPE001) shows that the average shear moduli calculated from the tensile tests and measured from the Arcan tests agree within the uncertainties,

The relative size of the uncertainties in both tensile and Arcan measurements are generally higher than for the stiff materials but the data are reliable to within ± 20%. This compares favorably with the results discussed in Section 2.2. The uncertainty in the overall average of the Arcan test data for the acrylic and polyurethane specimens is around twice the uncertain y in the overall average shear moduli calculated from tensile data. The data for polypropylene (Table 8) show the opposite with the Arcan data showing more consistency. In common with the epoxy and acetal specimens, the average modulus determined by the Arcan test is less than that determined by the tensile method. The size of the difference is 5-10% of the tensile derived, value.

The moduli for the materials, with the exception of the polyurethane, were determined from the slope of the stress-mean strain curves. The tensile curves, being smooth, allowed determination of the moduli between 0.05% and 0.25% strain as specified in ISO 527-1 [25]. Owing to the poor quality of the stress/strain curves at low strains, the modulus from the Arcan tests was usually calculated at slightly higher strains (eg between 0.2 and 0.5 % or 0.3 and 0.6 % being typical). The viscoelastic nature of the materials could lead to the slope of the stress/strain curve being slightly lower at these higher strains and this may account for some of the discrepancy between the moduli determined by the two methods. The discrepancy between the shear moduli measured in tensile and Arcan tests maybe larger for compliant materials than for stiff materials as the degree of curvature of the stress/strain curves for the compliant materials may be larger and it is more likely that the region where modulus is determined is at higher strain.

The modulus of the compliant polyurethane is the most difficult to determine and this is reflected in the high uncertainties in moduli determined by both tensile and Arcan tests. As the stiffness of these specimens is extremely low, the loads measured during the tests are very small and noise in the load signal is significant. The Arcan stress/strain curves show discontinuities rather than the smooth curves measured during tensile tests. The rate of strain in the Arcan tests is not constant and is another likely source of error. Given these problems, the moduli measured by the two tests are remarkably consistent.

5.2.4 Discriminating between specimens with different properties

The moduli measured using the Arcan test suggest that it is reliable to within ± 20 %. Where moduli differ by more than this the method can discriminate changes in material properties. For example, polyurethane specimens were manufactured from three different cartridges have different properties. Arcan tests show that those produced from cartridges HPD and HPE have similar shear moduli (approximately 60 and 70 MPa respectively). The bulk adhesive specimens manufactured from cartridge HPC have a significantly higher shear modulus (>150 MPa). This value falls far outside the range of moduli measured for the specimens manufactured from the other cartridges. The specimens from cartridge HPC have different material properties to the others. This was confirmed through tensile tests which showed similar differences.

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5.2.5 Influence of specimen thickness

Modulus measurements of bulk Arcan specimens made from the two epoxy adhesives and the polyurethane adhesive at different thicknesses (2 and 4 mm) show no significant influence of specimen thickness on the measured modulus. This is also the case for tensile measurements of these materials [19. This conclusion could not be made for the acrylic as the scatter of modulus values, measured using both the Arcan test and the tensile test, for specimens from different sheets was too large.

5.3 THE MEASUREMENT OF MODULUS OF ARCAN JOINT SPECIMENS

The shear displacements measured when determining the moduli of adhesives in Arcan joints are small. If the bondline thickness is around 0.5 mm then a shear displacement of 5pm is approximately 1 % strain. The accuracy in the measured strain when determining modulus will be thus be lower than in bulk specimen tests where the gauge lengths and, hence, the shear displacements are several times larger.

It was more difficult to achieve an acceptable contact between the extensometer needles and joint specimens than it was for bulk specimens. The metal adherends are harder and the needles do not embed as easily. The needles had to be sharp to prevent them from slipping on the surface of the joint specimens. Extensometer needles for joint specimen tests were manufactured from hardened steel to retain their sharpness for longer periods.

5.3.1 Stress/strain curves

Typical stress-strain curves obtained from modulus measurements using joint specimens are shown in Figures 34-37. The stress-strain curves are similar to those obtained from bulk specimens. Tests on the least stiff adhesives (acrylic and polyurethane) show the discontinuities at low load seen in the bulk tests. The stress/mean-strain curves measured for stiff adhesives (Figures 34 and 35) become smooth at lower strains than those measured for the compliant specimens (Figures 36 and 37). The strains measured by the individual extensometers can differ considerably. This maybe due to difficulties in determining small displacements accurately but could also be caused by the specimen twisting (the extensometer is sensitive to small out of plane movements). The moduli determined by the individual extensometers are marked on the figures. These can be very different and the mean-strain data should be used to obtain more acceptable data.

5.3.2 Measured Moduli

Moduli measured for the Epoxy specimens (nominal strain rate of 1% per minute) are shown in Table 13. These have been corrected for the deformation of the adherends. Sets of moduli measured in different tests on each specimen are summarised by the mean modulus and uncertainty calculated from the standard deviations at the 95 % confidence level. The moduli measured show a high degree of scatter but are comparable to those measured using bulk specimens (around 1000 MPa). The data measured for a series of measurements in a test are repeatable as shown by the relatively small uncertainties in the shear moduli for a single test. However, it is difficult to reproduce results by repeating the test after remounting the specimen. This lack of reproducibility in testing a single specimen is shown most clearly in Table 13 by the tests performed on the specimen HTE 120. Nine test results are presented where the modulus measured varies from 860 to 1100 MPa (a range of

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approximately 25% of the measured value). This is similar to the level of reproducibility determined using bulk epoxy specimens. The mean moduli calculated from the joint tests on the epoxies agree with the bulk Arcan values to within experimental error. The over all level of uncertainty is also similar at around ± 20% of the measured value.

The measured moduli of the acrylic F241 joint specimens are shown in Table 14, these specimens were manufactured at the same time as the bulk specimens M02-376 to M02-381. The moduli were determined from the slope of the stress-strain curves at low strain. Corrections for the adherend deformation for these low modulus adhesives are negligible and the data are presented uncorrected.

The adhesive moduli determined for the acrylic joints (240 ± 80 MPa) are significantly higher than the moduli determined from the comparable bulk specimens (170±60 MPa) despite the wide uncertainties in both values. This is the only adhesive where a significant difference between bulk and joint specimens was seen. The acrylic F241 is the fastest curing of the four adhesives under investigation in the current work. It was the only one of the adhesives for which the conclusion that the tensile properties are invariant with specimen thickness could not be made. Thus, it is likely that the properties of the adhesive in joint specimens are not the same as in the bulk specimens.

The modulus values for the polyurethane joint specimens are summarised in Table 14. The values measured are similar to the values measured for bulk specimens and show comparable scatter. The discontinuities in the stress-strain curves discussed for bulk specimens are also present in the tests on joints. There are similar problems in obtaining a constant strain rate for compliant specimens at low strains.

5.4 SUITABILITY OF TEST METHODS FOR MODULUS MEASUREMENT

V-notch shear tests have been used to determine the shear moduli of plastics and adhesives with moduli between 60 MPa and 1200 MPa. The values are generally consistent within the measurement uncertainties with those expected from tensile test measurements. Comparisons with moduli determined in TAST and torsion tests are needed to complete the evaluation of methods for determining low strain shear moduli.

The Iosipescu test gives reasonable values for the moduli of stiff materials. However, the requirement for thick specimens and the potential problems likely to arise when loading compliant specimens via their edges limit the usefulness of this method for studying bulk adhesives.

The Arcan test gives values for the modulus which are reliable to within ± 20 % for both bulk specimens and joint specimens provided that the average of the stain measurements from both sides of the specimen is used. The reproducibility of modulus for a single specimen is low (± 20%) but compares well with the accuracy of test data reported in the literature. The quality of the measured stress/strain curves, particularly for compliant materials, is poor at low strains. Improvements to the loading assembly and the Arcan shear extensometer could increase the accuracy of this test.

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6. MEASUREMENT OF STRESS-STRAIN TO FAILURE USING THE ARCAN TEST

6.1 STIFF BULK SPECIMENS

Stress/strain curves to failure for the acetal, l-part epoxy AV119 and 2-part epoxy TE251 are shown in Figures 38,39 and 40 respectively. The curves were measured at a nominal strain rate of l% per minute. Data for the epoxies were determined from tests using either strain gauges or the Arcan extensometer, the strain measurement methods are indicated on the figures.

6.1.1 Tests using the Arcan shear extensometer

The stress-strain behaviour to failure measured using the Arcan extensometer gives good agreement for different specimens of each of the three materials. Figure 38 shows that the breaking stress and strain measured for two acetal specimens agree very closely (40 % strain and 55 MPa stress). In each test the extensometer readings agreed well throughout the entire strain range. Figure 39 shows that the AV119 specimens have yield points around 9 % strain and the yield stresses are around 46 MPa and vary by less than 5 %. This level of reproducibility is similar to the reproducibility of the yield points in tensile tests [2]. The breaking strains vary between 15 and 30 % strain, well beyond yield. The individual extensometer readings do not diverge until after the yield point. Figure 40 shows that the TE251 failed very much earlier then the other two ‘stiff’ specimens, generally around 3-5 % strain and have maximum stresses between 20 and 25 MPa. This is the most brittle material and does not yield in any of the bulk Arcan or tensile tests performed as part of this study. The stress-strain curves seem to show more variation in behaviour than the other materials, probably due to the differences in the x-axis scales of the stress-strain plots.

Bulk specimens of both the epoxies having thicknesses of 2 mm and 4 mm have been measured to failure. No significant differences in test behaviour between the two thicknesses were observed.

Failure of the bulk adhesive specimens was initiated at the edge of the notch close to the notch roots. The crack grows rapidly between this point and the opposite side of the specimen. Typical specimens after failure are shown in Figure 41. The crack initiation points near the two notch roots, shown in Figure 42, are at the regions of tensile stress concentrations predicted by finite element calculations. Failure of these bulk adhesives is thus governed by tensile stresses rather than shear stresses.

6.1.2 Failure tests using strain gauges

The three specimens of the epoxy AV119 with strain gauges mounted all failed earlier than the specimens without gauges and all failed before the yield point (Figure 39). The breaking stresses are all lower than the specimens measured using the Arcan extensometer. The epoxy TE251 specimens show better agreement between the two techniques (Figure 40), most likely because brittle failure occurs at low strains well before the material yields even without the gauges. Strain gauges are not suitable for measuring the stress /strain behaviour of adhesives to failure in the notched-specimen shear tests.

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6.2 COMPLIANT BULK SPECIMENS

The compliant bulk specimens were tested to failure at a strain rate of 4 % per minute. The strains to failure were significantly greater than for the stiff materials. The large deformations of the specimens at failure were responsible for a number of problems in the measurement of strain.

6.2.1 Problems with strain deterrnination at large deformations

Measurement of the stress-strain behaviour of the bulk specimens manufactured from compliant materials (eg polypropylene, acrylic and polyurethane) is complicated by the poor performance of the Arcan extensometer at large displacements of the extensometer levers. At very large strains, the arc through which the lever arm moves becomes large and it is unlikely that the response of the extensometer remains linear at large arc angles. Additionally, the displacement transducers may run out of range and the contact force on the needle points will decrease as the displacements increase, eventually losing contact. To counteract this the measured displacements should be kept small. Therefore, during failure tests on the compliant specimens the extensometer was reset while the tests were running. This was done by lifting the needles from the specimen, using the cam, and returning them to the central position (and small displacements) each time some additional 20 % strain was applied to the specimens. Extensometers were also reset when strain readings appeared to deviate. In some tests it was necessary to ‘reset’ the extensometer several times.

Resetting the extensometers in this way may generate changes in the extensometer gauge length as the new contacts on the specimen may not be the same distance apart as previously. There is no current method for determining the extensometer gauge length in situ and, hence, uncertainties in the strain measurement will be increased. Repeated measurements of the gauge length, using the blanking piece, have shown that in normal circumstances each gauge length is usually repeatable to within 5 % of the measured values. When the extensometer has been perturbed to large lever deflections the repeatability will probably be worse.

As the deformation of the specimen grows the extensometer gauge moves relative to the centre of the specimen. The top grip of the specimen is fixed whilst the extensometer is mounted on the lower grip which moves. The point of contact of the needles on the specimen will move down the central notch each time the extensometer is ‘reset’. At sufficiently high deformations (estimated as about 8 mm of relative grip movement or 50 % strain), the point of contact is close to the notch root and not within the centre of the notched region where the shear stress is uniform, depending only on the load and specimen dimensions. The strain is then measured in regions where the stress field is not uniform and will not give an accurate value for the shear strain.

At large deformations the line joining the notch roots rotates relative to the axis of the force, deflected by the diagonally opposite tensile/compressive forces acting along the notch edges. The line joining the roots is no longer parallel to the direction of pull, this is clearly observable by 40 % strain. Thus, the notch roots are offset with respect to each other and the extensometer will no longer straddle the ‘centre line’ of the specimen. The strain measured will no longer be a representative shear strain.

The combined effects of specimen rotation and movement of the extensometer gauge section

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towards the notch roots will take the strain measurement outside the central section of the specimen, at very high strains (greater than 50 Y.). Figure 43 shows a failed polyurethane specimen, the two lines of dots show the location of the extensometer gauge section after each reset. Although the extensometer moves only in the vertical plane, the lines are curved due to the ‘rotation’ of the specimen. The specimen has almost recovered its original shape after removal of the load but at the failure loads the distortion of the specimen would straighten these lines. The final strain measurements are being made beside the notch root rather than in line with it. These strains will be meaningless.

6.2.2 Out of plane distortions of the test specimens

At high strains some of the bulk acrylic and polyurethane specimens appeared to buckle, out of plane, before failure. The out of plane distortion of the specimen could be severe with the top grip of the loading stage deflected by several mm from the plane defined by the lower grips. Generally, no ‘buckling’ is apparent in the first 20 % strain. The ‘buckling’ may be due to slight misalignments between the lower (rigid) pull rod and the upper (’loose’) pullrod. However, the lateral deflection experienced by the pull rod is far greater than any slight misalignment which may exist. Figure 44 shows a failed acrylic specimen which had buckled during testing. The distortion is clearly visible. The finite element calculations performed on the bulk Arcan test specimen did not consider stresses normal to the plane of shear. Stiffer specimens (with higher moduli or greater thickness) will be more resistant to buckling.

To restrain the specitnen from ‘buckling’ a method of clamping the top grip in plane with the lower grip was developed. Two 3 mm thick steel plates were clamped to either side of the top grip using G clamps. The ends of the plates bracket the top of the lower grip preventing out of plane movement. Shim was used to set the gap between the plates slightly greater than the grip widths to avoid clamping the two sets of grips together. The surfaces of the lower grip and steel plates were greased to minimise friction. Modulus measurements from tests with and without this clamping arrangement showed that any frictional forces at low strain were insignificant. The effects of friction on the measured load at high strains are not known. This method, whilst not ideal, does reduce the distortion of the specimen at large strains.

Figure 45 shows stress/strain curves (estimated from extrapolated strains, see section 6.2.3) for bulk polyurethane specimens cast from the same cartridge (HPE). The specimens from sheet 001 were tested without the additional clamps, those from sheets 002 and 003 were clamped to resist buckling. Sheets 001 and 003 were 2 mm thick, sheet 002 was 4 mm thick. The low strain moduli of all the specimens were similar. , However, there are large differences between the stress/strain curves from the two sets of tests (clamped and unclamped). The clamped specimens experience higher stresses than the unclamped specimens throughout the measured strain ranges. The clamped specimens fail at slightly higher stresses than the unclamped but at lower strains. The 4 mm thick specimen showed much less tendency to buckle than the 2 mm thick specimens, probably due to its greater stiffness against out of plane bending. The differences between the two sets of curves may be due to buckling occurring in the unclamped specimens at low strains increasing the measured extensions and /or reducing the measured load. Alternatively, friction between the grips and the clamps may increase the load measured.

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6.2.3 A method for extrapolating large strains from the crosshead movement

Despite these precautions, strain measurements at large deformations can be very unreliable. The individual extensometers readings can show discontinuities in slope and diverge significantly. Figure 46 shows the measure stress/strain curves to failure of a bulk Arcan specimen of Acrylic F241 as determined by each extensometer and the mean of these readings. The strains measured at the maximum stress by the extensometers on sides 1 and 2 of the specimen are 140 % and 40 % respectively. Clearly the mean strain at the maximum stress (90 %) must be considered extremely suspect. Significant differences between the strain measurements at large deformations from the individual extensometers indicate that the strain measurement is extremely unreliable. Where the individual strain measurements are smooth and agree well, strain measurements will be more reliable.

As an aid to data presentation, a method for determining strain from the crosshead movement at large deformations was adopted in this work. The rate of strain was calculated between 10 % and 20 % strain, where the two extensometers were in good agreement. Above 20 % strain, the additional strain is determined from the elapsed experimental time and the calculated rate of strain. In Figure 46 the extrapolated strain is shown as a thick solid line. The extrapolated strain at the maximum stress is approximately 60 %, between the extensometer 2 and the mean values. The strain measured by extensometer 1 is most likely to be extremely inaccurate in this test.

In many of the tests performed the measurement of strain at large deformations (above 20 %) was unreliable. The extensometer readings diverged substantially and problems in strain measurement at high strains occurred with either of the extensometers. Buckling of the test specimens, despite the restraints, undermined the contact between the extensometer needles and the specimen. If one or both of the needles is not in firm contact with the specimen then the ‘strain’ measurement will be inaccurate. The extrapolation method allows the deformation of the specimen to be estimated where the high strain extensometer data have to be written off. The material strength measured in the test regardless of the quality of the strain data is likely to be a much more useful value for designers than the strain at failure.

6.2.4 Stress/strain behaviour of compliant specimens

Figure 47 shows the measured stress/strain behaviour to failure of the polypropylene specimens. Two tests were performed at each of the strain rates 2 % and 20 % per minute. The curves agree very well. The maximum stress increases with the testing rate. The breaking strains of the specimens are in excess of 35 % but the maximum stresses occur around 20 % strain. These was no evidence of these specimens buckling. However, the shear modulus of the polypropylene (around 500 MPa) and the relatively large thickness of the specimens (4.5 mm) give a high resistance to out of plane forces and may thereby prevent buckling.

The polyurethane data shown in Figure 48 were measured in tests performed without the clamps. These tests suffered from excess ‘buckling’ during the latter portions of the test. The out-of plane distortions are registered by the displacement transducers as in-plane deflections and the measurement of strain in this region of the curve is extremely unreliable. The solid lines in Figure 46 are derived by the extrapolation method described earlier. The dotted lines show the measured data. The agreement between the extrapolated curves and the measured curves is generally good to around 50 %, breaking down above this. The

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agreement between the stress-strain curves for the different specimens is reasonable for each of the two methods of calculating strain. The maximum stresses vary between 9.8 and 12.5 MPa. The maximum stresses occur at measured strains between 150 % and 230 % (between 120 % and 190 % for the extrapolated strains), increasing with the maximum stress. These strains are extremely high.

—

Through clamping the test fixture to resist lateral movements it is possible to improve the quality of the measured strain data. Figure 49 shows three specimens from the same sheet (HPC001) tested using restraining clamps. The clamps reduced the buckling and the individual extensometer readings remained in good agreement. The extrapolated and measured readings agreed well (only the measured strains are shown) which is generally not the case for unclamped specimens. The three stress-strain curves are very consistent. Maximum stresses are higher (around 14 MPa) but the strains at these stresses are lower (100 to 120 %) than for the tests shown in Figure 48. The specimens tested in Figure 49 all had significantly higher moduli than those shown in Figure 48 (160 MPa to 70 MPa).

Figure 50 shows stress-strain curves to failure for 5 bulk specimens cut from sheets of acrylic F241 of nominally the same thickness, manufactured at the same time from the same batch of material. The moduli of these specimens lay in the range 170±30 MPa. Clamps were used to prevent the specimens from buckling but these were only partially effective. The measured (dashed) and extrapolated (thick) data are shown. The measured data vary considerably at high strains, the maximum stresses being at strains between 40 and 90 %. The data extrapolated from 20 % strain onwards being much more consistent, failure strains are between 40 and 60 %.

Estimates of the strain from extrapolated data at high strains appear to give reasonable and consistent curves for both the acrylic and the polyurethane bulk specimens. Direct measurements can be significantly in error due to buckling and rotation of the specimen. Estimating strain by the extrapolation method will give values for the deformation that the material can bear. It is extremely unlikely that adhesive joints will be designed for applications where the strain in the adhesive exceeds a small fraction of the deformation at failure. The extrapolation will only be accurate if the strain rate remains at the calculated value over the entire test.

6.2.5 Failure modes

Figures 41 and 43 show typical compliant specimens after failure. Failure of the compliant test pieces originates at the notch edge near the notch root. This is the location as the locus of failure of the stiff specimens, the principle stress concentrations on the notch edges. The crack propagates in the same direction although, in these specimens, the specimen tears and the growth of the tear is slow enough to be followed from the decrease in load on the specimen. Some tests, eg Figure 43, were stopped before complete failure. The acrylic specimens show the large distortions of the specimens produced by the buckling (Figures 41 and 44). These distortions are not observable for the polyurethane specimens (Figure 43)

— as the material recovers when the load is removed. The lighter regions around the centre of the specimen show the distribution of high strain areas.

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6.3 STRAIN RATE EFFECTS IN BULK SPECIMENS

The local rate of strain in the gauge section of the Arcan shear specimen is not constant throughout the test. As discussed in section 5.2, the initial strain rate in tests on compliant specimens may be extremely low as slack in the load assembly is taken up. However, the rate of strain may also change at the end of a test to failure as the adhesive yields.

The stress/strain plot for a viscoelastic polymer is curved with diminishing gradient (or modulus) at higher strains. As the stress in the gauge section is greater than the stress outside of the gauge section the material, the strain will be greater and the effective modulus less. As the deformation of the specimen increases, an increasing proportion of the deflection occurs in the gauge section rather than in the rest of the specimen and grips. The effective strain rate in the gauge section will rise as the specimen is sheared along the stress/strain curve. As a consequence of the increasing strain rate the modulus of the material in the gauge section will increase above what it would have been if the local strain rate had remained constant. Where a material yields significantly before failure, the local strain rate in the gauge section could be many times greater than the initial rate. The higher strain rate may increase the yield/failure stress. This argument applies to both bulk specimens and joint specimens.

To estimate the likely influence of changing strain rate, bulk specimen tests were performed at different strain rates. Two of the adhesives were chosen, the AV119, which is the stiffest of the adhesives but yields before failure, and the polyurethane, which is the most viscoelastic.

Bulk specimens ofAV119 (cut from the same sheet) were tested to failure in the Arcan test using the Arcan extensometer at 4 different strain rates (1, 5, 10 and 20 % per minute). The shear stress-strain curves to failure are shown in Figure 51. The early parts of the stress- strain curves (up to around 35 MPa /5 % strain) are very similar despite the large differences in strain rates. The higher the strain rate, the larger the maximum stress experienced by the specimen. Increasing the strain rate from 1 to 20 % per minute increases the maximum stress by approximately 10 %.

The stress-strain behaviour of the polyurethane is very sensitive to the rate of strain. Three bulk specimens cut from the same sheet (HPC 003) were tested to failure at 4.5,9 and 13 % strain per minute. These specimens were clamped to restrain any buckling. The stress-strain curves are plotted in Figure 52 show significantly different behaviors, the higher the strain rate the ‘stiffer’ the specimen as expected. With increasing strain rate the maximum stress increases whilst the maximum strain decreases. In contrast to the EpoxyAV119 (Figure 51) and the polypropylene (Figure 47), the difference in the strain rates has a significant effect at low strains. The curves are very different at low strains and much of the gap between the curves is already present at 20 % strain (where the extensometers are assumed to be measuring reliably). The large differences between the curves at relatively low strains is due to the viscoelastic nature of the polyurethane where the modulus is much more sensitive to rate at small times when the bulk of relaxations in the material occur than at large times when most relaxations have finished. It is the early part of the stress/strain curve where the strain rate in the polyurethane is most difficult to control. However, as the modulus of polyurethane is already low, most of the deformation occurs in the gauge section rather than elsewhere and the increase in the local strain rate with increasing strain is smaller than for a stiffer material.

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The extrapolated strain is plotted in Figure 52 as thick lines and the measured strains as dashed lines, for two of the tests (4.5 % and 13 % per minute) there is reasonable agreement between measured and extrapolated. The extrapolated data show much better consistency than the ‘measured’ data and are less likely to give an extremely inaccurate value for the strain than the extensometers. The relative sizes of the values of the strain at maximum stress for each specimen follow the relative magnitudes of the total crosshead displacement.

6.4 JOINT SPECIMENS

6.4.1 Features of joint tests

The gauge length for joint specimens is much smaller than for bulk specimens and the specimen, being mostly metal, is much stiffer. Thus, displacements measured by the extensometer are much smaller and there is no need to reset the extensometer during tests. The strain measurement will be in the central, uniform section of the stress distribution throughout the test. There is no buckling observed. As Figure 53 shows, the individual extensometer readings tend to agree well throughout the test even at high strains. Therefore, the joint test would appear to be most likely to give accurate stress-strain curves to failure.

However, it is difficult to maintain a constant strain rate throughout the test. As the effective modulus of the adhesive in the joint tends to decrease throughout the test, the strain distribution between the adherends and the adhesives alters with the effect of increasing the rate of strain experienced by the adhesive. This is particularly significant when the adhesive yields. The strain rate at the start of tests is usually extremely low as slack in the loading assembly is taken up. This leads to the discontinuities in the stress- strain curves at low loads seen in some of the modulus measurements and to potentially misleading estimates of the strain rate. The cross-head speed needed to achieve the required strain rate at low loads during a modulus measurement maybe too high to achieve the same rate further on in the test. For example the speed required to achieve a strain rate for the polyurethane joints of 4 % strain per minute at 1 % strain gave a strain rate of 12 % per minute above 10 “/0 strain. Data produced under these conditions are not directly comparable with bulk specimens where a 4 % per minute strain rate was maintained throughout the test since the polyurethane is very rate sensitive.

There are problems gripping the specimens at high loads. The surfaces of the steel adherends are much harder than the surfaces of the adhesives/plastics and the roughened faces of the grips do not indent as far. When the load is high, the friction between grip and adherend may not be sufficient to prevent the specimen from slipping. The specimen will tend to rotate slightly, increasing the displacement values (and strain) measured by the extensometer. The rotation of the specimen may also increase the peel forces on the adhesive and lead to premature failure. Examination of the gripped surfaces of epoxy adhesive joints loaded to failure revealed gouges in the steel surface in a pattern consistent with a rotation of the specimen. Such gouges were not present on joints which had merely been loaded to low stresses in modulus measurements.

When bulk and joint tests to failure are compared, as with AV119 specimens in Figure 53, it is obvious that the stress-strain curves are similar at low stresses but diverge at higher loads. The maximum stress experienced by each specimen is similar (around 47 MPa) but the strains are very different (around 9 % for the bulk, 30 ‘% for the joint). The bulk specimen gives a smooth stress-strain curve whilst the joint specimen gives a disjointed

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curve. The two are, however, similar until the stress exceeds 15 MPa (a load of around 900 N) where the divergence suggests that the specimen starts slipping. This implies that, for the apparatus described here, materials where the maximum load is less than 900 N can be tested in the joint configuration without the problem of the specimen slipping. Joint specimens of the polyurethane and, possibly, the acrylic can be tested as constructed.

The maximum loads applied when testing higher strength adhesives could be reduced through testing joints with lower bonded areas (ie by reducing the adherend thickness or the bond length). FE analysis has shown that halving the bondlength from 10 mm to 5 mm has very little effect on the stress distributions in the centre of the bond and that the end stress concentrations are altered little. Some shorter joint specimens of the TE251 and acrylic F241 were tested.

As a further attempt to eliminate slip, aluminium strips were bonded onto the steel adherends to improve the friction between the grip and the specimen. A steel adherend with a bonded aluminium strip is shown in Figure 55. These could not be bonded perfectly flat and required grinding to obtain flat surfaces which can be loaded into the Arcan jig. A high proportion of specimens (about 50 %) broke during this operation. These aluminium strips were bonded to the steel adherends of specimens of both epoxies and used to measure the behaviour of the materials to failure. An alternative approach is to use aluminium adherends, which being softer should be gripped more firmly. One drawback of this is the increased deformation of the adherends during tests.

6.4.2 Stiff Adhesives

The stress strain curves to failure forAV119 specimens prepared in different ways are shown in Figure 56. Bonding aluminium strips to the AV119 joints improves the agreement between the bulk and joint results. However, as Figure 53 shows, the curves start to diverge at around 30 to 35 MPa stress and the maximum stress (44 MPa) experienced by the joint specimens is some 3 MPa or so less than the bulk specimens (47 MPa). This may be within the material variability as the yield stresses measured for bulk specimens varied by a similar amount. The strain at which the maximum stress occurs in one of the joint specimens is significantly higher than that generally measured for the bulk specimens (12 % as opposed to 8 %) which may indicate that the specimens can still rotate in the grips to some extent. The curves measured using aluminium adherends diverge from the bulk test curves significantly around 30 MPa (1800 N) and the maximum stresses (approximately 47 MPa) occur around 15-20 % strain. This indicates that the gripping using aluminium, while improved greatly over steel, is still not adequate for testing AV119 specimens with the current dimensions (bond length 10 mm, bond width 6 mm).

The TE251 specimens were more brittle and many of the specimens broke during machining operations to remove excess adhesive, to grind flat bonded aluminium strips or when loading specimens into the Arcan jig. Consequently, only 1 measurement of a stress/strain curve to failure was obtained using a specimen with aluminium strips bonded on to improve gripping. This test is shown in Figure 57 together with data from typical bulk tests, a normal joint test (steel adherends and 10 m long bond), an specimen with aluminium adherends and a short bond length specimen (5 mm long bond). The data from the bulk, the improved joint specimen and the short specimen show reasonable agreement when the variability of the bulk specimens shown in Figure 40 is taken into consideration. The normal joint specimen seems to diverge from the bulk specimen around 15 MPa stress (900 N load),

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much like the AV119 specimens. The aluminium specimen does not withstand high stresses. This is a typical feature of many of the joint tests performed on the TE251 for which the stress/strain curves and stress at failure were very variable.

Some of the joint specimens seem to withstand higher strains than the bulk TE251 specimen and even show a ‘yield’ point. However, it is more probable that failure of the specimen occurs as a slow peel rather than as a sudden failure. In which case a smooth fall in load would be seen, together with increasing displacements, as the specimen is pulled apart. The tensile peel stress concentrations at the end of the bondlines are crucial to the performance of the brittle TE251 in the Arcan joint test. The variation of maximum stresses in the joint tests may be due to variations in stress concentrations due to adhesive fillet geometry. The results show that it is possible to prepare Arcan joint specimens that can withstand higher strains than bulk Arcan specimens.

6.4.3 Compliant adhesives .-.

-.

.

The polyurethane has a much lower modulus than the other adhesives, consequently loads needed to fail polyurethane joint specimens will be low and it is unlikely that the specimens will rotate in the grips. Normal joint specimens were used to determine the stress-strain behaviour to failure of this adhesive. Figure 58 shows the stress/strain curves to failure for several polyurethane joints. There is reasonably good agreement between the curves, and the maximum stresses are very similar (around 10.4 to 11.5 MPa). The failure strains are between 70 and 95 %. The higher maximum stresses occur at higher strains. The rate of strain experienced by the adhesive in the joint is not constant during the test and there were difficulties in selecting a crosshead speed for testing. At the crosshead speed needed to apply strain at a rate of 4% per minute at low strains for modulus measurements, the strain rates at larger strains are 10-15% per minute. One specimen (M06-451) was run at a much lower crosshead speed to give a rate of 4% per minute at large strains. The differences between this stress-strain curve and the others are surprisingly small at large strains. This specimen survived to a higher strati (95 %) and stress (11.5 MPa). The reasons for the relatively small differences between the tests performed at low and high strain rates are unclear and more tests would be required to determine the cause (or if the result for 4% per minute is reproducible).

The adhesive in the joint specimens M06-450, M06-451 and M06-453 was obtained from cartridge HPE. Figure 59 shows stress-strati curves for these joint specimens together with typical stress vs extrapolated strain curves for unclamped (HPE 001B) and clamped (HPE 003C) bulk specimens. The stress-strain curves for the joint and the clamped bulk specimens show good agreement. I

—

Figure 60 shows the stress-strain behaviour to failure of acrylic joint specimens. These agree well over the first 40% of measured strain but there is a wide variation in strains at failure (maximum stress) from 50 to 90 %. The curved shape of the stress-strain near the maximum stresses on the plot suggests that the adherends are slipping in the grips. The tests show data from specimens having two nominal bond lengths, 5 mm (specimens 393 and 395) and 10 mm (383-389). The maximum load sustained by the 5 mm long specimen 393 was around 685 N, below the levels of load at which epoxy joints appeared to slip, whilst the 10 mm long specimen 389 sustained 1550 N, far in excess of the loads where the stress-strain curves for epoxies appeared to diverge from the bulk specimen behaviour. The stress-strain curves for the two specimens 393 and 389 are very similar out to quite high strains (60 %) and it

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is specimen 393, at half the load of 389, which curves down first. The curvature in these curves may be due to slow peeling of the adhesive form the joint surface.

The stress-strain curves to failure of the acrylic joint specimens do not agree well with the stress-strain curves measured (or extrapolated) for the corresponding bulk acrylic specimens. This is shown in Figure 61 where the curve for a representative bulk specimen (381B) is plotted alongside two typical joint specimens (389 and 395). Bulk specimen 381B has lower stresses throughout the strain range. The low strain moduli of the joint specimens are significantly higher than the moduli of the corresponding bulk specimens. The stress-strain curves reflect these differences. Figure 58 also compares the typical joint specimens with bulk specimens (327C and 330C) taken from sheets of bulk acrylic having higher moduli which are comparable with those of the joint specimens. There is good agreement between with the joint specimens although the maximum strains sustained by the bulk specimens are lower.

The differences in the strains at failure seen for the polyurethane and acrylic joint specimens are probably due to variations in the shapes of the ends of the bondlines. FE analysis has shown that the size of the tensile peel stress concentrations is determined by bond end shape. The adhesive fillets of all of the specimens were filed concave using a miniature round file. However, no efforts were made to measure and control the exact shape of the adhesive fillet. Variations in the geometry of the adherends occurred often, in many cases the ends of the bonded faces were at different levels and some of the corners between bonded face and the notch edge on the adherends were rounded rather than sharp. Stress concentrations are likely to vary significantly between joint specimens leading to a large range of failure strains. This is perhaps most noticeable with the acrylic specimens but is likely to have a large effect on the other materials. A more systematic study of the effects of adhesive fillet geometry is required. Better control over adherend machining and adhesive fillet shape should lead to more consistent test data.

6.4.4 Failure modes in joint specimens

The Arcan joint specimens all appear to fail at the adhesive-metal interface. One side of the failed joint is ‘clean’ whilst the other side has a complete layer of adhesive. Figure 15 shows a typical polyurethane joint after failure. The failure occurs at similar stresses to the failure of the bulk specimens. The FE analysis of the Arcan joint specimen predicts that tensile stresses will be high at the end of the bond near to the adhesive-metal interface. This suggests that the joint will fail initially through tensile failure of the adhesive at the interface. As the test progresses the locus of the tensile stress concentration will follow the edge of the bonded region and the specimen will continue to peel. Therefore, failure is likely to be accompanied by a fast, but not instant, fall in load as the length of the load supporting bond line falls and a large increase in strain which the tests seem to show.

Most tests peeled along one interface only, whereas the FE analysis predicts equal peel stresses on opposite sides at either end of the bond which would lead to peeling from both ends. Only 1 or 2 specimens failed in this manner. The FE analysis assumes that the geometry, particularly the notch root radius, is the same at both ends of the specimen. In a real test piece this is unlikely and most probably the stress concentration at one end will be greater than at the other. Failure will then tend to occur along one interface only. The failure strains of joint specimens are very variable, this is probably a consequence of the

.

variations in adhesive end fillet shapes produced when preparing the specimens.

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6.5

6.5.1

Strain


SUITABILITY OF THE TEST METHOD FOR STRESS/STRAIN TO FAILURE

Strain measurement

gauges should not be used to determine strain to failure in the Arcan test. They initiate-pre-mature failure in brittle specimens and are unable to function at the large strains

.

experienced by the more ductile materials.

The shear extensometer is theoretically capable of measuring extremely high strains (100 % or more). However, in practice strain measurements on Arcan bulk specimens beyond around 20 % strain can be unreliable due to the reasons outlined in Section 6.2.

● Resetting of the extensometer and excessive lever deflections. ● In-plane ‘Rotation’ of the notch roots out of the axis of applied force. ● Out of plane distortion (’buckling’) of compliant specimens.

Divergence between the strain measurements from individual extensometers indicates unreliable strain data. Higher strains (up to 40 or 50 %) can be measured more reliably for stiff specimens, which can resist lateral distortions, than for compliant specimens.

It is possible to estimate strains in the bulk Arcan test using the crosshead movement. This is necessary where the strain readings become unreliable. To do this the rate of strain applied over a reliable part of the stress-strain curve (eg 10 % to 20 %) should be calculated. This rate, assuming a constant rate of specimen deformation, should be used to extrapolate from a reliable measured strain (eg from 20 %). Extrapolated strains tend to give more consistent stress-strain curves for the acrylic and polyurethane than the ‘measured’ data (which can be extremely unreliable).

The displacements applied in joint tests are much smaller than in bulk tests and the three problems outlined above are insignificant. However, there is a problem with the metal adherends slipping in the grips at high loads leading to in-plane rotation of the specimen which gives erroneously high strain readings. This problem could be eliminated by modifying the grips and clamping arrangement for joint specimens.

6.5.2 Failure modes

The V-notched shear tests do not fail in shear and therefore do not measure ultimate shear properties. The failure in the bulk Arcan test is always initiated at a tensile stress concentration on the edge of the notch near the root, outside of the gauge section. Failure in the Iosipescu specimen is also initiated at tensile stress concentrations.

Failure in the Arcan joint specimen is due to tensile peel stresses generated at the end of the bondline. The size of the peel stress depends on the shape of the end of the adhesive bondline. A concave shaped fillet has the lowest stresses. Depending on the symmetry of the two notch roots the failure can start at either or both of the bondline ends. The peeling can lead to complete removal of the adhesive from one of the bonded surfaces ie to an adhesive failure rather than a cohesive failure.

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6.5.3 Reliability of stress-strain curves

The Arcan test on bulk specimens employing the shear extensometer works well in the case of stiff specimens (such as AV119) where the maximum stress occurs before -20 % strain which are ductile enough to avoid premature failure at tensile stress concentrations. The values of the maximum stresses and yield strains measured for materials which yield before 30 % strain, such as AV119 or polypropylene, are repeatable to within about 5 %. The actual strain at failure (post yield) is much less repeatable but this is probably a less critical value for designers.

Specimens of brittle materials, such as the TE251, tend to fail prematurely in the bulk specimen tests, since the tensile stress concentration near the notch root is not relieved by local yielding. Failure strains measured in joint tests are higher and yield stresses can be determined. Stress concentrations in the joint specimens appear to be less critical at low strains.

Tests on bulk specimens of compliant materials, such as the polyurethane or the acrylic, in which the maximum stress occurs far beyond 20 % are much less reliable than tests on stiff materials where stress/strain curves appear reliable to around 40 % strain. Direct strain measurements above 50% are doubtful and extrapolated strains should be used. Bulk tests can give an indication of the strength of the material and an estimate of the deformation the material will withstand. This is probably sufficient for design purposes as most practical joints will be designed for situations where the maximum stress never exceeds a fraction of the material strength and any strains will be small.

Increases in the local strain rate in the gauge section, eg when the adhesive yields, can lead to higher measured yield/failure stresses. For some materials this effect is relatively small, for example AV119. For compliant materials, such as the polyurethane, the stress-strain behaviour to failure is more influenced by the strain rate.

Strain measurement at high strains in joint tests is much more reliable than in bulk tests, provided that the gripping of the metal adherends can be improved to eliminate slip. Stress/strain curves for compliant adhesives with maximum stresses occurring beyond 20 % are fairly consistent. The strengths measured for the acrylic and polyurethane are comparable with bulk tests.

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7. RECOMMENDATIONS ON THE USE OF NOTCHED-SPECIMEN SHEAR TESTS

7.1 IMPROVEMENTS TO TEST METHODS

The poor quality of the stress/strain curves at low loads reduces the reliability of modulus measurements, particularly for compliant adhesives. This could be overcome by employing a much more rigid loading assembly. Alternatively, the mass of the lower grip could be increased. Thus when the force is sufficient to ‘lift’ the lower grip from the lower pin position to the upper pin position and cause discontinuities in the stress/strain curve, the required strain for modulus measurement should have been reached. Effectively. the discontinuity will be moved to a higher strain.

The buckling of low stiffness test specimens at high strains is a serious limitation to the method as it stands. The test could be improved by making the loading assembly more rigid to lateral movements, eg by building the grips into a rigid frame with linear bearing guides, to restrict movement of the grips to the vertical direction only. This should eliminate both out-of-plane specimen distortion and twists of the specimen and grips about the vertical axis which can interfere with the measurement of strain.

Tests could be run at a constant strain rate by employing strain control. The crosshead speed could be linked to the strain readings from the extensometer and varied to keep the rate of strain constant. However, this would only be useful at small deformations or in joint tests where the measured strains are reliable and the extensometers do not require resetting.

Any redesign of the extensometer should be addressed at:

● reducing slack in the levers which leads to changes in the extensometer gauge length; ● adding height adjustment to the extensometer to allow the needles to remain in the

centre of the specimen throughout tests at large deformations; ● reducing sensitivity to movements in any other plane but the vertical; ● improving the contact with the specimen at larger displacements.

The joint test could be improved by adding features to prevent the specimen from slipping or rotating in the grips. One way could be to add further bolts or wedge pins to counter this. Alternatively, the Iosipescu test could be used for joint specimens, adherend slippage will not be a problem and the edge loading arrangement should pose no difficulties with steel adherends. However, a different extensometer would be required and there may be problems accessing both faces of the specimen for strain measurements.

The shape of the end of the adhesive bondline is important in causing stress concentrations and thus determining the strain at which joint specimens fail. This should be controlled more closely to give more consistency between tests. Adherends for the joint tests should be manufactured to tighter tolerances to ensure that the bonded faces are symmetrical and of equal length. The faces should be flat and parallel to guarantee a constant bondline thickness. The comer of the specimen between notch edge and bonded face should be sharp rather than rounded. The end of the bond should be concave to minimise tensile peel stress concentrations and future test specimens should be machined to the same shape.

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7.2 USE OF THE TEST

The suitability of V-notch shear tests for adhesives can best be assessed in comparison with other test methods such as torsion [1] or TAST [2] and with specifications for the accuracy of the data required for design purposes. Comparisons between test methods will be performed in a later report [3]. It is likely that the test method and the shear extensometer can be further improved to increase the reliability of the data generated. The use of the tests to measure shear data, recommended from the current work, follow.

7.2.1 Modulus Measurements

Using the V-notch shear tests in their present design gives shear moduli accurate to ± 20 % which does not seem to be a high accuracy but is comparable with data in the literature. Obtaining the shear modulus from tensile tests is much easier. The main difficulty is obtaining an accurate value for Poissons ratio. However, as for Poissons ratio for a polymer will fall between 0.3 and 0.5, an estimate of 0.4 for Poissons ratio will give the shear modulus as Youngs modulus divided by 2.8, to within approximately ± 10 %. This is comparable with the accuracy of the V-notch shear tests. Tensile tests will not give shear stress/ strain data to failure.

For measuring moduli using the V-notch shear tests, tests on bulk specimens are preferable to joint tests as the displacements are larger and, in theory, strains in the adhesive can be determined more accurately.

● Stiff Materials (G >1 GPa)

Either the Iosipescu or the Arcan test can be used. Strain measurement can be either strain gauges or shear extensometers.

● Compliant Materials (G <1 GPa)

The bulk Arcan test using the shear extensometer is recommended.

7.2.2 Stress-strain to failure

Joint tests are preferred over the bulk notched-specimen tests provided that the specimen clamping can be modified to prevent the specimen from slipping in the grips at high loads. Shear extensometers should be used to measure strains, although at large deformations strains in bulk specimens can be estimated from the crosshead movement.

● Adhesives where the maximum stress occurs before 20 % strain

Arcan tests using bulk specimens can be employed but premature failure is likely when testing brittle materials. This test is useful for discriminating between ductile and less ductile adhesives.

● Stiff adhesives where the maximum stress occurs before 40% strain

Arcan tests using bulk specimens can be employed but extensometer measurements

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should be checked for divergence.

● Compliant adhesives where the maximum stress occurs beyond 20 % strain and stiff adhesives where the maximum stress occurs beyond 40 % strain

Joint tests should be used.

(Tests on bulk specimens can give rough estimates of the material strength and the level of deformation it can sustain.)

8. ACKNOWLEDGEMENTS

This work forms part of a programme on adhesives measurement technology funded by the Department for Trade and Industry as part of its support of the technological competitiveness of UK industry. Other DTI funded programmed on materials are also conducted by the Centre for Materials Measurement and Technology, NPL as prime contractor. For further details please contact Mrs G Tellett, NPL.

The authors wish to thank Mr A Pearce, Dr P Tomlins, Mr D Hua and Mr A Ahmadnia (NPL) for their assistance with the shear measurements. Bulk and joint test specimens were prepared by Ms M Girardi and Ms N Linklater (TWI).

9.

1.

2.

3.

4.

5.

6.

7.

REFERENCES

Thomas R. and Adams R. Test methods for determining shear property data for adhesives suitable for design. Part 2: The torsion method for bulk and joint test specimens. MTS Adhesives Project 1, Report No 7, March 1996.

Vaughn L. and Adams R. Test methods for determining shear property data for adhesives suitable for design. Part 3: The thick-adherend shear test method. MTS Adhesives Project 1, Report No 8, March 1996.

Dean G. D., Adams R., Duncan B. C., Thomas R. and Vaughn L., Comparison of bulk and joint specimen tests for determining the shear properties of adhesives. MTS Adhesives Project 1, Report No 9, March 1996.

Iosipescu N., New accurate procedure for single shear testing of metals, J. of Materials, vol 2, No 3, 1967, pp 537-566.

ASTM D30.04, Standard test method for shear properties of composite materials by the V-notched beam method, 1992.

Weinberg M., Shear testing of neat thermoplastic resins and their unidirectional graphite composites, Composites, vol 18, No 5, 1987, pp386-392

Wilson D. W., Evaluation of the V-notched beam shear test through an interlaboratory study, J. Composites Tech. and Res., vol 12, No 3, 1990, pp 131-138.

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20.

Broughton W. R., Kumosa M. and Hull D., Analysis of the Iosipescu shear test as applied to unidirectional carbon fibre reinforced composites, Composites Science and Technology, vol 38, 1990, pp 299-325.

Conant N.R. and Odom E.M., An improved Iosipescu shear test fixture, J. Composites Tech. and Res., vol 17, No 1, 1995.

Brinson H. F., Wightman J. P., Dillard D.A., Lefebvre D. and Filbey J., Test specimen geometries for evaluating adhesive durability, Proceedings of the 19th International SAMPE Technical Conference, October 13-15, 1987, pp152-164.

Goldenberg N., Arcan M. and Nicolau E., On the most suitable specimen shape for testing shear strength of plastics, Proceedings of ASTM Symposium on Plastics Testing and Standardisation 247, pp 115-121

Arcan M., Hashin Z. and Voloshin A., A method to produce uniform plane-stress states with applications to fibre reinforced materials, Experimental Mechanics, April 1978, pp 141-146

Banks-Sills L. and Arcan M., A compact mode II fracture specimen, Fracture Mechanics: 17th Volume, ASTM STP 905, American Society for Testing and Materials, Philadelphia, 1986, pp 347-363.

Voloshin A. and Arcan M., Pure shear moduli of unidirectional fibre reinforced materials, Fibre Science and Technology, vol 13, 1980, pp 125-134.

Weissberg V. and Arcan M., A uniform pure shear testing specimen for adhesive characterisation, Adhesively Bonded Joints: Testing, Analysis and Design, ASTM STP 981, W.S.Johnson Ed, American Society for Testing and Materials, Philadelphia, 1988 pp28-38.

Grabovac I. and Morris C.E.M., The application of the Iosipescu shear test to structural adhesives, J Applied Polymer Sci. vol 33, 1991, pp 2033-2042.

Wycherley G.W., Mestan S.A. and Grabovac I., A method for uniform shear stress- strain analysis of adhesives, J. of Testing and Evaluation, Vol 18, No 3, 1990, pp 203- 209.

Duncan B.C. and Tomlins P. E., Measurement of strain in bulk adhesive testpieces, MTS Adhesives Project 1, Report No 2, DMM(B) 398;-October 1994.

Dean G.D. and Duncan B. C., Tensile behaviour of bulk specimens of adhesives, MTS Adhesives Project 1, Report No 3, DMM(B) 448, May 1995.

Ifju P. and Post D., A special strain gauge for shear testing of composite materials, Proceedings of the 1991 SEM Spring Conference on Experimental Mechanics, Society for Experimental Mechanics, June 1991.

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21. Kassapoglou C. and Adelmann J. C., KGR-1 thick adherend specimen evaluation for the deterrnination of adhesive mechanical properties, Proceedings of the 23rd International SAMPE Technical Conference, October 21-24, 1991, pp162-176.

22. Walrath D.E. and Adams D. F., Iosipescu shear properties of graphite fabric/epoxy composite laminates, University of Wyoming, Report No UWME-DR-501-1O3-1, 1985.

23. Duncan B.C, Girardi M. and Read B. E., The preparation of bulk adhesive samples for mechanical testing, MTS Adhesives Project 1, Report No 1, DMM(B) 339, January 1994.

24. Dickerson T. L., Finite element deformation and stress analyses of Arcan test specimens, TWI Report 34114/3/94, May 1994.

25. ISO 527 Part 1, 1993, Determination of tensile properties: General Principles.

I —

BCD CMMTb56.001 29 April 1996

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LIST OF TABLES

Table 1:

Table 2:

Table 3:

Table 4:

Table 5

Table 6:

Table 7:

Table 8:

Table 9:

Table 10:

Table 11:

Table 12:

Table 13

Table 14:

Iosipescu test data reported by Weinberg [6]

Iosipescu test data reported by Broughton et al [8]

Round robin reproducibility data reported by Wilson [7], the strength data has been re-analysed to exclude outlying laboratories.

Moduli of a fibre reinforced material reported by Voloshin and Arcan [13]

Materials used and tensile test data

Iosipescu test moduli for epoxy adhesives

Arcan moduli for epoxy adhesives measured using strain gauges

Moduli measured for acetal and polypropylene using bulk specimen tests

Moduli of AV119 bulk specimens measured using the Arcan shear extensometer

Bulk specimen shear moduli data for TE251

Bulk specimen shear moduli data for polyurethane

Bulk specimen shear moduli for acrylic F241

Shear moduli of epoxy joint specimens

Shear moduli of polyurethane and acrylic joint specimens

—

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.

Table 1: Iosipescu test data reported by Weinberg [6]

Material I Resin I Composite

shear shear shear shear modulus strength modulus strength

(GPa)1 (MPa)1 (GPa)l (MPa)1

Graphite/ Epoxy 7.1 (± 8 %) 94 (± 10 %)

polycarbonate 1.65 (± 62 %) 38 (± 5 %) 4.7 (± 20 %) 90 (± 11 %)

polysulphone 1.62 (± 34 %) 43 (± 4 %) 4.6 (± 13 %) 99 (± 14 %) ,

PMMA I 0.72 (± 46 %) 20 (± 46 %2) 4.3 (± 8 %) I 74 (± 6 %)

nylon 6,6 I 0.42 (±26%) I 66 I 4.7 (± 13 %) ] 120 (± 4%)

polystyrene I 1.3 (± 22 %) 15 (± 11 %2) 5.0 (± 17 %) I 61 (± 9 %)

polyethylene 0.38 (± 18%) 24 (± 3 %) 3.3 (± 17 %) 67 (± 6 %) terephtalate

polyethylene I 0.15 (± 46 %) 11 (± 16 %) 1.4 (± 17 %) I 32 (± 3 %)

1 All quantities are expressed as the mean value ± the 95% confidence band (expressed as a % of the mean).

2 Brittle failure mode hence larger uncertainties.

Table 2: Iosipescu test data reported by Broughton et al [8]

Material Shear Modulus (GPa) 95% Confidence band expressed as % of mean

Glass/polyester 4.68 11

AS4/3501-6 6.41 4

XAS 914C 5.48 7

T300 R23 4.76 7

APC-2 5.72 6

page 45 of 67


Table 3: Round robin reproducibility data reported by Wilson [7], the strength data has been re-analysed to exclude outlying laboratories.

Specimen Shear 9570 Shear 9570 Modulus confidence Strength confidence (GPa) band (%) (MPa) band (%)

Carbon/ Epoxy 0° 6,24 ± 1.33 21 105 ± 15 14

Carbon/ Epoxy 90° 5.21 ± 1.61 31 58 ± 14 25

Carbon/ Epoxy 0/90° 5.41 ± 0.85 16 94±5 5

Kevlar/ Epoxy 0° 2.36 ± 0.58 25 79±8 9

Kevlar/ Epoxy 90° 2.11 ± 0.57 27 15±4 27

SMC R-50 4.60 ± 0.77 17 83 ± 18 22

Table 4: Moduli of a fibre reinforced material reported by Voloshin and Arcan [13]

Orientation Shear Modulus (GPa) 95 % confidence band (%)

longitudinal 3.63 ± 0.39 11

transverse 3.48 ± 0.34 10

Table 5 Materials used and tensile test data

Material Tensile Poisson’s calculated Shear Modulus E Ratio v Modulus G (MPa) (MPa)

Evode 2-part epoxy TE251 2700 ± 175 0.34 ± 0.02 1000 ± 90

Ciba l-part epoxy AV119 3050 ± 150 0.38 ± 0.02 1100 ± 80

3M Polyurethane 3532 170 ± 20 0.38 ± 0.03 62 ± 10

Permabond Acrylic F241 600 ± 200 0.49 ± 0.03 200 ± 70

Polypropylene 1520 ± 70 0.42 ± 0.02 540 ± 70

Acetal 3040 ± 30 0.38 ± 0.02 1100 ± 60

-.

page 46 of 67


Table 6: Iosipescu test moduli for epoxy adhesives

AV119 Specimen Measured Shear TE251 Specimen Measured Shear Modulus (G ± 1sd) Modulus (G ± 1sd) MPa MPa

117 E 1150 ± 10 87 B 930 ± 10

118 C 1160 ± 10 87C 990 ± 20

120 D 1160 ± 30 II 90 E 990 ± 10

I II 90 F 990 ± 10

I II 86 B

I II 86 c 990 ± 10

mean (95%) 1160 ± 20 mean (95%) 980 ± 70 confidence level) confidence level)

Table 7 Arcan moduli for epoxy adhesives measured using strain gauges

AV119 bulk Measured Shear TE251 bulk Measured Shear specimen Modulus (G ± 1sd) specimen Modulus (G ± 1sd)

MPa MPa

117 D 1150 ± 10 87 D 980 ± 30

120 c 1220 ± 20 90 B 955 ± 10

90 c 1000 ± 10

mean (95%) 1190 ± 100 mean (95) 980 ± 50 confidence level) confidence level)

page 47 of 67


Table 8: Moduli measured for acetal and polypropylene using bulk specimen tests

Acetal I

specimen I G (MPa) I

Arcan 002B 1130 * 15

mean Arcan I 1060 ± 200 I

Polypropylene II specimen I G (MPa) II Tensile average 540 ± 70 II Arcan 002A 480 ± 20

450 ± 20

Arcan 002B 507 ± 25 487 ± 5

Arcan 002C 490 ± 30 492 ± 5

Arcan 002D 487 ± 5

mean Arcan 485 ± 40

page 48 of 67


Table 9 Moduli of AV119 bulk specimens measured using the Arcan shear extensometer

Tensile Specimen Modulus (MPa) Arcan Specimen Modulus (MPa)

M01-290 A 1100 ± 60 M01-290 B 1104 ± 34

M001-290 C 1070 ± 50 1170 ± 25

M01-291 A 1100 ± 60 M01-291 B 1095 ± 45

M01-291 C 1180 ± 60 1090 ± 30 1200 ± 40

M01-291D M01-291D 1050 ± 15

M01-286 A 1060 ± 50

M01-286 B 1070 ± 10 1040 ± 50 970 ± 30

1030 ± 70

M01-286 C 930 ± 80 1025 ± 20

M01-286 D 1085 ± 20 1010 ± 10

M01-286 E 1070 ± 30

Mean 1100 ± 60 Mean 1070 ± 140

-.

. .

page 49 of 67


Table 10: Bulk specimen shear moduli data for TE251

1

Tensile specimen I Modulus (MPa) . 1

I

HTE 108 A 1000 ± 40

HTE 109 A I 990 ± 30

HTE 023 A I 950 ± 50

HTE 081 A ! 1030 ± 80

HTE 102 B I 980 ± 60

HTE 103 A 1010 ± 40

mean I 990 ± 70

Arcan specimen I Modulus (MPa)

HTE 108 B I 1010 ± 20 1060 ± 30

HTE 108 C I 950 ± 20

HTE 108 E I 860 ± 80

HTE 109 B 1010 ± 10 1110 ± 10

HTE 109 C I 940 ± 10

HTE 109 D I 950 ± 20 985 ± 20

HTE 023 E I 860 ± 25

HTE 081 C I 1080 ± 15

HTE 102 C I 930 ± 10

HTE 102 E I 790 ± 20

HTE 103 E 950 ± 20 900 ± 30 880 ± 50 920 ± 20

Mean I 950 ± 170

—

page 50 of 67


Table 11: Bulk specimen shear moduli data for polyurethane

Tensile Specimen Calculated Shear I Bulk Arcan I Shear Modulus Modulus (MPa) Specimen (MPa)

HPD001 A I 62.0 ± 6.2 I HPDOO1 C 45.6 ± 5.8 64.1 ± 8.9

HPD001 B 61.5 ± 2.2 61.3 ± 6.2

~

HPE001 A 68.9 ± 7.3 73.3 ± 5.0

=

Mean HPD&HPE I 65 ± 15 I Mean 62 ± 28

HPC001 A

160 ~

.

—

page 51 of 67


Table 12: Bulk specimen shear moduli for acrylic F241

Tensile Calculated Bulk Arcan Shear Modulus Specimen Shear Modulus Specimen (MPa)

(MPa)

M02-239 A 222 ± 5 M02-239 B 171 ± 15 166 ± 3 168 ± 9 170 ± 25

M02-242 A 180 ± 10 M02-242 B 182 ± 10 174 ± 14 178 ± 18 179 ± 5

M02-243 A 170 ± 12 M02-243 B 166 ± 15 172 ± 10 179 ± 10 189 ± 22

M02-326 A 272 ± 18 M02-326 B 231 ± 11 257 ± 12 246 ± 12 219 ± 36 249 ± 26 246 ± 17

M02-326 C 258 ± 7 259 ± 30

M02-327 A 263 ± 14 M02-327 B 265 ± 3 248 ± 4

M02-327 C 269 ± 12 254 ± 7 246 ± 12 203 ± 12

U02-330 A 320 ± 20 M02-330 B 274 ± 5 265 ± 6

M02-330 C 248 ± 17

M02-330D 260 ± 20 246 ± 23

page 52 of 67


—.

M02-376 A 200 ± 15 M02-376 B 166 ± 11 198 ± 15 135 ± 25

M02-376C 162 ± 13

M02-377A 163 ± 10 M02-377 B 226 ± 26 176 ± 11 212 ± 50 174 ± 11 182 ± 5

M02-377 C 182 ± 18

M02-379 A 183 ± 15 M02-379 B 176 ± 11 194 ± 15 169 ± 18

150 ± 25

M02-379 C 153 ± 30

M02-381 A 184 ± 12 M02-381 B 131 ± 14 179 ± 12 140 ± 10

M02-381 C 152 ± 30

Mean 376-381 185 ± 30 Mean 376-381 170 ± 60

-.

I —

page 53 of 67


Table 13 Shear moduli of epoxy joint specimens

AV119 TE251

Specimen Modulus (MPa) Specimen Modulus (MPa)

M03-304 1100 ± 30 M05-314 835 ± 10 1065 ± 30 930 ± 30 1040 ± 15 930 ± 20

M03-298 1000 ± 80 M05-315 940 ± 100

M03-253 940 ± 30 HTE 124 1120 ± 40

M03-292 880 ± 200 HTE 120 910 ± 80 1070 ± 20 870 ± 30 1130 ± 30 860 ± 35 1065 ± 15 930 ± 40

970 ± 20 1100 ± 20 950 ± 15

1030 ± 30 940 ± 20

M03-208 1035 ± 35 960 ± 30

M03-303 980 ± 10

M03-293 910 ± 40 1000 ± 40

M03-294 1180 ± 30

Mean 1020 ± 160 Mean 950 ± 160

page 54 of 67

. . . NPL Report No CMMT(B)56

-.

Table 14: Shear moduli of polyurethane and acrylic joint specimens

Polyurethane 3M 3532 Acrylic F241

Specimen Modulus (MPa) Specimen Modulus (MPa)

M06-342 57 ± 15 M04-383 247 ± 28 76 ± 9 234 ± 15 62 ± 11 210 ± 20

220 ± 20 218 ± 18

M06-344 45 ± 5 M04-384 273 ± 22 53 ± 5 260 ± 10 55 ± 7

M06-347 71 ± 12 M04-385 270 ± 16 274 ± 16

M06-450 64 ± 20 M04-386 220 ± 20

M06-453 72 ± 3 M04-393 313 ± 50 74 ± 2 274 ± 10

M04-395 220 ± 17 160 ± 10

Mean 63 ± 20 Mean 240 ± 80

I —

page 55 of 67


APPENDIX I

ASSESSMENT OF THE UNCERTAINITES IN DETERMINING SHEAR MODULUS FROM ARCAN TESTS PERFORMED USING THE SHEAR EXTENSOMETER

The uncertainties are combined using the sum of squares methods. All uncertainties in mean quantities are calculated from the standard deviations at the 95 % confidence level

Calculation of Shear Modulus (G) from shear stress (6) and shear strain (E)

G=~ E

(4)

The uncertainty in G (u~) is calculated from the uncertainty in stress u. and in strain u:

(%)2 = (%2 + (5)2 (5) a e

page 56 of 67


displacements:

UNCERTAINTIES IN STRAIN MEASUREMENTS

Principle for Measuring Displacements

dm

Y x _l

i c I

d

PIVOT TRANSDUCER

●

I I

N PI P2 II T1 T2

Calculation of Lever Ratio -

X and Y are approximately

d=+dm (6)

distances measured with traveling microscope resolution 0.01 mm I

—

T = (T1+T2) ~ _ O’1 +~2) 2 2

(7)

X = T-P Y = P-N

20 mm. Y/X is approximately 1. Resolution uncertainty of traveling microscope taken as 0.02 mm

page 57 of 67


UNCERTAINTIES IN MEASURED LEVER LENGTHS

in T, UT == 0.03 mm; P, UP = 0.03 mm; N, UN = 0.02 mrn

therefore the uncertainties in the lever arm lengths X and Y are

Ux == 0.05 mm; Uy == 0.04 rnrn

The uncertainty in the lever ratio r (=Y/X),

u, = 0.003

Typical values measured when calibrating lever ratios (5 repeats of each measurement):

Extensometer Lever r uncertainty at 95% confidence level

1 A 1.009 0.002

1 B 1.008 0.004

2 A 1.008 0.003

2 B 1.008 0.003

e GL

o

dA dB

The uncertainty in the displacement transducer reading is assumed to be & 0.5% of the reading (based on the displacement transducer calibration).

page 58 of 67


Displacements measured by the transducers are A and B, as the lever ratio r is = 1, dA = A and d~ = B.

Shear displacement, D = dA - d~ = ArA - Br~

From measurements on bulk specimens it was observed that: d~ = %dA

asB= %A a D = %A, assuming r = 1. .

More precisely

D = ArA - %Ar~ = A(rA - %r~) from calibration data

for extensometer 1

= D = A( (1.009 ± 0.002) - %(1.008 ± 0.004) )

= A (0.337 ± 0.005) .

for extensometer 2

a D = A( (1.008 ± 0.003) - %(1.008 ± 0.003) ) —

= A (0.336 ± 0.004)

Calculations are based on the

Measured strain values are approximately ± 1.5%.

assumption that r = 1.00 and therefore D = 0.333A.

therefore around 1% too high with an uncertainty of

From measurements on joint specimens it was observed that: d~ = %dA

for extensometer 1

s D = A( (1.009 ± 0.002) - %(1.008 ± 0.004) ) —

= A (0.673 ± 0.005)

for extensometer 2

s D = A( (1.008 t 0.003) - %(1.008 ±+ 0.003) )

I

= A (0.672 ± 0.004)

Calculations are based on

Measured strain values approximately ± 1.5%.

. .

the assumption that r = 1.00 and therefore D = 0.667A.

are therefore around 1% too high with an uncertainty of

page 59 of 67


UNCERTAINTIES IN GAUGE LENGTHS

Bulk Specimen

A typical needle separation for both extensometers is 3.0 mm, this is repeatable to within ± 0.15 mm

Therefore the uncertainty in the gauge length is around 5%.

Joint Specimen

A typical bondline thickness is 0.5 mm, this is repeatable along the specimen on either side to within ± 0.03 mm

Therefore the uncertainty in the gauge length is around 6%.

UNCERTAINTIES IN DISPLACEMENT TRANSDUCER OUTPUTS

Transducers are calibrated to ± 0.5 %

Transducers have a resolution of ± 0.1 pm

Typical displacements recorded in tests (0.5% strain)

Bulk Specimen Joint Specimen

d= 15pm d = 2.5pm

uncertainty = 0.7 % uncertainty = 4%

total uncertainty in transducer outputs

bulk specimen test = 0.9 % Joint Specimen test = 4 %

OVERALL UNCERTAINTY IN STRAIN MEASUREMENT

Bulk Specimen

uncertainty in gauge length = 5 %, uncertainty in lever ratio = 1.5 %, uncertainty in transducer = 0.9 %

s uncertainty in bulk strain = 6 %

Joint Specimen

uncertain y in gauge length = 6 %, uncertainty in lever ratio = 1.5 %, uncertainty in transducer = 4 %

a uncertainty in joint strain = 7.4 %.

page 60 of 67

.


UNCERTAINTIES IN STRESS MEASUREMENTS

The stress (o) is calculated from the applied force (F) measured by the load cell and the cross-sectional area of the specimen between the notches:

stressa= ~, A = separation of notches x specimen thickness A

The load cells are calibrated to be accurate within 0.1 %, uncertainties in

(9)

the load cell calibration are therefore negligible.

Typical cross-section:

Bulk specimens (measured by calipers): Joint specimens (measured traveling microscope)

length = 12 ± 0.1 mm; length = 9 ± 0.1 mm; thickness = 2 ± 0.02 mm; thickness = 6 ± 0.05 mm;

by callipers and

A = 24 ± 0.3 rnmz (uncertainty = 1.3%) A = 54 ± 0.8 rnmz (uncertainty = 1.4%)

MODULUS UNCERTAINTIES FOR BULK SPECIMEN TESTS

The uncertainty in the stress measurements are approximately 1.4 %

The uncertainty in the strain measurements are approximately 6 %

The uncertainty in modulus measurements = 6.2 %

MODULUS UNCERTAINTIES FOR BULK SPECIMEN TESTS

The uncertainty in the stress measurements are approximately 1.4 %

The uncertainty in the strain measurements are approximately 7.4 %

The uncertainty in modulus measurements = 7.5 %

The uncertainties in modulus measurements for bulk and joint specimen tests are very similar and the bulk specimen test does not appear to have any significant advantages over the joint specimen test. This is due to the poor repeatability of the needle separation and, hence, the extensometer gauge length. To improve the accuracy of the test the extensometer gauge length needs to be much more stable.

page 61 of 67


APPENDIX II

ARCAN TEST ON JOINT SPECIMENS - CORRECTION FOR DISPLACEMENT OF THE ADHERENDS

d

I

I

L

I I

ai--

d2

dl -

-*

h—

Separation of measuring points = L Displacement in adhesive = dz (extensometer gauge length)

Thickness of adhesive layer = h Displacement in adherends = dl

page 62 of 67


Measured displacement = d = 2dl + dz

Apparent shear modulus Ga=~.h

True shear modulus = G=~.h 4

Shear modulus of metal adherends = G~

- ~.(L-h) ‘m - 2d,

G 2d, —= AMQL.1 +— Ga dah~ 4

~ .4 24 _ h .ti,

Gm 4- u(L-h) (L-h) ~

re-arranging (5) gives

2dl _ G (L-h)

~ Gn” h

(lo)

(11)

(12)

(13)

(14)

(15)

By substituting (6) into (4) and re-arranging the true shear modulus (G) is obtained in terms of the measured shear modulus (G.), the adherend modulus (G~), the extensometer gauge length (d) and the adhesive bondline thickness (h).

G. G = Ga.

Gm+G=(l -;)

If typical values are taken;

then G can be calculated in

(16)

d = 3.0 mm, h = 0.5 mm for q joint specimen with steel adherends (G~ = 27 GPa) – ‘

terms of the measured value (Ga)

eg Ga = 1000 MPa (eg an epoxy) G = 1.065 G. G= = 200 MPa (eg the acrylic) G = 1.012 G.

The corrections The corrections are negligible.

to the measured moduli range up to 7-8% of G=

.

for the stiffest adhesives. due to adherend deformations for the acrylic and polyurethane adhesives

page 63 of 67


LIST OF FIGURES

Figure 1:

Figure 2:

Figure 3:

Figure 4:

Figure 5:

Figure 6:

Figure 7:

Figure 8:

Figure 9:

Figure 10:

Figure 11:

Figure 12:

Figure 13:

Figure 14:

Figure 15:

Figure 16:

Figure 17:

Figure 18:

Figure 19:

Notched-beam (Iosipescu) and notched-plate (Arcan) test methods, directions of loading are indicated by arrows.

Arcan bulk test specimen dimensions as used in the present work.

Illustration of Iosipescu test fixture.

Strain gauged Iosipescu and Arcan bulk test specimens

Loading assembly for Arcan test showing adaptors for tensile test machine.

Frames for mounting an Arcan specimen in the test assembly. The method of clamping test specimens is shown.

Photograph of a shear extensometer for the Arcan test which was designed and constructed for this work.

Side view of Arcan extensometer. The needle points (N), pivot (P) and thumbscrew (T) define the lever lengths.

Arcan extensometer mounted on Arcan test frame. The spring (S) and hinge (H) allow the levers to move forward to contact the specimen. The cam is used for engaging the extensometer.

Shear stress distribution in bulk Arcan testpiece, calculated by finite element methods.

Shear stress distributions calculated by FEA between the notch roots of bulk Arcan specimens having notch root radii of 1.0 mm, 1.5 mm and 2.0 mm.

Shear stress distribution calculated by FEA across the width of the bulk Arcan testpiece between at the centre of the line joining the notch roots.

Principal stress distribution along the length of the bulk Arcan specimen gauge section as calculated by FEA.

Principal stress distribution along notch edge calculated by FEA.

Arcan joint specimen after failure. The shaped metal adherends are symmetrical. The adhesive has peeled cleanly from one face revealing the grit blasted bonding surface.

—

Jig for manufacturing joint specimens.

Finite element mesh used in FE analysis of the Arcan joint.

Shear stress distribution along centre and edges of the bondline, calculated for 5 mm long bond with flat spews with a nominal shear stress of -30 N.

Effect of mesh size on calculated stress concentrations, calculated by FEA for the left bond edge of a 5 mm long bond with flat spews with a nominal shear stress of -30 N.

page 64 of 67

Figure 20:

Figure 21:

Figure 22:

Figure 23:

Figure 24:

Figure 25:

Figure 26:

Figure 27:

Figure 28:

Figure 29:

Figure 30:

Figure 31:

Figure 32:

Figure 33:

Figure 34:

Figure 35:

Figure 36:


Effect of bond length on stress distributions, calculated by FEA for the left bond edge of a 5 mm and 10 mm long bonds with flat spews with a nominal shear stress of -30 N.

Effect of fillet shape on shear stress distributions predicted by FEA.

Tensile peel (direct) stress distribution, calculated by FEA, along centre and edges of the bondline, calculated for 5 mm long bond with flat spews with a nominal shear stress of -30 N.

Effect of fillet shape on peel stress distributions, calculated by FEA.

Effect of bond length on peel stress distributions, calculated by FEA for the left bond edge of a 5 mm and 10 mm long bonds with flat spews with a nominal shear stress of -30 N.

Epoxy AV119, stress/strain data measured in a bulk Arcan test using a strain gauged specimen.

Epoxy TE251, stress/strain data measured in a bulk Arcan test using a strain gauged specimen.

Epoxy AV119, stress/strain data measured in a bulk Arcan test using the Arcan shear extensometer.

Epoxy TE251, stress-strain data measured in a bulk Arcan test using the Arcan shear extensometer. Agreement between individual extensometers is poor.

Acrylic F241, stress/strain data measured in a bulk Arcan test using the Arcan shear extensometer. Agreement between the individual extensometers is good.

Polyurethane 3M 3532, stress/strain data measured in a bulk Arcan test using the Arcan shear extensometer. Agreement between the individual extensometers is good.

Polyurethane 3M 3532, stress/mean-strain data measured for during two tests (involving several measurements) using extensometer.

a single specimen the Arcan shear

Polyurethane 3M 3532, plot of strain data vs time measured during a bulk Arcan test using the Arcan shear extensometer.

Polyurethane 3M 3532, plot of stress/time measured during a bulk Arcan test using the Arcan shear extensometer.

Epoxy AV119, stress/strain data measured in an Arcan joint test using the Arcan shear extensometer.

Epoxy TE251, stress/strain data measured in an Arcan joint test using the Arcan shear extensometer.

Acrylic F241, stress-strain data measured in an Arcan joint test using the Arcan shear extensometer.

page 65 of 67


Figure 37:

Figure 38:

Figure 39:

Figure 40:

Figure 41:

Figure 42:

Figure 43:

Figure 44:

Figure 45:

Figure 46:

Figure 47:

Figure 48:

Figure 49:

Figure 50:

Figure 51:

Polyurethane 3M 3532, stress-strain data measured in an Arcan joint test using the Arcan shear extensometer.

Acetal, stress-strain curves to failure measured using the bulk Arcan test and the Arcan shear extensometer.

Epoxy AV119, stress-strain curves to failure using the bulk Arcan test. Strain measurements were made using the Arcan shear extensometer (dotted lines) and strain gauges (thick lines).

Epoxy TE251, stress-strain curves to failure using the bulk Arcan test. Strain measurements were made using the Arcan shear extensometer (solid lines) and strain gauges (thick dotted lines).

Arcan test specimens after failure.

Epoxy (TE251) specimen after failure. Cracks have initiated on the notch edges close to notch roots where FEA predicts tensile stress concentrations.

Polyurethane specimen after failure. The two lines of dots curving down the specimen show the extensometer contacts during the test. The final contacts are alongside the notch root. This specimen has recovered the deformations applied during the test. Failure was initiated near the notch root.

Acrylic specimen after failure. The buckling is evident from the distortion of the larger half of the specimen.

Polyurethane, stress-strain curves illustrating the effects of buckling. The bulk Arcan specimens were manufactured from the same cartridge and tested both with (solid lines) and without (dashed lines) restraining clamps. Sheets HPE001 and HPE003 were 2 mm thick, sheet HPE002 was 4 mm thick.

Acrylic F241, stress-strain curves to failure measured for a bulk Arcan specimen. The individual extensometer readings are shown as thin dotted lines, the mean measured strain as a thick, dashed line and the extrapolated strain as a thick, solid line.

Polypropylene, stress-strain curves to failure measured using the bulk Arcan test and the Arcan shear extensometer. Tests were performed at two strain rates (2 and 20%. per minute).

Polyurethane, stress-strain curves for bulk Arcan specimens tested without restraining clamps. The dashed lines show the measured strains and the solid lines show the extrapolated strains. —

Polyurethane, stress-strain curves for bulk Arcan specimens tested with restraining clamps.

Acrylic F241, stress-strain curves for bulk Arcan specimens tested with restraining clamps. The dashed lines show measured strains and the thick solid lines show extrapolated strains.

Epoxy AV119, stress-strain measurements to failure using the bulk Arcan test and Arcan shear extensometer. Tests were performed at different initial strain rates (1 to 20 % per minute).

page 66 of 67


Figure 52:

Figure 53:

Figure 54:

Figure 55:

Figure 56:

Figure 57:

Figure 58:

Figure 59:

Figure 60:

Figure 61:

Polyurethane, stress-strain curves for bulk Arcan specimens to failure measured at three different strain rates (4.5, 9 and 13%. per minute). The dashed lines show the measured strains and the thick, solid lines the extrapolated measurements.

Acrylic F241 Arcan Joint, stress-strain curve to failure showing good agreement between individual extensometers.

Epoxy AV119 Arcan test, stress-strain curves for a bulk and a normal joint specimen showing poor agreement at high loads.

Aluminium strip bonded to a steel adherend to improve gripping.

Epoxy AV119, comparison of stress-strain curves for a typical bulk specimen (thick solid line), normal joint specimens with steel adherends (thin dashed lines), improved joint specimens with aluminium strips (thin solid lines) and joint specimens with aluminium adherends (thick dashed lines).

Epoxy TE251, comparison of stress-strain curves for typical bulk specimens (thick solid lines), a normal joint specimen with steel adherends (thin dashed line), an improved joint specimen with aluminium strips (thick, short dashes), a normal joint with reduced, 5 mm long bond (thick, long dashes) and a specimen with aluminium adherends (thin solid line).

Polyurethane Arcan Joints, stress-strain curves measured at 10-15% per minute (solid line) and 4 % per minute (dashes).

Polyurethane, comparison of stress-strain curves measured for joint specimens and bulk specimens (clamped and unclamped) manufactured from the same cartridge.

Acrylic F241 Arcan joints, stress-strain curves measured with normal, 10 mm, length bonds (thin dashed lines) and shorter, 5 mm, length bonds (thick solid lines).

Acrylic F241, comparison between typical Arcan joint specimens (solid lines) and a typical bulk specimen manufactured at the same time (long dashes) and bulk specimens with comparable modulus (- 250MPa) to the joint specimens (thick short dashes).

I —

page 67 of 67

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Distance from top bond end, mm

Figure 24: Effect of bond length on direct stress distributions, calculated by FEA, along

the Ieft edge of 5 and 10mm long bonds with flat spews at a nominal -30 MPa shear stress

Figure 25: Epoxy AVl19, stress/strain data measured in a bulk specimen Arcan test using strain gauges

27/02/96 830AV119.xls Chart 2 27/02196 576HTE25.XLS Chart 1

AV119 bulk Arcan specimen test

‘---sidel

‘------ side2

— average

o 0.001 0.002 0.003 0.004 0.005

shear strain

I

TE251 bulk Arcan specimen

,. .,.

strain gauge S2 ,.’.

. .

, .-

,. ,.

,, #,-

— average

‘--- sidel

‘------ side2

I I

o 0.001 0.002 0.003 0,004 0.005 0.006 0.007

shear strain

Figure 27: Epoxy AV119, stress/strain data measured in a bulk Arcan specimen test using the shear Figure 26: Epoxy TE251, stress/strain data in a bulk specimen Arcan test using strain extensometer. gauges

27/02/96

1.8

1.6

1.4

1.2

6

g’

: S! % % ~ 0.8 w

0,6

0.4

0.2

0

4U2ACRLY.XLS chart 2

,, . .

Acrylic F241 bulk Arcan specimen test

moduli (G) are between 0.003 and 0.006 strain .“

.,

,,

,“

.,

,’

--- side 1 G = 163 MPa

side 2G= 178MPa

,. — average G. 176 MPa

o 0.002 0,004 0.006 0.008 0.01 0.012

strain

Figure 29: Acrylic F241, stress/strain data measured in a bulk Arcan specimen test using the shear extensometer.

27/02196 961 HTS25.XLS Chart 2

i+

7

6

5

4

3

2

1

-43

------ side lG=1020MPa

‘--- side 2G= 740MPa

— average G. 910 MPa

,’

!’

I ..-”

.’ t“ .’ /f

! .’ ,’ f!

.’ .’ II . .

.’ ft ,’ /

,’ ,“

,,

/’

.’ .’

“ .“

>’ ,’

>’ ,.

,’ ,’

,’

,’

,’ ,’

>’

>’ >’ Epoxy TE251, bulk Arcan specimen test

!’

moduli (G) calculated between 0.002 and 0.005 strain ,’

!’

‘/ I I I I I I I I I I

-0.001 0 0.001 0.002 0,003 0,004 0.005 0.006 0.007 0.008 0.009 0.01 O.Olt

shear strain

Figure 28: Epoxy TE251, stress/strain data measured in a bulk Arcan specimen test using the shear extensometer. Agreement between the individual extensometers is poor particularly at small strains.

27/02/96 HPD001.XLS Chart 1 27/02/96 0116HPU.XLS Chart 2

1

0.9

0.8

0.7

0.6 w

g m m g 0.5 5 a :

0.4

0.3

0.2

0.1

0

Test 2

Polyurethane bulk Arcan specimen HPD001 D

Two repeat tests shown Test 1

discontinue”

1

discontinuity

I 1 I I I I I I I

1

0.9

0.8

0.7

0.8 1?

!& (0 I 0.5 G % 2 to

0.4

0.3

0.2

0.1

0

/ .,’ . . . ,. .

., .’

.“ Polyurethane bulk Arcan specimen test ;“-

,’ .-

moduli (G) determined at 0.01 strain “ , .“ ,’

,’ ,’

.’ ,“

.’ ,“

.“ .’

— .’

J’ .’

.-. . ...’ ‘

.“” . . . ,., ,.-

,“ ,’

.’ ,“

.“ .“

.’

.’ ,’

,’ .’

.’ :

: :

— average G. 72.5 MPa

‘–-- sidel G.63.9MPa

‘------ side 2G=79.OMPa

I I I I I [ I I I

o 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0,016 0.02

shear strain o 0.002 0,004 0.006 0.006 0,01 0.012 0.014 0.016 0.018 0.02

shesr etrain

Figure 31: Polyurethane 3M 3532, stress/mean-strain data measured for a single specimen during two repeat tests (each involving 3 or more measurements),

Figure 30: Polyurethane, stress/strain data measured in a bulk Arcan specimen test at 4% strain/min using shear extensometer.

m

co

m

co z 0

z m

0

0 0 0

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meqs

CD

0 0 0

-f 0

2 0.

0 0

0

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0

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Lo

m

0 m

m

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28/02/96 0504HE25.XLS Chart 2 28/02/96 725AV119.XLS Chart 2

4

3.5

3

2,5

6-

g W

32 G s ;

1.5

1

0,5

0

Epoxy TE251 Arcan Joint Specimen

I ,’

,’

“ / .’

/

j ,’ (

/’ ,

,,/

,’ /’ ,.,

// ,’

.’ /’

Y ./ ‘

,, / : . . ,. .,

:: /’ . . :. ,

— average G. 960 MPa

~~ side 1 G =745 MPa

- - side 2G=1335MPa

moduli (G) determined between 0,001 and 0,0025 strain I I I I I I I

o 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0,004

shear strain

Figure 35: Epoxy TE251, stress/strain data measured during an Arcan joint test. . .

;

‘/‘ /, ~ ~

Epoxy AV119 Arcan Joint Specimen ~ (( .:”

,. ,,. ,>.’

~>” ..’. ,; , ,{

,“ {’ .“ /

,’/’ ,, , ,“ ,

,< .“ \,

,. ,,”

,.

‘------ side lG=1360MPa

““ side 2 G = 820 MPa

— average G .1025 MPa

if ~ moduli (G) determined between 0.001 and 0.003 strain

I I I I

o 0.001 0,002 0.003 0.004 0,005

shear strain

Figure 34: Epoxy AV119, stress/strain data measured during an Arcan joint specimen trot.

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45

40

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30 3

g In ~ 25 z s 2 U)

20

15

10

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AVARCF.XLS Chrut 3

f -. -.. ..5 .

Thick lines - strain measured by strain gauges

Dotted lines - strain measured by shear extensometer

I I I I I

o 0.05 0.1 0.15 0.2 0.25 0.3

shear strain

28f0’2196

60

50

40

3 n. ~ W In ~ 30 5 % 2 01

20

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HPBSHR.XLS Ch.rt 1

-=,,. . - . . . .---G-==- Acetal Bulk Arcan Specimens -.-,2 y>z--—

strain rate = 1 % strain/rein . ..>’ ;;/y ,,

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,- .’

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I I I I I I I

o 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

shear strain

Figure 39: Epoxy AV119, stress/strain curves to failure measured using the bulk Arcan Figure 38: Acetal polymer, stress/strain curves to failure measured using the shear specimen test. The nominal strain rate was 1% strain/min. extensometer. Temperature was 23C and strain rate 1 % strain/min.

1

—

i“; 1

‘IEz51A.xLs Chd 3

TE251 bulk Arcan specimen tests

. . . ----- . ---- /--”’ ,.- ,,-

solid lines - measured with shear extensometer

dotted lines - measured with strain gauges

I I I I I I I I I

o 0,005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

shear strain

Figure 40: Epoxy TE251, stress/strain curves to failure measured using the bulk Arcan specimen test. The nominal strain rate was 1% strain/min.

(edw

) sseJ&

JS

8q

S

. .

. . . . . . . . . . . . . .

--.-—----

------- .-.

..- . . . . . . . . . . . . . . ...-...-..---”-’ “.

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—=.

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2 \.,

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7 n

.-., . “=. \ G

2 .g

i?

2

I

u N

w

0

0

22/07/96 HPA002 Chart 2

25

20

15

10

5

0

0

------- - -/”” ----

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/’.’” , ‘.

,, i /,’-

_--— —— -- .’ ‘,

‘.

,,

,, ,,

/: ,,

l— Polypropylene - bulk Arcan specimen tests

t

-—— O02B 2?40

— 002C 2’% . . . . . . O02A 20% —.. — 002D 20%

0.1 0.2 0.3 0.4 0.5 0.6 0.7

shear strain

Figure 47: Polypropylene, stress/strain curves to failure measured using the bulk Arcan

specimen test and shear extensometer. Two strain rates were used (2 and 20 %/min).

05/03/96 PUBULKAF.XLS Chart 1

-—

; $ . , , w ,

. . . . . . . HPCOO1 B (rate = 4 %/min)

— HPC001C (rate = 4 %/min)

— - HPC001D (rate = 3 %/min)

I

I I I

o 0.2 0.4 0.6 0.8 1 1.2 1.4

shear strain

Figure 49: Polyurethane 3532, stress/stiain curves for buk Arcan specimens tested using

restraining clamps. Extrapolated and measured strains agreed well and only measured

strains are plotted.

Polyurethane bulk Arcan specimen tests

jf

,

— HPCO03B extrapolated (rate = 4,5%)

-- -‘ -”- measured strain

— HPCO03C extrapolated

‘------ measured strain

{ ‘— HPCO03D extrapolated (rate = 13%)

“ “’ ..- measured slrain

all specimens cut from the same sheet I I I I 1 I I I o

0 0.2 0,4 0.6 0.8 1 1.2 1.4

shear strain

Figure 52: Polyurethane 3532, stress/strain curves to failure for bulk Arcan specimens tested using restraining clamps at different strain rates.

/~

.“.. %

8“ --%% ““”” -.. izy”” ,-----

f

-- ..”,,,.,” .---- .~; .:...

,I,t’”- -’--’ -- -——-———------------ ---------------- ---. < \

.\\

I

AV119 bulk Arcan specimen tests, Sheet M01-286

at different strain rates

.- —- 1 %/min

-—”-”.- 2 %/min

. . . . 5 %/min

- — 10 %/min

— 20 %/min

I I I I I

o 0.05 0.1 0,15 0,2 0.25 0,3

shear strain

Figure 51: Epoxy AV119, stress/strain measurements to failure for bulk Arcan test specimens at different strain rates. All specimens were obtained from the same sheet

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m

m

0

ml

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0

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06/03/96

TE251 Arcan specimen tests

— bulk specimen

— bulk specimen

. . ..-.

—---- —

——

Aluminium strips bonded to specimen

normal joint specimen 10 mm long bond

Aluminium adherends

joint specimen 5 mm long bond

I I I I I I I I tl

0 0.01 0.02 0,03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

shear strain

Figure 57 Epoxy TE251, comparison of typical stress/strain curves measured during tests on bulk Arcan specimens and several configurations of the Arcan joint specimen.

. . . . . ,. . . . .,, ‘..

. .

‘.

-‘ - “ --- normal joint specimen

--- normal joint specimen

— Aluminium strips bonded to specimen

— Aluminium strips bonded to specimen

---- Aluminium adherends

---- Aluminium adherends

— bulk specimen

AV119

I I

Arcan Joint Specimens

I I I I I

I

o 0.05 0.1 0.15 0,2 0.25 0,3 0,35 0.4

shear strain

Figure 56: Epoxy AV119, comparison of stress/strain curves measured using normal joint specimens, joint specimens with aluminium strips bonded to steel adherends, specimens with aluminium adherends and a typical bulk specimen.

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0

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< ‘. \

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(edw

) eeelle Jeeq

s

0 m

lo w

0

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(edw) sse4

3s Je

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Documents

NPL Report No CMMT(B)56 - adhesivestoolkit.com