Characterization of UHMWPE Laminates for High Strain RateApplications

Frederick Philip Cook

Thesis submitted to the Faculty of theVirginia Polytechnic Institute and State University

in partial fulfillment of the requirements for the degree of

Master of Sciencein

Engineering Mechanics

Scott W. Case, ChairJohn J. Lesko

Romesh C. Batra

January 4, 2010Blacksburg, Virginia

Keywords: UHMWPE, Time-temperature superposition

Copyright 2010 by Frederick Philip Cook

Characterization of UHMWPE Laminates for High Strain Rate Applications

Frederick Philip Cook

ABSTRACT

The research presented in this thesis represents an effort to characterize the properties ofultra-high molecular weight polyethylene (UHMWPE). As a composite of polymers, theproperties of UHMWPE are time-dependent. It is desired by research sponsors to knowthe properties of the material at high strain rates, in order to simulate the use of thesematerials in computer models. Properties believed to be significant which are investigatedin this research are the tensile properties of lamina and laminates, and the interlaminarshear properties of laminates. The efficacy of using time-temperature superposition to shifttensile properties of the composite is investigated, and a novel apparent shear strength test isproposed and demonstrated. The effects of processing the material at various temperaturesand pressures are also investigated.

This research was conducted with support from the Army Research Laboratory, Aberdeen,Maryland. Note: The opinions expressed herein are the views of the author and should notbe interpreted as the views of the Army Research Laboratory.

Acknowledgments

I gratefully acknowledge the inspiration and direction of my advisor, Dr. Scott Case, whohas been endlessly patient with me during the research and development of this thesis overthe last two years. I would also like to thank Dr. Jack Lesko and Dr. Romesh Batra fortheir guidance and support. Thanks to all the other students in the MRG, whom it was apleasure to work alongside. Thanks also to Beverly Williams, Mac McCord, Dave Simmons,Lisa Smith, Pat Baker, Dr. Ishwar Puri, and all the other faculty and staff of the EngineeringScience and Mechanics Department for their friendship and support over the years. Eachand every one of you played a part in me completing this thesis, and I thank you.

Thank you to the good folks at the Office of Recovery and Support, as well as to Tom Tillarfor your friendship in the years since April 16, 2007. There have been many high’s and low’ssince then, and you have all been there for me throughout that time.

Thanks also to my parents, Richard Cook and Phyllis Cook for their love and encouragementover the years. Additionally to Marnie Rognlien, my swimming friends (the tubby bitches),my friends on the VT Triathlon team, and my brothers in Pi Kappa Alpha for giving methe moral support and strength to complete this thesis.

It is impossible to mention everyone, or for this simple acknowledgement to express my loveand gratitude to all those who have helped me over my tenure at Virginia Tech. This schooland town have made me the person I am today, and I will have ’Ut Prosim’ in my heart forall of time.

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Contents

List of Figures vi

List of Tables viii

1 Introduction 1

2 Time-Temperature Superposition and High Rate Response of Thermoplas-tic Lamina and Laminates 3

2.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.3 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3.1 Lamina Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3.2 Laminate Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3.3 Lamina Specimen Preparation . . . . . . . . . . . . . . . . . . . . . . 6

2.3.4 Laminate specimen preparation . . . . . . . . . . . . . . . . . . . . . 7

2.4 Testing Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4.1 Creep Compliance Testing . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4.2 Lamina/Laminate Testing . . . . . . . . . . . . . . . . . . . . . . . . 8

2.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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3 The Effect of Processing Conditions on the Properties of ThermoplasticComposites 21

3.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.3 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.3.1 Laminate processing procedure . . . . . . . . . . . . . . . . . . . . . 22

3.4 Testing Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.4.1 Creep Compliance Testing . . . . . . . . . . . . . . . . . . . . . . . . 23

3.4.2 Laminate Tensile Testing . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.4.3 Differential Scanning Calorimetry (DSC) Testing . . . . . . . . . . . 24

3.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.5.1 Creep Compliance Testing . . . . . . . . . . . . . . . . . . . . . . . . 25

3.5.2 Laminate Tensile Testing . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.5.3 Differential Scanning Calorimetry Testing . . . . . . . . . . . . . . . 26

3.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4 Apparent Shear Strength of UHMWPE Composites 32

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5 Conclusions 39

5.1 Tensile Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

5.2 Shear Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.3 Processing Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.4 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Bibliography 42

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List of Figures

2.1 Lamina specimens cut from consolidated lamina panel. . . . . . . . . . . . . 6

2.2 An untested laminate specimen (upper) and a failed laminate specimen (lower)with extensometer tabs attached. . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3 These extensometer tabs were made of aluminum. A: 12.7, B: 4.7, C: 1.6, D:1.8, E: 1.0, all mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.4 MTS load frame with temperature chamber and lamina sample with exten-someter attached. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.5 Temperature of the specimen as the environmental chamber is cooled. Cham-ber thermocouple refers to the temperature the environmental chamber reads.Free thermocouple refers to the temperature of a thermocouple freely hangingdirectly adjacent to the sample. Specimen thermocouple refers to a thermo-couple glued to the specimen. . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.6 A laminate specimen equipped with a thermocouple, gripped and with theextensometer attached. The ”free thermocouple” is seen positioned next tothe specimen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.7 Lamina stress at failure vs. strain rate at various temperatures. . . . . . . . 11

2.8 Unshifted creep compliance of a laminate sample. . . . . . . . . . . . . . . . 12

2.9 Shifted creep compliance of a laminate sample. . . . . . . . . . . . . . . . . . 12

2.10 Shift factors associated with the master curve in Figure 2.9. . . . . . . . . . 13

2.11 Shift factors for three samples of laminate taken from creep compliance data. 13

2.12 Shift factors for lamina taken from creep compliance data. . . . . . . . . . . 14

2.13 Creep compliance of a laminate sample subjected to two consecutive tem-perture sweeps. Solid lines are data from the first temperature sweep, andsymbols are data from the second temperature sweep. . . . . . . . . . . . . . 15

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2.14 A lamina specimen was designated as a normal failure (upper) and a laminaspecimen designated as a shear failure (lower). . . . . . . . . . . . . . . . . . 16

2.15 Strength master chart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.16 Strain at failure master chart. . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.17 Lamina and laminate shift factors for S3000 unidirectional and SS 3124 cross-ply composites compared with shift factors from dynamic testing and stressrelaxation testing of a Dyneema SK66 fiber, another UHMWPE, performedby Govaert et al. (1993). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1 Temperature profile and DSC measurement for a sample from Panel 4. . . . 24

3.2 Unshifted creep compliance of a laminate sample. . . . . . . . . . . . . . . . 25

3.3 Shifted creep compliance of a laminate sample. . . . . . . . . . . . . . . . . . 26

3.4 Shift factors associated with the master curve in Figure 3.3. . . . . . . . . . 27

3.5 Creep compliance shift factors of panels at various processed at various con-ditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.6 Laminate strength vs. processing pressure. . . . . . . . . . . . . . . . . . . . 29

3.7 Laminate strength vs. processing temperature. . . . . . . . . . . . . . . . . . 29

3.8 DSC vs. temperature for a sample from Panel 4, with the area approximatingthe enthalpy of fusion for the sample highlighted in red. . . . . . . . . . . . . 30

3.9 DSC vs. temperature for a sample from Panel 4, with the area approximatingthe enthalpy of recrystallization highlighted in red. . . . . . . . . . . . . . . 31

4.1 Apparent shear strength test with assumed stress state. . . . . . . . . . . . . 33

4.2 The [0,90]4S specimens were peeled back to create a τ31 stress at one end thatresulted in an interlaminar shear failure. . . . . . . . . . . . . . . . . . . . . 33

4.3 Typical load, displacement vs. time curve. . . . . . . . . . . . . . . . . . . . 34

4.4 Global-local model. (Used with permission of Gautum Gopinath, 2010) . . . 35

4.5 τ31 vs. -σ33 for SS 1214 laminates, with linear best-fit line. . . . . . . . . . . 36

4.6 τ31 in Pa for global model. (Used with permission of Gautum Gopinath, 2010) 37

4.7 Contour plot of σ33 from local model developed at the edge of the grippedsurface where the stress concentration appears. (Used with permission ofGautum Gopinath, 2010) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

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List of Tables

3.1 Processing conditions for various panels. . . . . . . . . . . . . . . . . . . . . 23

3.2 Enthalpy, melting and recrystallization temperatures, and calculated crys-tallinity, Xc for each sample tested. . . . . . . . . . . . . . . . . . . . . . . . 27

4.1 Geometric dimensions of global model. (Used with permission of GautumGopinath, 2010) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.2 Elastic moduli of the composite. (Used with permission of Gautum Gopinath,2010) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

viii

Attribution

The whole of this document was written by Frederick Philip Cook. In Chapter 4, Tables 4.1and 4.2, and Figures 4.4, 4.6, and 4.7 are the work of Gautum Gopinath, who completed themodeling work for this project. All other figures are original work.

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Chapter 1

Introduction

Ultra-high molecular weight polyethylene (UHMWPE) has been a very challenging materialto work with and characterize. The main focus of the majority of our work with laminaand laminates has been on the tensile properties of the material in the fiber direction. Weapproached the project as we would to characterize any other material, using standard com-posite material testing techniques and equipment. This proved to be a very naive approach,and we had a much more difficult time developing practical, repeatable techniques than wehad expected.

Over a period of several months, many different techniques for determining tensile propertieswere proposed and tested. After some time, we found a degree of success testing thinlaminates at high grip pressures using hydraulic testing equipment. During the research,however, much was learned about the material that led us in many other directions. Testingof thicker specimens at lower grip pressures resulted in interlaminar failure, which led to thedevelopment of an apparent shear strength study, described in Chapter 4. Observations ofdifferences in standard processing techniques at different manufacturing facilities led to astudy of the effect of processing conditions on the properties of the composite, described inChapter 3.

The main focus remained on tensile properties, and a time-temperature superposition (tTSP)approach was taken in order to use properties generated on our hydraulic equipment andscale it up to high strain rates. Again, this proved much more difficult than we had antici-pated. tTSP is a common technique, and work done by Alcock and others to shift polymercomposite failure data and create nice, smooth master curves led us to the same approach(Alcock et al., 2007). Our data produced anything but nice, smooth master curves, andother methods of determining shift factors were examined. Creep compliance testing usingdynamic mechanical analysis equipment produced predictable, repeatable results, thoughstill no nice, smooth master curve. Nonetheless, the efficacy of using creep compliance datato shift tensile failure data is examined in Chapter 2.

1

2

The three papers presented in this document are in various phases of publication. The Effectof Processing Conditions on the Properties of Thermoplastic Composites will be publishedat the 2010 Society of Experimental Mechanics (SEM) Annual Conference in June, 2010.Time-Temperature Superposition and High Rate Response of Thermoplastic Lamina andLaminates will also be published at the SEM Annual Conference, though it will be combinedwith another paper in which similar work is completed on UHMWPE fibers. Apparent ShearStrength of UHMWPE Composites will not be published, but the work completed here willbe used to further develop models of UHMWPE composites.

Chapter 2

Time-Temperature Superposition andHigh Rate Response of ThermoplasticLamina and Laminates

2.1 Abstract

High strain rate response of ultra high molecular weight polyethylene (UHMWPE) laminaand laminates is of interest to support computational modeling of applications such as impactdamage. In this study, the efficacy of utilizing time-temperature superposition principles(tTSP) to determine the high strain rate response of UHMWPE composites is investigated.Constituent and composite tensile properties are measured at various temperatures andstrain rates. Testing was completed at thermo-rheologically equivalent temperature-strainrate combinations to evaluate the effectiveness and limitations of the shifting approach.Analysis of modulus and failure data indicated that at the composite level, tTSP may notbe applicable, while at the lamina level Weibull analysis does not contraindicate this methodand high strain rate properties are estimated.

2.2 Introduction

The UHMWPE composites are used in applications requiring lightweight materials that arevery strong and have favorable properties at high strain rates. The two primary UHMWPEfibers and laminates for high strain-rate applications are Dyneema and Spectra and Spec-traShield, manufactured by DSM and Honeywell respectively. Developing strain rate de-pendence properties at the strain rates is not possible using standard servo-hydraulic testequipment.

3

4

One of the original developers of the Spectra fiber, Prevorsek, first determined the propertiesof Spectra 1000 fibers to be strain rate dependent by doing testing at rates from 10−2 and102 s−1 and demonstrating that elastic modulus increases with strain rate (Prevorsek etal., 1989). Tan, et al. (2006) used a Split Hopkinson bar setup to develop constitutiveproperties of aramid fibers (also used in high-strain rate applications) at high strain ratesand traditional tensile test methods at quasi-static strain rates. They found that Twaronaramid fibers are strain-rate sensitive, both in constitutive properties and failure mechanism(Tan and Zeng, 2006).

More recently, Koh et al. (2007) performed high strain rate testing of Spectra Shield lami-nates using a tensile split Hopkinson bar setup. Samples consisted of 7mm wide, 25mm gagelength [0,90] Spectra Shield product. They found that up to strain rates of 400 s−1, thereis an increase in stress and stiffness at failure and a decrease in strain with an increase instrain rate. At strain rates greater than 400 s−1, this trend reverses, reducing failure stressesand stiffness, and seeing an increase in failure strain. The study cites SEM images of brokenfilaments that undergo ductile and shear failure, as opposed to the brittle failures seen below400 s−1 (and above quasi-static strain rates )(Koh et al., 2007).

Time-temperature superposition (tTSP) is based on the principle that the properties of cer-tain materials at various strain rates may be estimated by performing tests at corresponding(but differing) rates and temperatures. It is used on time-dependent materials to estimatetheir properties on time scales much longer or shorter than is convenient or possible toperform testing on. Stiffness or compliance data taken at different temperatures may beshifted left or right along a time scale to create a master curve. This master curve canthen be used to predict the behavior of the material at time scales or strain rates that arenot physically achievable on test equipment (Brinson and Brinson, 2008). While it is onlydirectly applicable to amorphous polymers, tTSP has also been applied to failure mecha-nisms by Miyano et al., who successfully shifted composite strength data over time to predictlong-term durability in graphite fiber/vinyl ester (GFRP) laminates (Miyano et al., 2004).

In 2007, Alcock et.al. investigated the effects of temperature and strain rate on the me-chanical properties of highly oriented polypropylene (PP) tapes and all-PP composites. Theauthors analyzed strain rate and temperature effects on tensile modulus and strength anddeveloped master curves for each. In this case, it was found that different shift factors wereused to shift modulus and strength, indicating that different mechanisms contribute to thesefailures. These master curves were used to predict the constitutive behavior of the tapes andcomposites at various strain rates, including those that are difficult to achieve with physicaltesting. The author notes that the PP composite is a complex system, and is difficult tomodel as a thermo-rheologically simple material (Alcock et al., 2007).

Alcock used the Arrhenius equation to determine aT , the factor relating time and temper-ature. The Arrhenius equation is generally accepted as applicable to amorphous polymers(Ariyama et al., 2004), though Alcock has applied it to a more complex polypropylene com-posite. To apply the Arrhenius equation, a constant thermal activation energy, Ea, is defined.

5

The shift factor between two temperatures is therefore defined as

ln(aT ) =Ea

R

(1

T− 1

Tref

)(2.1)

where R is the universal gas constant, Tref is the absolute reference temperature, and T isthe absolute temperature at which the shift factor is to be computed (Alcock et al., 2007).

In this paper, temperature and rate dependence is investigated for UHMWPE lamina andlaminates, and time-temperature superposition is applied to estimate high strain rate prop-erties.

2.3 Sample Preparation

For this paper, Spectra S3000 fibers, S3000 unidirectional material (not commercially avail-able), and SpectraShield 3124 laminates were used. The unidirectional samples are referredto as lamina, and the SpectraShield 3124 samples are referred to as laminates.

2.3.1 Lamina Processing

Sheets of Honeywell pre-preg, the precursor to the commercial SpectraShield product, werecreated in a unit processing testing lab, with all Spectra fibers oriented in one direction andevenly distributed on a sheet of silicone paper, then sprayed with the proprietary Kratonmatrix. To create lamina panels, two of these sheets were placed with the fibers facingeach other, then pressed at 65 ◦C and 2.75 MPa, and held for 10 minutes. This new panelwas removed from heat and pressure, and one of the silicon sheets was carefully peeled off,leaving the now two-ply lamina on the other silicon sheet. This sheet is combined withanother identically prepared sheet and placed back in the press. To create four-ply lamina,these two sheets were pressed at 116 ◦C and 19 MPa, and held for 10 minutes, then removedfrom heat and pressure. To create 8-ply lamina, two sheets of two-ply were pressed at 65 ◦Cand 2.75 MPa, and held for 10 minutes, and then two resulting 4-ply sheets were combinedand pressed at 115.6 ◦C and 18.19 MPa, and held for 10 minutes, then removed from heatand pressure.

2.3.2 Laminate Processing

Sheets of the Honeywell commercial SpectraShield 3124 product (which has a lay-up of[0,90]) were cut to the desired dimensions, most commonly 30.5 cm x 30.5 cm using a small

6

computer-controlled circular saw. These sheets were then stacked in the desired configura-tion. In order to save time during processing, multiple panels were consolidated in a singlepress cycle by placing a sheet of silicon paper between each. Panels were placed betweentwo aluminum sheets (approximately 0.6 cm thick) and placed between the platens of thepress. Panels were pressed at 4.79 MPa (with the sides unconstrained) and heated fromroom temperature to 125 ◦C. Once the temperature stabilized there, the pressure was in-creased to 19.15 MPa and the temperature brought to 128.9 ◦C. The panels were held at thistemperature for approximately 15 minutes, and then were allowed to cool under pressure,which also took approximately 15 minutes. Once cooled to room temperature, the pressurewas relieved and the panels were removed.

2.3.3 Lamina Specimen Preparation

The as-processed lamina panels had dimensions 22.9 cm x 22.9 cm. Each panel was markedalong the top at two-centimeter intervals. At each interval, a razor blade was pressed intothe panel, then run down the length of the panel parallel to the fibers. On a well-constructedpanel in which the fibers are all parallel, the razor blade runs easily between adjacent fibersand creates a straight-edged specimen as shown in Figure 2.1.

Figure 2.1: Lamina specimens cut from consolidated lamina panel.

7

Aluminum V-notched extensometer tabs were then glued to each resulting 22.9 cm x 2 cmspecimen approximately 2.5 cm (or one inch, the gauge length of the extensometer) apartusing Measurements Group M-Bond 200 adhesive and catalyst.

2.3.4 Laminate specimen preparation

The as-processed laminates were cut using a water-jet cutting machine. A CAD programdirected the machine to cut the 30.5 cm x 30.5 cm panels into 2.5 cm x 15.2 cm specimens.At Virginia Tech, aluminum extensometer tabs were glued to each specimen approximately2.5 cm apart using an M-Bond 200 adhesive and catalyst. Figure 2.2 shows a failed laminatespecimen and an untested laminate specimen, both with extensometer tabs attached. Figure2.3 shows a schematic of the extensometer tabs.

Figure 2.2: An untested laminate specimen (upper) and a failed laminate specimen (lower)with extensometer tabs attached.

2.4 Testing Procedure

2.4.1 Creep Compliance Testing

A TA Q800 DMA was used to conduct creep compliance tests of lamina and laminates.Specimens were cut from larger panels using a razor blade to dimensions of approximately20 mm by 6 mm. They were gripped using the thin film gripping mechanism on the DMA,with a gauge length of approximately 8 mm. A creep compliance temperature sweep programwas set up for which the temperature of the sample was varied from -80 ◦C to 40 ◦C at 10 ◦C

8

A

B

C D

E

D

Figure 2.3: These extensometer tabs were made of aluminum. A: 12.7, B: 4.7, C: 1.6, D: 1.8,E: 1.0, all mm.

increments. At each temperature, the chamber and the sample were allowed to equilibratefor 10 minutes, then a tensile load of approximately 15 N (corresponding to a stress ofapproximately 6 MPa) was applied and the elongation of the sample was measured for 10minutes.

2.4.2 Lamina/Laminate Testing

Lamina and laminate tensile tests were conducted using an MTS hydraulic piston-actuatedload frame equipped with a Russells Technical Products model RD-3-LN2 liquid-nitrogencooled temperature chamber (with a Watlow Series 922 controller), shown in Figure 2.4. Forroom temperature testing, the temperature chamber was not activated, and the temperaturefor all tests was between 24.5 ◦C and 25.5 ◦C. A 25 mm gage-length MTS 632.27E-20 exten-someter was used for strain measurements, and was calibrated prior to each testing session.An 88-kN capacity MTS Force Transducer 661.20 load cell was used for load measurements.MTS 647 Hydraulic Wedge Grips were used to grip each specimen at a grip pressure of 74MPa. An in-house NI LabVIEW program was used to control the load applied to the sampleduring each test and to collect data.

For the sub-ambient tests, several samples were outfitted with thermocouples. These thinspecimens had very little bending stiffness, and had to be handled carefully to prevent themfrom being damaged. Because the material was difficult to bond to, the thermocouple andadhesive were clamped to the specimen to ensure adhesion. This clamping would visiblydisrupt the fibers in that region. Therefore, thermocouples were not used on specimensused to collect tensile strength data. Because of the thermodynamics of the temperaturechamber, the chamber set-point, the chamber temperature, and the specimen temperaturevary somewhat, a short study was conducted to determine what to set the chamber set-point to and how long to give each sample to equilibrate. This study found that with thetemperature chamber set point at -9 ◦C, the temperature of the sample after approximately

9

Figure 2.4: MTS load frame with temperature chamber and lamina sample with extensometerattached.

eight minutes was nominally 3 ◦C for the laminate specimens. For the lamina specimens,a similar study was conducted, and it was determined that with the chamber temperatureset point at -2 ◦C, the temperature of the sample after approximately three minutes wasnominally 5.5 ◦C. Figure 2.5 shows the lamina sample and the chamber temperatures as thechamber is cooled. Figure 2.6 shows the ”free thermocouple” and ”specimen thermocouple”referred to in Figure 2.5.

For all room temperature tests, the temperature during testing was between 24.5 ◦C and25.5 ◦C.

2.5 Results

The initial test plan was to develop a master curve and shift factors from tensile testing alone.To complete this, a matrix of lamina tests at a wide range of sub-ambient temperatures andvarious strain rates was developed. As Figure 2.7 shows, though overall trends may beevident, there was too much scatter in each data set to accurately construct a master curve.Similar scatter exists in the tensile modulus and strain at failure data.

The next iteration of our tTSP plan was to perform a creep compliance temperature sweep to

10

Figure 2.5: Temperature of the specimen as the environmental chamber is cooled. Chamberthermocouple refers to the temperature the environmental chamber reads. Free thermocouplerefers to the temperature of a thermocouple freely hanging directly adjacent to the sample.Specimen thermocouple refers to a thermocouple glued to the specimen.

Figure 2.6: A laminate specimen equipped with a thermocouple, gripped and with theextensometer attached. The ”free thermocouple” is seen positioned next to the specimen.

11

Figure 2.7: Lamina stress at failure vs. strain rate at various temperatures.

construct a master curve and shift factors, and then apply those shift factors to tensile data.The assumption here is that the same mechanisms governing the behavior of the material increep are governing the tensile properties, specifically elastic modulus, tensile strength, andstrain at failure.

An unshifted plot of laminate creep compliance data is shown in Figure 2.8 below. In Figure2.9, the data from Figure 2.8 has been shifted such that the initial slopes of each set of creepcompliance data line up. The accompanying shift factors are shown in Figure 2.10.

This same method was used for several other lamina and laminate samples, resulting in theshift factors shown in Figure 2.11 and Figure 2.12.

One concern was that the specimen was being physically changed by the creep compliancetest. To examine this, a creep compliance temperature sweep was performed twice on asingle specimen, and the compliance values at different temperatures were compared. As theresults in Figure 2.13 show, the sample does exhibit any change in creep compliance beyondthe normal variance between independent tests.

12

Figure 2.8: Unshifted creep compliance of a laminate sample.

Figure 2.9: Shifted creep compliance of a laminate sample.

13

Figure 2.10: Shift factors associated with the master curve in Figure 2.9.

Figure 2.11: Shift factors for three samples of laminate taken from creep compliance data.

Additionally, there was concern that some of the lamina specimens exhibited behavior con-sistent with shear failure instead of normal failure (Figure 2.14). Round-robin observation

14

Figure 2.12: Shift factors for lamina taken from creep compliance data.

was used to create a subset of four samples from the population of twenty room-temperaturetests, and eleven samples from the population of twenty sub-ambient tests. It was determinedfrom k-sample Anderson-Darling test on the strength, strain at failure, and strain rate thatall the room temperature samples were from the same population, so all the data was pooledfor analysis purposes. For the sub-ambient tests, the strengths and strains at failure of thefour specimens exhibiting normal failure were not found to be from the same population asthe non-normal failure specimens. These eleven samples were therefore preferably used foranalysis.

To verify that these creep compliance shift factors can be applied to tensile data, such astensile strength, tensile stress at failure, and tensile modulus, samples were tested at roomtemperature and at a lower temperature corresponding to a two-decade shift in the data,and at a strain rate 100 times slower. The two resulting data sets should be comparable ifthe assumption of creep compliance equivalence is valid.

To determine the temperature shift corresponding to a two-decade strain rate shift, Ea wascalculated for several ranges of temperature for each set of shift factor data. Based on theseEa values, it was determined that a temperature shift of 20 ◦C for laminate specimens and21 ◦C for lamina specimens would result in the desired strain rate shift. These shift factorsrepresent an activation energy, Ea, of 21.9 kJ mol−1 for laminate samples and 27.5 kJ mol−1

for lamina samples.

Figures 2.15 and 2.16 contain strength and elongation at failure data from many different

15

Figure 2.13: Creep compliance of a laminate sample subjected to two consecutive temperturesweeps. Solid lines are data from the first temperature sweep, and symbols are data fromthe second temperature sweep.

lamina and laminate tests. All data not from room temperature has been shifted usingthe creep compliance shift factors determined for that material. Laminate strength has beenestimated from lamina strength based on the strength of the number of fibers in the directionof loading.

Note that the solid line is a linear regression of log(shifted strain rate) and strength of 53specimens tested at a variety of strain rate-temperature combinations. The dashed linesrepresent a 68% prediction interval on future values of strength. The error bars on each datapoint represent one standard deviation of uncertainty (Montgomery and Peck, 1982).

2.6 Discussion

One of the largest sources of uncertainty for the lamina and the laminate tensile testing wasin trying to use the temperature chamber to bring the sample to the desired temperature.The material has been demonstrated to have anisotropic thermal conductivity (Skow, 2007).If heat is more readily absorbed (and diffused) in the direction of the fibers than in thetransverse direction, then it is possible that a thermocouple glued to the outside of a specimen

16

Figure 2.14: A lamina specimen was designated as a normal failure (upper) and a laminaspecimen designated as a shear failure (lower).

Figure 2.15: Strength master chart.

17

Figure 2.16: Strain at failure master chart.

does not conduct heat from the specimen at the same rate that the specimen conducts heatfrom the test fixture. If this is the case, the temperature measured by the thermocouple isnot necessarily the temperature of the entirety of the specimen. Due to the nature of ourtesting setup, during the period over which the chamber temperature is allowed to equilibrateand the specimen is assumed to be cooled to this temperature, it is being gripped by theMTS grips, which are actuated by a hydraulic line filled with heated oil. This uncertaintyis accounted for in the error bars on the strain rate measurements.

Another source of uncertainty in the laminate tests was that some of the specimens appearedto have failed in shear rather than in the normal direction. It is well documented in com-posites that there is a critical off-angle at which a unidirectional specimen is dominated byshear failure instead of normal failure. Because of the extremely large ratio of fiber stiff-ness to matrix stiffness in our composite, the off-angle is very small, and it is impossible toalign the specimen in the grips such that every test results in a normal failure. Round-robinobservation among three researchers familiar with distinguishing normal and shear failurein composites, but not familiar with this specific material system, was used to categorizesamples as either normal or shear failures. Samples for which two or more of the researchersplaced in the normal category were designated as normal failures. For the room-temperaturelamina specimens, only four out of twenty were determined to be normal failures. However,the k-sample Anderson-Darling testing showed that these samples and the remaining sam-

18

ples were from the same population, so the samples were combined for analysis (Scholz andStephens, 1987). The eleven sub-ambient temperature samples determined to be normal fail-ures were found not to be from the same population as the remaining samples, so these wereused preferably for analysis. This human decision matrix introduces an unknown amount ofuncertainty into the data set. However, some type of qualitative observation and decisionprocess is necessary due to the impossibly low margin for error that the very small criticaloff-angle introduces into the experiment.

It is also noteworthy that the laminate strength estimates shown in Figure 2.15 based on lam-ina properties are lower than the laminate strengths from room temperature testing. Thereare two readily apparent possibilities here, the first being that the matrix and transversefibers of the laminate are contributing to the strength of the composite. The second, andmore likely possibility is that the shear failures described above are reducing the apparentstrength of the composite. These shear failures are not seen when testing laminate samples.

Another source of uncertainty is the process of shifting the creep compliance data to createmaster curves. When Alcock et al. (2007) shifted their failure data, they cautioned thatusing an Arrhenius activation energy constant to describe the tTSP behavior of his poly-propylene composite was a simplification of a more complex material system. They furthercautioned that only amorphous polymers can be directly shifted using an activation energyapproach to tTSP, and that although their shifted data appears valid, large deviations arelikely present in strain-rate ranges far from those tested (Alcock et al., 2007).

For our study, we present a similar caution. Because of the variability in recorded materialproperties, even after accounting for other uncertainties, we cannot definitively declare thatthe method described herein for shifting strain rates using tTSP is valid. Though the methodof determining shift factors using creep compliance data results in an apparent first-ordershift factor-temperature plot that is observed to follow behavior that may be described asrheologically simple, the master curve is not a single, smooth curve such as an amorphouspolymer would exhibit (Brinson and Brinson, 2008). Furthermore, the extension of strengthand strain at failure data to strain rates several orders of magnitude faster than the strainrates at which the tests were performed (Figures 2.15 and 2.16) may have very large devia-tions from tests actually performed at those strain rates.

The issue of imperfect tTSP master curve fitting has been observed and explored by otherresearchers working on UHMWPE as well. Govaert et al. (1993) completed testing onDyneema UHMWPE samples. They noted a linear-elastic component in creep compliancetests, followed by a nonlinear plastic deformation phase. In an earlier work, Leblans et al.(1989) note that applying a tTSP model to the elastic portion of a creep compliance curveis applicable for small deformations and short time periods. Figure 2.17 shows shift factorsfor lamina and laminate specimens from this study along with those presented by Govaert etal. for Dyneema SK66 fibers. Note the general agreement in shape between the shift factorsfrom the linear region of the stress relaxation curves, the dynamic testing data, and the datafrom this study. Both materials are an UHMWPE used for the same application.

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‐6

‐4

‐2

0

2

4

6

8

10

12

14

2.00 2.50 3.00 3.50 4.00 4.50 5.00

a T

1/T,10‐3K‐1

Govaert(fromdynamicquan==es)

Govaert(stressrelaxa=on,linearregion)

Lamina(S3000Unidirec=onal)

Laminate(SS3124)

Figure 2.17: Lamina and laminate shift factors for S3000 unidirectional and SS 3124 cross-plycomposites compared with shift factors from dynamic testing and stress relaxation testingof a Dyneema SK66 fiber, another UHMWPE, performed by Govaert et al. (1993).

This method of shifting the creep compliance data was not developed until after the laminaand laminate testing was carried out. Originally, the changes in temperature for whichthe tests were designed to have equivalent strain rates were 20 ◦C for laminate specimensand 21 ◦C for lamina specimens. These temperatures used the shift factors given by theTA analysis program. Further investigation of the method by which the program shifts thecreep compliance data led us to the conclusion that we could more accurately predict the shiftfactors if we shift the creep compliance data by hand instead of relying on the TA program.The new temperature change values were calculated as 24 ◦C for laminate specimens and 23◦C for lamina specimens. These changes in temperature are small compared to the overalluncertainty in the temperature of the sample during the test, but nonetheless contribute tothe overall uncertainty of the results.

2.7 Conclusions

There is potential in using time-temperature superposition values from creep compliancetesting to shift tensile properties. Assumptions about how to measure the shift factors madeprior to the tensile testing were later challenged, which resulted in changing the shifted strainrates. These shifted strain rates did not align as well with the room temperature rates, soa direct comparison of the data sets is not possible. In the future, it is recommended that

20

a more exhaustive test of this method be undertaken, involving several different rate andtemperature combinations. To be statistically significant, a large number of tests must beperformed at each rate-temperature combination.

It was also observed that as levels of complexity are added to the material system, the rate-dependent behavior of the material becomes more complex. A lamina system containingonly fibers in one direction is much simpler than a cross-ply system. The up-turn in creepcompliance over time for each temperature is much less pronounced for lamina than forlaminate, resulting in a smoother master curve that the researcher may have more confidencein. The data presented in this study should be further investigated and validated by usingthe various composite system models to utilize fiber data to predict lamina and laminateproperties.

Chapter 3

The Effect of Processing Conditionson the Properties of ThermoplasticComposites

3.1 Abstract

Ultra high molecular weight polyethylene (UHMWPE) composites are produced from Spec-traShield 3124 to evaluate the influence of the magnitude of the temperature and the pressureused to consolidate the composite on selected mechanical and physical properties. In thisstudy, properties of composites processed using higher and lower temperatures and pressuresthan is the industry standard are examined. Differential scanning calorimetry (DSC) is usedto examine differences in crystallinity, and dynamic mechanical analysis (DMA) is used toexamine creep compliance response of the different materials. Tensile properties of the com-posites are also evaluated. Results indicate that higher processing pressure may result inmore favorable tensile properties, but that the impact of processing temperature is relativelysmall.

3.2 Introduction

Ultra-high molecular weight polyethylene (UHMWPE) composites are used in applicationsrequiring lightweight materials that are very strong and have favorable properties at highstrain rates. The two primary UHMWPE fibers and laminates for high strain-rate ap-plications are DSM (Dyneema) and Honeywell (Spectra and SpectraShield). The typicalprocedure for processing these materials involves stacking the desired number of sheets ina cross-ply lay-up, then placing them in a hydraulic press with heated platens for a certain

21

22

period of time at a specified pressure and temperature (Bhatnagar, 2006). Raising the tem-perature allows the matrix material to flow and adhere to the fibers. The pressure appliedhas been linked to composite interlaminar and fiber shear properties (Bhatnagar, 2006).

In this study, testing is completed on samples of UHMWPE processed five different pres-sure/temperature combinations. The tests completed include tensile strength, a creep com-pliance temperature sweep, differential scanning calorimetry, and a punch-shear test. Tensileproperties of UHMWPE are very important to the designers of systems utilizing the material(Bhatnagar, 2006).

Creep compliance may be used in time-temperature superposition (tTSP) to estimate prop-erties at high and low strain rates from intermediate strain-rate testing. tTSP is based onthe principle that the properties of certain materials at various strain rates may be esti-mated by performing tests at corresponding (but differing) rates and temperatures. It isused on time-dependent materials to estimate their properties on time scales much longeror shorter than is convenient or possible to perform testing on. Stiffness or compliance datataken at different temperatures may be shifted left or right along a time scale to create amaster curve. This master curve can then be used to predict the behavior of the materialat time scales or strain rates that are not physically achievable on test equipment (Brinsonand Brinson, 2008). tTSP has also been applied to failure mechanisms by Miyano et al.,who successfully shifted composite strength data over time to predict long-term durabilityin graphite fiber/vinyl ester (GFRP) laminates (Miyano et al., 2004). While tTSP is notapplied in this text, differences in tensile creep compliance values are examined and may beindicative of tensile properties at different strain rates.

Differential scanning calorimetry (DSC) involves measuring the amount of energy needed toraise the temperature of a sample of a material by a certain amount. The results of thesetests are used to measure the crystallinity of a sample (Kong and Hay, 2002).

3.3 Materials

3.3.1 Laminate processing procedure

Sheets of the Honeywell commercial SpectraShield 3124 product (which has a lay-up of[0,90]) were cut to the desired dimensions, most commonly 30.5 cm x 30.5 cm using a smallcomputer-controlled circular saw. These sheets were then stacked in the desired configura-tion. In order to reduce processing time, multiple panels were consolidated in a single presscycle by placing a sheet of silicon paper between each. Panels were placed between twoaluminum sheets (approximately 0.6 cm thick) and placed between the platens of the press.Panels were pressed at 4.79 MPa (with the sides unconstrained) and heated from room tem-perature to close to the desired processing temperature. Once the temperature stabilizedthere, the pressure was increased to the desired processing pressure and the temperature

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brought to the desired processing temperature. The panels were held at this temperaturefor approximately 15 minutes, and then were allowed to cool under pressure, which also tookapproximately 15 minutes. Once cooled to room temperature, the pressure was relieved andthe panels were removed.

Five different sets of panels were processed in this way, such that panels processed as shownin Table 3.1 were created.

Table 3.1: Processing conditions for various panels.

Temp ◦C Pressure, MPaPanel 4 127 19.4Panel 5 127 23.7Panel 6 128 15.5Panel 7 136 18.3Panel 8 122 18.3

3.4 Testing Procedure

3.4.1 Creep Compliance Testing

A TA Q800 DMA was used to conduct creep compliance tests of lamina and laminates.Specimens were cut from larger panels using a razor blade to dimensions of approximately20 mm by 6 mm. They were gripped using the thin film gripping mechanism on the DMA,with a gauge length of approximately 8 mm. A creep compliance temperature sweep pro-gram was set up for which the temperature of the sample was varied from -80◦C to 40◦Cat 10◦C increments. At each temperature, the chamber and the sample were allowed toequilibrate for 10 minutes, then a tensile load of approximately 15 N (corresponding to astress of approximately 6 MPa) was applied and elongation of the sample was measured for10 minutes.

3.4.2 Laminate Tensile Testing

For gathering tensile properties, the as-processed laminates were cut using a water-jet cuttingmachine at ARL-Aberdeen. A CAD program directed the machine to cut the 30.5 cm x 30.5cm panels into 2.5 cm x 15.2 cm specimens. At Virginia Tech, aluminum extensometer tabswere glued to each specimen approximately 2.5 cm apart using an M-Bond 200 adhesive andcatalyst.

24

Laminate testing was done using an MTS hydraulic piston-actuated load frame. A one-inchgage-length MTS 632.27E-20 extensometer with a one-inch gauge length was used for strainmeasurements, and was calibrated prior to each testing session. A 22-kip capacity MTSForce Transducer 661.20 was used for load measurements. MTS 647 Hydraulic Wedge Gripswere used to grip each specimen at a pressure of 74 MPa. An in-house NI LabVIEW programwas used to control the load applied to the sample during each test and to collect data.

3.4.3 Differential Scanning Calorimetry (DSC) Testing

A Netzsch STA 449F1 was used to complete differential scanning calorimetry on a sampleof each material. For each test, a baseline run was completed with only the sample holdersin place. Each sample test was run shortly after the completion of the baseline to keep theconditions as closely matching as possible. Figure 3.1 shows the temperature profile thesample was subjected to. The intention of the test was to completely melt the sample byraising the temperature to 150 ◦C, measuring the entropy required to do so, and then coolit to room temperature, 20 ◦C, such that it fully crystallizes. It was then melted and cooledonce more, so that the heat needed to melt the sample was again measured. A rate of 1 ◦Cper minute was used for both heating and cooling.

Figure 3.1: Temperature profile and DSC measurement for a sample from Panel 4.

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3.5 Results

3.5.1 Creep Compliance Testing

An unshifted plot of laminate creep compliance data is shown in Figure 3.2 below. In Figure3.3, the data from Figure 3.2 has been shifted such that the initial slopes of each set of creepcompliance data line up. The accompanying shift factors are shown in Figure 3.4.

Figure 3.2: Unshifted creep compliance of a laminate sample.

As Figure 3.3 shows, the master curves do not transition smoothly between temperatures.The sample becomes more compliant as testing time increases. To determine shift factors,the slopes of the creep compliance curves at the beginning of the test are aligned.

This same method was used for several specimens for each set of processing conditions. Theresulting shift factors are shown in Figure 3.5.

3.5.2 Laminate Tensile Testing

The tensile strength of each sample set is shown in Figure 3.6. The strain at failure of eachsample set is shown in Figure 3.7. For panels five through eight, each point represents theaverage of ten tests, with error bars representing one standard deviation. For panel four, 18tests are represented.

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Figure 3.3: Shifted creep compliance of a laminate sample.

3.5.3 Differential Scanning Calorimetry Testing

To measure crystallinity, the equation

Xc = (∆Hf − ∆Hc)/∆H◦f (3.1)

was used, where ∆Hf is the enthalpy of fusion of the original sample, ∆Hc is the enthalpyof crystallization as the sample is cooled, and ∆H◦

f is the enthalpy of fusion of a 100%crystalline sample (Kong and Hay, 2002).

Each of these values was measured by integrating the area under the DSC-temperature curveas the material melted or crystallized. These areas are shown in Figures 3.8 and 3.9. Table3.2 shows the enthalpy, melting and recrystallization temperatures for each sample tested,as well as the calculated percent crystallinity, Xc.

3.6 Discussion

It was determined from k-sample Anderson-Darling testing that tensile strength of eachsample set was from the same population as the panel processed under standard conditions.This analysis in addition to the similarity of the creep compliance data indicates that thetensile properties do not differ significantly at the various processing conditions.

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‐2

0

2

4

6

8

10

‐60 ‐50 ‐40 ‐30 ‐20 ‐10 0 10 20 30 40

ShiftFactor

Temperature,°C

Figure 3.4: Shift factors associated with the master curve in Figure 3.3.

Table 3.2: Enthalpy, melting and recrystallization temperatures, and calculated crystallinity,Xc for each sample tested.

∆Hf Tm ∆Hc Tc ∆H◦f T ◦

m Xc

J/g ◦C J/g ◦C J/g ◦C %Panel 4 8.90 142.2 -5.85 122.8 6.00 132.7 50.9%Panel 5 8.42 142.0 -5.78 122.3 5.22 133.7 50.9%Panel 6 2.18 142.6 -1.70 122.3 0.95 133.6 51.1%Panel 7 2.96 142.6 -1.81 122.5 1.98 133.5 58.0%Panel 8 11.44 142.6 -7.59 122.6 8.92 133.9 43.2%

Panels four, five, and six were all processed at approximately the same temperature, andhave very close to the same computed crystallinity. Panel seven, which was processed at ahigher temperature, exhibits a higher degree of crystallinity, while panel 8, processed at alower temperature, exhibits a lower degree of crystallinity.

It is also noteworthy that the DSC measurements have many fluctuations close to the orderof the increases seen during the melt phase of the test, bringing into question the precisionof the test.

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Figure 3.5: Creep compliance shift factors of panels at various processed at various condi-tions.

3.7 Conclusions

The lack of discernable difference in the tensile creep compliance or tensile strength, proper-ties driven by fiber behavior, indicates that the fibers are largely unchanged by the heat andtemperature applied during processing. This indicates that the changes are in the matrixrather than in the fiber, as any degradation in the fiber would likely have been evident in thetensile test results. Therefore, the higher crystallinity for the samples processed at a highertemperature is likely due to an increased degree of melting in the matrix material duringprocessing. Likewise, the lower crystallinity for the samples processed at a lower temperatureis likely due to less melting taking place in the matrix material at that temperature.

Future studies of the processing conditions should examine interlaminar shear strength ofsamples produced at different temperatures, as the matrix material largely determines theseproperties (Bhatnagar, 2006). Additionally, through-thickness shear strength, which it isalso believed is driven by matrix properties, should be examined in samples processed atdifferent temperatures and pressures (Bhatnagar, 2006).

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Figure 3.6: Laminate strength vs. processing pressure.

Figure 3.7: Laminate strength vs. processing temperature.

30

Figure 3.8: DSC vs. temperature for a sample from Panel 4, with the area approximatingthe enthalpy of fusion for the sample highlighted in red.

31

Figure 3.9: DSC vs. temperature for a sample from Panel 4, with the area approximatingthe enthalpy of recrystallization highlighted in red.

Chapter 4

Apparent Shear Strength ofUHMWPE Composites

4.1 Introduction

During attempts to define a testing procedure for determining the tensile strength of UHMWPElaminates, the extremely low interlaminar shear strength of the composite was observed. Itwas determined that all but extremely thin samples delaminated prior to fiber-direction ten-sile failure of the composite. Only specimens as thin as [0,90]4S or thinner could be testedusing our MTS load frames. Any thicker samples would only be loaded (and fail) on theouter-most 0◦ plies, or the composite would simply pull out of the grips if the outer-mostplies were 90◦. Part of this problem was remedied by increasing the grip pressure, ([0,90]4Stensile tests are conducted at a grip pressure of over 110 MPa) which led us to suspect thatwe could extract a shear strength measurement that would vary with the grip pressure.

The expectation was that because the ratio of the fiber stiffness to the matrix stiffness wasso high, a relatively uniform shear stress was applied to the interface between the outermost90◦ ply and the loaded 0◦ ply, and the resulting failure was an apparent shear strength,shown in Figure 4.1.

Initial testing at low and moderate grip pressures showed that the force needed to causean interlaminar shear failure in the grips monotonically increased with the grip pressure.This testing technique was referred to as the grip-slip technique, and a study was completedinvestigating a variety of grip pressures to explore this failure mode.

After testing was completed, a finite element analysis (FEA) model was created with theparameters of the test and using all known fiber and matrix properties, to try to 1) determinewhere the failure occurred (fiber shear, matrix shear, fiber/matrix interface, etc.) and 2)quantify that failure under specific loading conditions.

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33

Figure 4.1: Apparent shear strength test with assumed stress state.

4.2 Procedure

Composite specimens were used for this test, consolidated from SpectraShield 1214 materialat the Army Research Laboratory (ARL) using standard ARL processing techniques. Thespecimens were water-jet cut to 2.5 cm x 15.2 cm dimensions, and brought to Virginia Techfor testing. Some specimens were cut to have a [0,90]4S lay-up (long direction being theprimary direction) and some were cut to have a [90,0]4S lay-up. The [90,0]4S specimens weretested as-is, with no further sample preparation completed. The [0,90]4S specimens had theouter-most 0◦ ply peeled back from one end of the specimen as shown in Figure 4.2. It isassumed that an identical stress state exists for both cases, with failure resulting from theτ31 stress.

Figure 4.2: The [0,90]4S specimens were peeled back to create a τ31 stress at one end thatresulted in an interlaminar shear failure.

The stroke rate was 0.01 in/sec for all tests. Each specimen was gripped at a set pressure,then pulled uniaxially until failure. Failure was defined as the load reaching a peak, followedby a sharp drop-off. The shear stress at failure was defined as the load at failure divided by

34

the area in the grips. A typical load and displacement vs. time curve for a test is shown inFigure 4.3.

Figure 4.3: Typical load, displacement vs. time curve.

Following the experiment, a global-local model of the experiment was built in ABAQUS,a commercial FEA software package, with the same dimensions and loads, and with thebest-known material properties at the time. The global-local approach was used because itallows us to determine the stresses in a composite structure and then apply those stresses toa smaller critical element with a finer mesh. This leads to a better understanding of the localstresses resulting in global failure. Figure 4.4 shows the global-local model setup, where thegrip pressure, G is 2.84 GPa and the traction P is 383 GPa. The dimensions of the globalmodel are described in Table 4.1. The elastic properties assigned to the global model aredescribed in Table 4.2.

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Figure 4.4: Global-local model. (Used with permission of Gautum Gopinath, 2009)

Table 4.1: Geometric dimensions of global model. (Used with permission of GautumGopinath, 2009)

h1 (mm) h2 (mm) l1 (mm) l2 (mm)

0.222 0.0478 54.9 76.2

4.3 Results

The experimental results collected in Figure 4.5 indicate a clear dependence of apparentshear strength on applied normal stress. A linear fit to the data of the form

τ13 = −Aσ33 +B (4.1)

with parameters A = 0.059 and B = 179.

In trying to explain the experimental results, several approaches were taken. The Tsai-Wufailure criterion was approximated as

F33σ233 + F3σ33 + F55τ

231 = 1, (4.2)

but was found to not be as good of a fit as a simple linear model. It was also thought thatthe linear dependence could be explained by friction between the layers, but a follow-upexperiment in which a sample was displaced to approximately 80% of the expected failurestroke, then relaxed showed that the interlaminar region was intact, indicating that a shearfailure did not occur until a critical value of apparent shear stress was reached.

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Table 4.2: Elastic moduli of the composite. (Used with permission of Gautum Gopinath,2009)

E1 (GPa) 22.8E2=E3 (GPa) 0.800G12 = G13=G23 (GPa) 2.28ν12=ν13 0.084ν23 0.129

Figure 4.5: τ31 vs. -σ33 for SS 1214 laminates, with linear best-fit line.

The global model developed to try to better explain our experimental results did not confirmour expectation of a uniform shear stress along the interlaminar boundary. Instead, a stressconcentration existed at the edge of the grip area in the global model, shown in Figure 4.6.A stress contour map from the local model of this stress concentration is shown in Figure4.7.

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Figure 4.6: τ31 in Pa for global model. (Used with permission of Gautum Gopinath, 2009)

Figure 4.7: Contour plot of σ33 from local model developed at the edge of the gripped surfacewhere the stress concentration appears. (Used with permission of Gautum Gopinath, 2009)

4.4 Conclusion

The primary assumption on which our experiments were based was the uniform state ofshear stress along the interlaminar interface. The modeling, however, indicates that thereis more of a stress concentration at the discontinuity than previously thought, and that thislocation is likely where failure initiates. If this is indeed the case, then the data from ourApparent Shear Strength experiments is of little value in determining material properties orfailure criteria at this point.

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However, the low strain-rate experimental and modeling data of a hemispherical impactorpushing into a consolidated composite, not presented here, shows that the initial failuremechanism is delamination starting from the location of the impactor and pushing outwardtoward the composite boundary. As more advanced models are developed of this behavior, itmay be that the data from this study can be used to corroborate that mechanism of failure.

Chapter 5

Conclusions

The full characterization of the high strain-rate properties of UHMWPE composites is wellbeyond the scope of this thesis. Furthermore, an understanding of the mechanisms thatdrive the properties of this material are just beginning to be understood.

5.1 Tensile Properties

This research describes the development of a generally repeatable tensile testing procedurefor laminates using a hydraulic testing machine and temperature chamber. The lamina testis somewhat unreliable due to the tendency of the sample to fail in shear at times. Thistesting technique provides a method of measuring stress and strain at failure of lamina andlaminate samples at various strain rates and temperatures. Creep compliance testing wasalso described as a method to determine shift factors with which to relate strain rate andtemperature.

It is observed that there are many sources of uncertainty in this form of testing, not the leastof which is the inherent variability in the observed properties of the samples themselves.Additional uncertainty was derived from the creep compliance testing, determination of shiftfactors, application of creep compliance shift factors to failure data, and testing conditions(strain rate and temperature) of the lamina and laminate tests themselves. Despite this,trends are still evident across a large spectrum of shifted strain rate data for strength andstrain at failure. What is lacking is room-temperature testing at equivalent high strain ratesto verify the data in this paper.

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40

5.2 Shear Properties

It is known that the shear properties of UHMWPE composites play an important role intheir performance in the end-use application of the material. Quantifying these propertiesin laboratory testing has been difficult. The Apparent Shear Strength is thus named be-cause while intuition tells us that we are measuring a shear strength while performing thistest, modeling points to the well-documented phenomenon of a stress concentration at thesingularity where the plies separate, similar to that seen at a crack tip in any material.Most standardized composite interlaminar shear strength testing requires the material tohave some stiffness in bending, which our thin samples do not. Regardless, we know thatinterlaminar shear strength is an important property, and more innovative testing methodsshould be evaluated.

5.3 Processing Conditions

UHMWPE composites were manufactured for this thesis at both the ARL facility in Ab-erdeen, MD and the Honeywell facility in Petersburg, VA. At these two facilities, differentstandards were used for the processing pressure and processing temperature. Much work hasgone into creating a high degree of orientation of the molecule chains to create the UHMWPEfibers. This high degree of orientation and resulting high molecular weight give the fiber itsincredibly high strength-to-weight ratio that makes it ideal for its end application. If anyof this orientation is sacrificed during processing (due to heating), it was expected that itwould degrade the favorable properties of the composite. To ensure optimal consolidation,the processing temperature selected is to be greater than the melting temperature of thematrix, but lower than the melting temperature of the fiber. There is a small range to workwith here, and the different processing facilities work at different areas within that range.

No difference in tensile properties at the various temperatures and pressures indicates thatthe fibers are unchanged. However, the other important property set, laminate shear prop-erties, were not examined and should be evaluated for these materials as well.

5.4 Future Work

There is still a great deal of work that needs to be completed to determine the propertiesneeded to properly model UHMWPE composites. The efficacy of applying fiber shift factorswhich have been collected by other researchers to model lamina and laminate data shouldbe evaluated. The tTSP techniques applied to lamina and laminate samples in this workexhibit behavior that may be too complex to accurately predict properties at higher strainrates. The exclusion of the matrix material during fiber testing should simplify the tTSP

41

procedure, and may yield more accurate and precise results. Because it is known that theproperties of the fibers drive the overall tensile behavior of the composite, the use of fibershift factors to shift lamina and laminate data should be investigated.

There is also a need to verify the lamina and laminate tensile data gathered for this material.Split-Hopkinson testing has been used by researchers on other materials (Tan and Zeng, 2006)to gather fiber and laminate data at high strain rates. Similar testing may provide betterinsight into the effectiveness of predicting high-strain rate behavior of UHMWPE.

Further development of interlaminar shear tests should be completed. At room temperatureand strain rates achievable on hydraulic testing machines, the tensile properties of the fibersare so much greater than the properties of the matrix that is is hard to imagine that thematrix material plays a critical role in the overall strength of the specimen. At high strain-rates and lower temperatures, this is not necessarily the case, and the behavior of the matrixand the interlaminar behavior of the composite should be examined under these conditions.

During the processing conditions study, DSC testing was completed to calculate the crys-tallinity of the different specimens. It was expected that any differences of strength inmaterials consolidated under different conditions could be explained by a change in crys-tallinity in the composite, or specifically in the fiber. The DSC testing was completed ona machine with other capabilities, and was not done as accurately as may be possible on adedicated DSC machine. This may provide a more accurate look at the crystallinity of thematerial.

Because it has been observed that there are differences in the performance of the materialwhen different processing pressures are used (Bhatnagar, 2006), but no difference in tensileproperties was found, it is likely that these performance differences are derived from a changein interlaminar shear or through-thickness shear properties of the material. Additional in-vestigation in these areas should be completed.

There is still much that is not understood about the behavior of UHMWPE composites,especially at high strain rates. The inability to perform meaningful tests at these high strainrates greatly complicates the efforts to determine the performance of the material there.Hopefully this work provided a starting point in a standardized lamina and laminate testingprocedure. Further investigation of the application of tTSP to the material still has thepotential to develop a good understanding of the behavior of the material at a range ofstrain rates, provided the properties can be verified with meaningful high strain-rate testing.

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