Static and impact behaviour of metal foam cored sandwich beams · The sandwich beam construction under consideration consists of Alporas ... Xt 720 MPa “ Matrix Tensile Strength,

Static and impact behaviour of metal foam cored sandwich beams

S. McKown & R. A. W. Mines Department of Engineering, University of Liverpool, England

Abstract

The sandwich beam construction under consideration consists of Alporas Aluminium foam core and continuous glass fibre reinforced polypropylene skins laid up in a cross ply form. The experimental study concerns the static and low velocity impact behaviour of sandwich beams subject to three point bend loading. The beams fail mostly by either core shear or by upper skin compression failure. Gross beam failure modes under impact were similar to the static case, however localised deformation was larger for impact loading. General discussion of experimental results will be given with a view to the

1 Introduction

The core material of the sandwich beam is Alporas Aluminium foam supplied by Shinko Wire Company [1]. The skin material is a cross-ply lay-up of a glass fibre polypropylene matrix composite. This has the trade name Plytron and was supplied as a uni-directional pre-preg roll by the manufacturer [2]. Table 1 gives material properties of the foam and composite obtained from either the manufacturer or named author. A rapid cold stamping method of construction was employed for all the beams under investigation. Reyes and Cantwell [3] have investigated this method for the production of both thermoplastic-based fibre metal laminates and thermoplastic-based sandwich structures. All the beams under investigation have the composite skins laid up in the cross ply configuration [0,90,0,90], with the 0 degree layer on the outside of the beam top and bottom skins. Adhesive between the core and the skins, in the form of a semi-transparent sheet of modified

Keywords: impact, metal foam, sandwich beam, thermoplastic composite subsequent numerical simulation of beam tests using LS-DYNA.

.

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

polypropylene, ensured a bond strength an order of magnitude greater than the tensile failure strength of the Alporas core. The skin core interface has been studied in detail by Kiratisaevee and Cantwell [4]. The beam behaviour under quasi-static and impact loading is investigated in this paper, in the 3-Point bend configuration as shown in fig. 1.

Table 1: Sandwich beam core and skin material properties.

Alporas Property Value Source Relative density 8 – 10% of Al Average measured Tensile Elastic Modulus 1.0 GPa Andrews [5] Tensile Yield Strength 1.7 MPa Andrews [5]

Plytron Property (uni-directional) Value Source Density 1.48 g/cm3 Manufacturer [2] Elastic Modulus, E11 (Fibre dir.) 28 GPa “ Elastic Modulus, E22 (Matrix dir.) 4 GPa “ Shear Modulus, E12 (in plane) 1.4 GPa “ Fibre Tensile Strength, Xt 720 MPa “ Matrix Tensile Strength, Yt 11 MPa “ Fibre Compressive Strength, Xc 366 MPa “ Matrix Compressive Strength, Yc 45 MPa “ Shear Strength, S 19 MPa “

2 Quasi-static analysis of sandwich beams

The general trend for all 3-Point loaded beams under investigation is an initial indentation of the upper skin under the indenter with little global bending up to failure of the upper skin. The failure point is evident from fig. 2, were there is a sudden dip in the load. The indentation continues, along with progressive crushing of the core and global bending of the beam, corresponding to the post failure region of fig. 2, were a relatively flat plateau exists, increasing slightly as the displacement increases. Core shearing becomes evident in some cases at displacements greater than 15mm. Fig. 3 shows a series of images of the progressive damage of the core, including tensile rupture which can be seen to the left side of the indenter.

Figure 1: Sandwich beam construction for 3-Point bend tests.

Upper Skin (Cross Ply 0°,90°,0°,90°)

Lower Skin (Cross Ply 0°,90°,0°,90°)

B

c Foam Core

Span, S

Diameter 20mm

Beam length, L

c+2t

t


38 High Performance Structures and Materials II

2.1 3-Point bend tests

The 3-Point bend quasi-static testing of the sandwich beams consisted of a series of 3 separate investigations as given in table 2. The first series looked at beams with a core thickness of 10mm, whilst the second and third series investigated the effect of different spans and foam damage in beams with core thickness 20mm. An Instron 50kN testing machine provided the loading for the static 3-point bend tests. The indenter, of diameter 20mm, was driven down at a velocity of 1mm/min, with load displacement data collected at a sample rate of 1 point per second. The beams were centrally positioned on lubricated supports also of diameter 20mm. The arrangement is shown in Fig. 1. Series 3 consisted of 8 tests performed on beams of span 100mm with the 20mm thick core. The majority of the beams had a strain gauge on the upper and lower skins located 5mm from the centre line of the beam, positioned to record the longitudinal strain in the beam skins.

Table 2: Quasi-static and impact test series beam dimensions.

Series c (mm) B (mm) L (mm) S (mm) t (mm) Aspect ratio, S/c Quasi-static 1 10.0 20.0 240.0 180.0 1.8 18 2 20.0 30.0 varied varied 1.8 varied 3 20.0 30.0 200.0 100.0 1.8 5 Impact 20.0 30.0 200.0 100.0 1.8 5

Span 100mm (c=20mm)

Span 200mm (c=20mm)

Span 340mm (c=20mm)

Span 180mm (c=10mm)

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Indenter displacement (mm)

Load

(N)

Figure 2: Typical quasi-static load displacement results, showing upper skin failure points A-D.

A

B

C

D


High Performance Structures and Materials II 39

10mm displacement Cell wall cracking outside crush zone, near left-hand support

20mm displacement Shear first visible after 15mm displacement

30mm displacement

Figure 3: Quasi-static test of 20mm thick core showing damage progression at the indicated location.

2.2 Cell-size effects

Quasi-static tests of the 10mm core thickness beams with cell sizes of the order of the structure size were loaded in the 3-point bend configuration. The average cell size for the 8% dense Alporas has been measured to be 4.5mm [5]. It can be seen from fig. 4 that the deviation from this can be large, as a defect of diameter 10mm exists in this particular beam core. The foam size effect had a significant effect on the response of the structure to loading. Leading to a reduced load holding capability and premature failure of the beam, as seen in fig. 4.

2.3 Effect of span on the progressive failure of the beam

Following on from the 10mm core beam testing, the core depth, c, was increased to 20mm and similar 3-Point bend quasi-static tests were conducted. The aim

Core damage initiation



being to increase the size of the structure relative to the cell size of Alporas, and thus, produce a repeatable structural response under loading. The effect of reducing span size on the progressive failure of the beam was investigated. In order to induce tensile failure in the core, beams of reducing span were tested to increase the shear contribution. The beam span was in the range of 100mm to 340mm, with a constant width and overhang of 30mm. Fig. 5 shows that the global bending behaviour of the beams differs for the 100mm span, in that plastic hinges form on the lower skin at the support region. This is referred to as a ‘mode B’ type deformation [6]. Whereas a ‘mode A’ deformation occurs for the spans larger than 200mm, in which the plastic hinges form at the upper skin either side of the indenter. The beams with spans of 340 and 200mm showed no visible evidence of shear type failure in the core. However for the 100mm span, there was some evidence beyond indenter displacements of 15mm, well after upper skin failure at around 3mm. Fig. 6 shows schematically the location of the shearing type failure in the core and its progression as the indenter displacement increased. The general trend of the load versus indenter displacement, fig. 2, follows earlier discussions of showing an increase in load up to upper skin compression failure, followed by a plateau region of core crushing.

2.4 Core damage investigation under static loading

In order to further investigate the core progressive damage, quasi-static tests focused on the 100mm span beam in which it was shown that high tensile and compressive stresses are developed. Visible evidence of this in the static tests was shearing of the core after upper skin failure. This grew from regions of cracking of the cell walls outside of the crush zone, visible from about 10mm indenter displacement. Large scale shearing of the core then progressed from anywhere between 15 and 30mm of indenter displacement. The relatively large range over which core failure initiates points to localized defects reducing material strength. Fig. 3 shows a series of photographs of beam progressive damage, typical of the tests. The sample shown has a large core defect near to the right-hand support. However, unlike the 10mm depth core, this defect is not the origin of the core failure, this occurs near to the left-hand support in this case.

3 Low velocity impact test of sandwich beams

3-Point Bend impact of the sandwich beams was performed using a 2 metre drop tower. Indenter masses of 1.825kg and 3.75kg were dropped from heights in the range of 1 to 2m covering a velocity range of 4 to 6 m/s. The impact energy covers the range of 15 to 50J, as summarized in table 3. A Laser Doppler Accelerometer, LDA [7], measured the velocity of the impact mass, and by the respective differentiation and integration the inertial loading and displacement of the beam could be derived. For some of the tests a high speed camera (running at 2000 frames per second) was used to capture the impact event. This data proved useful, for



example, matching the load displacement curve to the video frames one could confirm the dip in load was due to the upper skin failure, consistent with the quasi-static results. In one instance, core failure and progression was captured by the video data.

Table 3: Summary of indenter data for the impact tests.

Test id Impact Velocity m/s

Impact mass kg

Initial Energy (K.E), J

Max displacement mm

Series 1 4.14 1.825 15.6 11.1 Series 2 5.92 1.825 32.0 22.0 Series 3 4.26 3.75 34.1 20.0 Series 4 5.17 3.75 50.1 32.0

†Average values for each test series quoted

Figure 4: Core damage propagation from the defect (Quasi-static test of 10mm depth core).

Figure 5: Comparison of beams at spans 100 to 340mm (20mm core depth).

Cell defect



Figure 6: Schematic of core damage progression (20mm core, span 100mm).

3.1 Beam failure characterisation

The deformation and progressive impact behaviour was similar to the quasi-static test results. Upper skin failure and progressive crushing of the core were followed by core shear failure over the impact time period of about 5 to 13ms. Nominal strain rates in the core and skin were estimated to be 10 s-1 in the impact region. Fig. 7 shows a typical history profile of the lower skin strain gauge and the LDA derived load, in this case for test 1(a), one of the lower energy impacts. The initial sharp dip in strain coincides with a dip in the load, as seen in the quasi-static tests, and is the point of upper skin failure. Static data is shown for comparison on fig. 7. The impact data shows a higher peak load and larger displacement to upper skin failure compared to the static case.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 1 2 3 4 5 6 7 8 9 10 11

Displacement, mm

Load

, kN

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

Stra

inStatic test

LDA load (Impact test)

Lower skin strain gauge(Impact test)

Figure 7: Impact test load displacement compared with quasi-static result. Strain gauge shows upper skin failure point for the impact test.

The energy absorbed up to upper skin failure as a fraction of the total energy absorbed by the beam is depicted in fig. 8 as a function of the total energy lost by the indenter in deforming the beam to its maximum displacement. The trend indicates that the fraction of energy absorbed by the upper skin falls from 45% to around 20% with a 3-fold increase in indenter energy from 15 to 45J. The core contribution to energy absorption must therefore increase, which is evident from the increased size of the crush zone in the higher energy impact test results.

Upper skin failure point

Crush zone



0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0 5 10 15 20 25 30 35 40 45 50 55

Total energy loss of indenter (K.E. + P.E.), J

Frac

tion

of in

dent

er e

nerg

y ab

sorb

ed

up to

ski

n fa

ilure

Series 1 (a-c)Series 2 (d-f)Series 3 (g-i)Series 4 (j)

Figure 8: Energy absorption of the beam up to upper skin failure expressed as a

fraction of the total energy lost by the indenter over the impact event.

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35 40 45 50 55

Initial indenter K.E. (or approx. energy absorbed by beam), J

Max

imum

bea

m d

ispl

acem

ent,

mm

Series 1 (a-c)Series 2 (d-f)Series 3 (g-i)Series 4 (j)

Figure 9: Maximum beam displacement for the impact tests as a function of

the indenter initial K.E.

3.2 Core damage investigation under impact loading

Figure 9 shows a linear relationship over the investigated range of indenter energies for the maximum displacement of the beam. This refers to the maximum distance by which the indenter deforms the upper skin before coming to rest, and beginning its rebound. The lower skin deforms by a smaller distance due to crushing in the core. The high speed video data allows, amongst other things, the ability to track the location of the skins during the impact and thus estimate the amount of deformation the core experiences. Table 4 gives the minimum separation of the upper and lower skin, and hence the maximum amount by which the core was compressed in the impact region. Only the higher energy beam tests, series 3 & 4, were filmed with the high speed camera. The core crush is expressed as a percentage of the initial thickness of the beam, being 23.5mm. The effect of core failure on the beam behaviour became apparent for the higher energy tests of series 3 and 4. These beams showed evidence of shear failure similar to the quasi-static cases.



For one case the shear failure was captured by the high speed camera. The initial position and progression of the damage was monitored and correlated to the indenter displacement and lower skin strain. Fig. 10 shows the strain profile on the lower skin as a function of indenter displacement. The typical profile is straining to a maximum value followed by a drop in strain as the beam is unloaded on indenter rebound. However, for the beams showing core shear failure there is a noticeable change in gradient of the strain. For test series 3(h), in which core failure was captured on video, the indenter displacement at which the strain profile changes corresponds to the video footage showing initiation of core failure at the same indenter displacement. The location and indenter displacement of core failure covers a range similar to the quasi-static results as summarised in table 4.

Table 4: Video data analysis of core crush and core failure location in terms of distance from beam centreline and beam displacement.

Core failure Beam Id

Max. core compressio

n (mm)

†Core crush (%) distance from beam

centre line (mm) initiates at indenter

displacement of (mm) Series 3 (g) 9.4 40.1 - - Series 3 (h) 8.3 35.3 15-25 19 Series 3 (i) 6.8 28.9 15-25 16 Series 4 (j) 10.1 43.1 20-30 27.5 Static - - 20-35 15-30

† Calculated from nominal beam thickness of 23.5mm before impact

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Series 1 (a)

Series 2 (e)

(h)Series 3(i)

Series 4 (j)

0.000

0.0020.004

0.006

0.0080.010

0.012

0.014

0.016

0.0180.020

0.022

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32Indenter displacement, mm

Low

er sk

in st

rain

Figure 10: Impact test core failure inferred from lower skin strain and indenter

displacement relationship.

4 Concluding remarks

Defects and a large cell size variation in the Aluminium foam structure have a considerable effect on the behaviour of the sandwich beams under quasi-static and low velocity impact loading.

Change in curve trend indicates core failure



Beams with 10mm core thickness showed catastrophic failure in some cases due to the cell size being of the same order as the structure. An increase in the core thickness to 20mm produced a more consistent set of results for quasi-static and impact tests in terms of the upper skin failure load and displacement. Core tensile failure was investigated by reducing the beam span to 100mm. The range of indenter displacements, 15-30mm, over which core failure initiated was similar for both quasi-static and impact cases. This would suggest that any strain rate effect in the aluminium foam, tending to locally increase the core strength in the impact region, is swamped by the initiation of core failure at a weak point in the material structure. Again, the weak point refers to the large cell size variation and structural defects such as concentrations of impurities or regions of merged cells. The concern of large cell size variations and defects brings into question the use of continuum material models used in any numerical simulation of the beam tests. An alternate approach to modelling the core would be to generate a geometrical approximation to the foam structure, at much greater computational cost, for beams that have dimensions of the order of the cell size. As the beam dimensions increase, the defect induced beam failure diminishes and so the continuum material models become valid.

Acknowledgements

This research has been funded by the EPSRC Grant number GR/N35618/01.

References

[1] Alporas data sheet, Shinko Wire Company, Amagasaki, Japan, 2001. [2] LEAP Systems data sheet, BI Composites Ltd., England, 2001. [3] Reyes V., G. & Cantwell, W.J., The mechanical properties of fibre-metal

laminates based on glass fiber reinforced polypropylene. Composites Science and Technology, pp. 1085-1094, 2000.

[4] Kiratisaevee, H. & Cantwell, W.J., The Fracture Behaviour of Aluminium Foam Sandwich Structures Based on Fiber Reinforced Thermoplastics, Journal of Sandwich Structures and Materials, vol. 5, no. 1, pp. 53-75, 2003.

[5] Andrews, E., Sanders, W. & Gibson, L.J., Compressive and tensile behaviour of aluminium foams, Materials Science and Engineering, pp. 113-124, 1999.

[6] Ashby, M.F., Evans, A., Fleck, N.A., Gibson, L.J., Hutchinson, J.W., Wadley, H.N.G., Metal Foams-A design guide, B-H, Oxford 2000.

[7] Birch, R.S., Jones, N, Measurement of impact loads using a laser Doppler velocimeter, Proceedings of the Institute of Mechanical Engineers, Part C: Mechanical Engineering Science, vol. 204, no. 1, pp. 1-8, 1990.



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Static and impact behaviour of metal foam cored sandwich beams · The sandwich beam construction under consideration consists of Alporas ... Xt 720 MPa “ Matrix Tensile Strength,