behavior of cfrc/al foam composite sandwich beams under three-point bending

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DOI: 10.1002/adem.201300055
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Behavior of CFRC/Al Foam Composite Sandwich Beamsunder Three‐Point Bending

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By Martin Vcelka, Michelle Dunn, Yvonne Durandet, Christopher C. Berndt and Dong Ruan*
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Cellular hybrid structures, such as carbon fiber reinforced extended to include the use of woven glass fiber epoxy face
composite (CFRC) sandwich beams, are increasingly beingused in load-critical structural applications.[1] Outstandingmechanical attributes such as high strength to weight ratio andsuperior bending stiffness coupled with good energy absorp-tive capabilities, make composite sandwich beams idealcandidates for such applications. The automotive sector isone of several industries that embrace this type of compositebeam. It has placed a focus on this type of structure to addresschallenges such as the reduction in overall vehicular mass andthe increase in occupant safety. The current generation oflightweight sandwich structures usually consist of some typeof cellular core and various combinations of composite facesheets. Load carrying and energy absorption capability dependnot only on factors such as the strength and stiffness of the facesheets, but also on the core compressive and shear strength andmore importantly, the bond between the constituents.[2] Withcomposite face sheets, these factors increase to include fiberorientation, matrix composition, and interlaminar effects.During manufacture or application, the latter may result inareas of delamination and can cause a structure to failprematurely.[3] This is of concern in many load criticalapplications, as, in some cases, delamination can be one ofthe failure modes during deformation.[4] Therefore, it isimportant to understand the effect of defects (delamination)on the failure mechanisms and energy absorption of sandwichbeams.
Previous research using combinations of polymeric foamcores with aluminum face sheets and metallic foam cores withmetallic face sheets has shown that sandwich beams fail due tocompeting failuremechanisms: face indentation, core shearing,face wrinkling, and face microbuckling.[5–9] Much of this workinvolves the use of mechanism maps. This work has been

[*] Dr. D. Ruan, M. VcelkaFaculty of Engineering and Industrial Sciences, SwinburneUniversity of Technology, John Street, Hawthorn, VIC 3122,AustraliaE-mail: [email protected]. M. Dunn, Dr. Y. Durandet, Prof. C. C. BerndtIndustrial Research Institute Swinburne (IRIS), SwinburneUniversity of Technology, Hawthorn, VIC 3122, Australia

[**] The authors thank the Cooperative Research Centre forAdvanced Automotive Technology (AutoCRC) and SwinburneUniversity of Technology for their support.

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sheets with polymeric and aluminum foam cores.[10–12]

The inclusion of defects within sandwich structureswith metallic cores and various metallic face sheets hasexamined the ability of these structures to retain their loadcarrying capabilities.[13] Imperfections that cause a face sheetto de-bond can result in an appreciable reduction in thestiffness of the structure.[14] Several studies have experimen-tally examined this type of defect and treat the delaminationas an interfacial fracture at the interface; i.e. between theface and core.[15–17]

In the current study, composite sandwich beams comprisingwoven carbon fiber epoxy face sheets with aluminum foamcores were employed. Bonding defects of specific sizes wereintroduced to elucidate the modes of failure and the resultingenergy absorption under three-point bending. Two types ofspecimens were manufactured: fully bonded beams andpartially bonded beams with defects of specific size andgeometry; i.e. delaminated areas.

Beams with the largest delamination areas were usedfor a further set of experiments involving various indentersizes. Given that woven carbon–epoxy composite facesheets in these types of structures fail in a brittle mannerand do not hold together after the fibers have broken, themost important failure mode to study was face indenta-tion.[18] As such, indenter sizes were chosen to be smallenough in diameter to ensure this type of failure mode waspresent.

1. Preliminary Tests

1.1. Ultrasound TestingThe samples were inspected using ultrasound to verify that

there was no bond between the face sheet and the aluminumfoam in the partially bonded specimens. The phase andamplitude of the ultrasonic response received by the probewasrecorded and plotted. Theoretically, if there is a good bondpresent, the energy from the ultrasonic wave will attenuatethrough the aluminum foam and the returned amplitude willbe small. If there is no bond between the aluminum foam andthe face sheet, the energy of the ultrasound wave will only beattenuated by the carbon fiber and not the aluminum foam,hence the returned amplitude will be large. Typical phase–amplitude graphs are shown in Figure 1a and b. The phase–amplitude graphs were further analyzed by calculating themean value of the amplitude and plotting this value against the

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Fig. 1. Phase–amplitude graph for the returned ultrasound signal. The graph is in polar coordinates with themagnitude represented by the distance from the origin and the phase represented by the angle from the x-axis. (a)A typical response for a well bonded area. (b) A typical response for a non-bonded area. (c) The average amplitudefor the returned ultrasound signal. The non-bonded areas are represented as a square wave.

M. Vcelka et al./Behavior of CFRC/Al Foam Composite Sandwich Beams

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known location of the defect. An example of this is includedin Figure 1c. It can be seen that there is a strong correlationbetween the location of the defect and the peak values from theultrasound inspection, differentiating between the bonded andthe non-bonded sections of the sample.

Table 1. Specimen dimensions used for three-point bending experiments.

SamplesDefectposition

Length (L)[mm] (�0.5)

Width (b)[mm] (�0.2)

Face sheetthickness (tf)[mm] (�0.02)

Height (c)[mm] (�0.2)

a–c N/A 350 40 0.9 50d–f Upper face 350 40 0.9 50g–i Upper face 350 40 0.9 50j–l Lower face 350 40 0.9 50m–o Upper face 350 40 0.9 50p–r Lower face 350 40 0.9 50b1–c1 Upper face 350 40 0.9 50d1–e1 Upper face 350 40 0.9 50

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1.2. Compliance TestingThe three-point bending tests were con-

ducted on a 250 kN MTS universal testingmachine with a crosshead speed of 5� 10�3

mmin�1. All the samples were tested in twoconfigurations: (i) defect between the upperface and core, and (ii) defect between thelower face and core. Sample dimensions areshown in Table 1. Repeated series of testswere conducted by using three samples at atime with the same defect configuration.Displacements of both the upper face sheetand the lower face sheet were measured bythe MTS sensor and motion potentiometer,respectively.

Quasi-static three-point bend testing ofnon-reinforced aluminum foam beams andface sheets was performed separately toascertain the deformation and energy absorp-tion of the respective composite sandwichbeam constituents. For all the non-reinforcedaluminum foam beams (no face sheets), thedeformation was dominated by upper faceindentation followed by cell wall tearing atthe lower face, emerging as a crack. Local cellcrushing under the roller was observed as thedominant deformation mode in the first partof the curve in Figure 2 leading up to the peakforce at 6mm. This type of deformationis common in aluminum foam and wasalso observed by McCullough et al.[19] whotheorized that the individual cells crush bybending of their cell edges, not by stretching.The origin of the crack appeared at the lowerface, directly under the roller. The direction ofthe crack, as it propagated, was up toward theindenter. A similar observation was made by

Arun et al.[20] who investigated the deformation of sandwichpanels with polyurethane cores. Quasi-static three-pointtesting of the carbon face sheets showed that the samplesbehaved in an elastic-brittle manner. In three-point bending, alarge deformation was observed until the onset of failure,

Span (ds)[mm] (�0.1)

Defect length (DL)[mm] (�0.2)

Indenterdiameter

[mm] (�0.1)

150 0 19150 10 19150 20 19150 20 19150 40 19150 40 19150 40 38150 40 10

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Fig. 2. Three-point bending force-displacement curve of 10% relative density aluminum foam. The samplethickness is 50mm.


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which resulted in complete separation of the sample halves.At the onset of failure, the matrix reached the critical limitof interlaminar stress and a crack evolved across the widthof sample, resulting in a softening of the structure and aninitiation of fiber breakage. The developing broken fiberscontinued to increase in number as the crack propagated untila complete structural failure was observed.

2. Results

Three point-bending structural tests of the compositesandwich beams reveal that the deformation mechanismswere dominated by face indentation followed by core shear.Although the same block of 10% relative density aluminumfoam was used to manufacture all the samples, there was adensity difference in the samples. The relative density of thesamples varied from 8.5 to 12.2%. To normalize the raw data,the load curves were divided by the mass of each sample to

Fig. 3. Three-point bending response of composite sandwich beams. Samples a–c had no defect inclusions.Samples d–r had defects of specific size. All sample thicknesses were 50mm.

obtain a specific force, which was used toobtain specific energy values.

In all three fully bonded, non-defectivesamples, the loading roller produced a localmulti-axial stress state, which resulted in thecollapse of the upper face sheet. Figure 3shows the initial face sheet failure, whichoccurred between 8.80 and 9.05 kNkg�1 forthe fully bonded samples. This resulted in asingular hinge line beneath the roller. As theupper face sheet failed, the underlying foamcontinued to deform creating the subsequentload drop, which occurred at 4.16mm upperface sheet displacement (UFSD). A second setof hinges emerged at the upper surfaceadjacent to the roller and was measured tobe equivalent to twice the roller diameter. Anupturn in the curve after 4.16mm UFSD isrepresentative of the transition between a

ADVANCED ENGINEERING MATERIALS 2014, 16, No. 1 © 2013 WILEY-VCH Verlag GmbH & Co. KG

localized instability (core crushing) and aglobal force distribution (beam bending). At13mm UFSD, a further set of hinge linesemerged andweremeasured to be 5mm fromeach side of the central loading point. Theload continued to increase as the deformationmode of the beam changed to pure bending.This is due to the fact that the foam hadreached its densification strain and could notcompact further. The crosshead displacementwas limited to 35mm to prevent the upperface sheet touching the sides of the indenter.

Figure 3 shows that there is little differencein the overall deformation response in all thetests. What is interesting, however, is thatwhile the initial peak forces are within 8% ofeach other, the strains at which these occur,appear to be greater for the defective samples.The sandwich beams with 10mm defects in

the upper face produced an average initial specific peak force(SPF) of 8.80 kNkg�1 at 2.62mm UFSD (Figure 4a). This is anaverage of 0.62mm further in displacement of the upper facesheet or 15% greater than that of the non-defective compositesandwich beams.

Aswith the composite sandwich beamswith 10mmdefects,the samples with 20mm defects at the upper face exhibitedonly slightly lower peak forces, with an average initial SPF of8.76 kNkg�1 at 2.85mm. The curves in Figure 4b, for thesampleswith the defects in the upper face, resembled the shapeof the non-defective samples; albeit, with a slightly prolongeddisplacement at initial peak load that was followed by asharper drop in force. The samples with the defect on the lowerface, however, revealed a more compliant curve at initial SPF.The sharp drop in force after the initial SPF is not as apparent inthese curves as it was for the upper sheet defect samples.

The final set of samples, with the largest defects (40mm),again produced a similar initial SPF of 8.79 kNkg�1 at 4.1mm

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Fig. 4. Three point bending experimental results: (a) three-point bending force-displacement curves of samples with 10mm defects; (b) three-point bending force-displacementcurves of samples with 20mm defects; (c) three-point bending force-displacement curves of samples with 40mm defects; (d) photograph showing 50mm thick sample andpictorial representation of deformed composite sandwich beams.


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UFSD. Figure 4c demonstrates that the initial part of the curves,produced by the samples with defects on the upper face,exhibit “a toe”. The toe arises because the upper face sheet isunable to be locally supported by the adhesive bond, which

Fig. 5. Three-point bending response of composite sandwich beams with different indenter sizes (10, 19,38mm diameters). All sample thicknesses were 50mm.

only exists in the non-defective samples. Theslope of the curve, after the toe, resembledthat of the other samples, but as with the20mm defect samples, the occurrence of theinitial SPF was at a greater displacement(average of 4.15mm). The load drop after theinitial maximum, when the top face sheethas failed, remained in line with the otherobservations.

For samples with 40mm defects in thelower face, no toe existed, but interestinglyneither did the sharp drop in force afterthe initial SPF. It appeared that the initialpart of the curves follow that of the non-defective sample average. The curves tend todiverge from the non-defective average as4 kNkg�1 is reached and then increase andshow a softening response. A graphic repre-sentation of the collapse can be seen inFigure 4d.

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For further comparison, three-point bending tests wereperformed using two other roller sizes. Figure 5 showsaveraged results of samples tested (two samples per condition)with the 40mm upper face sheet defect. The smallest indenter

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Table 2. Energy absorption values of composite sandwich beams.

SamplesDefect size[mm] (�0.2) Location

Averageenergy

absorbed[J] (�0.1)

Average specificenergy absorbed[J kg�1] (�1.0)

a–c 0 N/A 73.06 390d–f 10 Upper face 70.75 374g–i 20 Upper face 74.95 380j–l 20 Lower face 83.52 370m–o 40 Lower face 66.39 378p–r 40 Lower face 83.56 380b1–c1 40 Upper face 68.24 358d1–e1 40 Upper face 62.18 314


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(10mm diameter) produced similar initial stress in the upperface sheet to that of the medium roller (19mm diameter),however, with a smaller volumetric strain distribution. As theupper face sheet collapsed, a set of hinge lines in the shape of a“V” formed directly around the 10mm indenter. The hingelines were defined by a sharp corner predominantly caused byinterlaminar failure of the matrix, with some fiber breakage.The onset of face sheet failure commenced at 7.30 kNkg�1

and the formation of the “V” was observed at 7.35 kNkg�1.This was followed by a small load drop, as observed in thepreceding experiments but the extent of this drop was muchsmaller than that found in the 19mm diameter tests. Localizedcell crushing was the dominant failure mode from this point.This can be seen by the somewhat flatter curve. It was alsoobserved that the vertical penetration of the indenter wasgreater into the beam for the same crosshead displacement, i.e.the lower face to upper face dimension in the highest crushzone was 20mm which was the smallest of all the tests carriedout. Thus, the 314 J kg�1 average specific energy absorption(SEA) values were also the smallest.

The largest roller (38mm diameter) produced an initialupper face sheet failure at 5.9 kNkg�1. This failure wasdifferent to the others observed as it did not form the typicalhinge lines observed earlier. The face sheet deformed toform the shape of the roller causing matrix cracking andinterlaminar face sheet failure but without the sharper edges.The shape of the curve is quite different to the other seriesof tests due to the absence of the large load drop that followsthe formation of the hinge lines. Cell crushing followed byshear cracking were the dominant modes of collapse duringthe final stages of deformation. The average SEA for this seriesof tests was found to be 358 J kg�1.

The highest SEAvalues were observed in the non-defectivesamples tests (using the 19mm indenter), with an average of390 J kg�1. The SEA for the defective samples ranged from 374to 380 J kg�1. The remaining results can be found in Table 2.

3. Conclusions

The deformation and energy absorption of defective andnon-defective composite sandwich beams under three-point

ADVANCED ENGINEERING MATERIALS 2014, 16, No. 1 © 2013 WILEY-VCH V

bending were investigated experimentally. The results indicat-ed that there was approximately 10% difference in the initialSPF in the sample range examined. The displacement at whichthe initial SPF occurred in the partially bonded defectivesamples was at least 15% greater than that of the fully bondednon-defective samples. Observations revealed a sharper dropin initial SPF for beams with defects in the upper face thanthose with defects in the lower face. Face indentation followedby cell crushing were the dominant deformation modes ofcollapse. The non-defective samples exhibited the highest SEAof the samples examined in this study.

4. Experimental

4.1. Beam ManufactureThe composite sandwich beams were manufactured from woven

carbon fiber epoxy face sheets (GMS Composites, Melbourne,Australia) bonded to aluminum foam cores (Alporas©, GLEICHAluminiumwerk GmbH & Co. KG, Germany). The fully bonded, non-defective, composite sandwich beams were manufactured by stackingfour 200 gsm 300K twill weave (2� 2) prepregs, using a high strain EP-620 Epoxy resin, onto both sides of a 50mm thick aluminum foamblock. The stacking sequence of the plies resulted in the warp fibersbeing oriented along the length of the samples and the weft along thewidth of the samples.

The aluminum foam exhibited a nominal relative density of 10%and an average cell size of approximately 3mm. The beamsincorporating the defects were manufactured using the above method,however, with the inclusion of a 50mm thick release film. The film wascut into strips of 10, 20, and 40mm and placed between the face sheetand the aluminum foam core in a central location. Care was taken toensure that full coverage from one side to the other was achieved.

The sandwich beamswere cured at 150 °C for 30min under 0.7MPa.The face sheet thickness used was 0.85mm, which was dictated by thesheet lay-up and corresponded to four plies. The volume fraction of thecarbon fiber was 40%� 1%.

4.2. Ultrasound InspectionA BondMaster 1000eþ (Olympus) ultrasonic flaw detection device

was used with a Broadband Pitch-Catch probe. Pulses of ultrasoundwere passed into the sample at frequencies ranging from 20 to40 kHz. The probe was mounted on an automated ball screwassembly and the spring-mounted probe ran laterally along thesample. Readings were recorded every 1mm to permit the detectionof the smaller non-bonded areas.

4.3. Three‐Point Bending TestsThe three-point bending testing rig consisted of a 10, 19, and 38mm

diameter rollers with two 25mm wide rectangular bars, which wereused for loading and support, respectively.

Received: February 21, 2013Final Version: May 28, 2013

Published online: July 24, 2013

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behavior of cfrc/al foam composite sandwich beams under three-point bending

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