Composite Structures 94 (2012) 617–624

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Composite Structures

journal homepage: www.elsevier .com/locate /compstruct

Failure behaviour of foam-based sandwich joints under pull-out testing

Khanh-Hung Nguyen a, Yong-Bin Park a, Jin-Hwe Kweon a,⇑, Jin-Ho Choi b

a Department of Aerospace Engineering, Research Centre for Aircraft Parts Technology, Gyeongsang National University, Jinju, Gyeongnam 660-701, South Koreab Department of Mechanical Engineering, Research Centre for Aircraft Parts Technology, Gyeongsang National University, Jinju, Gyeongnam 660-701, South Korea

a r t i c l e i n f o

Article history:Available online 1 September 2011

Keywords:Sandwich structuresFoam coreProgressive failurePull-out test

0263-8223/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compstruct.2011.08.027

⇑ Corresponding author.E-mail address: jhkweon@gnu.ac.kr (J.-H. Kweon).

a b s t r a c t

Sandwich materials have been used widely in various fields. However, the failure behaviour of the jointsthat connect the sandwich structures is not fully understood. In this paper, foam-based sandwich jointswere studied under pull-out conditions through experiments and numerical analyses. Two differenttypes of joints, with and without inserts, were examined. The experiments showed that the failure modesof sandwich joints were a combination of different failure modes, such as core shear failure, the debond-ing of face and potting material, composite face failure, and local failure. A finite element analysis usingMSC.MARC was also conducted to predict the failure load of sandwich joints. Two methods were appliedto predict the first drop of the sandwich joints: progressive failure analysis and the damage zone method.The former resulted in an acceptable prediction for a preliminary design, and the latter was shown to pre-dict the first peak of loading curves very well.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

A sandwich is a special class of composite materials. Generally,a sandwich consists of a lightweight thick core and two faces withhigh stiffness and strength. The faces are identical in shape andmaterial and can be metallic or nonmetallic. In all cases of con-struction, the bending moment is carried by tension and compres-sion in the outer faces, and the core supports the transverse loads.The core separates and stabilises the faces against buckling underin-plane compression, torsion, or bending. Typical properties ofthe sandwich materials are a high strength-to-weight ratio, goodenergy and sound absorption, and low production costs. Two com-mon cores used in sandwich structures are foam and honeycomb.The behaviour of the core itself under static and dynamic loadingcan be found in several publications based on tests or/and numer-ical analysis, such as Andrews et al. [1], Deshpande and Fleck [2],Johnson et al. [3], Fortes and Ashby [4], Lamb et al. [5], and Heimbset al. [6], to name a few.

The behaviours of sandwich structures have been thoroughlyinvestigated, and one of the interesting research areas is relatedto joints made with these materials. There are several types of jointsbetween sandwich structures. In general, the joints require pottingmaterial and/or inserts to connect bolts for transferring loads. Ananalytical model was developed by Thomsen and Rits [7] to inves-tigate the structural performances of metallic honeycomb sand-wich plates with inserts. He pointed out the stress concentrationin the regions close to the inserts. Some guidelines were also de-

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scribed for the design of sandwiches with inserts. Demelio et al.[8] conducted an experimental investigation of the static and fati-gue behaviour of sandwich composite panels jointed by fasteners.The investigation also included a preliminary study that investi-gated the best set-up of parameters for drilling composite panels.Song et al. [9] experimentally studied the behaviour of honeycombcore sandwich joints under pull-out and bearing loading conditions.In another study, Feldhusen et al. [10] commented that there was alack of a consistent joining technology (easy, quantifiable) and pro-posed a mechanical technology for joining sandwich elements.

Finite element analysis has been applied to study the behaviourof sandwich joints. Bunyawanichakul et al. [11] developed a numer-ical model to predict the failure load of Nomex core sandwichjoints. The prediction method required several tests of the constit-uent material properties. The composite surface was modelled bytaking into consideration the damage initiation and propagation.The buckling phenomenon in the honeycomb structure was mod-elled with a nonlinear material with damage using SAMCEF soft-ware. The prediction of the failure load matched well with theexperimental results. Heimbs and Pein [12] developed a finite ele-ment model using spotweld elements to simulate the joining be-tween inserts and other parts of the sandwich structures. Thespotweld model did not physically represent the real progressivefailure process in the model where post-damage behaviour was ob-served during the test. However, the method showed good agree-ment with the tests.

From the literature survey, the authors found that the main focusof the research on sandwich joints was not the failure behaviour ofthe joints. The failure prediction methods for these structures havenot been well established. The objective of this paper was to inves-

Table 2Properties of the constituents (Unit: MPa).

Property Prepreg Potting material Core(rohacell)

G30–500PWfabric/5276-1

Magnobone6398 A&B

Epocast1618D/B

51WF 71WF

Tensile E11 62,050 75 105E22 60,670

Compressive E11 61,360 2068 2414E22 58,600

Tensile strength X11 924 41.4 1.6 2.2X22 820.5

Compressivestrength

X11 854 68.95 34.5 0.8 1.3X22 854

Shear modulus G12 468.8 24 42Shear strength or lap

shear strengthS12 128 32.4 4.8 0.8 1.3

618 K.-H. Nguyen et al. / Composite Structures 94 (2012) 617–624

tigate the failure behaviour of sandwich joints with foam core underpull-out loading. Three types of specimens were tested. Because thefirst loading drop is very significant, it is usually considered as thedesign allowable. Besides testing, finite element analysis was alsoapplied to predict the design allowable of the joints. Two methodswere used in the prediction of the first drop: the progressive failureanalysis method and the damage zone method.

2. Experimental testing of sandwich joints

2.1. Testing

There were three types of sandwich joints. The first one was thejoint with an insert, and the other two were the joints without in-serts. The two joints without inserts had different foam cores. Com-posite faces were used in all of the sandwich specimens. Thecomposite prepreg was the G30-500PW carbon-epoxy fabric/5276-1 from CYTEC. Two faces had the same stacking sequence of[45/0]. The thickness of a laminar after curing was typically0.2 mm. Each specimen was co-cured in an autoclave and cut intoa square shape with an edge dimension of 120 mm. The thicknessesof specimens with and without inserts were typically 20.8 and11.4 mm, respectively. Two types of potting material were used:Magnobone 6398 A&B and Huntcell Epocast 1618D/B. Details ofthe specimens and the properties of the composite prepreg, pottingmaterials, and foam cores are given in Tables 1 and 2, respectively.

The schematic configurations of specimens under loading areshown in Fig. 1. An Instron machine with a loading rate of1.27 mm/s was used for testing. The specimen support and experi-mental set-up are shown in Fig. 2. The upper plate of the supporthad a circular hole with an 80 mm diameter and prevented thespecimens from moving upward during loading.

2.2. Experimental results

2.2.1. Joint with insert (Cowl3)A typical load–displacement curve for the Cowl3 joint is shown

in Fig. 3. As shown in the figure, there were several drops in theload–displacement curve. At the beginning, the curve shows a lin-ear segment up to the first peak. When the first peak is reached, theload suddenly drops almost in half and then increases towards thesecond peak. There is another drop at the second peak. The loadfalls again by more than 50% and then increases to the third peak,where it gradually decreases.

To investigate the failure modes of the joint, the specimens werecut across the centre section for observation. A typical cross sectionof a tested specimen is shown in Fig. 4. As shown in the figure, thefailure of sandwich joints is a combination of several failure modes:(a) core shear failure, (b) debonding of the face and potting, and (c)other local failures, including debonding of the upper face and pot-ting. The core is broken with a long crack with an angle of approx-imately 45� from the loading direction. The crack starts at theintersectional region between the core and the potting material.This region experiences stress concentration because of the suddenchange of material and geometry. These failure modes are shown inall the tested specimens of the Cowl3 joint. Two cracks occurred in

Table 1Details of the sandwich joints.

Joint ID Foam core(rohacell)

Pottingmaterial

Coreheight(mm)

Insertheight(mm)

No. ofspecimen

Cowl3 51WF Magnobone 20 10.9 7Cowl111 51WF Epocast 11 N/A 7Cowl112 71WF Epocast 11 N/A 7

the core but not in symmetry around the centre line in the crosssection. The asymmetry of the cracks is attributed to the imperfec-tion of materials that is included during the specimen manufactur-ing. However, considering the complexities of the manufacturingprocess and the consistency among the test results, the obtainedfailure modes are believed to be acceptable.

The failure mode at the first drop in the load–displacementcurve was identified by testing an additional specimen. After thefirst drop occurred, the test was stopped manually. The specimenwas then taken out and was cut to check the failure mode. The firstpeak, denoted by (1) in Fig. 3, was found to correspond to failuremode (a): the core shear failure. The second drop (2) of the load–displacement curve was related to failure mode (b): the face/pot-ting debonding, which can be seen during the test. The local fail-ures (c) are also detectable during the test but are not clearlyobserved in some specimens.

2.2.2. Joints without inserts (Cowl111 and Cowl112)Typical load–displacement curves of joints without inserts,

Cowl111 and Cowl112, are shown in Fig. 5. As shown in the figure,each joint without an insert also shows two major drops in the sus-tained load. In both cases, however, the first peak is much smallerthan the second peak. The first peak occurs very early comparedwith the second peak. The second peak is always the highest peakand determines the maximum sustainable loads of the joints. It iscommon that the design allowable of a structure is often deter-mined by the first peak load of the load–displacement curve.Therefore, in the case of the joints without inserts, the designallowable is much lower than the maximum sustainable load.

The failure modes of the joints are also quite complicated. Theyinclude (a) core failure, (b) lower face cracking, (c) potting materialcracking, and (d) upper face failure, as shown in Fig. 6. Instead ofcutting the specimen through the section, the failure modes in thiscase were inspected by separating the faces and the core with asharp knife.

Similar to the Cowl3 case, the process of failure was verified bytesting an additional specimen. The first peak of the curve wasfound to correspond to the failure of the foam core. At the secondpeak, a lower face failure was observed during the experimentaltest. The third peak, therefore, was supposed to be the combinationof the potting material and an upper face failure. The failure peaks(1), (2), and (3), therefore correspond to (a) the core failure, (b)lower face failure, and (c) potting material and (d) upper face fail-ure, respectively.

The load at the first drop and the maximum load of the jointsare summarised in Table 3. Cowl111 of the joints without inserts

Fig. 1. Schematic configuration of a specimen with (left) and without (right) an insert.

(a) (b)

Fig. 2. Test fixture and set-up. (a) Fixture. (b) Set-up.

Fig. 3. A typical load–displacement curve of the Cowl3 joint.

Fig. 4. Failure surfaces of the Cowl3 joint.

K.-H. Nguyen et al. / Composite Structures 94 (2012) 617–624 619

is different from Cowl112 only in the properties of the foam core.As the core tensile strength increased by 37.5% from 1.6 MPa(Cowl111) to 2.2 MPa (Cowl112), the first peak also increased by51.4% (from 895 to 1355 N). The increasing core strength, however,does not lead to a gain in the maximum load carried by the joint.On the contrary, the maximum load was reduced by approximately9.5%, from 3960 down to 3590 N. The final failure modes corre-sponding to the maximum load were caused by the combinationof the face failures and face-core debonding, which are not domi-nated by the core strength. As described, the first drop always cor-responds to the core shear failure. Therefore, it is obvious that theincrease of core strength strongly affected the first drop of theloading curves.

3. Finite element analysis

3.1. Finite element model for sandwich joints

The finite element model was built using a combination of shellelements and solid elements. In the model, two faces were mod-elled using shell elements. Beside the two composite faces, theother parts of the specimen were built with solid elements. To re-duce the analysis time, symmetric boundary conditions were ap-plied, and therefore, only a quarter of the real structure wascreated. A part of the upper surface of the model was constrainedin the upward (vertical) direction to simulate the contact betweenthe specimen and the upper supporting plate. The movement ofthe stroke in the test was simulated by applying an upward dis-placement to the bolt model. The finite element model for theCowl3 and Cowl111 joints are shown in Fig. 7.

Fig. 5. Typical load–displacement curves of the Cowl111 and Cowl112 joints.

Table 3Summary of the test (Unit: N).

Joint ID Coreheight

First drop(N)

Maximum load(N)

Design allowable(N)

Cowl3 20 mm 1707 1911 1707Cowl111 11 mm 895 3960 895Cowl112 11 mm 1355 3590 1355

620 K.-H. Nguyen et al. / Composite Structures 94 (2012) 617–624

The aim of the current finite element analysis is to predict thefirst peaks of the load–displacement curves of the joints. As men-tioned in the experimental section, the first peak is expected tobe related to the design allowable of the joints. In addition, the firstsignificant drop is also related to the considerable failure of thespecimen, and therefore, its service in the structures should bestopped. The prediction of the first peak is therefore valuable forthe design and application of the sandwich joints in industry.Two failure prediction methods are applied in this paper to predictthe load at the first drop: progressive failure analysis and the dam-age zone method. Each of these methods has advantages anddisadvantages and is discussed below.

3.2. Progressive failure analysis (PFA)

Progressive failure analysis has been applied widely to predictthe failure of composite laminates [13], composite pinned joints[14,15], sandwich joints with Nomex cores [11], and other struc-tures. In this method, the failure of each constituent is detectedat each time step of the analysis. The damage is taken into accountby reducing the stiffness of the material in the specific directionwhen the considered failure criterion is satisfied. The current anal-ysis used the maximum stress criterion to evaluate the failure ofthe composite faces, foam core, and potting material. The bolt is as-sumed to be an elastic material. The progressive failure analysis, ingeneral, can predict the failure of the structure progressively.

Fig. 6. Failure modes of the sandwich

However, the reduction of the stiffness of a material frequentlyleads to instability of the numerical analysis and sometimes causesdivergence of the calculating process.

In the current analysis, all materials are assumed to be linear upto the point of failure. When failure occurs, the element stiffness isdegraded. In this study, the selective immediate degradation meth-od available in MSC.MARC is used to allow the stiffness to be re-duced to 1% and 10% of the original stiffness. The reduction to10% was recommended in MSC.MARC version 2005, and the valueof 1% is suggested in the latest version. In this study, we want toinvestigate the effects of these two values. Six failure indices areevaluated using the failure criteria in Eq. (1) as follows:

FI1 ¼r1=Xt if r1 � 0�r1=Xc if r1 < 0

�

FI2 ¼r2=Yt if r2 � 0�r2=Yc if r2 < 0

�

FI3 ¼r3=Yt if r3 � 0�r3=Yc if r3 < 0

�

FI4 ¼ r12S12

��� ���FI5 ¼ r23

S23

��� ���FI6 ¼ r31

S31

��� ���

ð1Þ

where Xt, Xc, Yt, Yc, Zt, and Zc are the maximum allowable stresses intension and compression in three directions and S12, S23, and S31 arethe maximum allowable shear stresses in-planes 12, 23, and 31. Inthe case of the composite material, the 1, 2, and 3 direction are thefibre, matrix (in-plane transverse) and out-of-plane (transverse)direction. In the case of isotropic material, these directions coincidewith the global X, Y, and Z directions in Fig. 7.

joint without inserts (Cowl111).

(a) (b)

(c) (d)

Fig. 7. Finite element model for Cowl3 and Cowl111. (a) Cowl3 including bolt and insert. (b) Cowl3 excluding bolt and insert. (c) Cowl111 including bolt. (d) Cowl111excluding bolt.

K.-H. Nguyen et al. / Composite Structures 94 (2012) 617–624 621

3.3. Damage zone method

The damage area method was used by Sheppard et al. [16] topredict the failure of single-lap joints to overcome the singularityat the end of the adhesive layer. Based on the idea of the damagearea, the failure area index method (FAI) [17], which consideredthe effect of the failure index, was developed to predict the failureof pinned joint specimens. Nguyen et al. [18] modified the FAImethod to consider the damage volume and the effect of geometryand the failure index in the prediction of composite-to-aluminiumsingle-lap bonded joints. In this study, we consider the damagezone method as the one that evaluates the failure of a structureby using the damage area, the damage volume or a modificationof those parameters.

The damage zone method requires at least one joint test. Thedamage area, volume or modification calculated from a finite ele-ment analysis of this joint is used to predict the failure load ofthe other specimens that have similar geometry or material. Theprocess is as follows:

(1) Test one joint to record the failure load and mode.(2) Analyse the joint under an experimental failure load using

an appropriate analysis tool such as the finite elementmethod.

(3) Use the appropriate failure criterion and the relevant mate-rial allowable to calculate the damage zone size in the joint,which is considered as the reference in the current study,and choose a value to be the critical value.

(4) Use the critical damage zone size calculated in the previousstep to predict the critical loads of joints with similar geom-etry and material.

In this method, failure criteria are also important. The maxi-mum stress criterion is also applied. However, only the maximum

failure index of the six failure indices described in Eq. (1) was con-sidered. In the third step of the damage zone method, the damagevolume is generally used. However, we found that if only the dam-age volume is of interest, the prediction method is not robust, andthe effect of geometry should be taken into account [18]. Therefore,besides the damage volume (DV) in the specimen, the damage vol-ume ratio (DVR) is also calculated in the third step as follows:

DVR ¼ DV=Core Height ð2Þ

In Eq. (2), core height is considered because it is the main un-ique geometrical parameter in the joints of interest. The DV andDVR are used independently to predict the first peak of the load–displacement curve in this study.

3.4. Results of failure prediction

3.4.1. Progressive failure analysis (PFA)Results of the progressive failure analysis of the Cowl3,

Cowl111 and Cowl112 joints are shown in Fig. 8. In the figure,PFA denotes the progressive failure analysis. As noted previously,two values of the residual stiffness were applied after failure: 1%and 10% of the initial stiffness. Besides the experimental curve,each figure includes the results from the analysis with these twovalues. As shown in the figures, the drop in the curves occurs muchearlier compared with the experimental results. The slope of thecurve from the test is slightly smaller than the slopes from theanalyses. The nonlinearity in the test curve is believed to implyplastic deformation inside the foam material. This phenomenon,however, was not considered in the current analysis. If plasticityis taken into account, the nonlinearity of the curve may be de-picted. From the current analysis results, only the load–displace-ment curve in the case of the Cowl112 proceeded much furtherafter the first drop. In the other two joints, the analyses stoppedsoon after the drop.

(a)

(b)

(c)

Fig. 8. Load–displacement curves determined by progressive failure analyses. (a)Cowl3. (b) Cowl111. (c) Cowl112.

Fig. 9. Predicted failure loads using progressive failure analysis.

Table 4DV and DVR at the first drop of loading.

Joint ID First drop (N) DV (mm3) DVR (mm2)

Cowl3 1707 1175 58.75Cowl111 895 450 40.9Cowl112 1355 501 45.5

622 K.-H. Nguyen et al. / Composite Structures 94 (2012) 617–624

In all three cases, when 1% residual stiffness is applied, theload–displacement curves drop vertically right after the first peak.This phenomenon is closer to the observation from the test. Thepredicted peaks, however, were not very different when two values(1%, 10%) of the residual stiffness were tried.

The results using the progressive failure analysis are summa-rised in Fig. 9. As shown in the figure, the PFA prediction underes-

timates the design allowable loads compared with the test results.When the residual stiffness is set to be 10% of the initial stiffness,the maximum deviation is 23% in the Cowl3 joint and the mini-mum deviation is 11% in the Cowl111 joint. Similarly, when theresidual stiffness is assumed to be 1% of the initial stiffness, themaximum and minimum differences are 26% (Cowl3) and 12%(Cowl111), respectively. Even though the analysis slightly underes-timates the test, these predicted results are acceptable for the pre-liminary design, especially in the case of joints without inserts.

3.4.2. Damage zone methodThe progressive failure analysis often encounters numerical dif-

ficulties during the calculation because of the abrupt change in thematerial properties. The damage zone method, however, does notdegrade the material stiffness of the failed elements. The volumeof the failed parts of the joint, instead, is collected to calculatethe critical damage zone. As noted, the current paper uses a dam-age volume and a damage volume ratio in the joint to represent thedamage zone. These values calculated in the models after the firstdrop are given in Table 4.

The damaged regions of Cowl3 after the first peak are shown inFig. 10. All failed elements are coloured, and the un-failed elementsare shown in grey. Only core failure was found in the modelaccording to the maximum stress criterion. As shown in the figure,the foam fails because of transverse compressive and transverseshear stresses. The compressive failures are found along the edgeof the constrained region of the upper face.

The predicted failure loads when a CDV of 1175 mm3 and aCDVR of 58.75 mm2 are used, which are the DV and DVR of Cowl3at the first drop of the loading curve, respectively, are shown inFig. 11. The failure loads predicted by progressive failure analysisare also plotted in the same figure for comparison. As shown inthe figure, the prediction of the failure load for Cowl3 is perfectlycorrect because it was chosen as the reference. The values obtainedusing the damage zone methods are always higher than those fromthe progressive failure analyses. Clearly, the CDVR is more accuratein prediction than the DVR. This implies that the difference ofgeometry should be considered in the damage zone method.

When the joints without inserts (Cowl111 and Cowl112) arechosen as the references, the prediction gives more accurate re-

Fig. 10. Damage zones of Cowl3 after the first drop. (a) FI3 P 1 (transverse compression). (b) FI5 P 1 (transverse shear r23). (c) FI6 P 1 (transverse shear r13).

Fig. 11. Failure load predicted using the DV and the DVR of Cowl3.

K.-H. Nguyen et al. / Composite Structures 94 (2012) 617–624 623

sults, as shown in Fig. 12. The reason for this could be related to thedifference in configuration between the Cowl3 joint and the otherjoints.

The deviation between the test and the predicted values aresummarised in Table 5. The maximum deviation from the damagezone method, which used the DV, is 19.5% when the DV of Cowl3 isused as the CDV to predict the first drop of Cowl112. When theDVR is considered, the evaluation of the first peak is much moreaccurate, with a maximum deviation of 4.17%. It is obvious thatthe geometrical differences affected the results of the damage zonemethod. In this case, the core thickness is shown to be a meaning-ful parameter for the geometry.

4. Conclusions

In this paper, the failure analysis of foam core sandwich jointsunder pull-out loading was conducted by testing and numericalanalysis. Failure modes of the sandwich joints were first identifiedthrough testing. The test results showed that the joints always failvia through-thickness shear cracking in the foam core and that this

causes a sudden drop of the load–displacement curve. Accordingly,the objective of the numerical analysis was to capture the first

(a)

(b)

Fig. 12. Failure load predicted using the DV and the DVR of Cowl111 and Cowl112.(a) Cowl111 as the reference. (b) Cowl112 as the reference.

Table 5Deviation (%) of the failure loads by the damage zone method from the test.

Jointpredicted

CDV (mm3) CDVR (mm2)

1175(Cowl3)

450(Cowl111)

501(Cowl112)

58.75(Cowl3)

41.28(Cowl111)

45.90(Cowl112)

Cowl3 0.00 �8.76 �8.13 0.00 �3.85 �2.90Cowl111 16.06 0.00 1.19 4.17 0.00 1.07Cowl112 19.50 �1.45 0.00 4.15 �1.64 0.00

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peak loads of the load–displacement curves. Two methods wereused to predict the first peak loads: progressive failure analysisand the damage zone method. The current progressive failureanalysis slightly underestimated the peak load, with a deviation

of 11–26% from the test. A damage zone method was also appliedto predict the same peak load. When the difference in geometrywas considered, the method could predict the peak load very well,with a maximum difference of 4.15% from the test results whencore height was taken into account.

Acknowledgments

This work was supported by the Priority Research Centers Pro-gram through the National Research Foundation of Korea (NRF)funded by the Ministry of Education, Science and Technology(2010-0029689). The authors also gratefully acknowledge thefinancial support of the Defence Acquisition Program Administra-tion and Agency for Defence Development under the ContractUD100048JD.

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