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International Journal of Impact Engineering 63 (2014) 129e139

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International Journal of Impact Engineering

journal homepage: www.elsevier .com/locate/ i j impeng

Study of foam density variations in composite sandwich panels underhigh velocity impact loading

Rasoul Nasirzadeh, Ali Reza Sabet*

Composites Department, Iran Polymer and Petrochemical Institute, PO Box 14965/115, Tehran, Iran

a r t i c l e i n f o

Article history:Received 24 August 2012Received in revised form27 May 2013Accepted 13 August 2013Available online 22 August 2013

Keywords:High velocity impactComposite sandwich panelsFoam density

* Corresponding author. Tel.: þ98 21 44580024; faxE-mail addresses: a.sabet@ippi.ac.ir, sabetreza2002

0734-743X/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.ijimpeng.2013.08.009

a b s t r a c t

This study investigates the effect of foam density variations in sandwich structure under high velocityimpact loadings. The structure consists of composite facing made from glass fiber woven roving rein-forced unsaturated polyester resin and rigid polyurethane foam core with density of 37, 49, 70, 95, 105and 240 kg/m3. Smooth bore gas gun in velocity range of 100e150 m/s and 10.7 g semi-spherical tip steelprojectile was used for high velocity impact tests. Results showed 49 kg/m3 foam density attainedhighest performance in term of ballistic limit velocity and energy absorption for fully perforated spec-imens. Optimized foam core density resulted in projectile yawing and side impact to back face and higherenergy absorption. SEM analysis and morphological study revealed, low ballistic performance in lowdensity foam core (below 40 kg/m3) in the sandwich structure may be associated with foam’s low cellwall thickness and strut. Similar analysis for the 40e70 kg/m3 foam core densities showed increase infoam’s cell wall thickness with no significant change for the strut and also highest strut thickness for theabove 70 kg/m3 foam density with no change in cell wall thickness.

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1. Introduction

Polymer matrix composite laminates and sandwich structuresoffer many advantages compared to traditional materials, such assteel, aluminum and other metals. Nowadays these materials arewidely used in transport, marine, civil, military and aerospace in-dustries. Lightweight, high specific stiffness and strength, corrosionresistance, and high fatigue life characteristics are among theiradvantages. Such properties make them a good candidate for theprotection of aerospace structures from foreign objects impact [1].

In fact, sandwich panels are capable of offering substantialshielding and good energy absorption characteristics. Behavior ofsandwich structure subjected to an impact loading has been thefocus of many studies [2e8]. Sandwich structures as a part ofaircraft body may undergo projectile impact, such as, tool drops,runway debris, bird strikes, hailstorms and ballistic loading. Thisresults in damage to the structure in the form of face sheetindentation, core crushing, visible penetration or perforation, andinvisible internal delamination or debonding. All these types ofdamages will result in strength and stiffness reduction of thestructure [9]. Penetration and perforation resistance of sandwichpanels at high impact velocity are then required to qualify different

: þ98 21 44580054.@yahoo.com (A.R. Sabet).

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panels made of different skin materials (aluminum, fibre-rein-forced, polymer) and many wide varieties of core materials,including trusses, honeycombs, corrugations and foam-like mate-rials, with either open or closed cell, offering a high specificstrength and energy absorbing ability. In the recent years severalexperimental studies have been carried out to characterize theimpact damage in sandwich panels. Some of these studies are onhigh velocity impact behavior of sandwich structures [10e16].

Most studies focuses on determination of failure load, ballisticlimit velocity, perforation energy and damage induced in com-posite sandwich panels subjected to projectile impact and also toimprove the damage resistance characteristics of composites [17].The facing sheet of composite sandwich structure under transverseimpact load undergoes significant damage such as fiber breakage,matrix cracking and delamination followed by penetration of theimpactor into the core [18]. Vinson [19] identified the face sheets asbeing the main load bearing element in a composite sandwichstructure and an optimal design of such structures must take intoconsideration the face sheet response to static and dynamic loads.Weihong Hou et al. [20] studied ballistic performance, quasi-staticand impact perforation of metallic sandwich structures withaluminum foam core. They discussed the effects of several keyparameters, i.e. impact velocity, skin thickness, thickness anddensity of foam core and projectile shapes, on the ballistic limit andenergy absorption of the panels during perforation. Villanueva andCantwell [21] investigated delamination and longitudinal splitting

Table 1Physical and mechanical properties of material used.

Name Type Density(kg/m3)

Tensilestrength(MPa)

Tensilemodulus(GPa)

Elongation atbreak (%)

Polyester resin Isophthalic 1.2 56 3.6 1.2Plain weave

woven rovingE glass 2.54 2900 73 3.4

Table 2Foam mechanical properties.

Properties

Foam densities (kg/m3) 37 49 70 95 105 240Tensile strength (MPa) 1.0 1.8 2.6 3.1 3.5 6.2Compressive strength (MPa) 0.4 1.0 1.3 2.6 3.0 8.9Elastic modulus (MPa) 35 70 90 155 164 360Strain at break (%) 3 2.7 2.5 2.1 2 1.2

Table 3Composite sandwich panel matrix of variables.

Sandwichpanel

Foam density(kg/m3)

Panel totalthickness (mm)

Panel totalweight (g)

1 0 124 70.12 37 124 75.83 49 124 77.84 70 124 81.15 95 124 85.16 105 124 87.67 240 124 107.9

All composite sandwich panels contain identical three layered facings.

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of woven and unidirectional glass fiber composite skins sandwichstructure with an aluminum foam core in the fiber/polypropylene-based systems In contrast, the woven glass fibre/polypropylene-based sandwich structures exhibited smaller amounts of delami-nation after high velocity impact testing. They reported that, thealuminum foam in both systems exhibited a localized indentationfailure followed by progressive collapse at higher impact energies.Avila [22] investigated by varying core density in the sandwichstructure, as in functionally graded sandwich structures, damagemechanism also changes as a function of density distribution.Goldsmith et al. [23] experimentally studied the perforation ofdifferent types of honeycomb cores with metallic/plastic covers inthe wide range of the striking velocities. He discussed the conceptof ballistic limit in relation to composites and sandwich structureand examined the behavior of post-perforation speeds with initialvelocities. V Skvortsov et al. [24] considered elastic and kinematicresponse of a sandwich panel with FRP composite laminate facesand foam core, during impact. They investigated matrix cracking,core crushing, debonding, delamination, and fiber failure and en-ergy absorption. Velmurugan et al. [25] studied the response ofthree sets of sandwich plates (Woven roving mat/Epoxy/Foam,Chopped strand mat/Epoxy/Foam, andWoven roving mat/Choppedstrand mat/Epoxy/Foam) to projectile impact in the velocity rangeof 30e100 m/s. They reported ballistic limit, residual velocity, andthe energy absorption change for different types of cores. Kepler[26] looked in to penetration and damage patterns of sandwichpanels and showed that the most important contributions are frommembrane-state fiber stretching, core compression, and frictionbetween core material and impactor. He also asserted that lessercontributions are from delamination, core fracture, and debondingbetween core and back face sheet. He used simple physical modelsto estimate energy absorption contributions. Goldsmith and Sack-man [27] studied the effects of several parameters, e.g. impact ve-locity, boundary conditions and bonding strength between thehoneycomb core and aluminium facing, on the energy dissipationduring perforation. Hoo Fatt et al. [28e30] developed analyticalsolutions for the ballistic limit of a honeycomb core sandwich platesubjected to normal impact by projectiles with different shapes, inwhich the overall bending and stretching of the plates wereconsidered. Although extensive work has been published ondifferent aspects of foam-filled sandwich structures, High velocityimpact response, in particular optimized foam density is one areawhich has received less attention. The main aim of this study is toinvestigate significance of foam density in a sandwich structurewith composite facing toward high velocity projectile impact.

2. Experimental

2.1. Materials

Sandwich panels consist of two composite facing and a foamcore. Each composite facing was made from three layered E-glassplain weave woven roving (400 g/m2) obtained from CamalyafTurkey and isophthalic unsaturated polyester resin from Busheherchemical Co Iran. Methyl ethyl ketone peroxide (MEKP) from Iran

Peroxide Co and cobalt naphthenate from AkzoNoble (Netherlands)was used as initiator and the accelerator to cure the compositefacings. Resin for making composite facings was degased by placingit in a vacuum oven at 23 �C prior to laminating the compositepanels. All composite panels were cured at room temperature for24 h followed by post curing at 80 �C for 5 h. Composite facingswere constructed by hand lay-upmethod. Table 1 presents physicaland mechanical properties for the facing materials. The poly-urethane foam core was prepared from two part system, i.e. Plur-acol 735 from BASF Co USA and Millonate MR-200 from Nipponpolyurethane industry Co Japan. All foam cores were casted in byfirst placing the composite facings in specially designed mold priorto casting the polyurethane foam and then the two part poly-urethane foam cores were mixed in a mechanical mixer at 100 rpmunder controlled temperature. The foam was casted in place underfree rise condition and at room temperature and left for 24 h for fullsetting followed by 3 h post curing at 80 �C. Polyurethane foam corewere prepared in density of 37, 49, 70, 95,105 and 240 kg/m3. Se-lection of such density range was because most foam cores used inaerospace falls in this range e.g. Elastopor and Autofroth from BASFCorporation, Zotek F from ZOTFOAMS plc (UK) and Ariex from 3AComposites. Different foam core densities were achieved by con-trolling the amount of foaming agent added at the mixing stage. Atleast five samples were prepared for each foam density to ensurethe required density and repeatability. The sandwich panels weremade in 12.5 � 12.5 cm size as defined by gas gun target holder. Allsandwich panels were left for at least 72 h for foam setting.

2.2. Characterization

Mechanical tests were performed on all foam core densities,providing information on tensile and compressive strength for thefoam core densities tested (see Table 2). Matrix of variables for allspecimens tested is depicted in Table 3.

Morphology study and scanning electron macroscopic analysiswere conducted on foam core cell using SME instrument(VEGA\\Tescan HV:2000KV),providing information on foam cellsize, cell wall thickness, foam cell strut characteristics, foam cellcrushing during projectile perforation. The analysis was also usedto investigate fracture pattern associated with front and back fac-ings in the sandwich panel after high velocity impact.

Fig. 2. Projectile.

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2.3. High velocity impact test

High velocity impact tests were carried out using a gas gun(Fig.1). The gas gun consists of 1.75m long smooth barrelwith insidediameter of 8.7 mm, a fast acting pressure release valve, a projectileloading unit, a supply gas vessel, a 500ml gas reservoir for each shotrelease, a target holder, two projectile velocitymeasuring units, andballistic paste to catch the projectile intact. Initial velocity of pro-jectilewasmeasured after itwaspropelled fromthe gunbarrel usinga chronograph F-1 model from Shooting Chrony Co Canada. Due tounpredictable line of flight of projectile after exiting the target, theresidual velocity for the projectile which perforated the specimenwas recorded using two sets of wide screen of thin-aluminum foilpanels. Rectangular thin-aluminum foilswith a 30 mmthickness andhaving dimensions of 300� 300mmwere attached to a rigid acrylicframes. Each set consists of two pieces of the aluminum thin foilswhich were placed close together with a gap of 5 mm. A very thinpaper was placed between the two foils to prevent prematuretriggering. Another pair of similar aluminum sheets was fixed at135 mm from the first set. An electric circuit was set up betweeneach set of thin foils which were then connected to a 20 MHz fastpulse meter from Autonics Co, to record the time of travel for theprojectile between the two sets, more details of the device are givenin Ref. [31].

Hemispherical nose shaped projectile was used for all impacttests. The projectile was hardened steel (Rc60) of 25.21 mm totallength, 8.7 mm diameter, and 10.7 � 0.05 g weight (Fig. 2). Initialvelocity of projectile (before impact) was calibrated and measuredfor helium gas at various gas pressures with a chronograph. Thecalibration curve showed linear behavior for various gas pressuresversus projectile velocities at low pressure range (see Fig. 3).Sandwich panel specimens preparedwere fixed in the target holderat all four edges via tightened bolt.

3. Result and discussion

Table 2 presents results for mechanical tests on foam coresdensity range tested (37e240 kg/m3). It showed increase in tensileand compressive strength as well as compressive modulus withincrease in foam density. However, strain at break showedconsiderable decrease, starting with 3% for foam density of 37 kg/m3 to about 1.3% for the foam with density of 240 kg/m3. Thisindicated more brittle nature for higher density foam cores.

In this study the effect of different foam core density in asandwich panel under high velocity impact load has been investi-gated. Fig. 4 shows residual velocity of projectile after perforationversus impact velocity. The figure indicates relatively better bal-listic performance by sandwich panel containing 49 kg/m3 foamcore density in term of lower residual velocity thought-out velocityrange tested. Almost all panels showed a linear behavior just above

Fig. 1. High velocity impact t

ballistic limit velocity (velocity at which projectile seized in thesandwich panel) followed by non-linear parabolic response ataround 100e125 m/s and then again a linear increase in residualvelocity with increase in impact initial velocity. Non-linearresponse in the curve may be directly related to non-tensile fail-ure mode in the panels [32,33]. Similar behavior shown by differentsandwich panels are directly related to the similar facing used interm of material, thickness and fiber architecture. Consideringsimilar initial impact velocity and comparison of residual velocityfor different foam core density sandwich panels reveals the sig-nificance of foam core presence in the panels. Reduced ballisticperformance in the sandwich panels with foam density in range of70e240 kg/m3 may be attributed to changes in the micro andmacro structure of the foam core. As the foam core density in-creases the struts thickness interconnecting foam cells increasesand simultaneously decreasing the foam’s cell wall, resulting inmore brittle nature of the foam. This brittle behavior may easily bededuced by considering the strain at break for different foamdensities tested and referred to earlier (see Table 2). This means lessresistance to foam crushing and consequently less resistance topenetration. On other hand, the low performance associated withlow density foam core (37 kg/m3) can be related to foam’s very thincell wall and struts thickness.

One important performance criteria associated with high ve-locity impact is the ballistic limit velocity, throughout this study thecriteria used for ballistic limit velocity is, the velocity at which theprojectile seizes in target [34]. Fig. 5 depicts ballistic limit velocitiesfor the composite panels with different foam core densities. Thefigure shows highest ballistic limit velocity for the specimens withfoam core density of 49 kg/m3, this is despite the fact that they allhad identical composite facing in term of material and thickness.The lowest ballistic limit velocitywas for sandwich panel with foamcore density of 240 kg/m3. This change of behavior may be directlyattributed to more brittle nature of the foam core with the density

esting device (gas gun).

Fig. 3. Projectile velocity calibrations at various helium gas pressures.

Fig. 5. Ballistic limit velocity versus sandwich panel foam core density.

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of 240 kg/m3. Fracture study in the later part of this report willconfirm more brittle behavior associated with this foam coredensity. Fig. 5 also shows ballistic limit velocity for other foam coredensities. Considering data scatter presented with error bar (stan-dard deviation from average of five tests conducted), the ballisticlimit velocities for other specimens are in the same level.

Fig. 5 also presents estimated ballistic limits, which werecalculated using the initial and residual impact velocity for fullyperforated specimen by utilizing the following relationship.

12mV2

r ¼ 12mV2

i � 12mV2

50 (1)

Estimated ballistic limit for EV50 ¼�V2i � V2

r

�1=2for Vr > 0

(2)

The above equation assumes non-deformable projectile andconservation of energy [34]. Vi is the initial impact velocity, Vr is theprojectile residual velocity, EV50 is the estimated ballistic limit

Fig. 4. Residual velocity versus impact velocity.

velocity and m is the mass of projectile. Sandwich panels mass loss(debris) for partial and fully perforated impact tests were minute(much less than 1% of specimen’s total weight). The figure showsrelatively good correlation between estimated ballistic limits andexperimentally determined values for most specimens tested, someof the variations may be due to contact phenomenon or other en-ergy absorbing parameters such as fiber volume fraction and heatduring perforation of projectile or data scatter.

Fig. 6 presents residual velocity versus different foam coredensities for fully perforated impact tests on composite sandwichpanels. The testswere carried out at four different impact velocities.Result shows with increase in initial velocity the projectile residualvelocity increases too and the curve move to upper level. Study ofindividual case reveals that, initially as foam core density increasesfrom 37 kg/m3 the residual velocity reduces to a minimumvalue forfoam core density of 49 kg/m3 and then it increases to near plateaubehavior within core density range tested. This behavior wasobserved in all initial impact velocity range tested but with lessintensity i.e. foam core density of 49 kg/m3 showing better energyabsorption.

Fig. 7 presents energy absorption in the sandwich panels fordifferent foam core densities at various impact velocities. The en-ergy absorption reported here were obtained using initial impactvelocity and residual velocity after perforation using Eq. (3). Theterm in the expression have their usual meaning.

E ¼ 12m�V2i � V2

r

�(3)

Fig. 7 shows better energy absorbing performance for thesandwich panel containing foam core density of 49 kg/m3, followedby foam core density of 70 kg/m3. The figure depicts lowest energyabsorption for the specimenwith no foam core. Energy absorptionsassociatedwith other specimens having different foam core densityare all approximately in the same level with small variation. Thisvariation may be due to slight changes in fiber volume fraction offacings in the sandwich panel or different energy absorptionmechanism in each specimen. One peculiar behavior which wasnoticed in all specimens is the energy absorption obtained forhighest impact velocity tested (151.4 m/s). The figure shows, theenergy absorption associated with this impact velocity is almost18% higher than energy absorbed in other impact velocities. Thiswas consistent regardless of foam core density. Although thishigher value may be explained by the data scatter for most

Fig. 6. Residual velocity versus foam core density of specimens. Fig. 8. Clustered column compares specific energy absorption values at different foamdensity.

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specimens, but, however, the result for the specimenwith 49 kg/m3

foam core density is undeniable and cannot be solely due to datascatter. The higher energy absorption for 151.4 m/s impact velocitymay also be due to the fact that, at higher impact velocity thespecimen experience less deflection in particular in the back facing.This results in less time for the projectile to wedge through the twocomposite facings i.e. more fiber stretching, therefore fiberbreakage and consequently higher energy absorption. Other factorswhich may explain higher value is the fact that, at higher impactvelocity the sandwich panel experiences higher strain rate, thisresult in higher tensile and flexural modulus in the compositefacings [35e37] and therefore higher resistance to fracture. Also,higher strain rate, the foam core experience more brittle naturewhich results in higher fracture surfaces which means higher en-ergy expenditure to overcome while projectile perforate foam core,similar findings were reported by Refs. [38e40].

In order to assess the sandwich panels based on minimumweight design, the energy absorption for all specimens wasnormalized to their corresponding composite sandwich panel

Fig. 7. Clustered column compares energy absorption at different impact velocities forvarious foam core densities tested.

weight. Fig. 8 presents specific energy absorption versus foam coredensity for four different impact velocities. The figure shows, as thefoam core density increases above 49 kg/m3 the specific energyabsorption decreases significantly, this clearly present weightpenalty involved for the foam core above 49 kg/m3. Also, the spe-cific energy absorptions for the spaced composite panel (i.e. withno foam core) and sandwich panel with foam core density of 37 kg/m3 showed lower performance compared to panel with foam coredensity of 49 kg/m3. Study of specific energy absorption fordifferent impact velocities for each foam core densities reveal nosignificant variations within the first three velocity range tested.But however, the highest specific energy absorptions were associ-ated with the highest impact velocity tested (151.4 m/s), thispeculiar behavior was consistent for all foam core density rangtested.

4. Damage assessment

Damage assessment for different specimens reveal highestrupture area for sandwich panel containing foam core density of49 kg/m3, this rupture area is related to back facing. Study ofrupture area indicates that in these specimens, the projectile hasyawed during perforation, meeting the composite back face prac-tically side way (see Fig. 9A and B). Fig. 9A shows composite backface for the sandwich panel containing foam core density of 49 kg/m3, consideration of rupture extension clearly shows side impact ofthe projectile. Whereas in Fig. 9B, the extent of fiber fracture clearlyreveals head on impact by the projectile (i.e. no yawing of theprojectile). The fracture areas for the front composite facing in allspecimens are almost identical regardless of their foam core den-sity (see for example Fig. 10A and B). The figures show almostsimilar fracture pattern for the composite sandwich panels having49 and 240 kg/m3 foam core densities respectively.

Morphological study of the foam structure for different densityfoams revealed that, the foam cell sizes are all approximately equalin diameter, but cell wall thickness and strut size vary in differentfoam densities. In low density foam (37 kg/m3), the thickness offoam cell wall as well as the struts are very thin. But in the mediumdensity foams (49e70 kg/m3), the foam cell wall increases with nochange in strut thickness. In high density foams (above 70 kg/m3),due to low level of foaming agent, densification of PU foam are

Fig. 9. Back face damage area for sandwich panels with foam core density of 49 and 240 kg/m3.

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observed, this result in, a bigger struts size and also foam cell wallthinning. This gives rather denser foam structure and better me-chanical strength. However, the main factor which plays role inhigher energy absorption is associated with projectile trajectorywhile perforating the sandwich structure. The result shows in lowfoam densities (0 and 37 kg/m3) the projectile after penetrating thesandwich structure encounter very low resistance therefore itexperience no trajectory change in the line of flight path and meetthe second facing almost normal to the flight path and perforate thefacing. However, in sandwich structures with medium range foamdensities (49 and 70 kg/m3), due to their regular foam cell structureand uniform distribution of the foam mass between the struts andcells wall. During perforation, the projectile encounter resistanceand compression of the foam core occurs resulting in some devia-tion from its trajectory path i.e. projectile yawing. This results in

Fig. 10. Front face damage area for sandwich panel

side impact of the projectile to the second facing. Similarly, in thehigh density foam core specimens (70e240 kg/m3) under highvelocity impact, the projectile after perforating the front facingexperience more resistance from foam core side wall compared tofoam cells. This is due to higher foam densities while perforatingthe sandwich structure a head of projectile during perforation,therefore no yawing occurs. In fact in the process of perforation twoparameters are competing, one is the strength of foam core and theother being the densification of the foam’s structure. Here one canconclude that, the resistance shown by less dense foam density isgood enough to deviate the projectile from its normal impact tra-jectory. This argument can be explained by the fact that, in themedium density foam cores due to projectile deviation fromnormal trajectory path while perforating the foam core, the pro-jectile yaw, impacting the second facing laterally. This means

s with foam core density of 49 and 240 kg/m3.

Fig. 11. Clustered column compares back face rupture length values for different foamcore densities.

Fig. 12. The fracture patterns for foam core density of 37 kg/m3(A), 49 kg/m3(B), 70 kg/m3(C), 105 kg/m3 (D) and 240 kg/m3(E).

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higher impact surface area, resulting higher energy absorption andlower residual velocity. However, in cases where projectile impactthe second facing at normal angle i.e. no trajectory deviation ofprojectile, impact surface area is only as much as projectile nose(semi-spherical shape) and resulting in much lower surface areathan side impact. Normal impact in higher density foam corespecimens may also be due to the fact that, as the projectileperforate the foam core the foam crushing result in densification ofthe supporting wall consequently not allowing the projectile toyaw. This means higher stress as well as less resistance by thesecond facing and therefore lower energy absorption and higherresidual velocity after perforation. Fracture study of perforated areain high velocity impact tests revealed extensive deboning betweenfoam core and the back face composite sheet for the specimenswhich experienced trajectory change in projectile flight path. Thiscan be attributed to vast stress wave propagation as a result of sideimpact. In these specimens, energy absorption associated withelastic deformation is far greater than specimens with no deviationin trajectory path. This may also be related to side impact of pro-jectile. Also the number of fiber fracture is more in these specimens.From fracture study one can conclude that if foam core in suchstructures are selected in a way that the projectile experience tra-jectory change, by choosing high tensile strength reinforcement forthe back face greater energy absorption can be obtained. Figs. 9 and10 clearly showing back and front face damage areas for the foamcore for 49, 240 kg/m3 densities respectively. Fig. 9A shows thefracture length extension for the sandwich specimen with foamcore density of 49 kg/m3 being greater than other specimens. Also,the deboning area is much larger, thought not shown in the picture.

Fig. 11 shows clustered column for the measured rupture area inthe composite back facing for the sandwich panels with differentfoam core density. The figure shows the highest rupture length ofapproximately 79 mm in the back facing for the specimen withfoam core density of 49 kg/m3 at impact velocity of 115.4 m/s,followed by rupture length of 35mm and 40 mm for the specimenswith foam core density of 37 and 70 kg/m3 respectively. Fig. 11 alsoshows that for the specimens with foam core density 0, 95, 105 and240 kg/m3 the rupture length in the back facing is significantlyreduced. This clearly indicates that the projectile meet the backfacing at normal impact in these specimens. This may be attributedto more brittle nature of foam density in foam cores with density of95, 105 and 240 kg/m3 (see strain at break for corresponding foam

cores in Table 2). Further evidence of more brittle nature of lastthree higher foam core densities can be found in less sensitivitytoward impact velocity variation shown in the figure. Further studyof Fig.11 also depict rupture length at three impact velocities for theback facing of different foam core density specimens. The figureshows, apart from foam core density of 49 kg/m3 which revealhighest rupture length, the rest of specimens have approximatelysame rupture length. In order to investigate the crushing behaviorin different foam core densities, sandwich panels were sectioned atthe impact area using a diamond blade cutter. The fracture patternsfor different foam core densities are presented in Fig. 12AeE. Thefigures indicate that, in the specimen with foam core density of49 kg/m3 (12B) there are evidence of extensive debonding betweenfoam core and back facing, followed by severe delamination at thecomposite back facing and fiber fracture. The figure shows almosttotal delamination at the composite back facing. This pattern cansolely be due to projectile yawing during perforation as a result offoam core resistance. Fracture pattern for the foam core density of37 kg/m3 (Fig. 12A) is very similar to foam core density of 49 kg/m3,

Fig. 13. Foam core morphology by Scanning Electron Microscopy (using magnification 100 mm): A) Foam core density of 37 kg/m3, B) Foam core density of 49 kg/m3.

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but, with less debonding and delamination extension at the com-posite back facing. Study of fracture pattern for specimens withfoam core densities above 105 kg/m3 reveals very smooth foamcore crushing with some fiber fracture in the back facing andalmost no debonding. Sever delamination failure of the compositeback facing observed in the sandwich panel with foam core densityof 49 kg/m3 is due to the fact that, the projectile meet the backfacing side way as a result of yawing. It was also observed that, inhigher foam core densities the foam core experienced disintegra-tion near back facing, this may be attributed to brittle nature of thefoam core (see Fig. 12E).

Figs. 13e15 present scanning electron microscopic images fordifferent density foam core structure. Considering identicalchemical and processing condition for all foam densities, the cellsizes are all approximately 810 mm in diameter. However the cell’s

Fig. 14. Foam core morphology by Scanning Electron Microscopy (using magnification

wall thickness and the strut thickness vary in different foam den-sity. Study of Fig. 13A shows the lowest cell wall thickness ofapproximately 15 mm associated with foam core density of 37 kg/m3. Similar investigation for foam density of 49 kg/m3 indicates cellwall thickness of almost three fold (45 mm). This is depicted inFig.13B. The cell geometry for this density is more similar to regularhexagon. This regular structure together with higher cell wall andthe associated strut thickness are postulated to be the reason forbetter ballistic performance and projectile yawing during highvelocity impact perforation. SEM analysis for foam core density of70 kg/m3 (Fig. 14A) reveals reduced cell wall thickness to about30 mm, however, the strut for this foam density is increased.Furthermore the cell geometry takes irregular shape which resultsin lower resistance of foam cell network structure toward impactload. Figs. 14B and 15A,B are associated with foam core density of

100 mm): A) Foam core density of 70 kg/m3, B) Foam core density of 95 kg/m3.

Fig. 15. Foam core morphology by Scanning Electron Microscopy (using magnification 100 mm): A) Foam core density of 105 kg/m3, B) Foam core density of 240 kg/m3.

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95, 105 and 240 kg/m3 respectively. As the foaming agent in thesespecimens is less compared to other foam core densities, the foamcells tend to shrink and agglomerate. These two characteristicsresult in gradual decrease in foam cell wall thickness and simul-taneous increase in the strut thickness. The strut thickness is 70, 80and 361 mm for foam core density of 95, 105 and 240 kg/m3

respectively. Although one would expect increase in foam strengthwith increase in density, but, however, this expectation failed tomaterialize, low cell wall thickness acted as weak point resulting ineasy foam cell failure in high velocity impact tests. During impact,fracture starts from these weak points and propagate through thefoam structure by circumventing the struts. Putting this simply,would be to say the foam became more brittle with increase infoam density.

Fig. 16. Scanning Electron Microscopic images for different density foam core sect

Further study of foam core region associated with to 125 m/simpact test is presented in Figs. 16e18. As stated above, the regularshape for the foam structure in term of foam cell wall thickness andthe struts apparently plays a vital role in energy absorption andballistic performance of the structure. In Fig. 16A, due to low cellwall as well as strut thickness for the 37 kg/m3 density, we witnesssome elongation of foam cells in direction of projectile movement.This phenomenon was not observed in foam density of 49 kg/m3

(see Fig. 16B). This may be attributed to regular collapse of the cellsin latter foam density. Fig. 16B also clearly shows projectile yawing.Fig. 17A and B represents SEM images for perforation section of thespecimens with foam core density of 70 and 95 kg/m3. The figureshows reduced cell wall thicknesses compared to foam density of49 kg/m3, however, the struts thickness has increased slightly. This

ions after impact test for foam core density of 37 kg/m3(A) and 49 kg/m3(B).

Fig. 17. Scanning Electron Microscopic images for different density foam core sections after impact test for foam core density of 70 kg/m3 (A) and 95 kg/m3 (B).

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has resulted in lower strength, showing some area of foam crush-ing, fracture propagation and back facing/foam core delamination.

Fig. 18A and B represents sectioning at projectile perforationarea for the sandwich panel with foam core density of 105 and240 kg/m3 respectively. The figures depict very thin-walled foamcell and thick section struts. Fracture study reveals very brittlenature in this foam density, this causes total crushing of the foamand some cracks in form of disintegration at impact. Consideringfracture study of various density foam cores reveals, regular cellnetwork together with high cell wall thickness as well as uniformfoam density throughout the structure results in good impactperformance on sandwich structures.

5. Conclusions

This study successfully investigates the effect of variation inrigid foam core density in a composite sandwich panel under highvelocity impact loading.

Fig. 18. Scanning Electron Microscopic images for different density foam core sect

Following conclusion can be drawn from the results.

1. Foam core density of 49 kg/m3 showed highest energy ab-sorption in foam density range tested in term of lowest residualvelocity in perforated tests.

2. Highest ballistic limit velocity was also associated with samefoam core density.

3. Study of specific energy absorption revealed better ballisticperformance was related to sandwich panels with foam coredensity below 70 kg/m3.

4. Damage assessments for the perforated area showed severprojectile yawing in the foam core with highest energy ab-sorption causing projectile side impact to the back compositefacing.

5. In the high density foam core tested, no projectile yawingabsorbed (projectile perforating the composite back facing atnormal angle), this was attributed to lower foam cell wallthickness and higher strut thickness

ions after impact test for foam core density 105 kg/m3 (A) and 240 kg/m3 (B).

R. Nasirzadeh, A.R. Sabet / International Journal of Impact Engineering 63 (2014) 129e139 139

6. Foam cell wall thickness and strut play vital role in crushingbehavior and consequently energy absorption.

7. Higher foam core density showed total foam collapse in brittlemanner with disintegration and consequently low energyabsorption.

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