instrumented impact testing of composite sandwich panels

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DOI: 10.1177/073168448900800304

1989 8: 270Journal of Reinforced Plastics and CompositesW.K. Shih and B.Z. Jang

Instrumented Impact Testing of Composite Sandwich Panels

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270

Instrumented Impact Testing ofComposite Sandwich Panels

W. K. SHIH AND B. Z. JANG

Composite Research Center201 Ross Hall

Materials Engineering ProgramAuburn University, AL 36849

ABSTRACT

The impact response of various composite sandwich panels has been investigated. Thesecomposite sandwich panels, consisting of various fabric facesheet materials and differentdensities of poly(vinyl chloride) (PVC) foam cores, demonstrate exceptional low-energyimpact resistance. The macroscopic failure modes, microfailure mechanisms, and energyabsorbing characteristics of these composite sandwich panels were studied by using in-strumented impact test, light microscopy, and scanning electron microscopy. The impactresistance of composite sandwich panels was found to be mainly controlled by thefacesheets and relatively independent of the density of the PVC foam core, provided thefacesheet material is tough enough The impact failure mechanism of sandwich panelscontaining less tough facesheets was found to change from facesheet-dominated to foam-core-dominated behavior, when the PVC foam core increased from low density to

relatively high density. The energy absorbed by a sandwich panel made of low-densityfoam core was about 15%-100% greater than the sum of the energies absorbed by its

separate constituents This result indicates that the impact energy of a composite sandwichpanel cannot be predicted by the rule-of-mixtures law. This percentage deviation increasedas the density of the foam core increased.

INTRODUCTION

A COMPOSITE SANDWICH panel comprises a lightweight foam, honeycomb,or corrugated core sandwiched between two composite facesheets. Such acombination offers exceptional specific stiffness and strength, buoyancy, dimen-sional stability, and thermal and acoustical insulation characteristics. The mainapplications of these composite sandwich panels are in building components, in-sulated structures, marine construction and aerospace vehicles.A considerable amount of literature dealing with the theories for the dynamics

of composite sandwich structures of different geometries has been reviewed indetail by Bert and Egle [1]. A governing equation for freely vibrating sandwichbeams was derived by Ditaranto and Blasingame [2,3]. Bert [4] developed asimplified model to determine non-linear compressive and tensile-strain curves

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for sandwich beams. A theory to obtain non-linear and linear stress-strain curvesfor composite sandwich beams subjected to pure bending loads was derived bySchoutens [5,6]. The elastic response of sandwich plates was studied by Pagano[7]. Whitney [8] used an extension of an existing laminated plate theory in thestress analysis of sandwich plates. Optimization studies of composite sandwichpanels under compression have been performed by Hirano [9] and Vinson [10].More recent developments include the use of thin walled braided tubes as thecore material of the sandwich panels [11].Due to the nature of the application, the most frequent damage to these

materials results from low-energy impact. However, the impact resistance of thesandwich panel has drawn only limited attention from the composite researchers.Slepetz [12] studied the damage tolerance of composite sandwich structures sub-jected to ballistic and laser impact. Several other researchers [13,14,15] have in-vestigated the surface damage by impact and the effect of core stiffness on damageresistance for sandwich panels. ’t Hart [16] used various techniques to examinethe visible damage, fiber fracture, and damage area of impact-loaded sandwichpanels. Bernald and Lagace [17] evaluated the effects of low energy impact on thesandwich panels with various core materials and core thickness. The damagemodes occurring in the facesheets, the cores, and the facesheet-core interfaceswere examined [17]. Very few studies [18] that utilized the technique of in-strumented impact testing to study the fabric-composite facesheets, the plastic-foam cores, and the corresponding sandwich panels were reported. This tech-nique provides both impact load-time and energy-time traces experienced by thetested specimens and has proved to be a powerful tool for studying the impactresponse of composites [19,20].The objective of this study was to investigate the effects of low velocity impact

on the sandwich panels with various fabric-composite facesheets and plastic-foam-core densities, by using instrumented impact testing. In particular, the

macroscopic and microscopic failure modes and energy absorbing characteristicsof these panels were examined as a function of the constituent material proper-ties.

EXPERIMENTAL

The facesheets of the sandwich panels were made of a variety of plain-wovenfabrics as specified in Table 1. The epoxy and hardener used in this work were

Ciba Geigy 507 and 956, respectively. The core materials were 0.5&dquo; thick Klege-cell rigid PVC foams (supplied by Klegecell Corp., Texas) with two different den-sities (56.1 and 240.5 kg/m3). The panels were fabricated by compression mold-ing. The stacking sequence from bottom to top of the molding was polypropylene(PP) film coated with epoxy, fabric impregnated with epoxy, foam core coatedwith epoxy on both sides, epoxy impregnated fabric, and epoxy coated PP film.This molding configuration was then placed in an 8.5&dquo; &dquo;

x 8.5&dquo; frame windowmold. The entire sandwiched mold was then compression molded at a pressureof approximately 50 lbs and at 60°C temperature for 45 minutes. The mold wasopened after a postcure procedure that lasted two hours. The protective PP films




272

Table 1. Materials and suppliers.

were then removed to get a sample with smooth surfaces. Each composite sand-wich panel was further cut and trimmed into four 4&dquo; x 4&dquo; square samples forinstrumented impact testing. Several separate facesheet samples (single-layerfabric/epoxy composite sheets) were made using similar compression moldingprocess without bonding to the core materials. For comparison purposes,facesheets alone and core materials alone were also cut into 4&dquo; x 4&dquo; size for im-

pact testing.The instrumented impact tester used in this study was Dynatup Model 730. The

striking tup of this drop-weight system is instrumented. Both load-time and

energy-time traces can be acquired and recorded by an IBM PC/AT during theimpact event. The maximum load, the energy absorbed at maximum load,energy after maximum load, total absorbed energy, and deflection data can be ob-tained in a matter of seconds [19].

RESULTS AND DISCUSSION

Impact Response of Single-Layer Fabric/Epoxy Composite Facesheets

The impact results for various single-layer fabric-epoxy composites made ofgraphite (Gr), Kevlar (Kv), polyester (PET), and polyethylene (PE) fibers areshown in Figures 1 to 4. Figure 1 illustrates the penetrated fracture appearanceof these four single-layer composites at a lower impact velocity (2.85 m/s) usinga 10-kg impactor. Both graphite and Kevlar-49 composite sheets show a diamond-shape circumferential fracture and cross-type radial fracture along the two wovenfiber directions. No further deformation was seen outside the diamond area. Thefracture patterns of both the polyester and polyethylene fabric based compositesheets are quite different. Specimens made of these tough materials bulged cir-cularly prior to fracture. In addition, certain extent of interfacial debonding or awhitened zone outside the circular area was also observed. The interfacial

debonding in PET single-layer fabric/epoxy composite occurred as a cross-shaped whitened area along the two woven fiber directions. The interfacial




273

Figure 1. The fracture appearance of single-layer (a) Gr, (b) Kv, (c) PET, (d) PE fabric/epoxycomposites impacted at 41 joules.

Figure 2. Impact load-time (sohd lines) and energy-time (broken lines) traces of single-layer(1) Gr, (2) Kv, (3) PET, (4) PE fabnclepoxy composites impacted at 41 joules.




274

Figure 3. The fracture appearance of single-layer (a) Gr, (b) Kv, (c) PET, (d) PE fabric/epoxycomposites impacted at 150 joules

Figure 4. Impact load-time (solid lines) and energy-time (broken lines) traces of single-layer(1) Gr, (2) Kv, (3) PET, (4) PE fabnclepoxy composites Impacted at 150 joules.




275

debonding in PE composite was more extensive, and whitening occurred alongall the radial directions of the entire specimen.The impact load-time and energy-time traces of these four types of specimens

are shown in Figure 2. The incident impact energy is the instantaneous kineticenergy, E = 1/2mv2, of the dropping weight just before striking the specimen.Since the mass of the dropping weight was 10 kg and incident velocity was 2.85m/s, the incident energy was 41 joules. Graphite and Kevlar single-layer com-posite specimens each absorbed only about 5 % of the incident energy. Thepolyester (PET) single-layer composite absorbed about 25 % and the PE single-layer composite absorbed about 60 % of the incident energy.At a higher impact velocity (4.5 m/s) and with a heavier impactor (15 kg), i.e.,

higher incident energy, the fracture patterns of these four single-layerfabric/epoxy composites (Figure 3) are similar to those at the lower impactvelocity using a lighter impactor (Figure 1). The only difference in fracture ap-pearance in the case of higher impact incident energy was a somewhat higherextent of protrusion and interfacial debonding. The impact response of these fourtypes of specimens at the higher impact incident energy, 150 joules, are illus-trated in Figure 4. The maximum load remains practically unchanged as the im-pact incident energy increases. However, the total absorbed energy decreasesabout 30% for Gr composites, 25% for Kevlar composites, 20% for PET com-posites, and 10 % for PE composites. A detailed study on the response of fibrouscomposites subjected to low-energy impact was reported by Jang and coworkers[20].

Impact Response of Foam Cores

The impact fracture patterns of PVC foam cores of different densities are illus-trated in Figure 5. Figures 5(a) and 5(b) show the fracture appearance of low den-sity (56 kg/ml) foam core using circular anvils of different sizes (1 &dquo; and 3 &dquo;,respectively). Both specimens were cut through clearly by the tup. Apparentlythe size of the circular anvil has little effect on the fracture patterns of this low

density foam core. This is not the case in the high density (240 kg/m3) PVC foamcore. If a one-inch cylindrical support was used during impact test, only half ofthe thickness of the foam was cut through to form a circular hole and the rest ofit was cut out in such a way that a dog-bone-like fracture was observed [Figure5(c)]. On the contrary, a rectangular circumferential fracture and x-shape radialfracture pattern was observed in the penetrated specimen [Figure 5(d)] when a 3

&dquo;

cylindrical support was used during impact test.Figure 6 shows the load-time and energy-time traces of these two different

foam cores using two different sizes of cylindrical supports during impact tests.The absorbed energies in the low density foam cores tested with two anvil sizesdemonstrate no difference, which is consistent with the observation of the frac-ture appearance described above and shown in Figure 5. The total energy ab-sorbed by high density foam core when using a 1 &dquo; circular anvil is higher thanthat when using a 3 &dquo; circular anvil. The difference in absorbed energy basicallyresults from the magnitude of friction energy between the tup and the foam core




276

Figure 5. Photographs of penetrated low-density PVC foam core using (a) 1 &dquo;, (b) 3&dquo; circular

supports, respectmely, and of penetrated high-density PVC foam core using (c) 1 &dquo;, (d) 3&dquo; &dquo;

circular supports, respectively

Figure 6. Impact load-time (solid lines) and energy-time (broken lines) traces of low-densityPVC foam core using (1) 1 &dquo;, (2) 3&dquo; circular supports, and of high-density PVC foam core us-mg (3) 1 &dquo;, (4) 3 &dquo; supports respectmely.




277

after cut-through. In other words, the friction between the tup and foam core isstill large enough to consume more kinetic energy of the impactor if it is acircular cut-through as is the case when using a one-inch circular anvil. Such athrough circular hole offered good contact (friction) between the tup and foamcore. In the case when using 3 &dquo; circular anvil, the rectangular circumferentialfracture permitted the impactor to readily go through so that the friction force be-tween the tup and the foam core was smaller. The load-time traces in Figure 6 ofthese two situations indicate different values of level-offs resulting from the fric-tion forces. The energy-time traces also indicate that, if we neglect the contribu-tion of friction energy after the load levelled off, the absorbed energies for thesetwo cases of high density foam core, using different anvil sizes, will be more orless equal. Note that the friction between the tup and low-density foam is verysmall so that there is little frictional contribution to energy absorption.

Impact Response of Composite Sandwich Panels

The fractured specimens of the sandwich panels containing the low densityfoam core and four different facesheet materials are shown in Figure 7. The

macroscopic failure modes of graphite and Kevlar facesheets, both at top andbottom of the sandwich panels, are very similar to those of the correspondingsingle-layer composite specimens. Again a diamond-shape circumferential frac-ture and cross-type radial fracture pattern along the two woven fiber directionswas observed in these cases. The fracture behavior of the top facesheets of PETand PE sandwich panels are analogous to that of the corresponding single-layercomposites. In other words, the PET top facesheet was penetrated and a cross-shape pattern of interfacial debonding was found. The PE top facesheet was also

Figure 7. Fracture appearance of composite sandwich panels containing low-density PVCfoam core, and (a) Gr, (b) Kv, (c) PET, (d) PE facesheets, respectively.




278

Figure 8. Impact load-time (solid lines) and energy-time (broken lines) traces of compositesandwich panels containing low-density PVC foam core, and (1) Gr, (2) Kv, (3) PET, (4) PEfacesheets, respectwely.

penetrated and plastically deformed along all the radial directions. The PET back(bottom) facesheet was penetrated and accompanied with extensive radial plasticdeformation. The PE back facesheet was also penetrated and cross-type inter-facial debonding along two woven fiber directions was observed.The impact traces reflecting the impact resistance of these four types of

materials sandwiched with low density foam cores are shown in Figure 8. Notethat the load-time and energy-time traces of Gr and Kv sandwich panels are closeto each other. The doublet in the load-time trace corresponds to failure of the topfacesheet and the back facesheet, respectively. The valley between two peaks cor-responds to the penetration of the low-density foam core. The absorbed energiesfrom the energy-time trace corresponding to these three points are designated asEi, E, and E, where 1 and 2 stand for peaks 1 and 2, and v stands for valley.Therefore, Ei represents the energy required to start the damage on the topfacesheet. E, is the total energy of penetration of the top facesheet including somecontribution from crushing of the core beneath. E2 stands for the total energyrequired to damage the top facesheet, to crush the core, and to start the damageon the back facesheet. The energy absorbed by the low-density core material issmall and is assumed to be approximately the same for each of the facesheets[18].The load-time traces of these specimens reached zero value, or the energy-time

traces levelled off soon after failure of the back facesheet. This indicates that thefriction between the tup and the low-density foam core is negligible. Representa-




279

Figure 9. SEM micrographs of low-density PVC foam core (a) before, (b) after impactdamage




280

tive microstructures of the low-density foam core before and after crushed areshown in Figure 9. Figure 9(a) illustrates the ordered closed-cell structure of thelow-density foam core. The micrograph for the crushed surface of the low-densityfoam core [Figure 9(b)] shows that there is no obvious orientation induced duringthe penetration of the impactor. Again this supports the finding that the frictionbetween the impactor and the low-density core is small.

Figure 8 shows that PE sandwich panel absorbed the highest impact energy anddemonstrated the highest impact resistance. PET sandwich panel absorbed lessenergy than the PE sandwich panel. Gr and Kv sandwich panels absorbed theleast energy and had the lowest impact resistance. Since most of the load wascarried by the facesheets, it is not surprising that the order of impact resistanceof sandwich panels is the same as that of the single-layer fabric/epoxy com-posites. Evidence from the SEM micrographs (Figure 10) further support thisobservation. Figures 10(a) and 10(b) are the fracture patterns for Gr and Kv sand-wich panels, respectively. The fracture mechanisms are fiber pull-out and fiberbreakage, with very little damage (deformation) propagated to the neighborhoodof the penetrated zone area. The epoxy coated on the fabric near the penetratedzone remains intact. Since no significant plastic deformation occurred to bothfibers and resin and with only little friction existing between the tup and the low-density foam core, the energy absorbed in composites containing these less toughfibers is therefore low. In contrast, the fracture patterns of PET and PE [Figures11(c) and 11(d), respectively] illustrate that the energy absorbing mechanisms arenot merely the fiber pull-out and breakage. The interfacial debonding mechanismis also important in dissipating the incident impact energy. This is what causesthe whitening phenomenon observed on the PET and PE sandwich specimensafter impact loading. Significant extent of microcracking in the matrix resin isalso observed. Both PE and PET fibers appear to have been deformed to a con-

siderably higher extent. Higher impact resistance of PET or PE sandwich panelthus should be attributed to higher toughness of the fibers and significant inter-facial debonding.The fracture patterns of four types of sandwich panels made of different

facesheets with the same high-density foam core are illustrated in Figure 11. Thefracture appearance of Kv and Gr sandwich panels look very similar to their cor-responding sandwich panels of low-density foam core. The interfacial debondingareas of both PET and PE sandwich panels are smaller than those in the cor-responding low-density-core sandwich panels.The impact response of these high-density-core sandwich panels are shown in

Figure 12. The doublet behavior of load-time trace is not as clear as in the low-density-core sandwich panels (Figure 8). The main reason is because the highdensity foam core plays an important role in carrying the load during the impactevent. In other words, the load carried or energy absorbed by this core is signifi-cant and is not negligible. The evidence is given in Figure 13(a), which shows themicrostructure of the high-density foam core before being crushed. Note that themagnification is the same as the low-density foam core (Figure 9), thus a smallercell size represents a higher density. Figure 13(b) illustrates the highly orientedcrushed high-density foam core. This indicates that the friction between the tup




281

Figure 10. SEM fracture patterns for composite sandwich panels containing low-densityPVC foam core, and (a) Gr, (b) Kv, (c) PET, (d) PE facesheets, respectively.




282

Figure 10 (continued). SEM fracture patterns for composite sandwich panels containinglow-density PVC foam core, and (a) Gr, (b) Kv, (c) PET, (d) PE facesheets, respectively




283

Figure 11. Fracture appearance of composite sandwich panels containing low-density PVCfoam core, and (a) Gr, (b) Kv, (c) PET, (d) PE facesheets, respectively

Figure 12. Impact load-time (sohd lines) and energy-time (broken lines) traces of compositesandwich panels containing high-density PVC foam core, and (1) Gr, (2) Kv, (3) PET, (4) PEfacesheets, respectively.




284

Figure 13. SEM micrographs of high-density PVC foam core (a) before, (b) after Impactdamage




285

and the high-density foam core is large. The energy dissipated by the frictionforce is significant, therefore, the load-time trace did not decline as much as thetop facesheet failed after the first peak. A shoulder or hump instead of a valleywas found in most of these impact tests. This shoulder corresponds to the failureof the high density foam core. The second peak then followed the shoulder,which is an indication of the starting damage on the back facesheet.

In the sandwich panels made of less tough facesheets such as Gr and Kv, twopeaks were not obvious because most of the load was carried by the core. Whenthe top facesheet failed, the core carried most of the load. The load-time tracetherefore dropped slightly and rose again to form a hump. As the toughness of thefacesheet increased (e.g., PET and PE), the doublet behavior became moreobvious. This is analogous to the low-density-core sandwich panels. However, thecontribution of high density core is still not negligible so that a shoulder existsbetween these two peaks. The fracture mechanism of this tougher facesheet sand-wich panel is slightly different from that of the less tough facesheet sandwich.When the top facesheet fails, the foam core carries the load, resulting in a

shoulder formation. As soon as the foam core fails, the load-time trace declinesand increases again due to the interaction with the back facesheet. Thus therewould be a more clear doublet with a shoulder in between as seen in the PE sand-wich panel. Although the fracture mechanism changed when the low-densityfoam core was replaced with a high-density one, the fracture patterns of thefacesheets are basically the same in both cases. Figures 14(a) and 14(b) show theSEM micrographs of the fracture patterns of Gr, Kv, PET, and PE facesheets, re-spectively, on high-density foam cores. Again the interfacial debondingmechanism is observed in the PET and PE sandwich panels which will improvetheir impact resistance.

Core Density Effect

Figures 15-18 show the effects of core density on the load-time and energy-timetraces of various composite sandwich panels. As seen in Figure 15, there are sev-eral orders of magnitude increase in maximum load and total absorbed energy ofgraphite sandwich panels, if the high density foam core is used. A similar

behavior, as shown in Figure 16, was observed in Kevlar sandwich panels. Themaximum load and total absorbed energy of PET composite sandwich panelsshown in Figure 17 are all double when the low density foam core is replaced bythe high density one. Figure 18 exhibits that the increases in maximum load andtotal absorbed energy in PE based sandwich are relatively small as the lowdensity core is changed to the high density one. This interesting observationindicates that the impact resistance of composite sandwich panels is mainly con-tributed by the facesheets, if the facesheet material is tough enough. The effect offoam core density on the impact resistance is insignificant. This is again consis-tent with the previous observation that the doublet feature in load-time trace is

very clear if the impact event is facesheet-dominated. Figure 18 shows two strongpeaks in the load-time trace of high density core-PE sandwich panels. Thereforethe impact response of PE sandwich panels is facesheet-dominated, whatever thedensity of the foam core being used.




286

Figure 14. SEM fracture patterns for composite sandwich panels containing high-densityPVC foam core, and (a) Gr, (b) Kv, (c) PET, (d) PE facesheets, respectively




287

Figure 14 (continued). SEM fracture patterns for composite sandwich panels containinghigh-density PVC foam core, and (a) Gr, (b) Kv, (c) PET, (d) PE facesheets, respectively




288

Figure 15. Impact load-time (solid lines) and energy-time (broken lines) traces of compositesandwich panels containing Gr facesheets, and (1) low-density, (2) high-density PVC foamcores, respectively

Figure 16. Impact load-time (so/1d hnes) and energy-time (broken lines) traces of compositesandwich panels containing Kv facesheets, and (1) low-density, (2) high-density PVC foamcores, respectively




289

Figure 17. Impact load-time (solid lines) and energy-time (broken lines) traces of compositesandwich panels containing PET facesheets, and (1) low-density, (2) high-density PVC foamcores, respectively.

Figure 18. Impact load-time (sol1d hnes) and energy-time (broken lines) traces of compositesandwich panels containing PE facesheets, and (1) low-density, (2) high-density PVC foamcores, respectively.




290

Note that all the impact tests of low density core sandwich panels showeddoublet characteristics in the load-time traces. Thus the impact responses of allpanels are facesheet-dominated and the contribution from the foam core is

negligible. In the facesheet-dominated impact response, the effect of core densityon the impact traces is little if the doublet is intensive and obvious, such as canbe seen in PE sandwich panels. On the other hand, if the doublet becomes lessobvious and a hump or shoulder develops between these two peaks as the densityof the foam core is increased, then the mechanisms of impact failure haveswitched from facesheet-dominated event to foam-core-dominated event, such asin the Gr and Kv sandwich panels. The PET sandwich panel however seems tobe somewhere in between these two extremes. That is, both the facesheet andfoam core are important in carrying the load and in absorbing energy. Accordingto this argument, we may conclude that the failure mechanisms, either facesheet-dominated or foam-core-dominated, are important for the design of these com-posite sandwich panels. If the facesheet is always dominant during impact event,there is not much benefit to be gained by using high density foam core. In doingso, the lightweight advantage of the sandwich panels will be somewhat sacrificed.

Effect of Impact Velocity on Single-Layer CompositesSince most composite sandwich panels studied here were only penetrated at the

highest position with the maximum weight of the impactor, there was not enoughdata to discuss the impact velocity effect on the sandwich panels. However, the

Figure 19. Impact load-time (solid lines) and energy-time (broken lines) traces of single-layerGrlepoxy composites impacted at (1) 2.85 m/s and (2) 4.50 mls, respectively.




291

Figure 20. Impact load-time (solid hnes) and energy-time (broken lines) traces of single-layerKvlepoxy composites impacted at (1) 2.85 m/s and (2) 4.50 mis, respectively

impact velocity effect on the single layer fabric/epoxy composites used as thefacesheet materials should provide important information related to their cor-responding sandwich panels, especially for those facesheet dominant impactevents. Figure 19 shows the effects of two typical impact velocities at 2.85 m/sand 4.50 m/s on the impact response of the single layer graphite/epoxy com-posites. Figures 20, 21 and 22 are the similar comparisons for Kv, PET, and PEsingle layer composites at these two different velocities. In general, the maximumload and total absorbed energy decline as the impact velocity increases. This phe-nomenon may be attributed to the rate dependence of most polymer composites.A similar behavior is anticipated in composite sandwich panels. The load andenergy absorbed by sandwich panels will decrease if the impact velocityincreases.

Additivity_

,

It is of interest to determine if the impact energy absorbed by the compositesandwich panel equals the sum of the energy absorbed by its individual con-stituents when tested separately. Apparently one superior feature of compositesis that the performance of the composite as a whole is usually better than the per-formance of its separate constituents. Evidence from Figures 23 and 30 supportthat the energy absorbed by the sandwich panels is always higher than the sumof the energies absorbed by the separately tested constituents. Figure 23 illus-trates the corresponding energy traces of the low density core, the Gr facesheets,




292

Figure 21. Impact load-time (sol1d lines) and energy-time (broken lines) traces of single-layerPETlepoxy composites impacted at (1) 2.85 m/s and (2) 4.50 mls, respectively.

Figure 22. Impact load-time (solid lines) and energy-time (broken lines) traces of single-layerPElepoxy composites impacted at (1) 2.85 mls and (2) 4.50 m/s, respectively.




293

Figure 23. Impact energy-time traces of (1) low-density foam core, (2) single-layer Grlepoxycomposite, (3) composite sandwich panel containing (1) and (2)

Figure 24. Impact energy-time traces of (1) low-density foam core, (2) single-layer Kvlepoxycomposite, (3) composite sandwich panel containing (1) and (2).




294

and the Gr sandwich panel. Similarly, Figures 24, 25, and 26 are the energytraces of low density core and corresponding Kv, PET, and PE facesheets andsandwich panels. In general, the energy absorbed by the low density core sand-wich panels is about 15 % to 100% more than the sum of the energies absorbedby the two facesheets and the core. The energy traces of high density core, fourtypes of facesheets (Gr, Kv, PET, PE) and corresponding composite sandwichpanels are shown in Figures 27, 28, 29 and 30. The energy absorbed by the highdensity core sandwich panels is about 20% to 125% more than the sum of theenergies absorbed by their constituents. The energy absorbing mechanism is afunction of material and interfacial properties. The adhesive failure between thecore and the facesheets obviously contributes significantly to the energy absorb-ing capability of a composite panel.

CONCLUSIONS

The impact response of various composite sandwich panels, including themacroscopic and microscopic failure mechanisms, was investigated in relation tothe constituent material properties. This investigation leads to the following con-clusions :

1. The impact energy absorbed by a composite sandwich panel containingsingle-layer facesheets increases many fold compared to that of the foam core

Figure 25. Impact energy-time traces of (1) low-density foam core, (2) single-layerPETlepoxy composite, (3) composite sandwich panel containing (1) and (2)




295

Figure 26. Impact energy-time traces of (1) low-density foam core, (2) single-layer PElepoxycomposite, (3) composite sandwich panel containing (1) and (2)

Figure 27. Impact energy-time traces of (1) high-density foam core, (2) single-layer Grlepoxycomposite, (3) composite sandwich panel containing (1) and (2)




296

Figure 28. Impact energy-time traces of (1) high-density foam core, (2) single-layer Kvlepoxycomposite, (3) composite sandwich panel containing (1) and (2).

Figure 29. Impact energy-time traces of (1) high-density foam core, (2) single-layerPETlepoxy composite, (3) composite sandwich panel containing (1) and (2).




297

Figure 30. Impact energy-time traces of (1) high-density foam core, (2) single-layerPElepoxy composite, (3) composite sandwich panel containing (1) and (2).

when alone. Such a sandwich combination offers exceptional impact resis-tance and yet maintains its light-weight characteristics.

2. The impact response of composite sandwich panels is mainly controlled by thefacesheets and is practically insensitive to the density of the PVC foam core,provided the facesheet material is sufficiently tough (e.g., PET or PE fibers).

3. The impact failure mechanisms of the composite sandwich panels made of lesstough facesheet material such as Kv or Gr fibers tend to be foam-core-dominated, provided the PVC foam core possesses a relatively high density.

4. The impact response, including maximum load and total absorbed energy, ofthe single-layer composite facesheets declines as the impact velocityincreases.

5. The energy absorbed by the composite sandwich panels containing the low-density PVC foam core is about 15% to 100% greater than the sum of theenergies separately absorbed by the two facesheets and the foam core. Thispercentage deviation from the rule-of-mixtures prediction increases as thedensity of the foam core increases. However, this deviation will be insignifi-cant if tougher facesheets, such as PE fibers, are used.

ACKNOWLEDGEMENTS

We gratefully acknowledge the financial support of the U.S. Army ResearchOffice.




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instrumented impact testing of composite sandwich panels

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