impact energy absorption

Fibers and Polymers 2000, Vol.1, No.1, 45-54

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Impact Energy Absorption Mechanism of Largely Deformable Composites with Different Reinforcing Structures

Tae Jin Kang* and Cheol Kim

School of materials Science and Engineering, Seoul National University, Seoul 151-742, Korea(Received September 1, 2000 ; Accepted September 15, 2000)

Abstract : Impact behaviors of the large deformable composites of Kevlar fiber reinforced composites of differentpreform structures have been investigated. An analytic tool was developed to characterize the impact behavior of thKevlar composites. The image analysis technique, and deply technique were employed to develop energy balancequation under impact loading. An energy method was employed to establish the impact energy absorption mechanism of Kevlar multiaxial warp knitted composites. The total impact energy was classified into four categoriesincluding delamination energy, membrane energy, bending energy and rebounding energy under low velocity impactMembrane and bending energy were calculated from the image analysis of the deformed shape of impacted specimand delamination energy was calculated using the deplying technique. Also, the impact behavior of Kevlar composites under high velocity impact of full penetration of the composite specimen was studied. The energy absorptionmechanisms under high velocity impact were modelled and the absorbed energy was classified into global deformation energy, shear-out energy, deformation energy and fiber breakage energy. The total energy obtained from thmodel corresponded reasonably well with the experimental results.

Introduction

A number of works have been reported in the fields ofimpact phenomena of fiber reinforced composite materials.Especially, the damage resistance and damage toleranceunder impact loading are of the most importance ofcomposite materials characteristics because they are oftensusceptible to impact. Low-velocity impact can introducesevere internal damages to composite structures andsignificantly reduce their load-carrying capacity[1-3].Low-velocity impact damages may consist of matrixcracks, delaminations, and fiber breakage in a zone sur-rounding the impact point. Damage patterns are compli-cated such that the exact sequence of events involved andthe size of the damage cannot yet be analytically predicted.

Delaminations are the main cause of reductions inmechanical properties of the laminate and usually cannotbe detected by visual inspection except for in the case oftranslucent or transparent materials, thus, a lot of attentionhas been devoted to understanding how they developduring impact and how they propagate when subjected tofurther loading. After impact, many matrix cracks arepresent in a complicated pattern. It would be difficult topredict the final pattern of matrix cracks throughout thedamaged region, but this would not be necessary asmatrix cracks do not significantly contribute to reducingthe residual properties of the laminate. However, manyinvestigators believe that the entire damage process isinitiated by matrix cracks, which then induce delamina-tions at ply interfaces[4-6].

The response of a laminated composite to an impobject depends on the impact parameters of the impaand the material properties of the composites suchstacking sequence, interlaminar shear strength, tenand flexural properties of the composites. When a foreobject impacted on the laminates, the impact energy wabsorbed by the composite and damages such as delnations, fiber breakage, and matrix cracks occurredthe composite structure. The carbon fiber reinforccomposites or the glass fiber reinforced compositwhich are susceptible to impact damage because of brittle characteristics of the reinforced fibers, showelow impact energy absorption, and usually a complepenetration occurs by the impactor. The major enerabsorption mechanism with the carbon fiber reinforccomposite is fiber breakage mode. Damage mechanisof ductile fiber reinforced composites such as Kevlar Spectra fiber composites are quite different. The eneabsorption is mainly due to the delamination and lardeformation perpendicular to the impactor. Surface eneis needed to create newly delaminated surfaces, and senergy is also required to deform reinforcing fibers membrane deformation. Thus, the energy absorptmechanisms of ductile fiber reinforced composites aquite different form those of brittle fiber reinforcedcomposites[7].

Furthermore, the energy absorption capabilities brittle fiber composites are less than those of ductile fibreinforced composites, thus, the Kevlar and Spectra fibare widely used as an reinforcement fiber for the imparesistant composite materials. In consequence, undstandings of the behavior of largely deformable compoites such as Kevlar fiber reinforced composites und

45

46 Fibers and Polymers 2000, Vol.1, No.1 Tae Jin Kang and Cheol Kim

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When the impactor strikes the Kevlar plates, the platesabsorbs impact energy solely by the elastic deformation ifthe incident impact energy is lower than a certain value.As the impact energy increases, matrix cracking occurs,and the delamination starts to propagates until the maxi-mum delamination area has been reached. Thus, thedominant energy absorption mechanism is delaminationmechanism in this stage. If the impact energy is higherthan this stage, no additional energy absorption is possi-ble by delamination growth. The Kevlar plates absorbsadditional impact energy by the inelastic deformation ofthe plates, including bending and membrane deforma-tions called as bulging. If the impact energy is above thislevel, when the impactor velocity is high and the contacttime is not enough to transfer impact waves induced bythe impactor, then the localized damage including fiberbreakage and delamination occurs.

Delamination, fiber breakage, inelastic deformationabsorb impact energy, but it is not clear which one isdominant at each stage. It is not known how the relativeenergy absorption changes with increasing incidentimpact energy. It is not known whether the delaminationstill absorbs significant energy for the case of perforation,or if it can be neglected compared to the energy absorbedin fiber breakage. The main object of this paper is to fullyquantify the relationship between the damage mecha-nisms and energy absorption under low and high velocityimpacts.

Energy Balance Equation

To analyze the impact damage mechanism of a Kevlarcomposite, it is assumed that no fiber breakage occursunder low level of impact energy. When the impactorstrikes the specimen surface, the given impact energy bythe impactor can be classified into two quantities. Oneis the elastic energy which is stored elastically in thespecimen and transferred back to the impactor. Another isthe absorbed energy which is the sum of the absorbedenergy in the specimen by its damage initiation andpropagation, and the energy absorbed by the impactsystem in vibration, heat, inelastic behavior of the impac-tor or supports.

Thus, the following relationship holds under lowvelocity, low energy impacts.

Etotal = Ereb + Eabs (1)

where Ereb : rebounded energy, Eabs : absorbed energy, Etotal : total energy

Furthermore, the absorbed energy can be divided into

two classes; one is Eab, which is absorbed by the specimen while the other is Esys which is absorbed by the testing system itself. The total energy absorbed by tspecimen has three components Emb, Ebd and Edel, whichare respectively, membrane energy, bending energy, energy absorbed by the creation of damages in the maFor a ductile Kevlar composites, we assumed that fibbreakage did not occur, therefore, the terms which relato fiber breakage were ignored, and delamination eneEdel, dominated. Thus, the resultant equation for timpact energy is given as

Etotal = Ereb + Emb + Ebd + Edel (2)

Triangular elements were used to calculate the mebrane and bending energy. Membrane strain eneexpression and its derivative with respect to coordinategiven as :

(3)

where [Cm] is a tensile property matrix

Bending strain energy can be expressed as follousing the coordinate information of the three elemenneighboring the current element, the bending energy tecan be expressed as :

(4)

In this study, delamination energy Edel was obtained bya damage area analysis. If a composite laminate wimpacted by an object, the delamination initiated belothe impacted area and broad delaminations could fowith the increase of impact energy. Additionally, matrcracking could have occurred, but many researchreported that matrix cracking plays an important role the onset of delamination. No attempts were madedivide delamination and matrix cracking, since matrcracking is always associated with delamination and ratio of matrix cracking area and delamination areaconstant for a given system, or the energy absorptionmatrix cracking is often negligible. If the fracture toughness of the laminate is known, the energy neededdelaminate a given area can be calculated. The impfracture toughness i.e., the unit energy needed to delainate a unit area by impact, can be measured from relationships of impact energy and delaminated area.obtain the precise estimation of delaminated area, deply technique was adapted as it will be described laOnce the impact fracture toughness has been measuthe following equation is applicable.

(5)

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Impact Energy Absorption Mechanism of Largely... Fibers and Polymers 2000, Vol.1, No.147

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where Edel : delamination energy, K : fracture toughness under impact, Ai : delaminated area in the ith layer, n : number of plies

Finally, the total absorbed impact energy by impactloading is the sum of the delamination energy, bendingenergy and membrane energy.

Experimental

MaterialsKevlar 29 fiber was used for reinforcement as well as

for stitch fibers. Three kinds of preforms were used. Oneis multiaxial warp knitted preform(MWK) and another iswoven fabric as well as unidirectional angle ply werestacked and consolidated with epoxy resin. Multiaxialwarp knitted preforms can be called 'high performancemultiaxial warp knitted preforms because a Kevlar fiberwas used as stitch yarns. This multiaxial warp knittedfabric consisted of insertion yarns of four directions(warpand fill, bias yarns of +45/-45) with two types of stitches,i.e., tricot and chain stitches. And this multiaxial warpknitted fabric is defined as QTC(quadriaxial insertion yarn/tricot/chain stitch) preform. The unidirectional laminatewas stacked with a sequence of [(0/45/-45/90)]s. Forwoven laminates, 8 plies of fabric having a density of 21count per inch for warp and fill were stacked to obtain a 4mm thickness of composites. Thin teflon strips (<5 µm)were inserted between each plies to make ease ofdeplying. Laminates were press-claved at 120oC for 90min at 5 MPa pressure, then cooled to room temperature.

All impact specimens were prepared to have 4 mmthicknesses and 0.53-0.61 fiber volume fractions.

Tests and SpecimensThe in-plane Young's modulus as well as the in-plane

Poisson's ratio were determined by a tensile test, followingASTM D 638 M. An extensometer was also attached withrespect to the center line of the specimen, having a gagelength of 1 in. Shear properties of the composites wereobtained using a Iosipescu shear test fixture, which couldoffer a pure shear stress on the neutral plane of the testspecimen. A schematic diagram of a Iosipescu test fixtureis shown in Figure 1. Bending properties were obtainedby ASTM D 790a where a four point bending test methodwas used. Span-depth ratio was 16:1, while the crossheadspeed was fixed at 1 mm/mim.

Impact testThe instrumented impact test equipment used in this

study was a ITR-2000 impact tester. The general featuresof the testing apparatus are shown in Figure 1. The mate-rial specimen is a flat plate, approximately 127-mm(5-in.)

square, which is securely clamped over a 101 mm(4 indiameter annular anvil. The test consists of complepenetration of the specimen by a 12.5 mm(1/2-indiameter hemisphere probe which is guided through center of the annulus under conditions of near constvelocity. It is possible, with tough materials, to have thundesirable condition of large velocity variations of thprobe during penetration.

The probe is equipped with transducers for measurvelocity and the load interaction between the probe aspecimen. Photoelectric sensors are used to start anddata acquisition. Three thousand load data points collected during the 101 mm(4 in.) of probe travel whicis considered to be the impact event. The velocity aload transducers provide complete profiles of the defmation response of specimen from initial impact to finpenetration.

A thick composite laminate was first impacted at loimpact energy levels and a penetrant was injected durthe deplying process to enhance the image resolutionthe delaminated areas. After the injection of penetrant, laminates were deplied and the damaged area inducethe impact loading were measured. In order to measfracture toughness correctly, the impact specimenshave enough thickness and the incident impact enelevel had to be kept low to avoid involving verticadeformations and indentations.

In order to evaluate impact properties of the thcomposite laminate specimen, the following steps weperformed. Before impact, a number of points we

Figure 1. Instrumented impact tester.


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printed on the back surface of the laminates to obtain a 3-dimensional deformed shape after impact loading, aspreviously described[8-10]. Impacts were inflicted on thelaminates using an instrumented impact tester. Thediameter of the impact tip was 12.7 mm while the 100�

100 mm size specimens were clamped by an aluminumfixture by a pneumatic pressure of 600 kPa. Impactenergy was varied by altering the initial velocity of theimpactor. Once impacted, the image of the back surfaceof the impacted specimen was captured and analyzed toobtain the deformed coordinate.

Results and Discussion

Responses under Impact Loading A photograph of the cross-sectional area of the

impacted specimen is shown in Figure 2. Broad delami-nation and large deformation can be found in the entireregion of specimen (a), which was impacted at 7.0 m/sinitial velocity. Thus, it is reasonable to assume that themain energy absorption mechanisms are delaminationand global deformation including membrane and bendingdeformations because no fiber breakage can be found infigure (b), energy absorption mechanisms under highvelocity impact are fiber breakage, delamination and so

called, shear out process and global deformation. Tprinciple damage is apparently fiber failure, as a sustantial bundle has been detached from the matrix apushed along the direction of the striker motion. Thshape of the damaged region depends upon the weand lay-up of the specimen. A woven laminate showesquare-shaped damage zone on the distal side withextent of 1.5-2 times of the impactor diameter, while tQTC composite, resulted in a circular pattern.

Force-displacement curves under impact loading ashown in Figure 3 Since the impact velocity was not hienough to infiltrate full penetration of the compositenon-perforated impact occurred at 7.0 m/s velocity, whall the specimens were perforated at 11.0 m/s initvelocity. From this figure, it is clear that the unidirectional laminates showed more elastic features than otspecimens since the unidirectional laminates responmore quickly upon impact loading at low impact velocitUnidirectional laminates have much higher initiatioenergy than the propagation energy while other spemens showed otherwise. Both woven and QTC multiaxwarp knitted preform composites showed more ductfeatures than the unidirectional laminate. At high impavelocity, the force-displacement curves showed simitendency.

The impact time could be measured during the imptest and varied within the range of 6.4-7.1 ms for lovelocity impact while 3.6-4.1 ms for high velocityimpact. From the experimental data obtained from tforce transducer. Duration of impact phenomena MWK fabric reinforced composites were longer than thof UD and woven laminated composites.

The variation of total energy dissipated during impaare shown in Figure 4. The energy diagram includes bthe energy to induce damage as well as the energy dpated via recoverable deformation and other secondmechanisms such as the delamination energy, bendand membrane energy, etc.

Figure 3. Force-displacement curves under impact loading (7.0m/s with 6.5 kg).

Figure 2. Cross-section of impacted specimens (upper : lowvelocity, under : high velocity).

Figure 4. The variation of total energy dissipated during impac


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Delamination Area and Energy LevelFor a woven laminate, delamination initiated at the

center of impact, and propagated to the directions of warpand fill directions. Due to the existence of warp and fillreinforcements, impact damages which initiated by matrixcracks were propagated along them. For unidirectionallaminates which were fabricated up to 24 plies, thedirection of the delaminations always followed thedirection of the reinforced fibers in the lower ply of eachinterface. For example, in the -45o/+45o interface,delamination occurred in the +45o direction. And thisphenomena coincides with the results by Guynn andO'Brien[11], who observed that the delaminations alwaysfollowed the matrix cracks in the back ply of each uniqueinterface.

All the captured images of delaminated area are storedwith 640�480 pixels. Five identical specimens wereimpacted, of which three of each five specimens havedeplied to obtain the delaminated area images. Datashows that limits of delamination by impact exist. Sincethe impact specimen was clamped by an impact fixture asmentioned earlier, it is clear that delaminated areas havean upper limit extending up to the inner area of the clamp.Delaminated patterns of the QTC and woven specimenare shown in Figure 5. The delaminated areas induced by

the impact loading for the laminated composites of vaous types of preforms are shown in Figure 6 at varioimpact energy levels.

The absorbed energy of Figure 6 indicates the toabsorbed impact energy including membrane, bendand delamination energy. Since the initial slopes of Figu6 implies the delaminated area by unit impact enerimpact fracture toughness can be obtained by invertthe slopes. As shown in Figure 7, the fracture toughnof the multiaxial warp knitted composite is higher thathat of the woven laminate or unidirectional laminacomposites. Impact fracture toughness values were 3kJ/m2 for the QTC composites, and 1.22 and 1.39 kJ/2

for the unidirectional laminate and woven laminatrespectively. At the same level of given impact energmultiaxial warp knitted fabric composites, especialQTC composites showed the smallest damaged area. means that the QTC composite is capable of absorbhigher impact energy and constraining the damaged ato a relatively small region.

Figure 5. Delaminated patterns of the QTC and wovenspecimen (upper : QTC, under : woven, both impacted at 7.0 m/d initial velocity).

Figure 6. Delaminated area-absorbed energy relations various composites.

Figure 7. Impact fracture toughness.


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The interceptions of the lines on the energy axis indicateminimum incident energy levels needed to start the onsetof the delamination. For the QTC specimen, 11.5 J ofimpact energy were necessary to initiate delamination.However, the woven laminates and unidirectional lami-nates the impact energy was about 7-8 J. These resultscoincide well with other test results, since the QTCshowed higher interlaminar shear strength and thus higherenergy was needed to initiate interlaminar impact damagesuch as delaminations. Conventionally it is believed thatperforation takes place above this level of impact.Additional energy absorption is possible by means of fiberfracture. In contrast to glass composites, Kevlar compos-ites can absorb additional impact energy by the globaldeformation as well as fiber fracture mechanism. Thus,absorbed energy-delaminated area relationship of Kevlarcomposites is different from that of glass composite.

Membrane and Bending EnergyUsing the image analysis method, the membrane and

bending energy were calculated from the impactinduced deformed shape of the impacted specimen. Thereconstructed 3-D images are shown in Figure 8. Withthe coordinate information of the deformed specimenfrom the image analysis, the membrane and bendingenergies were calculated. Coordinates of transitionalsteps between non-deformed and deformed specimenswere generated by linear interpolation. Velocity profilesof the impactor were used to calculate the time scale forthe transitional steps.

Now, all the energy absorbed by each mechanism ofEreb, Edel, Emb, Ebd have been calculated. The impactenergy is divided into three major parts at low velocityimpact: the rebounding energy part, measured fromenergy impact data; the delamination energy part, whichneeded to initiate and propagate delaminated surface, andwhich was calculated by impact fracture toughness and

delaminated area measured by the deply techniqdeformation energy which were computed using imagof deformed specimens and material properties.

For high velocity impact, the image analysis techniqcould not be used because of the relatively small glodeformation of the specimen. Thus, the energy absorby global deformation under high velocity impact werobtained by static indentation tests.

A static indentation test was performed to the undaaged specimen until the same global deformation wachieved. Since additional delamination could occduring this static test, delamination energy was calclated by deply method as proposed in an earlier sectiThus, the energy absorbed by global deformation cancalculated.

Figure 8. Reconstructed 3-D images and example of energycalculation.

Figure 9. Breakdowns of energy absorption under low velociimpact (upper : QTC, middle : UD, lower : woven).


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Energy absorption by shear-out was estimated by firstdetermining the transverse fracture energy of the material.In order to obtain this, an out of plane shear test wasperformed by the Iosipescu shear fixture. This techniqueyielded values of 104 kJ/m2 for woven laminates. Conse-quently, the shear-put energy was calculated by multi-plying this fracture energy by the area of shear zone.Energy dissipated in delamination was estimated bymultiplying the delamination area with the impact fracturetoughness obtained in an earlier section on low velocityimpact. Energy absorbed by fiber breakage was calculatedby examining broken fiber width in each ply.

Thus, the total energy required to perforate the Kevlarcomposites investigated in this study was then determinedby adding the estimated energies associated with globaldeformation, shear-out, delamination and fiber break-age[12].

Energy Absorption Mechanism under Low VelocityImpact

The breakdown of absorbed energy under low velocityimpact is shown with different mechanisms of energyabsorption in Figure 9.

The total impact energy is the sum of the componentenergy of Edel, Ebd, Emb and Ereb. We did not consider theEsys term nor the energy absorbed by fiber breakage sincethe incident energy was not sufficient enough to cause thefiber breakage. The gaps between incident energy andthe summation of membrane and bending energy aresub-summations of Esys and the energy absorbed by fiberdamage or delamination. Delamination energy can alsoobtained by multiplying the delaminated area and impactfracture toughness. The delamination energies calculatedin this way was 36 J for QTC, 29 J and 42 J for unidirec-tional laminates and woven laminates, respectively. Notethat all the delamination energies are lower than the gapsin the figures. From Figure 11, sub summation of delami-nation energy, Esys and energy absorbed by fiber breakageare 48 J, 36 J and 46 J, respectively. These results indicatethat the energy absorbed by other mechanisms remainsmall compared with the energy absorbed by the majorenergy absorption mechanisms such as delamination andglobal deformation.

As reported in the previous section, the impact fracturetoughness of multiaxial warp knitted composites are sig-nificantly improved by the existence of through-the-thickness stitched loops. Although the fracture toughnesswas improved, the total energy absorbed by delaminationwas not increased as much since the delamination areawas drastically reduced with the multiaxial warp knittedcomposites. Woven laminate, which had the lowest impactfracture toughness, showed much greater delaminationenergy absorption because of the broad extent of delam-ination.

For non-perforating impact tests the incident enerwas fully transferred to the specimen at the point maximum impactor displacement. After the maximudisplacement, the specimen transfer elastically stoimpact energy back to the rebounding impactor. The raof rebounded energy to stored energy could be a paramof elastic/plastic properties. Ratios of rebounded eneto stored energy were 0.17 for QTC multiaxial warp knittecomposites, 0.35 and 0.14 for unidirectional laminate awoven laminate, respectively. This tendency is similar the results from force-displacement relationships.

The onsets of delamination were also shown in tabove figures. The multiaxial warp knitted compositeshowed retarded delamination onset. Delamination inated at 0.7 ms with 11 J of impact energy for QTC spemens, while 0.3-0.4 ms and 5-6 J were necessary forunidirectional laminate and woven laminates, respectiveThe results correspond well with the other test resusince the fracture toughness of the former specimenmuch higher than those of the latter two specimens.

For the QTC composites, the main modes of impaenergy absorption were deformation, bending amembrane mechanisms from the beginning of impaloading until maximum displacement was reached. Aftmaximum displacement had been reached, the impenergy stored by bending and membrane deformatwas transferred back to the impactor. Unidirectionlaminate showed greater elastic characteristics thother specimens, such as higher rebounding enelower displacement, and shorter total impact timWoven laminates showed less elastic features than other specimen. It showed higher deformation, longeimpact time, smallest rebounded energy. It absorbed menergy by means of membrane energy mechanism tother mechanism, thus resulting in relatively small bening energy absorption although the deformation is larsince the bending stiffness is lower than multiaxial waknitted composites.

Two kinds of additional experiments were performed verify the energy absorption mechanism. One wasdelamination simulated impact test which was done impacting a composite plate having thin Teflon films ieach interply. The other was a deformation restrictimpact test in which global deformation of the impactespecimen was restricted by the clamping fixture whihas a smaller diameter.

Delamination simulated specimens were made by insertion of thin Teflon films in each interply of wovenlaminate composites. A thin Teflon film(<5 µm) of 40mm radius was inserted in each interply to simuladelamination. All other parameters including cure temperature and fiber volume fraction were unchangeRestriction of global deformation was done by the fixtuof a smaller diameter.


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The resultant force-displacement curves for a 8 plylaminate with an embedded 80 mm diameter delamina-tion and that of fully laminated plates when impacted by a12.7 mm hemispherically tipped impactor are presentedin Figure 10.

The delaminated specimen has a lower initial slope, andslightly lower peak load but slightly increased deforma-tion. At the initiation of rebounding, up to the point ofmaximum deformation, the delamination simulated speci-men absorbed approximately 71% of the homogeneouslaminate energy and exhibit 74% of its peak load. Thesefigures provide an idea of the effect of delamination.

According to the results of the above energy calculatiothe woven laminate absorbs 33% of its maximum eneabsorption by delamination mechanism. Results of tdelamination simulated specimen showed good corspondence well with the previous calculated result.

Also, the effect of restraint on global deformation andelamination are shown in Figure 10. The peak load the two cases are about the same, but the restrained syexhibits a much lower deflection both at the maximuload and at the end of the test. Thus, the energy absorpof restrained specimen was up to 36% of unrestrainspecimen. Also, for the restrained specimen, eneabsorption by delamination had reduced. The eneabsorbed by delamination was up to 79% of unrestrainspecimen, 33.2 J. But this quantity is not sufficient explain the reduction of total absorbed energy. Thus, ireasonable to consider that the reduction of eneabsorption of restrained specimen is mainly due to treduction of global deformation.

Energy Absorption Mechanism under High VelocityImpact

As discussed earlier, energy absorption mechaniswhich are available in composite materials are fibbreakage, membrane deformation, bending deformatand delamination. For low velocity impacts, bendindeformation and delamination are the major enerabsorption mechanisms. At energies above the threshthe fracture zone are generally conical in shape with overall area of damage increasing towards the lowsurface of the composite. This type of conical-shaped fas well as in a number of metals.

In high velocity impacts, at the perforation threshoenergy, the spherical tipped impactor generally makeconically shaped shear plug as it penetrates the impspecimen and energy dissipates during this process. Tthe energy absorption mechanisms under high velocimpacts can be classified as global deformation, fibbreakage, delamination and shear-out.

The total energy required to perforate the impactspecimens are calculated and compared with thoseexperimental results in Table 1 (A) is the experimenvalue. The calculated energies by each of the ene

Figure 10. Effect of delamination and global deformation onthe energy absorption under low velocity impact ; (a) force-displacement curves of normal/delaminated specimens, (b)force-displacement curves of normal/restrained specimens.

Table 1. Comparison of calculated and experimental results of the energy absorption under high velocity impact (unit: J, standard deviation in bracket

Specimen (A)

Predicted(B)

Experimental(S.D)(C) Calculated (J)

Total Fiber breakage Global deformation Delamination Shear ou

BTC 176 182(5.08) 172(5.3) 91.5 14.2 24.6 42.7

QTC 184 188(6.85) 177(7.1) 94 10.3 22.6 50.1

UD 154 145(8.59) 137(9.1) 72 8.3 20.2 37.1

Woven 188 203(9.1) 182(7.4) 100 15.6 26.3 40.9


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absorption mechanisms are shown in column (B).As shown in this table, the experimental values have

slightly higher than the predicted energy level, sinceexcess energy absorbed by other mechanisms such asvibration and noise are neglected. Energy absorption inUD angle ply laminates showed the lowest values, sincethe fiber breakage energy of this specimen showed thelowest value.

QTC composite specimen showed higher shear outenergies than those of UD or woven laminates. An addi-tional impact test similar to tests done with low velocityimpacts has been performed. The results are shown inFigure 11 for the delamination simulated and restrainedtests, respectively. All the test configurations are the sameas the low velocity impact, except for the impact velocity.

From these figures it is clear that the delamination andglobal deformation have little effect on the energyabsorption. The results are reasonable because their con-tribution to energy absorption is relatively small comparedwith the energy absorption by fiber breakage and shearout processes.

The energy absorption of the delamination simulatedspecimen decreased about 4% of the undelaminated

specimen, while the restrained specimen showed a 1decrease in energy absorption. All of these results cocide well with the results in the previous section.

In contrast to the trends observed in the low velocimpact, the effects of varying the fixture diameter donot appear to vary the perforation energy significantly this impact energy level. The strong geometric dependeobserved in the low velocity impact may be attributeto the global deformation. Under high velocity impacconditions where the contact time between the impacand the target specimen is very short, it appears thattotal energy absorbing capability of the specimen is nutilized, and the impactor indeces a very localized forof target response. As a result, varying the specimgeometry does not show any significant effect on tenergy absorption, and the extent of damage incurrThis reasoning is also supported by the associadelamination area analysis, where no significant variatin the total delaminated area was noticed.

Conclusions

Impact properties of largely deformable compositeof Kevlar fiber reinforced composites with differenreinforcing structure have been investigated. An analytool was also developed to characterize the impabehavior of composites. The image analysis and detechniques were employed and energy balance equawas established.

An energy approach was employed to study the impbehavior and impact energy absorption mechanismslargely deformable composites under low and higvelocity impacts.

The multiaxial warp knitted composite showed highimpact fracture durability and bending properties, resultiin increased delamination and bending energy compawith unidirectional laminate. For woven laminates, sinthe delaminated area of the woven laminates was mlarger than that of multiaxial warp knitted composites,absorbed slightly more energy in delamination moalthough the impact fracture toughness of the wovlaminate was much smaller than that of the multiaxwarp knitted composites.

The onset of delamination was retarded in multiaxwarp knitted composites because of higher impact fractdurability than those of unidirectional laminate or wovecomposites. The multiaxial warp knitted fabric compositshowed reduced delaminated areas at the same incienergy level because the energy absorbed by delaminawas larger and not much lower than that of woven laminwith the presence of stitched loop fibers. This means tthe multiaxial warp knitted composite has the capabilof absorbing high impact energy while constraining thdamaged area into a relatively small region.

Figure 11. Effect of delamination and global deformation on theenergy absorption under high velocity impact ; (a) force-displacement curves of normal/delaminated specimens, (b)force-displacement curves of normal/restrained specimens.


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