energy absortion by kevlonorepoxy

Testing Methods for Energy Absorption of Kevlar/EpoxyDean D. Dubey* and Anthony J. Vizzini†

Gessow Center for RotorcraftDepartment of Aerospace Engineering

University of MarylandCollege Park, Maryland 20742

Abstract

Twenty flat-plate specimens and six tubes were crushed under quasi-static conditions. Theenergy absorbency of each specimen was measured resulting in a value of the specific sus-tained crushing stress (SSCS). The plates and tubes were manufactured from 49/CYCOM 919Kevlar/epoxy fabric and had the same layup providing a common laminate for comparison. Theflat plates were scored to facilitate the fracture of the laminate along the constraints during crush-ing. Two different widths of flat plates were tested to determine the effect of the end conditionsof the testing fixture on the observed energy absorbency. The results for the flat-plate speci-mens indicate that the energy absorbency per unit thickness is independent of the specimen testwidth while maintaining the crushing mechanism. Thus, the edges of the testing fixture providedno significant contribution to the energy absorbency. Furthermore, the results indicate that theflat-plate geometry provides comparable energy absorption values to those of the tube geometry.

Introduction

Safe operation of aircraft and rotorcraft requires that occupants survive crash events atvarious attitudes and velocities. Crew areas must not become lethal. A liveable volume must bemaintained, post-crash fires must not occur, and deceleration of the occupants must be limitedto prevent serious injury or death. Typically, a systems approach is taken to decelerate theoccupants. Initially, the landing gear stroke and then deform. Concurrently, the seat strokes. Inextreme conditions, the subfloor of the helicopter is sacrificed as a result of being crushed.

The Advanced Composite Airframe Program (ACAP) demonstrated that composite materialsare an effective part of a crashworthy design [1]. High energy absorbency per unit mass ispossible with composite materials if proper failure mechanisms are initiated and maintainedduring the crash event. Whereas metals absorb energy primarily through plastic deformation,composite materials absorb energy through a variety of failure mechanisms. For example,Kevlar reinforced composites absorb energy through a buckling failure mechanism similar tothe accordion buckling modes of metal structures. Graphite- and glass-reinforced compositesabsorb energy through successive failures involving delamination, intraply cracking, and fiberfracture. Because energy absorbency of a composite structure is directly dependent on the failuremode that occurs and the failure mode is a function of the laminate stacking sequence, the loadinghistory and environment, proper characterization ought to include off-axis crush tests [2].* Graduate Research Assistant† Associate Professor, member AHS

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An initial geometry used by researchers to study the energy absorption capabilities ofcomposite materials was the tube. This geometry is self stabilizing and allows testing ofrelatively thin-section laminates. The lack of edges along its length reduces the complexityof the boundary conditions and provides consistency throughout the cross section [2–4]. Usingthis specimen geometry, Farley observed various energy absorbent failure modes [3]. Moreover,he determined the influence of such factors as the friction of the crushing surface, strain rate,material, and stacking sequence. These tests, however, were limited to uniaxial crushing.

Fleming and Vizzini used truncated cone specimens to investigate the effect of side loads andspecimen taper on the energy absorbency of composite graphite structures [2]; Knack and Vizziniinvestigated these same issues for truncated Kevlar cones [5]. The tests on the truncated conesdemonstrated the effects of side loads; however, the cone geometry resulted in a combinationof failure modes which made further analysis difficult.

Jackson, et al developed a fixture to crush flat-plate specimens [6]. The fixture developedstabilized the flat plates during crushing to avoid plate buckling. Flat-plate specimens are easierto manufacture and are less expensive than tubes. Fleming and Vizzini adapted this fixture typeto allow the crushing of flat plates at specified angles of incidence [7]. The flat-plate specimensindicated a performance degradation due to off-axis loading conditions.

Although the tube specimen with its finite D/t ratio may be used to indicate the potentialenergy absorption of a given laminate, it remains a structural test. The diameter to thickness ratio,D/t, of composite tube specimens affects the energy absorption capability of laminates. For thintubes, corresponding to large D/t, specimen crushing can become unstable leading to specimencollapse [8]. Even for stable crushing situations, D/t may affect the measured energy absorbencyof composite specimens. Typically, Kevlar/epoxy composite specimens have a bilinear relationbetween energy absorption and D/t, while graphite/epoxy specimens have a non-linear relationbetween energy absorption and D/t [9]. An increase in energy absorption was observed in generalas D/t decreased, reaching as its limit the compressive yield strength of the material [10]. Thisincrease in energy absorbing ability is caused by the reduction in interlaminar cracking, whichreduces the characteristic damage length of the fibers. Thus, a minimum of bending occurs andmore of the strength of the material is realized.

These conclusions would indicate that the tube geometry should provide superior energyabsorption capability to the flat-plate geometry which has an infinite D/t. Because resistance tobending is presumed to be the cause of the variance of energy absorbency to D/t, then a flatplate that is stabilized by means of a fixture may reduce this observed difference in measuredenergy absorbency. Researchers have shown that testing of flat-plates for energy absorption canbe accomplished when they are stabilized in a fixture [6, 7, 11]. Moreover, the simplicity ofthe manufacture of the flat plate provides a low-cost alternative method to observe crushingphenomenon. Under off-axis loading, flat-plates experience the same load across the crush planewhereas a tube specimen experiences a varying load around its circumference.

It is desirable to be able to correlate test results from flat-plate specimens with tube specimens.Although previous tests done with specimens manufactured from unidirectional graphite/epoxyshowed that the plates readily cracked along the edge constraints [7,12], the effect of theconstraint on the damage mechanisms occurring in Kevlar/epoxy specimens was undetermined.

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To overcome these limitations, an experimental test program was conducted to first determinethe effect of the fixture by testing specimens with two different widths and then to compareequivalent laminates tested as flat plates and tubes. The figure of merit for comparing energyabsorbed for a specific structure is the incremental energy absorbed divided by the instantaneousmass consumed. Thus, the specific sustained crushing stress SSCS is defined as:

SSCS =Energy

�� V olume=

�P�l

�A�l=

�P

�A(1)

where �P is the average crushing load, A is the cross sectional area, and � is the densityof the material. The stacking sequence, (+45/–45/0/90)S, was used for the 49/CYCOM 919Kevlar/epoxy laminates. This stacking sequence was chosen for similarity with previous studies.

Experimental Test Matrix

A total of 26 (+45/–45/0/90)S specimens with various geometries were manufactured from49/CYCOM 919 Kevlar/epoxy bidirectional preimpregnated fabric. Of these, 20 were flat-platespecimens and six were tubes. Of the flat plates, ten were machined to 165 mm � 89 mm and tenwere machined to 165 mm � 64 mm. The tubes were 102 mm in height with an inner diameterof 114 mm. A steeple chamfer was machined into one end of each specimen to initiate damage.

A comparison is to be made first between flat-plate specimens of different widths. Bymaking this comparison, a SSCS per unit width and the energy associated with cracking alongthe edges of the specimen at the constraints can be determined. Next, equivalent laminates withtwo different geometries will be compared. Finally, the present testing method can be evaluatedfor broader application by including data of graphite/epoxy specimens that were tested in thesame manner [12].

Manufacturing

The flat-plate specimens were machined from larger 305 mm � 356 mm laminates witha stacking sequence of (+45/–45/0/90)S. The laminates were made from 49/CYCOM 919Kevlar/epoxy bidirectional cloth. The tube specimens were machined from two longer tubeswith an inner diameter of 114 mm. Tubes were laid up and cured on aluminum mandrels. Themanufacturer’s recommended cure cycle was followed for both the tubes and the laminates. Itconsisted of 1 h at 121 �C with a total pressure of 621 kPa.

The flat-plate laminates and the tubes were then machined into individual specimens. Amilling machine with a water-cooled diamond-grit cutting blade was used. From the threeplates, 20 flat-plate specimens were machined with one plate yielding six 165 mm � 89 mmspecimens, another plate yielding eight 165 mm � 64 mm specimens, and the third yieldingfour 165 mm � 89 mm specimens and two 165 mm � 64 mm specimens. From the two tubes,six tube specimens were cut to a nominal length of 102 mm. The edge of each of the flat-platespecimens that was towards the original center of the laminate was chamfered. Also the tubeswere chamfered along edges that were originally internal. These internal edges were chosen toinitiate crushing in consistent regions of the specimens.

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To chamfer the flat-plate specimens, they were held in a fixture on a milling machine and atwo-flute titanium-nitride end mill was used to machine a steeple chamfer of approximately 34�

as shown in Figure 1. At least two cutting passes were made per side with the tool to removethe material. Additional passes were made until the peak of the steeple chamfers was deemedacceptable. The end mill machined the Kevlar/epoxy without fraying the ends as commonlyoccurs. To chamfer the tube specimens, they were held with a circular indexing head so thattheir central axes were parallel to the milling machine table, as shown in Figure 2. Chamferingwas accomplished by positioning the tool to chamfer a certain thickness and then rotating thecomposite tube around its central axis by rotating the cylindrical indexing head. Dimensionalvariations in the tube specimens resulted in a less-precise machining of the chamfer than wasaccomplished with the flat-plate specimens.

The length, width, thickness, and mass of the flat-plate specimens and the length, diameter,thickness, and mass of the tube specimens were measured and averaged. These values wereused to determine the densities of the specimens. Measurements are provided in Table 1. Thelow coefficients of variation (CVs) indicate the relative uniformity of the specimens.

Experimental Testing

All specimens were tested on a 220 kip hydraulic testing machine. The flat-plate specimenswere crushed using the fixture shown in Figures 3 and 4 [7]. This portion of the fixture inFigure 3 is an extension of the hydraulic grip and allows for the testing of specimens of differentthicknesses with alteration of only the restraining plate. The edge constraints move with thespecimen eliminating that source of friction. One drawback of this fixture is that the specimenmust split along its length at the constraints provided by the guide rods as shown in the photographof tested flat-plate specimens (Figure 5). The ruler in the photograph is 152 mm long. The centerportion of the flat-plate specimen is crushed against the crush plane, illustrated in Figure 4, whilethe edges are gripped by the support rods to stabilize the specimen and pass by both sides ofthe crush plane. The crush plane is positioned between the bottom edge of the specimen andthe rod tip support plate while resting on the bottom portion of the test fixture.

The fixture was originally developed to crush flat specimens at an angle of incidence, �,as shown in Figure 4. In the present test program, the load incidence angle is set to 0�. Twoseparate fixtures were used, one for each specimen width. The fixture used for the 64-mm-widespecimens was the same as that used in Reference 7. It has a gap of 38.1 mm between the edgeconstraints. The fixture used for the 89-mm-wide specimens has a gap of 63.5 mm between theedge constraints. The edge constraints are 12.7 mm diameter hardened steel. During the courseof testing, the four rods of the fixture for the testing of 89-mm-wide flat-plate specimens werestrain gaged to determine to what extent the axial load was being carried through the fixture.The gages were oriented longitudinally midway along the length of the rods opposite the pointof contact with the specimen. The tubes were crushed between self-aligning platens. All of thespecimens were crushed at a constant stroke rate of 0.0635 mm/s. Load, stroke, and strain datawere recorded every 0.5 s using a computer data acquisition system.

During initial testing of the flat-plate specimens, cracking occurred along the constraints ofthe test fixture and the sides of the crush plane. The Kevlar/epoxy flat-plate specimens were

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resistant to tearing along the constraints and material bound between the support rods and alongthe crush plane resulting in Euler buckling of the specimens. Because of the friction causedby the binding, the measured load increased rapidly with stroke. To ease the tearing of thespecimen along the constraint rods and to prevent the specimen material from binding withinthe test fixture, the specimens were altered. The flat-plate specimens were scored along the pathof desired tearing adjacent to the location of the constraint rods. Scores were machined into thespecimens with a jeweler’s blade of thickness 0.154 mm. Scores were machined into both sidesof the flat-plate specimens and their approximate locations are shown in Figure 6. The distancebetween the score lines was set to the width of the crush plane. Eight of the 64-mm-widespecimens and all 10 of the 89-mm-wide specimens were tested with scoring.

Specimens were scored to varying depths from 1/5 to 2/3 of the thickness of the plate. Themachining of the score lines was difficult because of variations in the thickness of the specimen.The thickness varied by up to 0.25 mm as indicated by repeated positioning of the jeweler’sblade to the surface of the specimen. An average zero was determined. Scoring and testing ofspecimens was done sequentially. After each test was complete, the failure mode of the testedspecimen was assessed and, using this knowledge, the scoring depth of the next specimen wasadjusted as necessary.

Results and Discussion

The primary data obtained from a crushing test is the load versus stroke curve. A schematicrepresentation of such a curve for a composite material is shown in Figure 7 where three separateregions are highlighted. The first region is the initial linear-elastic response of the structureterminated by the initiation of damage. The peak load divided by the cross sectional areaprovides the initiation stress. In the second region, damage spreads across the entire specimenand transitions into the third region which is characterized by a sustained crushing load.

The load-stroke curves are inspected to determine the point at which the crushing hadtransitioned into a sustained mode. The total energy absorbed from that point until the end ofthe test or a chosen end point is then used to calculate the SSCS. For comparison purposes,the instantaneous load is converted into a specific crushing stress (SCS) by dividing it by thecrushing area and the material density.

In the flat-plate specimens, a transition region could not be identified. Instead the flat-platespecimens immediately began stable crushing, although the load extremes were much greaterthan those seen in the tube specimens. For all flat-plate specimens, a stroke of 17.8 mm wasassumed to be the start of the sustained crushing region for the calculation of the SSCS. Duringtesting of the flat-plate specimens, the specimens would bow ahead of the point of crushingof the specimen on the crush plane. At a stroke slightly greater then 50.8 mm, this bowingcurvature would approach the clamped condition of the gripped portion of the specimen. Thus,the sustained crushing region was taken to end at 50.8 mm of stroke. An exception was madefor the first four flat-plate specimens. An endpoint of 38.1 mm was used to calculate the SSCSbecause the total stroke of each of these tests was only slightly greater then 38.1 mm. TheSSCS was determined by numerically integrating the area beneath the load-stroke curve withinthe sustained crushing region and then dividing this energy by the mass of the crushed volume

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as prescribed by Equation 1. Table 2 includes the initiation stress and the SSCS of all thespecimens tested successfully. For each group of specimens the average is provided as well asthe coefficient of variation.

Three failure modes were exhibited by the flat-plate specimens, and they were a functionof scoring depth. One failure mode consisted of stable crushing with a constant amplitudesinusoidal buckling evident. Score depths for those specimens were 0.36 mm for the 64-mm-wide specimens and 0.53 mm for the 89-mm-wide specimens. After the load reached an initialpeak the specimen would bow out-of-plane above the crush plane. The chamfered edge againstthe crush plane would begin to move out of plane in the opposite direction of the bow. Thisresponse would be accompanied by slight “popping” sounds. As a buckle developed, the materialwould begin to fold back over itself. This behavior would be accompanied by occasional loudcracking noises and the appearance of white linear bands across the width of the specimen onthe crest of the buckle which indicated internal delaminations. Occasionally, outer plies woulddelaminate along the width of the crest of the buckle. Then, the process of a new buckleforming on the other side of the specimen would begin. Successive buckles would continueto form resulting in the deformation shown in Figure 5. This failure mode is consistent withdescriptions of previous testing on Kevlar/epoxy 1� cone specimens [5]. The buckles of the89-mm-wide flat-plate specimens were of larger amplitude and wavelength then the buckles ofthe 64-mm-wide flat-plate specimens. This behavior can be seen by comparing the frequency ofthe load-stroke curves for typical 64-mm-wide flat-plate and 89-mm-wide flat-plate specimensin Figures 8 and 9, respectively.

Another failure mode consisted of the specimen buckling or bending exclusively to one sideduring all or most of the test. This failure behavior was indicative of too great of a score depthresulting in a loss of stability. This failure mode was only seen in the 89-mm-wide flat-platespecimens and may indicate that the wider 89-mm specimens are less stable. Figure 10 showsa photograph of a specimen which exhibited this type of failure mode.

The final failure mode consisted of the specimen exhibiting sinusoidal buckling, but withbinding of the specimen in the test fixture resulting in a rapidly climbing load. The specimenwould deform along the constraints and the material within the constraints that had been strokedbeyond the crush plane would bend towards the center of the plate. Figure 11 shows a photographshowing both a 64-mm-wide flat-plate and a 89-mm-wide flat-plate specimen which experiencedthis binding failure mode. This failure mode was also occasionally accompanied by bucklingalong the constraints; these were most predominantly visible on 64-mm-wide flat-plate specimens.

Figure 12 shows a photograph illustrating the buckling of a 64-mm-wide flat-plate specimen.This failure mode was indicative of a shallow score depth. A typical load-stroke plot for a flat-plate specimen that experienced this binding failure mode is shown in Figure 13. The SSCSdepicted on the plot was calculated using the same sustained crushing region. Friction from thetest fixture at the crushing plane increases this measured value substantially. Thus, for this typeof failure mode, the SSCS value cannot be applied as a performance indication.

Evaluation of the strain gage data from the rods on the 89-mm-wide flat-plate test fixtureshowed only minor strain in the top portion of the test fixture. The strain versus stroke plotsfor typical tests with the bending, sinusoidal buckling, and specimen binding failure modes are

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shown respectively in Figures 14, 15, and 16. The strain readings are greatest for the specimenbinding failure modes case and indicated that the specimen was pushing against the constraintrods. This force resulted in a bending moment on the constraint rods. Loads applied by thespecimens to generate the maximum strains over the entire stroke for each failure mode are givenin Table 3. The loads were calculated by modeling the constraint rods as clamped-clamped beamswith a point loading acting normal to the constraint rods at the gage location. The loads indicatedare an order of magnitude less than the sustained crushing loads (about 6000N). This indicatesa low value of loading of the fixture during testing.

The effect of the score depth was substantial. A survey of SCS versus stroke curves isshown in Figure 17. Different score depths from zero to 0.71 mm illustrate the variety of failuremechanisms from binding to sustained crushing to bending. The effect on the measured SSCSvalue corresponds to the failure mode as shown in Figure 18. The three failure modes were easilyidentified and the specimens that exhibited either bending or binding within the test fixture areexcluded in further discussions. For the 64-mm-wide flat-plate specimens a total score depth ofapproximately 1/2 of the specimen thickness (0.53 mm on each side) resulted in a successful test.For the 89-mm-wide flat-plate specimens a total score depth of approximately 1/3 of the specimenthickness (0.36 mm on each side) resulted in a successful test. The variation of failure modes inthe 89–mm-wide specimens at a score depth of approximately 0.36 mm is due to the differencebetween the nominal score depth and the as-machined depth which could not be measured overthe entire specimen. Out of eight 64-mm-wide specimens, four were deemed to have failed inan acceptable manner. Of the ten 89-mm-wide specimens, four were also deemed acceptable.

From the SSCS data presented in Table 2, the values for the 64-mm-wide and 89-mm-wideflat-plate specimens are nearly equal. This would indicate that whatever the contribution of theconstraints is, it is small and can be neglected. This is only possible because of the dominantcrushing mechanism is maintained within the widths tested. Note also, that this similar behavioris achieved only after effectively scoring the specimens to facilitate the fracture at the constraints.

The crushing failure of the 114-mm-diameter tube specimens was consistent with that ofthe flat-plate specimens that failed in the preferred failure mode. Sinusoidal buckling failuremodes were exhibited around the circumference. Figure 19 shows a photograph of a typicalcrushed tube specimen showing sinusoidal buckling. The width of the buckles was generallya small fraction of the total circumference and the buckles were generally not in phase. Thus,the SCS versus stroke plot shown in Figure 20 does not exhibit the same sinusoidal patternevident in the successfully tested flat-plate specimens. Instead, during steady crushing the loadfluctuation is small.

Comparison of the SSCS for the flat-plate and tube specimens is made in Table 4. Alsoshown are SSCS values for previously tested graphite/epoxy specimens of the same geometryfrom Reference 12. The flat-plate and tube SSCS values for the Kevlar/epoxy specimens arewithin 2% of each other, while the graphite/epoxy specimens are within 10% of each other.Although the SSCS is reported to be a function of D/t for both graphite/epoxy and Kevlar/epoxylaminates [9], the effect in the Kevlar/epoxy specimens appears to be minimal. The correspondingD/t ratios are 52.7 and 47.4 for the Kevlar/epoxy and graphite/epoxy specimens.

Another comparison can be made with previous work done by Knack [5]. Knack tested 1�

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tapered Kevlar/epoxy cones of stacking sequence [+45/-45/0]S manufactured from preimpreg-nated tape. Tested at a load inclination of 0�, the SSCS values averaged 40.5 kN-m/kg. Thisvalue is almost 20% greater then the value obtained for the tubes of the present study. Thedifference in the SSCS values between the two studies can be attributed to layup differencesand thickness effects. One-third of the cone laminates were 0� plies, while one-fourth of thetube laminates were 0� plies. The greater percentage of the 0� plies in the cones helps accountfor the differences in SSCS between the two studies. Also, as noted in Reference 6, the SSCSvalues decreased 10–25% for graphite-Kevlar plate specimens when thicknesses were scaled bya factor of two. The Knack Kevlar/epoxy specimens consisted of six plies of preimpregnatedtape, while the present specimens consisted of eight plies of preimpregnated cloth. The Knackspecimens were substantially thinner than those of the present study. Therefore, if the scalingresults for the graphite-Kevlar plates holds true for the Kevlar/epoxy tubes and cones of thepresent and Knack’s study, then the differences in SSCS values can be partially explained bythe differences in thicknesses of the laminates.

Conclusions

Flat-plate and tube specimens manufactured of Kevlar/epoxy preimpregnated fabric withequivalent layup and thickness were quasi-statically crushed. The energy absorbency of thespecimens were measured and compared. In addition, the present data were compared withexisting data. Based on the experimental observations and the comparisons among the datagroups the following conclusions can be made.

1. Testing of flat-plate Kevlar/epoxy laminates with the present fixture is possible in spite ofthe material being resistant to tearing at the crushing plane.

2. Three distinct failure modes in the Kevlar/epoxy flat-plate specimens are observed: stablesinusoidal buckling, binding of specimens with test fixture because of shallow scoring, andbending of specimens to one side because of deep scoring. The failure modes are easilydistinguishable from each other both visually and via the load versus stroke curves. Onlysinusoidal buckling is deemed to be indicative of a stable crushing failure mechanism.

3. For the 64-mm-wide flat plate, a score depth of approximately 1/2 of the specimen thicknessleads to a successful test. For the 89-mm-wide flat plate, a score depth of approximately 1/3of the specimen thickness leads to a successful test for this layup.

4. The load applied to the fixture by the specimen during the testing is much less than themeasured crush loads.

5. The specific sustained crushing stress for Kevlar/epoxy flat plates with a layup of(+45/–45/0/90)S is not affected by the width of the plate using the present fixture.

6. Flat-plate and tube geometry SSCS values are similar for the present data. Because thematerial represents a testing challenge, this result indicates the capability of the presentfixture to determine the energy absorption capability of composite materials.

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The ability to test Kevlar/epoxy composite laminates consisting of fabric laminae with aflat-plate specimen has been shown utilizing the present fixture. Binding between the specimenand the test fixture can be eliminated for this test fixture by scoring the flat-plate specimensalong the constraint rods. Comparable SSCS values for the flat-plate and tube specimens werefound. The flat-plate specimen is a capable, cost-effective alternative to the tube geometry fortesting the energy absorbing capabilities of composite laminates.

Acknowledgments

The material used in this study was graciously provided by Dr. Steven Peake of CYTEC.The authors wish to thank Mr. James S. Harris for his help in this effort.

References

1. Sen, J. and Dremann, C., “Design Development Tests for Composite crashworthy HelicopterFuselage,” SAMPE Quarterly, Vol. 17, No. 1, Oct. 1985, pp. 29-39.

2. Fleming, D. and Vizzini, A., “Tapered Geometries for Improved Crashworthiness under SideLoads,” Journal of the American Helicopter Society, Vol. 17, No. 1, Jan. 1993, pp. 38-44.

3. Farley, G., “Energy Absorption of Composite Materials,” Journal of Composite Materials,Vol. 17, No. 5, May 1983, pp. 267-279.

4. Hull, D., “A Unified Approach to Progressive Crushing of Fibre-Reinforced CompositeTubes,” Composites Science and Technology, Vol. 40, 1991, pp. 377–421, .

5. Knack, J. and Vizzini, A., “Energy Absorption of Truncated Kevlar Epoxy Cones Under SideLoads,” Proceedings of the 35th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynam-ics and Materials Conference, Hilton Head, South Carolina, Apr. 1994, pp. 2831–2837.

6. Jackson, K., Morton, J., Lavoie, J., and Boitnott, R., “Scaling of Energy AbsorbingComposite Plates,” Journal of the American Helicopter Society, Vol. 39, No. 1, Jan. 1994pp. 17–23.

7. Fleming, D. and Vizzini, A., “The Energy Absorption of Composite Plates under Off-AxisLoads,” Journal of Composite Materials, Vol. 30, No. 18, 1996, pp. 1977–1995.

8. Thornton, P. and Edwards, P., “Energy Absorption in Composite Tubes,” Journal of Com-posite Materials, Vol. 16, No. 11, Nov. 1982, pp. 521–545.

9. Farley, G., “Effect of Specimen Geometry on the Energy Absorption Capability of CompositeMaterials,” Journal of Composite Materials, Vol. 20, No. 7, July 1986, pp. 390-400.

10. Farley, G., “Energy Absorption of Composite Material and Structure,” Proceedings of theAHS 43rd Annual Forum, St. Louis Missouri, May 1987, pp. 613–627.

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11. Lavoie, J. and Morton, J., “A Crush Test Fixture for Investigating Energy Absorption of FlatComposite Plates,” Experimental Techniques, Nov./Dec. 1994, pp. 23-26.

12. Dubey, D. and Vizzini, A., “Energy Absorption of Composite Tubes and Plates,” Journal ofComposite Materials, Vol. 32, No. 2, 1998, pp. 158-176.

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34°

17°

Composite Specimen

Aluminum Guide Plates

Clamp Fixtured on Mill

Grinder or Cutter held in Mill

900-1500RPM

Figure 1 Chamfering of flat-plate specimens

Dubey & Vizzini Testing Methods JAHS–1

Grinder or Cutter held in Mill

Circular Indexing Head Fixtured on Mill

34°

17°

Composite Specimen

Aluminum Mandrel

114 mm

Figure 2 Schematic section of chamfering of tube specimens


Compositespecimen

Holes for attachmentof gripping fixture

to testing machine grips

Subassembly 1

Rod tipsupport plate

Figure 3 Gripping portion of crushing fixture


CrushingPlane

Spacers

φ

Figure 4 Incline feature of crushing fixture


Figure 5 Photograph of crushed flat plate specimens


165

mm

Scoring

70 m

mGrip Inserts

SteepleChamfer

Support Rods

38.1 or 63.5 mm

Figure 6 Use of scoring to ease tearingof specimen along support rods


0 Stroke

0

Load

Damage Initiation

Linear Elastic (1)

Crushing (3)

Transition (2)

Figure 7 Schematic representation of a crushing test


0 25 50Stroke, mm

0

50

100

SC

S, k

N-m

/kg

SSCS = 35.7 kN-m/kg

Figure 8 Typical specific crushing stress (SCS) versusstroke for 64 mm flat plate (preferred failure mode)


0 25 50Stroke, mm

0

50

100

SC

S, k

N-m

/kg

SSCS = 31.5 kN-m/kg

Figure 9 Typical specific crushing stress (SCS) versusstroke for 89 mm flat plate (preferred failure mode)


Vise

Specimen

Figure 10 Photograph of crushed 89–mm-wideflat-plate specimen (bending to one side failure mode)


Figure 11 Photograph of crushed 64–mm-wide flat-plate and89–mm-wde flat-plate specimens (binding failure mode)


buckling

Figure 12 Photograph of crushed 64–mm-wide flat-platespecimen (binding failure mode) exhibiting buckling


0 25 50Stroke, mm

0

50

100

SC

S, k

N-m

/kg

SSCS = 52.1 kN-m/kg

Figure 13 Typical specific crushing stress (SCS) versus strokefor 89–mm-wide flat-plate exhibiting a binding failure mode


0 20 40 60Stroke, mm

-100

0

100

200

300

400

Str

ain,

mst

rain

Figure 14 Typical fixture strain versusstroke for bending failure mode



0

200

400

600

800

Str

ain,

mst

rain

Figure 15 Typical fixture strain versusstroke for sinusoidal buckling failure mode



0

500

1000

1500

2000

Str

ain,

mst

rain

Figure 16 Typical fixture strain versusstroke for binding failure mode


0 25 5000

50

100

0

No Score

0 25 5000

50

100

0

0.36 mm

0 25 5000

50

100

0

0.53 mm

0 25 5000

50

100

0

0.71 mm

Stroke, mm

SC

S, k

N-m

/kg

Figure 17 The effect of score depth on theload stroke behavior (64-mm-wide specimens)


0.0 0.2 0.4 0.6Score Depth, mm

0

20

40

60

80

100

SS

CS

, kN

-m/k

g

64 mm89 mm

Binding

Valid

Bending

Figure 18 SSCS and failure mode versus score depth


Figure 19 Photograph of crushed tube


0 25 50Stroke, mm

0

50

100

SC

S, k

N-m

/kg

SSCS = 33.5 kN-m/kg

Figure 20 Typical specific crushing stress (SCS)versus stroke for 114 mm diameter tube


Table 1 Kevlar/epoxy Average Measurements

Thickness,mm(CV)

Length,mm(CV)

Width,mm(CV)

InnerDiameter, mm

(CV)

Density,g/cc(CV)

64-mm-wide63.9

(0.1%)

89-mm-wide

2.11(1.8%)

165.6(0.2%) 89.3

(0.0%)

—1.41

(0.4%)

114 mm tube2.16

(1.2%)101.9(0.6%)

—113.8(0.1%)

1.43(1.3%)


Table 2 Initiation and Energy Absorption

Geometry Initiation Stress [MPa] SSCS [kN-m/kg]

Flat Plate64 mm

75.381.683.787.0

29.135.737.934.7

Average(CV)

81.9(6.0%)

34.3(10.8%)

Flat Plate89 mm

61.370.666.866.0

33.431.534.433.6

Average(CV)

66.2(5.7%)

33.2(3.8%)

Tubes114 mm

83.279.977.990.684.086.2

36.034.135.733.534.732.4

Average(CV)

83.6(5.4%)

34.4(3.9%)


Table 3 Loads exerted by the specimens on the bending rods

Failure Mode Maximum Strain, strain Load, NBending 313 97.7Sinusoidal Buckling 788 246.3Binding 1850 578.6


Table 4 Overall Comparison of SSCS

Flat-Plates [kN-m/kg](CV)

Tubes [kN-m/kg](CV)

PresentKevlar/epoxy

33.8(7.8%)

34.4(3.9%)

Previousgraphite/epoxyReference 12

77.6(16.1%)

86.0(2.1%)


energy absortion by kevlonorepoxy

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