Impact response of high density flexible polyurethane foam(2)

2.Material and experimental

Planar impact experiments with the polyurethane foam were performed with59-mm bore4-m long gas gun of the Laboratory of Dynamic Behavior of Materials at Ben-Gurion University.The8.9 (Æ0.1)-mm thick sheets of high density(r0¼409Æ4kg=m3,some 65e66%open porosity)flexible polyurethane foam were received from PLASAN Ltd.,Sasa,Israel.The structure of the studied foam,the dense packing of interconnected hollow spheres of100e150-micron diameter,is shown in Fig.1a.Prior the impact experiments,the foam was tested in quasi-static compression using a5587Instron testing machine equipped with the Instron2501150-mm compression platens and an Instron2601deflection sensor.The results of this test with a55mm thick(6layers of8.9-mm thick)foam sample are shown in Fig.1b.

The presently studied material has a stress-strain diagram that is typical for foams[1],with an inflection point at approximately 0.13MPa and collapse stress,determined as shown in the insert of Fig.1b,equal to0.07MPa.The initial,straight,segment of the diagram has a slope of about0.78MPa which may be interpreted as the foam’s Young’s modulus.

For8of10planar impact tests(marked as PFA to PFH in Table1) both the samples and the impactors were prepared from60Â60mm2square pieces cut from the foam sheets.The projectiles equipped with the foam impactors were prepared according to the following procedure:the foam square was glued(two-component DEVCON5min epoxy)to the front part of55-mm diameter,11.8-mm thick plane-parallel polymethylmethacrylate(PMMA)disk,and the foam surplus over the55-mm diameter was removed by turning.In order to provide shorting of the velocity and trigger electrical charged pins,the14-m thick aluminum foil ring was glued,using the same epoxy,to the front surface of the foam impactor.The back surface of the PMMA disk was glued with Loctite Super glue to the front edge of the hollow aluminum cylinder sabot.Finally the rear edge of the sabot was closed with a PMMA lid with an O-ring.In these eight tests and in the ninth test PFE1where the sample was made of 5.5-mm(instead of8.9-mm)foam layer,a similar sample assembly was used.The rear surface of the square foam sample was glued on a polyvinyl chloride(PVC)100-mm diameter and5-mm thick disc with a45-mm diameter central hole.In order to provide reflection of VISAR beam,a piece of14-m aluminum foil was glued on the rear surface of the sample.The disk with the glued sample wasfixed on the base ring of the double-tilt sample holder,whose parallel orientation to the front of the projectile had been preliminarily adjusted with an accuracy of0.1mrad.The schematics of these experiments are shown in Fig.2a.In the tenth experiment,aimed at measuring the dynamic tensile(spall)strength of the foam,the impact was produced by a free-of-foam PMMA disk(primary impactor)on the1-mm thick foam sheet(secondary impactor) separated from the sample by a spacer ring of5-mm thickness.As result,the foam sample(Fig.2b)was struck by a thin foam impactor.

The impact velocity,ranged from43.5to605m/s,was controlled by electrical charged pins.The uncertainty of the measurement of the impact velocity did not exceed1%of the measured velocity value.The impactor-sample misalignment controlled by the trigger pins did not exceed1mrad in all experiments.Depending on the impact strength,the velocity of the rear sample surface was moni-tored by VISAR with delay lines providing velocity constants of96.4, 224.0,and407.2m/s per fringe.The parameters of the ten planar impact experiments are listed in Table1.

3.Experimental results

The VISAR-recorded velocity histories obtained after symmetric planar foam e foam impacts are shown in Fig.3a e c.

Except for the waveform obtained after the weakest impact,the waveforms shown in Fig.3are characterized by a two-wave structure marked as P1and P2in Fig.3a.The PFE1test was per-formed in order to show that the presence of the P2wave is caused by interaction of the unloading wave generated at the rear surface of the sample,with the reloading wave generated at the interface between the foam impactor and the PMMA backing.The stress s-particle velocity u diagram and the time t-distance(Lagrangian)h diagram of Fig.4are to illustrate such interaction in the case of the PFE and PFE1tests.Since both the samples and the impactors are made of the same foam,the particle velocity u1behind the shock fronts P1propagating through both the sample and impactor with velocity U S is equal to one half of the impact velocity.Accepting for both shocks the same impact velocity equal to v0¼312m/s,yields for the particle velocity u1¼156m/s.The amplitude of this impact-generated shock is s1(Fig.4a).At the arrival of the shock at the sample free surface it acquires velocity equal to196m/s.In the absence of the PMMA backing,the rear surface of the foam impactor should be decelerated from312m/s to106m/s.The presence of the PMMA backing results in the reloading of the impactor material from state with stress s1to state with stress s2

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Fig.1.SEM image of the cross-section of the studied polyurethane foam(a),and stress-strain diagram obtained after quasi-static compression tests performed with a55-mm initial thickness foam sample.The strain rate in the test was_ε¼1:5Â10À3sÀ1.The insert shows the determination of the collapse stress.

E.Zaretsky et al./International Journal of Impact Engineering39(2012)1e7

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