Impact response of high density flexible polyurethane foam(4)

height with Pearson correlation coef ficient p ¼0.9998implying

more than four-fold compression of the foam at strong shocks.It is

apparent from Fig.6b that the difference between U LE and U S

decreases with the strength of the impact and for the particle

velocities higher than 200m/s,it becomes negligibly small,2e 3m/

s.The rise times of the waveform fronts (the time intervals between

0.1w 1and 0.9w 1)decrease from approximately t rt ¼126m s for the

weakest,43.5-m/s,impact to less than t rt ¼0.43m s for the strongest,605-m/s,one.The rise time of the shock front is usually associated with the material strength [14].It is plausible to assume that in the tests with relatively low impact velocity the rise time is controlled by the foam crush up,while at the stronger impacts the steepness of the shock front is controlled by the dynamic viscosity of the crushed (free of voids)foam.The linear dependence of U S on u yields the foam ’s principal

Hugoniot on the stress s -particle velocity u

plane Fig.3.Experimentally recorded waveforms obtained after "strong"(a)and "weak"(b)impacts.The corresponding impact velocities are shown along the waveforms.The waveforms recorded after stronger (72.5m/s and higher)impacts have a two-wave structure containing P1and P2waves propagating with different velocities.The 141-m/s waveform is shown in both (a)and (b)as a scale.The 311-m/s waveform (PFE test)of (a)is shown in (c)together with the velocity history recorded after impacts of similar velocity (314m/s,PFE1test)on the sample of 5.5-mm thickness.The values of the sample thickness are shown near the bottom part of the waveforms.

E.Zaretsky et al./International Journal of Impact Engineering 39(2012)1e 7

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