Characterization of low-impedance materials at elevated strain rates

Experimental characterization of materials at elevated strain rates of the order of 10(3) s(-1) is typically performed by Hopkinson-bar-type facilities. The specific nature of Hopkinson bar tests is that a specimen is loaded by waves. As both the input and output bars are often made from high-impedance metallic materials, the ratio between transmitted and reflected wave components is very poor in the case of low-impedance specimen materials such as rubbers and foams. That poor wave transmission brings with it a limited quality of the signal-to-noise ratio. Changing the bar materials (e.g. to polycarbonate) is of little help because of the related viscous effects. A further deficiency of the wave-driven Hopkinson bar test for many rubber and foam applications is the comparably small amount of compression that is achievable. Maximum compressions of 95 per cent and above, often expected for damping and energy absorption applications, are out of the range of most Hopkinson bar set-ups. In order to overcome these deficiencies, first a split Hopkinson pressure bar (SHPB) facility was equipped with polyvinylidenefluoride (PVDF) thin-film sensors. Second, a direct-impact set-up for low-impedance materials was developed and calibrated at the Ernst Mach Institute (EMI). With the PVDF gauges, the range of application both for the SHPB and for the new set-up was enhanced. This is demonstrated with an SHPB-based investigation of stress homogeneity in polyurethane (PU) specimens. The direct-impact facility made it possible to derive the stress-strain relations for PU-rubber up to strain rates in the range of several 10(3) s(-1). Maximum compressions of more than 95 per cent were achieved. The paper describes the limitations of a classic SHPB set-up applied to low-impedance materials. The problem of stress homogeneity is addressed, and a set-up for a new direct-impact facility is presented. As an example application, a Confor-Blue foam, typically used for automotive crash-test dummies, is characterized at strain rates between 160 and 1115 s(-1)