No abstract

The equation of state and the mechanical response (dynamic tensile strength and dynamic shear strength) of the shape memory alloy NiTi have been investigated using plate impact. The Hugoniot has been extended with additional data and a nonlinear behavior of the Hugoniot (shock velocity-particle velocity) has been noted. A bilinear representation has been proposed for the trend. These two behaviors were attributed to the shock-induced phase transformation from B2 to monoclinic. However, this phase transformation seems to have no influence on the dynamic tensile strength. A minimum impact stress value was found necessary to create the spallation in NiTi but the pull back stress remains near constant above this value. A negative strain-rate dependency was also noted on the spallation. The shear strength (τ) of NiTi appears to increase continuously with the impact stress. The evolution of τ behind the shock front seems to be linked to the phase transformation observed in determination of the Hugoniot. Indeed, It was observed that there was no change of τ until an impact stress value corresponding to the inflection point on the Hugoniot, followed by increase behind the shock front above this value.

The shock response of a two-dimensional glass-fibre–epoxy composite has been investigated in terms of orientation of the fibres to the loading axis. Results show that when shocked in the through thickness orientation (fibres normal to the loading axis), the resultant shock pulses are similar to many other monolithic materials, and the shock velocity–particle velocity is linear. Examination of the spall response as a function of specimen thickness and the release velocities has also led us to suggest that the shock pulse generates damage behind it, which accumulates as a function of pulse duration. In contrast, when shocked along a fibre direction, the shock pulses have a strongly ramped nature, which steepens to a shock at higher stress amplitudes. Analysis of the wave velocity–particle velocity behaviour has led us to suggest that a low amplitude stress pulse is transmitted along the fibres, with the higher amplitude remainder being transmitted in the ‘matrix’—i.e. the epoxy binder and fibres normal to the loading axis between them. Such a response has been observed in other fibre-based composites, and it is believed that this is due to the structural configuration of such materials.