On Thresholds for Dynamic Strength in Solids

The limits of elastic behaviour change with the nature of the impulse applied to a target and the size of volume interrogated by a measurement, since it is the pre-existing defects sampled within its rise that determine the response observed. This review considers a range of solids of different material classes and tracks the development of the strength of the material during shock loading, from yield at the Hugoniot elastic limit, across the weak shock regime, to its transition to strong shock behaviour. It is shown that at this stress, the weak shock limit (WSL), the shear component of the applied stress exceeds the theoretical strength of the material. Beyond this threshold, there are a number of new responses that confirm a transition from an inhomogeneous to a homogeneous state. Further, whilst strength rises across the weak shock regime, it saturates at the WSL. For instance, failure in shocked glasses transitions from localised fracture initiated at target boundaries to a global failure at this threshold at the theoretical strength. Sapphire′s strength asymptotes to the theoretical strength of the strongest direction in its lattice. Finally, the fourth-power dependence of strain rate upon stress appears to be a consequence of the homogeneous flow in the strong shock regime. This review suggests that µ/2π is a good approximation for the unrelaxed theoretical strength of solids at increasing stresses beyond the WSL. The methodology unfolded here represents a new means to experimentally determine the ultimate shear strength of solids.

Physicomechanical and design characteristics of surface high-entropy alloys

We investigated the phase composition of high-entropy shape memory alloys CoCuTiZrHf, NiCoTiZrHf, NiCuTiZrHf, ZrCuNiCoTi, TiNiZrHfCoCu after HVOF, thermomechanical treatment and thermal cycling. We have also developed criteria for calculating the energy capacity, enthalpy, entropy, theoretical strength of high-entropy alloys, taking into account the chemical composition and structural-phase state according to the results of X-ray phase analysis. We proposed a criterion for assessing the damageability of high-entropy alloys. The article describes calculations of energy consumption, entropy, enthalpy, theoretical strength of high-entropy alloys CoCuTiZrHf, NiCoTiZrHf, NiCuTiZrHf, ZrCuNiCoTi, TiNiZrHfCoCu both after HVOF and after thermomechanical treatment. After full-scale mechanical tests of surface-modified layers of high-entropy materials for bending with rotation, we obtained experimental values of cyclic durability and plotted fatigue curves. The calculated assessment of the cyclic durability was carried out according to the developed mathematical criteria. We calculated the damageability of high-entropy alloys and constructed the graphical curves of the dependence of the damageability on the cyclic life.