Dynamic Constitutive and Failure Behavior of a Two-Phase Tungsten Composite

The constitutive response and failure behavior of a W-Ni-Fe alloy over the strain rate range of 10-4 to 5 X 105 s-1 is experimentally investigated. Experiments conducted are pressure-shear plate impact, torsional Kolsky bar, and quasi-static torsion. The material has a microstructure of hard tungsten grains embedded in a soft alloy matrix. Nominal shear stress-strain relations are obtained for deformations throughout the experiments and until after the initiation of localization. Shear bands form when the plastic strain becomes sufficiently large, involving both the grains and the matrix. The critical shear strain for shear band development under the high rate, high pressure conditions of pressure-shear is approximately 1–1.5 or 6–8 times that obtained in torsional Kolsky bar experiments which involve lower strain rates and zero pressure. Shear bands observed in the impact experiments show significantly more intensely localized deformation. Eventual failure through the shear band is a combination of grain-matrix separation, ductile matrix rupture, and grain fracture. In order to understand the effect of the composite microstructure and material inhomogeneity on deformation, two other materials are also used in the study. One is a pure tungsten and the other is an alloy of W, Ni, and Fe with the same composition as that of the matrix phase in the overall composite. The results show that the overall two-phase composite is more susceptible to the formation of shear bands than either of its constituents.

The Rheology of Lubricants at High Shear Rates

A modified torsional Kolsky bar is used to shear a thin annular layer of lubricant held between concentric cylinders. The shear rates obtained range from 4 × 103 to 4 × 104 s−1; the duration of the shear pulse is about 400 microseconds. The shear stress history and the corresponding shear-rate history are deduced using standard torsional Kolsky bar techniques. This experimental technique provides the capability of measuring the shear stresses sustained by fluids at very high shear rates, while the short test duration ensures a minimal rise in temperature due to the associated shear heating. Results are presented for a synthetic lubricant (5P4E) and a mineral oil (HVI650). The lubricants are both observed to be rate-sensitive over the entire range of shear rates examined; the rate-dependence of the shear response is strongly non-Newtonian at high shear rates. Softening during the test is observed for both materials; this softening appears to be largely driven by a thermal mechanism.