A combined experimental and computational investigation of the behavior of oxygen-free high-conductivity copper under very high shear rates is presented. Pressure-shear plate impact is used for conducting constant strain rate tests and strain rate change texts in which the specimen is strained at shear rates up to 106s−1 for 1μs and then strained at substatially lower shear rates for another microsecond. The specimen is sandwiched between two hard elastic plates to impose conditions of simple shear at very high strain rates and constant hydrostatic pressure. Marked increases in flow stresses are observed at strain rates of 105s−1 and higher. Flow stresses decrease gradually after a sharp drop in strain rate in all strain rate change tests. Homogeneous equiaxed dislocation cells are found as the predominant substructure in the deformed specimens. Theoretical analyses of the nonlinear wave propagation within the specimen are carried out using a general internal variable formulation in which the hardening rate depends on the rate of deformation. The governing system of hyperbolic partial differential equations is solved using a finite-difference scheme; computational results are compared with the experimental results. Both small- and finite-deformation formulations are considered. Only the internal variable model which incorporates a strong rate sensitivity of strain hardening is successful in describing the observed response to the change in strain rate. The enhanced rate sensitivity at high strain rates is concluded to be related primarily to the rate sensitivity of strain hardening, not the rate sensitivity of the flow stress at constant dislocation structure. The generation and evolution of dislocation cells appears to be the dominant micromechanical process during the high- rate deformation of pure metals.

Pressure-shear impact investigation of strain rate history effects in oxygen-free high-conductivity copper

The present work examines the inertial effects on void growth in viscoplastic materials which have been largely neglected in analyses ofdynamic crack growth and spallation phenomena using existing continuum porous material models. The dynamic void growth in porous materials is investigated by analyzing the finite deformation of an elastic/viscoplastic spherical shell under intense hydrostatic tensile loading. Under typical dynamic loading conditions, inertia is found to have a strong stabilizing effect on void growth process and consequently to delay coalescence even when the high rate-sensitivity of materials at very high strain rates is taken into account. Effects of strain hardening and thermal softening are found to be relatively small. Approximate relations are suggested to incorporate inertial effects and rate sensitivity of matrix materials into the porous viscoplastic material constitutive models for dynamic ductile fracture analyses for certain loading conditions.

Inertial Effects on Void Growth in Porous Viscoplastic Materials

Experimental results are presented on the dynamic plastic response of OFHC copper at strain rates of 10 5 10 6 s -1 and temperatures up to 700°C Measurements in this previously unexplored regime of high temperatures and very high strain rates are made possible by recent extensions of the pressure-shear plate impact methodology to allow testing at high temperatures Athin foil of the specimen material is sandwiched between two pure tungsten carbide plates which have sucient high temperature strength to remain elastic under the temperature and loading conditions of the experiment This target assembly is heated by an induction heating coil To overcome possible misalignment of the impact face of the target due to thermal expansion of the target supports, a laser beam reflected from the rear surface of the target is used as an optical lever to detect changes in the orientation of the target Remote controls are used to make the necessary adjustments in the orientation of the target assembly to maintain its original alignment with the impact face of the flyer plate To withstand the high temperatures, the photoresist gratings which normally provide the diffracted beams used in recording the transverse velocity of the target assembly are replaced by titanium phase gratings produced by SEM lithography Over temperatures from 300 to 700°C and strain rates from 10 5 to 10 6 s -1 the flow stress of OFHC copper increases with increasing strain rate, and decreases with increasing temperature Numerical simulations of the experiments, based on popular constitutive models, exhibit lower flow stresses than those measured in the experiments The models also fail to predict the softening that is observed at large strains in the tests at strain rates of 10 6 s -1  1998 Elsevier Science Ltd All rights reserved

High-temperature pressure-shear plate impact experiments on ofhc copper

The dynamic Mohr–Coulomb behavior of silicon carbide (SiC) was inferred from symmetric pressure/shear plate‐impact experiments which entail planar impact of two SiC plates inclined at 15° to the impact direction. The transverse velocity of the free rear surface of the target plate was recorded using a laser Doppler velocimeter system, and the experiments were simulated using a postulated viscoplastic constitutive model that accounts for comminution and dilatancy. Model parameters were varied until the computed and measured velocity histories agreed. The results indicate that comminution occurred soon after loading, and thus the experiment measures the behavior of granulated material at shear strain rates of ≊105 s−1 and mean stress ranging from 1 to 9 GPa. A friction coefficient of 0.23 was obtained, which is about half the value for quasistatic compression of precomminuted ceramic reported in the literature. The simulation results were strongly affected by the values chosen for the friction coefficient a...

The strength behavior of granulated silicon carbide at high strain rates and confining pressure

T he pressure-shear plate impact technique is used to study material behavior at high rates of deformation. In this technique, plastic waves of combined pressure and shear stresses are produced by impact of parallel plates skewed relative to their direction of approach. Commercially pure alpha-titanium and 6061-T6 aluminum are tested under a variety of pressure and shear tractions by using different combinations of impact velocities and angles of inclination. A laser interferometer system is used to monitor simultaneously the normal and transverse components of motion of a point at the rear surface of the target plate. The experimental results are compared with numerical solutions based on an elastic/viscoplastic model of the material. Both isotropic and kinematic strain hardening models are used in the computations. The results indicate that unlike the normal velocity profiles, the transverse velocity profiles are sensitive to the dynamic plastic response and, thus, can be used to study material behavior at high strain rates. For the materials tested the results suggest that the flow stress required for plastic straining increases markedly with increasing strain rate at strain rates above 104s−1. Hydrostatic pressure of the order that exists in the tests (up to 2 GPa) does not affect the plastic flow in 6061-T6 aluminum and appears to have at most a minor effect on the deformation of the titanium.

Pressure-shear waves in 6061-T6 aluminum and alpha-titanium

A second-order accurate difference method for systems of hyperbolic partial differential equations

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A second-order accurate difference method for systems of hyperbolic partial differential equations