Dependence of dynamic fracture resistance on crack velocity in tungsten: Part 1. single crystals
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I.
INTRODUCTION
ThE study of the surface and subsurface features along a fracture path has yielded much of the current understanding of the fracture processes in materials. Characteristic features of ductile and brittle transgranular and intergranular fracture have been documented for many materials. ~ Usually, these features are considered representative of a given material under a set of mechanical and environmental conditions. Less well known are the magnitude and the rate of the local crack driving force, which affect the local response of the material associated with the surface features. Current ideas in the literature 2-6 in regard to dislocation processes at a crack tip have emphasized that the resistance to both micro- and macro-crack propagation is a function of the "shielding" properties of crack tip dislocations, which control the occurrence of a local K large enough to produce cleavage at the atomic crack tip. Since it takes a finite time to generate a dislocation across the crack tip dislocation free zone6'7 during a high rate of applied crack opening, local stress buildup may be faster than these dislocations can be activated. As a result, a substantially brittle crack propagates. In semi-brittle metallic materials, it is known 8'9 that the processes of cleavage and dislocation emission can occur simultaneously at the tip of a propagating crack. It is reasonable to expect that when crack speed is high, the emitted dislocations are under the influence of the crack stress for a short time, effectively giving rise to a short distance of travel per unit time (i.e., average dislocation speed is low). This results in a small local plastic strain and a low drag force on the propagating crack tip. The grown-in sources would also have little time to be under the influence of the moving crack stress field, if the crack speed is high. While a number of dynamic crack resistance experiments were performed in structural steels, ~~ LiF, 14 and Zn, ~s we are unaware of previous such measurement on bcc single crystals, where the dependence of flow stress on the rate of strain is high. Some comments relating surface features in tungsten single crystals over a range of fracture stresses at JOHN M. LIU, formerly with the State University of New York at Stony Brook, is now with Materials Evaluations Branch (R34), Naval Surface Weapons Center, Silver Spring, MD 20910. B.W. SHEN, formerly with the State University of New York at Stony Brook, is now with Semi-Conductor Processing Laboratory, Texas Instruments, Inc., Dallas, TX 75215. Manuscript submitted February 11, 1983.
METALLURGICALTRANSACTIONS A
different temperatures were made by Hull and Beardmore. 16 By means of Waller lines on the smooth part of the fracture surface, they estimated the crack velocity for different specimens. We report here some direct measurements of crack velocities on {100} in tungsten at room temperature. By examining the fracture surface features, we attempt to find the source of the crack propagation resistance at a range of crack speeds a
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