Abstract
The effect of prestrain on the propagation of cracks in [010] single crystals of tungsten has been studied. In the first series of experiments, crystals containing a spark-induced microcrack were deformed at 295°K to various stress levels both below and above the limit of proportionality. The brittle-fracture stress at 77°K increased as a function of the amount of prestrain. This increase has been rationalized on a model based on the concept that the blunting of the crack due to prestrain is equivalent to a compressive stress on the initial crack. Etch-pitting experiments supported the tensile results in that no dislocation multiplication at the crack tip was detected until stresses greater than the limit of proportionality were applied. A second series of experiments has been performed to investigate the effect of initial dislocation density on crack propagation by prestraining crystals various amounts at 295°K before introduction of the microcrack and subsequent fracture at 77° K. The fracture stress at 77°K was independent of the initial dislocation density, and the appearance of the fracture surface was dependent only on the stress at fracture.
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