Crack propagation in single crystals of tungsten

Abstract

The effect of pre-existing micro-cracks on the fracture of [010] tungsten single crysta's has been investigated. Cleavage cracks on (010) planes were introduced into the edge of the specimens by a spark-machining technique. Tensile tests at 77°K showed that the fracture stress was dependent on the crack length according to the Griffith criterion. At this temperature the specimens failed in a completely brittle manner. A surface energy of 6300 ergs cm⁻² was obtained in close agreement with the theoretical value for the true surface energy. Metallographic examination of the fracture surfaces showed that the markings on the surface depend on the stress level at fracture. One test was carried out to determine the effect of prestraining at 295°K on the subsequent fracture behaviour at 77°K. The effect of temperature on the fracture of specimens containing a constant crack length was determined between 77 and 478°K. As the temperature increased, the crack propagation mechanism changed from fast cleavage to slow cleavage propagation, and at higher temperatures to ‘ductile cleavage’ propagation. The effect of temperature is discussed in terms of the blunting effect of plastic deformation.