Intercrystalline cracking, grain-boundary sliding, and delayed elasticity at high temperatures

Abstract

The hypothesis of an interrelation between grain-boundary sliding and delayed elasticity in polycrystalline materials at high homologous temperatures is used to investigate the conditions conducive to microcracking. It is known that a material may exhibit cracking activity on attaining a critical delayed-elastic strain corresponding to a critical grainboundary sliding displacement. Experimental data on ice at temperatures >0.9T m are used to verify this concept. The new criterion is then extended to develop simple, selfconsistent equations describing the interdependence of stress, strain, time, temperature, and grain size in predicting the onset of structural degradation due to microcracking and hence possible failure by fracture or rupture. The merit of the theory lies in its ability to forecast explicitly a large number of commonly observed high-temperature phenomena, including superplasticity, brittle-ductile transition, and the stress and temperature dependence of the apparent activation energy for fracture. One derivation makes it clear that cracking occurs when a critical stress depending only on temperature (and independent of grain size) is exceeded. The near constancy of fracture strain in the quasi brittle range can also be predicted