A PHENOMENOLOGICAL MODEL FOR INTERGRANULAR FAILURE BY R-TYPE AND WEDGE-TYPE CAVITATION

Abstract

Equations to predict local intergranular failure (by ''r''-type and ''wedge''-type cavitation and the coupling between them) have been developed. The derivation has utilized physically based concepts such as thermal activation of the controlling processes, wedge cracking driven by grain boundary sliding, and cavity growth driven by diffusion. It has also been based upon phenomenological observations such as the variation in the steady-state creep rate with stress and temperature, incomplete healing of cavities under compression, and differences in life under ''slow-fast'' and 'fast-slow'' cycling. The model has been tested against data on the low-cycle fatigue life of 304 stainless steel under unequal ramp rates. The new equations simulate, for example, the differences in life produced by slow-fast, fast-slow, and equal ramp rate cycling in terms of their effects on infernal cavitation. Simulations have also been generated concerning creep crack advance by cavitation. Together with the new equations' ability to treat monotonic creep rupture, these comparisons demonstrate that the intergranular failure equations are capable of simulating a number of phenomena of importance in life prediction for high-temperature structures.