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

Equations to predict local intergranular failure (by ''r''-type and '' wedge''-type cavitation and the coupling between them) have been devel oped. The derivation has utilized physically based concepts such as th ermal activation of the controlling processes, wedge cracking driven b y grain boundary sliding, and cavity growth driven by diffusion. It ha s also been based upon phenomenological observations such as the varia tion in the steady-state creep rate with stress and temperature, incom plete healing of cavities under compression, and differences in life u nder ''slow-fast'' and 'fast-slow'' cycling. The model has been tested against data on the low-cycle fatigue life of 304 stainless steel und er unequal ramp rates. The new equations simulate, for example, the di fferences in life produced by slow-fast, fast-slow, and equal ramp rat e cycling in terms of their effects on infernal cavitation. Simulation s have also been generated concerning creep crack advance by cavitatio n. Together with the new equations' ability to treat monotonic creep r upture, these comparisons demonstrate that the intergranular failure e quations are capable of simulating a number of phenomena of importance in life prediction for high-temperature structures.