DWELL EFFECTS IN ISOTHERMAL AND THERMOMECHANICAL FATIGUE OF ADVANCED MATERIALS

Effects of dwell-times in the isothermal fatigue (IF) and thermal-mech anical fatigue (TMF) behaviors of six advanced high temperature materi als are investigated in this paper; two of which belonged to types SS 304L and SS 304 stainless steels, two tantalum alloys; T-111 and ASTAR 811C, pure nickel Ni 201 and a nickel based, single crystal superallo y PWA 1480. The SS 304 and 304L steels were found to be sensitive unde r tensile dwells, however, effect of dwell-times was found to saturate with the increase in strain range (inelastic or total) for all materi als examined. At lower strain ranges the effects of dwell-times were f ound to produce lower lives than at higher strains as in the case of A ISI SS 304 and two tantalum based alloys T-111 and ASTAR 811C. Trends in various normalized life curves were found to be near sigmoidal for ASTAR 811C, in which a typical inflection point could be found at near ly 0.2% inelastic strain range. Effects of dwell-times were more delet erious below that strain range and above 0.2% inelastic strain range t he effects of dwell-times were nearly the same. Mechanistic aspects un der different test conditions were summarized.