Conditions for Appearance of Internal Intergranular Cracking Type Fracture Under Creep-Fatigue

This Chapter presents analytical results based on the experimental results and observations of cracks in creep-fatigue tests of a stainless steel SUS304. Two fracture morphologies were observed in high-temperature creep-fatigue fracture: inner cracking type fracture due to coalescence of small internal intergranular cracks and surface cracking type due to growth of small surface intergranular cracks. The fracture morphologies are strongly dependent on loading conditions. The inner cracking type fracture occurs at low tensile strain rate and high compressive strain rate, as well as at high temperatures, on the other hand, the surface cracking type fracture occurs under the opposite conditions. Three-dimensional fracture mechanism mapsThree-dimensional fracture mechanism map were created for tensile strain rate, compressive strain rate, and temperature to visualize the relationship between loading conditions and fracture morphologies. For the same strain range at high temperatures, the fatigue life at fatigue cracking type fracture is the longest, followed by the fatigue life at surface intergranular cracking type fractureSurface intergranular cracking type fracture and the fatigue life at internal intergranular cracking type fractureInternal intergranular cracking type fracture.