Propagation of short cracks in alloy 800HT and in the steel 22CrMoV12-1 at high temperature under biaxial fatigue

For the alloy 800HT and the steel 22 CrMoV 12-1 mode I, mode II and biaxial in-phase fatigue tests were performed on thin-walled tube specimens at temperatures from room temperature to 900 C. Under pure torsion the fatigue life for the same equivalent strain range was found to be always longer than for tensile and biaxial loading. The investigation of surface replicas indicated that crack initiation occurred mainly on grain-boundaries. After initiation stage II crack growth was observed for the tensile and biaxial loading. In torsion, the growth rate was significantly lower and the cracks grew mainly in stage I. Above 800 C stage II crack growth was also found under torsion in Alloy 800HT. For low cycle fatigue the ΔJ integral if formulated in principal stresses and strains was found to be an appropriate parameter to describe crack growth for all loading conditions. (orig.)