Micro-crack growth modes and their propagation rate under multiaxial low-cycle fatigue at high temperature

Crack growth behavior and propagation rates in SUS316L steel, 1CrMoV steel, and Hastelloy-X in multiaxial low-cycle fatigue tests at high temperature were investigated. Crack growth mode in pure torsion tests changed from the principal strain plane to the maximum shear plane with increasing temperature or strain range. The dominant mechanism of micro-crack initiation, however, differed among the three materials. The dominant process of micro-cracking was considered to be slip in grains for SUS316L steel, whereas oxidation film cracking and grain boundary cracking were the mechanisms for 1CrMoV steel and Hastelloy-X. respectively. The appropriate parameter for evaluating the crack growth rate under multiaxial conditions is discussed in relation to the micro-crack growth mechanisms. Good correlation was obtained between the crack growth rate and the strain parameter based on the micro-cracking mechanism of each material.