Experimental and computational characterization of the effect of manufacturing-induced defects on high temperature, low-cycle fatigue for MarBN

Manufacturing-induced defects are a key source of crack initiation and component failure under high temperature cyclic loading. In this work, 3D X-ray micro-computed tomography and microstructural analysis of manufacturing-induced defects is presented for forged and cast MarBN martensitic-ferritic steel, along with high temperature, low cycle fatigue testing, for assessment of the comparative effects of two manufacturing processes. Forging is found to significantly reduce the volume fraction and complexity of manufacturing defects, compared to the cast material, resulting in approximately double the fatigue life. A voxel-based finite element methodology for experimentally-identified cast and forged manufacturing defects is presented, in conjunction with a multiaxial, critical-plane damage model, within a unified viscoplastic user-material subroutine. The effect of the complex morphologies of the manufacturing defects on high temperature fatigue crack initiation is thus quantified, highlighting the relative effects of the two different manufacturing processes.