Creep-fatigue assessment of martensitic welds based on numerically determined local deformation

Abstract

Abstract The low cycle fatigue and creep-fatigue behavior of base material and similar welded joints of martensitic steel Grade 92 were examined in this work. Several low cycle fatigue tests with different loading scenarios: fatigue tests with strain-controlled hold time, fatigue tests with strain-controlled amplitudes, and load-controlled hold times, were performed at 873 K. The tests were evaluated with regard to fatigue lifetime, peak stress, the evolution of relaxation stress or creep strain, and failure location. The effect of amplitude level, hold time modus and hold time length on the lifetime of the specimens was investigated using lifetime and damage assessment methods based on strain energy approaches. The numerical simulations for the determination of the evolution of local strains and stresses were done using a viscoplastic material model of Chaboche type, including a strain energy based damage formulation. For the parameter identification, tests were carried out on the specimens made of base metal, welded joint, weld metal, and with microstructure simulated specimens aiming to obtain representative microstructures of the fine grain, intercritical, and coarse grain heat affected zone. The numerical assessment of the local stress, strain, and damage distribution explains the smaller lifetime of the welded joint specimens.