Fatigue Life Estimation Method Based on Inelastic Behavior of Ni-Based Directionally Solidified Superalloy with Small Holes

Abstract

Low cycle fatigue tests at elevated temperature were conducted on test specimens with small holes made of a Ni-based directionally solidified superalloy, which are intended to be the cooling structures formed in components in fossil fuel power plants. The tests included cases with and without strain holding processes. The number of crack initiation cycles of the tests without hold processes for the one- and seven-hole specimens was about 1/30 that of the smooth one. The number of crack initiation cycles of the test with the compressive hold processes for the seven-hole specimen was smaller than that of the tests without hold processes while that of a tensile hold case was even smaller. The test results were evaluated based on the inelastic behavior around the center hole of the specimens, where the largest inelastic strain occurred, using finite element analysis that takes into account the inelastic anisotropy of material properties. The number of crack initiation cycles of the tests without hold processes and that with compressive hold processes correlated with the maximum tensile stress around the hole, while that of all the tests correlated with the frequency-modified strain energy by setting the appropriate material parameters based on the test results of smooth specimens.