A Prediction Method for Fatigue-Creep Interaction Life of High Strength Superalloy Based on Crack Propagation Behavior Rule

Abstract

A new life prediction method is proposed for a cast Ni base superalloy, René80, under fatigue -creep interaction (FCI) conditions based on its surface crack propagation behavior.
In the previous paper, we proposed that the frequency modified total strain energy parameter was useful for evaluating the failure life (N_f) and crack initiation life (N_c) of a high strength superalloy under FCI conditions. The tensile strain energy parameter was obtained from the total tensile strain energy (ΔW_TT) and the loading time (τ_TT) under tensile stress.
In this paper, not only the failure and crack initiation lives but also surface crack propagation behavior are found to be evaluated by this parameter under FCI conditions. The surface crack propagation rate (da/dN) is a function of the parameter and surface crack half length (a); da/dN=B'(ΔW_TTτⁿ_TT)^l·awhere B', l, n are material constants.
A new life prediction method taking account of surface crack propagation behavior and initiation life can be defined; N_2a=A(ΔW_TTτⁿ_TT)^m+B(ΔW_TTτⁿ_TT)^-l(log(2a)-log(2a₀))If the grain size (2a₀) and the number of cycles [N_c2a0=A(ΔW_TTτⁿ_TT)^m] needed for the surface crack length to grow to 2a₀ are known, the number of cycles (N_2a) in a certain crack length (2a) can be determined from the above equation.