Engineering Application of Enhanced C*-Q* Two Parameter Approaches for Predicting Creep Crack Initiation Times

Enhanced C*-Q* two parameter approaches were developed into engineering application where the creep crack initiation was defined by a critical distance of 0.2 or 0.5 mm. Enhanced C*-Q* approaches coupled the stress-regime creep strain rate and Fermi's function of creep ductility as well as constraint effect was used. Two methods were proposed to evaluate the creep crack initiation (CCI) times based on enhanced C*-Q* approaches. (1) Simple method I, i.e. directly applying engineering critical distance 4d into enhanced C*-Q* two parameter approaches to calculate the CCI times. (2) Four-stages method II, i.e. divide 0.2 mm into 4 stages, in each stage, critical distance d = 0.05 mm could be applied into enhanced C*-Q* approaches to obtain creep time, and then CCI times were total creep times in four stages. The estimation method of initial stress state and selection criterion of prediction model for plane stress and plane strain state was also proposed. By comparing CCI times between the predictions from method I, method II and R5 method, and finite element simulation and experimental data, it was indicated that method I (with average crack length a+1.5d) and method II could be accurate and conservative. Method I was more convenient than method II, while method II was more accurate. Hence, it was recommended to apply these methods according to actual condition or demand.