The variation in fatigue crack growth due to the thickness effect

In this study, to investigate the statistical variability of crack growth rates, fatigue tests are conducted on 7075-T6 aluminum alloy specimens under constant amplitude load. Experimental results show that the da/dN−ΔK curve has a sigmoidal relationship, and the variation in crack growth rates increases with decreasing thickness. To describe the sigmoidal da/dN−ΔK relationship, the Forman equation is modified, and to consider the variability of crack growth rates, a variable Z is added to the modified Forman equation. Log Z, the residual of the modified Forman equation, is a normal random variable and is independent of crack length and stress intensity factor range. In addition, the variance of log Z is the parameter that can estimate the variation in crack growth rates due to the thickness effect. Using the statistical properties and the quantitative estimation of log Z, crack growth process is simulated and the distribution of fatigue life is predicted. The results are compared to the prediction based on the Paris–Erdogan equation.