Influence of Mean Stress Intensity on Fatigue Crack Growth in an Aluminium Alloy

Abstract

The effects of the mean stress-intensity factor, K_m, and the range of the stress intensity, ΔK, on crack propagation phenomena in the Al-alloy RR 58 have been studied using contoured double-cantilever beam specimens providing a constant stress-intensity factor for all crack lengths.

Based on the experimental data available, a relationship of the following form, between the cyclic crack growth rate, da/dN, and the tensile loading levels, has been proposed:

where ΔK = (K_max - K_min); K_max, K_min and K_m are the maximum, minimum and mean values of the stress-intensity factor; ΔK_th is the threshold value of ΔK for crack propagation; K_1C is the critical fracture toughness in plane strain conditions; A and α arc constants. In tests at room temperature (21 °C) in laboratory air and at a loading frequency of 0.15 Hz, it was found that ΔK_th decreased with increasing values of K _m , α was equal to 1.36 and A equalled 3.16 times 10^-5(in/cycle).