Crack Initiation Under Low-Cycle Multiaxial Fatigue

This study is devoted to the determination of a criterion for predicting crack initiation under low-cycle multiaxial fatigue. Here crack initiation, N_i, is defined as the number of cycles needed by the final microcrack to reach one grain size. The tests which were carried out at room temperature on Type 316L stainless steel and Inconel 718 alloy included (a) reversed tension-compression (b) reversed tension-compression with a superimposed steady torque, (c) pulsated tension-compression with a stress ratio R_σ such that −0.5 < R_σ < 0 and (d) reversed torsion. Scanning electron microscope (SEM) observations were used to determine the number of cycles corresponding to crack initiation and the orientation of the microcracks.

From these observations a criterion combining the plastic shear strain range for the crack initiation plane, Δγ_p, and the maximum of the shear stress on this plane, τ, is proposed. It is shown that the predictive capability of this criterion is satisfactory for various experimental results featuring both Types A and B crack initiation. Using the proposed criterion and the experimental relation between N_i and the number of cycles at failure, N_f, in conjunction with a double linear damage rule, it is shown that this approach accounts for previously published experimental results involving two level loadings.