After reviewing previous attempts to quantify delay, a composite stress intensity parameter, (K_ℓmax/K_hmax) ∙ ΔK_ℓ; proposed to correlate the number of delay cycles in simple two-level variable-load amplitude tests for various combinations of stress levels. Its application is restricted in the present investigation to room temperature tests in an air environment. Delay (cycles) is defined herein as the period of apparent zero crack growth after the overload, and the stress intensity factors, K_ℓmax and K_hmax refer to the stress intensities accompanying the lower and higher load levels, respectively. It is hypothesized on the basis of experimental observations that material under different two-level block loading sequences, but having equal values of the parameter, will experience the same delay.

Good correlations for data on aluminum alloys, titanium alloys, and carbon steel taken from the work of several investigators are obtained using the parameter. An empirical model relating the delay to a power function of the parameter is suggested. Limitations of the model are pointed out and discussed. The engineering significances of the parameter with respect to the determination of crack propagation life and crack arrest conditions are discussed.