Random Load Fatigue Damage Accumulation in Mild Steel

Abstract

Using high-sensitivity eddy-current crack detection, quantitative division of fatigue life into two stages has been achieved for a variety of random and constant amplitude loading conditions. The divisions adopted were:Stage A, microcrack initiation and propagation, and Stage B, macrocrack propagation.The results of specimen sectioning and optical microscope work show that this behaviour is physically compatible with the observations of Forsyth (1)‡.It is established under random loading that for a given specimen configuration, the proportion of time spent in stage A behaviour for a given fatigue life remains unaltered for changes in the waveform irregularity factor. This statement applies to two series of tests for loading waveforms of unaltered fundamental p.s.d. (power spectral density) shapes and Gaussian amplitude probability density distributions. It is suggested that the signal maximum peak/r.m.s. ratio is a significant factor in determining the proportion of life spent in crack initiation.Cumulative damage predictions are made using Miner's hypothesis on a basis of positive peak stresses to failure for stage A and stage B lives, and overall fatigue life. The results confirm that Miner seriously underestimates the damage contribution at low stresses on fatigue life as a whole, and also for the crack initiation phase of life.