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The behaviour of a low carbon steel has been studied, in particular the initiation, growth and coalescence of fatigue microcracks on the surfaces of smooth specimens via surface replicas and photomicrographs. From the study, quantitative information on the initiation period, growth and coalescence of cracks, statistical distributions of crack length and crack depth, density of cracks, distribution pattern and characteristics of the major crack, were obtained. Knowledge of these parameters is critical for non-destructive inspection during service life and the application of fracture mechanics to life assessment. NOMENCLATURE 2a = surface crack length 2a,, = length of major crack 2am = mean crack length 2a, = knee of composite Weibull plot of crack length b = crack depth b / a = aspect ratio b, = mean crack depth b, = knee of composite Weibull plot of crack depth N = number of loading cycles N , = number of loading cycles for crack initiation N,= number of loading cycles to fracture R = stress ratio = o,,/umar o, = Ao/2 = stress range 6 = crack density AKth = threshold stress intensity factor range N / N , = loading cycle ratio

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