Abstract
The relationship between random microstructure and the statistics of small fatigue crack growth is illuminated by Monte Carlo simulations whose formulation has been described in previous work by the author. In this paper, more detailed analysis is presented of fluctuations in crack shape and the observed surface crack velocity, dc dN , using empirical laws of growth proposed elsewhere for Al 2219-T851 and Al 7075-T6. Unusually large values of dc dN are shown to follow retardation or arrest of the surface crack tips while the subsurface crack front has continued to advance. This relation implies that a physically based model of the statistics of dc dN must account for irregular crack shape to predict the distribution of dc dN and remaining lifetime accurately. The value of even rough measurements of surface crack depth in estimating remaining lifetime from nondestructive evaluation of a small crack is demonstrated. It is concluded that the statistics of dc dN require at least two independent random variables for their description. It is also proposed that experimental observations of the degree of irregularity of cracks of various sizes provide the most direct means to date of comparing the mechanics of the growth of surface and subsurface segments of the crack front.
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