Crack Opening Displacement and the Rate of Fatigue Crack Growth
CONNECTICUT UNIV STORRS United States
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Using a direct proportionality between the rate of fatigue crack growth and crack opening displacement above a threshold, it is shown that fatigue crack growth data for a wide variety of different materials can be accurately described in terms of the mechanical properties and two material constants the constant of proportionality A and the threshold stress intensity factor K sub th. Some 65 sets of data for tests at R approximately equal to O were analyzed by computer and it is shown that the approach is valid to growth rates up to about 0.0001 in.cycle, i.e. until the onset of crack propagation by dimple formation. It is found that can be related to the yield strain for crack growth in non-aggressive environments, and is increased by increasingly severe environments, while K sub th is decreased. These changes provide a measure of the severity of the environment. Crack growth rate in non-aggressive environments is shown to be independent of the yield stress and proportional to the strain intensity factor. above the threshold. The tabulation of A and K sub th values as a function of material, environment and loading conditions provides a systematic engineering approach to estimating rates of fatigue crack growth and in determining the residual lifetimes of flawed structures.