Prediction of fatigue crack growth rates: theory,mechanisms,and experimental results

A range of continuum-based crack growth laws are reviewed and it is concluded that they may be divided into those based on instantaneous values of cyclic crack-opening displacement which have a ∆K exponent of 2 and those based on damage or strain accumulation which have a ∆K exponent of 4 and, in addition, predict the onset of rapid crack acceleration as K→ _Ic. No laws showing how to introduce the effects of aggressive environments were found. A third type of law, based on energy-balance concepts within a crack tip process zone, can predict a gradient lying between 2 and 4 depending on the size of the reversed plastic zone. Comparison between theory and experiment suggests that the (∆K)⁴ type laws provide better agreement for data gathered in inert environments. It is thought that, when growth laws are being assessed by comparison with experimental observations, data gathered in inert environments should be used. In addition it is noted that when experimentally and theoretically determined exponents of ∆K are being compared it is the minimum value achieved by the exponent of ∆K which should be considered. The average exponent value, obtained by approximating a sigmoidal curve to a straight line, can deviate significantly from the minimum.