The material-dependent variability of fatigue crack growth rates of aluminium alloys in the Paris regime

Constant amplitude fatigue crack growth rates of a number of heat-treatable thin-sheet wrought aluminium alloys are presented in terms of the Paris equation with the stress intensity factor range normalized by a characteristic value, in the present case 10 MPa m1/2. There is a considerable overall variability of the crack growth rates at any given value of ΔK. It turned out that there are two markedly different groups of alloys. For the first group, the crack growth curves strongly focus at 10 MPa m1/2, whereas the Paris exponents vary considerably and are related to a material parameter obtained by dimensional analysis. For the second group, the crack growth rates at 10 MPa m1/2 are found to be always less and vary much more than for the first group. The observed behaviour is discussed indicating that for the first group plasticity-induced crack closure should mainly be responsible for the observed material dependence, whereas for the second group roughness-induced crack closure also plays an important role.