A small fatigue crack growth relationship and its application to cast aluminum

Small fatigue crack propagation in two cast aluminum alloys - W319-T7 and A356-T6 - was investigated by initiating cracks from micronotches. It was demonstrated that an underlying relationship describes the growth behavior of these fatigue cracks. This relationship is based on a parameter which is the product of monotonic and cyclic crack tip displacements and under conditions of small scale yielding reduces to the Paris law with an exponent of four. The value of this parameter can be calculated from the mechanical properties obtained from a simple tensile test and it can account for the effect of temperature, maximum stress, microstructure, heat treatment, alloying and load ratio on small fatigue crack growth rates. The crack growth rates are linearly related to the value of this parameter and the validity of this relationship can be supported by physical considerations.