Fatigue crack growth tests under random loading are performed and the behavior of crack closure and growth is investigated in detail. In particular, the effects of the characteristics and history length of random loading on the behavior of crack closure and growth are discussed. Moreover, single overloading, periodic overloading, and Hi-Lo(high-low) 2 step block loading tests are performed to compare the crack opening behavior under random and under simple variable amplitude loading.
Crack length and crack closure are measured by an unloading elatic compliance method, and an automated method is developed to determine the crack opening point from differential displacement signal. To improve measurement precision, a noise reduction method using a digital filter is employed. This method can be used efficiently even for random loading.
It is observed that crack opening point is nearly constant within a block of random loading and is higher than the value of constant amplitude loading corresponding to the highest load cycle. The crack opening behavior under random loading is mainly affected by the largest load cycle rather than the characteristics of random loading history. The fatigue crack growth rates under random loading can be well expressed in terms of the effective stress intensity factor range $\triangle{K}_{\mbox{eff}}$. The crack opening points rapidly increase after applying overload, and then decrease gradually. The maximum value of crack opening point under single overloading is agreed well with the crack opening value under random loading.
