Transient crack growth behaviors under two-step varying loads

Transient crack growth behaviors were observed on aluminum alloy A2017-T3 under two-step varying amplitude loads. The prediction of the crack growth rates by the linear cumulative rule is valid only in limited cases. Retardation of crack growth occurs at the lower level of the two-step load patterns, due to the deceleration and the subsequent acceleration of the growth rate. The equations governing these processes are formulated as functions of the stress intensity factors at high and low load levels. The effects of strain hardening and aging treatment on the transient behaviors are discussed, and a reasonable crack growth model is proposed.