High-temperature fatigue crack growth under transient overloading: application to Alloy 617

Abstract

A key problem in fatigue crack growth is the influence of load transients, such as overloads, on crack growth rates. This has been investigated in detail for materials operating near room temperature but is less well understood at high temperatures, where rate dependence becomes important. Here, finite element computations using an elastic-viscoplastic material model and an irreversible cohesive zone formulation are used to assess crack growth following high-temperature overloads. The model predicts that crack growth acceleration occurs in more rate-sensitive materials, while retardation occurs in less rate-sensitive materials. Retardation is associated with higher crack-tip viscoplastic strain following the overloads, which produces compressive residual stresses that disturb the crack-tip fields. Fatigue crack growth experiments were conducted at 800 $^\circ$C on Alloy 617, a nickel-base superalloy. The experiments show that a block of 20 overloads causes retardation at low loads and acceleration at high loads. The model predictions show good agreement with the trends in the experiments. The computations and experiments indicate that a transition between acceleration- and retardation-dominated behavior occurs at intermediate rate sensitivities, which are characteristic of many solid solution strengthened alloys.