Abstract
The well-known load frequency effect on creep-fatigue crack growth is explained by the interactions between fatigue and creep loading and is quantified using the concept of plasticity-induced crack closure. It is shown that the hold time during creep loading affects crack growth rates during subsequent fatigue cycles. Longer hold times lead to lower crack-tip opening stresses and faster crack growth rates during fatigue loading. To model the impact of hold time on crack opening stresses during fatigue loading, a strip-yield model was developed for creep-fatigue crack growth. The strip-yield model computes crack-tip opening stresses, which determine the effective stress intensity factor range and crack growth rate during the fatigue portion of each loading cycle. Maximum stress intensity factor is used to compute the crack growth rate during the creep portion of each cycle. The proposed strip-yield model is used to compute creep-fatigue crack growth rates for several structural materials, i.e., an Astroloy, aluminium alloy 2650 and 316 stainless steel. The model predictions of crack growth rates compare well with published experimental data for these alloys. This model achieves reliable predictions of crack growth rates and life prediction on components subjected to creep-fatigue loading at elevated temperatures by considering loading interaction effects.
Highlights
Failure by crack growth during creep-fatigue loading is an important failure mode for structural components in service at elevated temperatures in power plants or in the aerospace industry
The Piard et al data is perhaps one of the most relevant in terms of highlighting the interaction between creep and fatigue loading on crack growth rates. They measured the influence of hold time tH on crack growth rates during fatigue loadingfat
Piard et al showed in their experimental measurements that the longer the hold time, the faster cracks grow during the loading from Fmin to Fmax
Summary
Failure by crack growth during creep-fatigue loading is an important failure mode for structural components in service at elevated temperatures in power plants or in the aerospace industry. The objective of this paper is two-fold It proposes a modeling approach for the influence of hold time during creep loading on the crack growth rate during subsequent fatigue cycles. Creep-fatigue crack growth is predicted using a strip-yield methodology, devised to compute crack growth rates during the fatigue and creep portions of each loading cycle. Strip-yield methodology has been used successfully to predict fatigue crack growth rates in metallic components under constant and variable amplitude loading. This modeling approach is well suited to capture the effects of loading history on fatigue crack growth rates. SYM-CFCG is used to capture loading history effects on crack growth by quantifying how creep loading affects crack growth rates during subsequent fatigue cycles
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