Effect of Cyclic Loading on Ductile Fracture Resistance

The influence of cyclic loading on ductile fracture toughness was examined from limited experimental data available in the literature. The review indicated that, in the absence of global compressive loads during the unloading cycle (R ≥ 0), the total crack extension generally is the sum of the monotonic component (Δa_mono) and the cyclic component (Δa_cyclic). The former can be obtained from the monotonic J-R curve and the latter from a fatigue-crack-growth curve. When large global compressive loads are developed during unloading (R<0), the review indicated that simply summing Δa_mono and Δa_cyclic gives an under prediction of the actual crack extension in load cycling tests. Thus, it appears that the local compressive strains lower a material’s normal monotonic fracture resistance in a region immediately ahead of the crack following the load reversal. Although few data are available in this regime of cyclic loading, it is likely that the magnitude of the loss of monotonic toughness depends on frequency of unloadings, R-ratio, and material. For ferritic steels susceptible to strain aging, temperature and strain rate also are likely to be important. The review also examined the applicability of various fracture mechanics parameters, including those based on deformation plasticity and those based on incremental plasticity, in quantifying cyclic-loading effects. While incremental plasticity parameters (for example T*) appear promising, they may be time-consuming and costly to develop. Until such advanced parameters are available, it is likely that empirical correlations involving deformation J will have to be developed to handle cyclic loading effects. Finally, the significance of the findings reported herein relative to the performance of flawed nuclear piping subjected to cyclic loading is discussed. It appears that circumferential cracks are of greater concern in seismic events than are axial cracks because the former may experience negative R-ratios.