Use of CTOD as crack driving force parameter for low-cycle thermal fatigue

Repeated exposure to rapid temperature transients causes gradual damage in material. This is called thermal fatigue. Thermal fatigue is an important degradation mechanism in nuclear power plant components and can limit the plant lifetime where thermal loads are present, e.g., due to turbulent mixing or change in plant operating conditions. The effects of the thermal load cycles include residual stresses, hardening or softening of the material and, finally, crack initiation and growth. Traditionally, thermal fatigue crack growth rates are estimated from the stress intensity factors calculated from uncracked stress distributions and the Paris' law. In the low-cycle regime, the use of weight function based stress intensity factor solutions derived under linear elastic assumptions is questionable due to considerable plasticity. On the other hand, numerical contour integral techniques are ill-suited for thermal cyclic loading. In this work, the use of the crack opening displacement as the crack driving force parameter is evaluated through simulations of a low-cycle thermal fatigue experiments. The use of the crack tip opening displacement avoids the traditional limitations in the numerical evaluation of the J-integral. The unique relationship between the crack opening displacement and J-integral is derived and the crack driving force is used in a crack growth assessment. The results show that the crack driving force calculated from the uncracked stress distributions overestimates the crack driving force significantly (as compared to values calculated from the crack opening displacement). The crack growth rate calculated with the Paris' law is in good agreement with the experimental results, when the crack driving force is computed from the crack opening displacement.