Assessment of thermal fatigue damage caused by local fluid temperature fluctuation (part II: crack growth under thermal stress)

In order to assess the crack growth due to thermal fatigue caused by a local temperature fluctuation, it is important to consider the stress gradient in the thickness direction. Even if the stress fluctuation on the surface is sufficiently large enough to initiate fatigue cracks, the initiated cracks may stop growing due to the stress gradient in the thickness direction. In this study, not only the stress on the surface but also the stress in the body was investigated in order to evaluate the crack growth behavior. A trapezoidal temperature fluctuation and a sinusoidal temperature fluctuation were analyzed. It was revealed that the stress range on the surface caused by the former fluctuation could be larger than that caused by the latter. Based on the evaluated stress distribution the stress intensity factor (SIF) was derived by the weight function method. It was shown that, in a crack growth analysis under thermal stress fluctuation, the crack could penetrate the wall thickness when the threshold SIF ΔKth for crack arrest was not assumed. If the ΔKth was considered, cracks could stop growing under the sinusoidal temperature fluctuation, although it was still difficult for cracks to stop growing under the trapezoidal temperature fluctuation. The reason for the arrest of crack growth at the mixing tee and the crack penetration at the branched elbow was reasonably explained from the analyzed results.