Crack Growth Under Thermal Fatigue Loading (Effect of Stress Gradient and Relaxation)

Thermal fatigue is a critical problem in nuclear power plants. To prevent crack initiation, JSME has issued a guideline for design. In this study, the feasibility of incorporating crack growth analysis into the design and integrity evaluation was investigated. Two characteristics of thermal fatigue loading were considered. The first was the effects of stress gradient in the depth direction. It was shown that the steep stress gradient near the surface significantly reduced the stress intensity factor (SIF) of deep cracks. Assuming that crack growth is arrested by small SIF, it is possible to leave detected cracks unrepaired. Otherwise, the cracks should be removed regardless of their size. The other characteristic is the displacement controlled boundary condition. Through finite element analyses, it was revealed that the displacement controlled boundary condition reduces SIF, and the magnitude of the reduction in SIF depends on the crack depth and boundary length. It is concluded that, under thermal fatigue loading, the cracks that are detected in the in-service inspection have already been arrested if they do not penetrate the wall thickness. The crack arrest scenario is a reasonable method of assessing the integrity of cracked components.