Study on a unified criterion for preventing plastic strain accumulation due to long distance travel of temperature distribution

摘要

The prevention of excessive deformation by thermal ratcheting is important in the design of high-temperature components of fast breeder reactors (FBR). In an experimental study that simulated a fast breeder reactor vessel near the coolant surface, it was reported that the long distance travel of temperature distribution causes a new type of thermal ratcheting, even in the absence of primary stress. In this paper, we propose a simple screening criterion to prevent continuous accumulation of plastic strain derived from long distance travel of temperature distribution. The major cause of this ratcheting is the lack of residual stress that brings shakedown behavior at the center of yielding area. Because residual stresses are derived from constraint by the neighboring elastic region, we focused on the distance from the center of yielding area to the elastic region. Accordingly, the proposed criterion restricts the axial length of the full-section yield area, which is the double of the above distance. We validated the proposed criterion based on finite element analyses using an elastic-perfectly plastic material. As a result of the validation analyses, we confirmed that the accumulation of plastic strain saturates before second cycles in the cases satisfying the proposed criterion, regardless of the shape of temperature distribution.