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

Abstract

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.

Formulation of plastic strain distribution derived from long-distance travel of temperature distribution based on residual stress required for elastic shakedown behavior

“…In this subsection, we explain typical case with the continuous accumulation of the plastic strain due to long-distance temperature travel. As same with our previous papers (Okajima et al, 2016(Okajima et al, , 2017 (Okajima, 2016(Okajima, , 2019, we assume that thermal membrane stress is within the shakedown range in this paper. Because, JSME design and construction code (JSME, 2018) have prohibited excessive thermal membrane stress that exceeds the shakedown limit.…”

“…According to the FEA results on our previous papers (Okajima et al, 2016(Okajima et al, , 2017 (Okajima, 2016(Okajima, , 2019, in the case if neither magnitude nor travel distance of the temperature distribution were so massive, the accumulation of the plastic strain is finally saturated. In these cases, the saturated distributions of the plastic strain and residual stress showed following characteristics:…”

“…( 14) to the wider range of conditions than the existing method. When √ , which is screening criterion proposed on our previous paper (Okajima et al, 2017), maximum residual stress on surrounding region is nearly equal to . Fig.…”

“…We have focused on following 2 parameters as dominant parameters of this ratcheting mechanism (Okajima, 2016) (Okajima et al, 2017): a) Maximum value of membrane stress intensity, 〈 〉 , and b) the axial length of the full-section yield region, . In case if thermal membrane stress 〈 〉 is loaded at specific point in thermal transient, the residual stress whose magnitude is 〈 〉 is required to achieve the elastic shakedown behavior.…”

“…Meanwhile, we had studied on a screening criterion for this ratcheting mechanism (Okajima et al, 2016(Okajima et al, , 2017) (Okajima, 2016). In order to prevent continuous accumulation of plastic strain due to this ratcheting mechanism, we proposed the criterion for the axial length of the region with full-section yielding, , by referring to the definition of local membrane stress in current design codes.…”