A study on plastic strain accumulation caused by traveling of temperature distribution synchronizing with temperature rise

Abstract

The prevention of excessive deformation by thermal ratcheting is important in the design of high-temperature components of fast breeder reactors (FBR). This includes evaluation methods for a new type of thermal ratcheting caused by an axial traveling of temperature distribution, which corresponds to moving-up of liquid sodium surface in startup phase. Long range traveling of the axial temperature distribution brings flat plastic deformation profile in wide range. Therefore, at the center of this range, residual stress that brings shakedown behavior does not accumulate. As a result, repeating of this temperature traveling brings continuous accumulation of the plastic strain, even if there is no primary stress. In contrast, in the case with short range traveling, residual stress is caused by constraint against elastic part, and finally it results in shakedown. Because of this mechanism, we supposed that limit for the shakedown behavior depends on distance from the elastic part (i.e. half length of region with plastic deformation). In this paper, we examined characteristics of the accumulation of the plastic strain caused by realistic heat transients, namely, traveling of temperature distribution synchronizing with temperature rise. This examination was based on finite element analyses using elastic-perfectly plastic material. As a result, we confirmed that the shakedown limit depends not on the traveling range of the temperature distribution but the plastic deformation range, which was predicted by the elastic analysis. In the actual application, we can control the plastic deformation range by changing rate of the moving-up of liquid sodium surface.

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

“…We have focused on the axial length of the full-section yield area, which corresponds with double of the above distance. In the previous study (Okajima, 2016b), we confirmed that the saturation of the plastic strain accumulation depends not on the travel distance of the temperature distribution but on the axial length of the full-section yield area, which was predicted by the elastic analysis.…”

“…(1), was validated by a set of FEAs using elastic perfectly-plastic material and a ramp-shaped temperature distribution. We also validated the applicability of this criterion to the more detailed temperature model simulating the thermal transients nearby the coolant surface of the vessel (Okajima, 2016b). This temperature model had the feature that the increasing of the coolant level was synchronized with the increasing of the coolant temperature, and so we called this model the synchronizing model.…”

“…In our previous paper (Okajima et al, 2016a), we proposed a screening method of thermal ratcheting strain for the practical component design. The proposed method initially determines whether plastic strain is generated at all points Okajima, Wakai and Kawasaki, Mechanical Engineering Journal, Vol.4, No.5 (2017) [DOI: 10.1299/mej.…”

“…In the previous study (Okajima et al, 2016a), we focused on the axial length of the full-section yield area as the dominant parameter of the ratcheting mechanism in this paper. Figure 1 illustrates the effect of this parameter.…”

“…Igari et al (2000) proposed an evaluation method that considered the effect of travel distance. In addition, we have proposed and validated a screening criterion for this ratcheting mechanism (Okajima, 2016a(Okajima, , 2016b. In our previous studies, we assumed a reactor vessel wall near the coolant surface as the target equipment.…”