Study on thermal ratchetting of a three bar specimen without primary stress

Garatani, Kazuteru; Yamashita, Takuya; Tsukimori, Kazuyuki; Nakamura, Mitsugu*; Iwata, Koji

The thermal stress ratchetting is one of the important failure modes to be prevented in the design of FBR components. To avoid this kind of deformation, the Bree diagram expressed by the combination of primary and secondary stresses has been used in the design of FBR. However, even without primary stress, there is a possibility of ratchetting due to temperature dependence of yield stress when large variations of temperature exist locally. The purpose of this study is to confirm those phenomena by experiment and FEM analysis, and to prove this mechanism by a theoretical model. The following results were obtained. (1) The occurrence of compressive ratchetting deformation was demonstrated by a three bar thermal ratchetting test where the temperature of the central bar was varied cyclically from 20C to 500C. (2) The analysis of the three bar ratchetting test was performed by using the nonlinear cyclic hardening model (Ohno model) installed in FINAS. It is confirmed that the result of analysis represents the experimental ratchetting behavior well. (3) The ratchetting strain per one cycle : of a three bar thermal ratchetting model was derived by using the elastic-perfect plastic model in consideration of temperature dependence of yield stress. [=- : the first cycle a)] [=- : following cycle b)] : thermal expansion ratio, T : variation of temperature, E : Young's modulas, , ; :yield stress at higher and lower temperature respectively. The ratchetting occurs when the first term of right side is greater than the second term of b) equation. (4) The analytical solution by (3) represents well the initial ratchetting behavior of the test. This type of ratchetting occurs due to the temperature dependence of the yield stress of the material.