Ratcheting Responses of Strain Hardening Plasticity Models

Nuclear reactor components are often subjected to combinations of sustained and cyclic stresses due to the application of internal pressure and transient temperature loads. During the thermal transients, the interaction of such stresses could cause cyclic plastic deformation, which may either lead to shakedown or ratchetting behavior. Design criteria meant to guard against ratcheting are established in Section III of the ASME Code. The criteria are based on perfectly plastic material behavior. However, materials undergo strain hardening upon reaching the yield strength. For elastic-plastic analysis, NB-3228.4 does not contain any guidance on modelling of material hardening, leaving it to the analyst to justify their choice. Although it is widely known that some plasticity models are inadequate for modeling cyclic plasticity, there has been little published study of the effect various plasticity models have on the ratchet boundary. This paper compares the ratchet boundary obtained from bilinear and Chaboche kinematic hardening material models with that obtained from perfect plasticity. The investigation is carried out for simple examples such as the classical Bree problem and a thin plate under biaxial loading.