Ratcheting Models for Fusion Component Design

One of the primary failure mechanisms addressed by structural design rules for fusion components is ratcheting, the accumulation of strain with cyclic loads. If a component is loaded such that ratcheting occurs, failure can be expected in relatively short order, so design rules must ensure that the behavior is avoided. In this paper, we present finite element models for cyclic loading of typical fusion structures and compare the results to analytical models for simple geometries and design rules intended for more complex geometries. Both material and structural ratcheting is considered. For structural ratcheting, the 3Sm rule employed in the ITER Structural Design Criteria is found to be unduly conservative and the accompanying Bree rules are found, in some cases, to be non-conservative. Significant advantage can be gained from using fully plastic models to avoid ratcheting.