Ratchet analysis of structures under a generalised cyclic load history

This paper introduces a new approach based upon the Linear Matching Method in order to obtain the ratchet limit of structures subjected to an arbitrary thermo-mechanical load history. This method varies from the traditional Linear Matching Method ratchet analysis, where the cyclic load history is decomposed into cyclic and constant components, instead calculating the ratchet limit with respect to a proportional cyclic load variation, as opposed to an additional constant load. The shakedown and limit load boundaries are initially obtained for the given structure, followed by the utilisation of a bisection procedure in order to calculate an approximate ratchet boundary based upon a predefined magnitude of ratchet strain per cycle. The method also yields the total and plastic strain ranges based upon perfect plasticity, for low-cycle fatigue post-processing considerations. The effects of analysing the ratcheting mechanism of structures undergoing a cyclic primary load that varies proportionally with a cyclic secondary load can be seen to lead to modified and less conservative ratchet boundaries compared to the traditional Bree solution in which the thermal ratcheting requirement (NB-3222.5) of ASME III is based upon. This paper introduces the theory, numerical implementation and verification of the proposed method via a series of example problems.