Optimisation of the relaxed core stress in the ratcheting evaluation for load histories in elevated temperature design

In the present paper we investigate the possibility of optimizing the application of the procedure proposed by O'Donnel-Porowsky which introduces the concept of partial relaxation of the core stress and the possibility of analyzing load histograms with severe cycles in the ratcheting regime. The computer program utilized in this work is briefly described and we report the results of the calculations for two typical sample problems: the first considers only one block of three uniform cycles, the second considers seven blocks as described in Appendix A. The results indicate that the optimal choice of the relaxation stress is dependent on the creep-law adopted. When the load sequence is not specified the safety requires the analysis of all the permutation which is performed with the aid of the computer program; otherwise we must use the lowest value of the core stress in the load history to assure the conservatism of the method. A procedure which considers the actual relaxation stress for the time associated with each block and which eliminates the arbitrarity of σ1 is also described. (orig.)