The isochronous stress–strain method for creep evaluation in pressure vessels is a very effective and efficient alternative analysis method to the rigorous time dependent numerical approach. However, the isochronous data are generated from uni-axial load-controlled constant stress state. Its constraints or limitations have not been systematically studied for general or three-dimensional state of stress and variable loading conditions. In reality, pressure components are subjected to complex and combined loading conditions that may vary during operation, resulting in general state of stress and nonconstant loads. In this study, the accuracy of the isochronous stress–strain method for general state of stress and the concept and application of differential isochronous stress–strain data for slowly time-varying loads are brought up and investigated, wherever the time-varying loads can be approximated by piecewise constant step functions. By introducing the differential curve, the isochronous method is expanded into certain nonconstant loading conditions.

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