Incremental growth due to creep and plastic yielding of thin tubes subjected to internal pressure and cyclic thermal stresses

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Abstract
A theoretical investigation is made of incremental growth in a thin pressurized tube when the tube wall is subjected to large cyclic variations of temperature and temperature gradient, the mechanisms of deformation being plastic yielding during application and removal of the temperature gradient and creep during the high temperature part of the cycle. In a previous paper a simple model of the tube was developed and used to determine approximately when the mode of incremental growth known as ratcheting occurs and to estimate the ratchet strain per cycle, both when creep is ignored and when it causes complete relaxation of internal stresses. The above work is extended in the present paper to cases where only partial relaxation of internal stresses occurs and a method of evaluating the ratchet strain per cycle is presented. In addition, the method allows the increased creep strain due to non-linear interaction between internal stresses and the pressure stress to be calculated. This accompanies ratcheting when the relation between creep rate and stress is non-linear.