Straight Pipe Cyclic Analyses for Shakedown Verification Code Criteria

The analyses performed in this paper are exclusively for a straight pipe subject to a constant primary stress and a cyclic secondary stress, as a result of a bending moment on the pipe cross-section. The constant primary stress is, in each analysis, from a CONSTANT dead-weight load and a CONSTANT internal pressure. The cyclic secondary stress is due to cyclic thermal expansion in the pipe, which is the result of a cyclic top-to-bottom linear temperature difference on the pipe cross-section. This temperature cycling produces a secondary stress range C₂ ΔM Do / 2 I. This secondary stress range is either higher, or lower than the 3 * Sm stress limit from the ASME-Code. Twenty-four different cyclic elasto-plastic analyses have been performed for this paper. The same elastic perfectly-plastic material model is used for all twenty-four analyses. An attempt has been made to compare the results from the cyclic elasto-plastic analyses with the ASME-Code NB-3600 Rules and with the Efficiency Diagram developed in France for their Code. The main advantage of the Efficiency Diagram is that the decision whether shakedown occurs, or not, is not only a function of the secondary stress RANGE, but also of the CONSTANT primary stress that exists at that time. A relief to the current ASME-Code rules is suggested for the case of a low constant primary stress (less than Sy / 2).