The ASME Boiler and Pressure Vessel Code, Section Xi, Appendix K, contains criteria for evaluation of the acceptability of nuclear reactor pressure vessels when the upper-shelf Charpy energy level is less than 50 ft. -1bs (68 J). The evaluation is based on postulating a flaw in the reactor pressure vessel, and evaluating its stability under service loadings. The acceptance criteria for ASME level D service Loadings require that the remaining ligament after ductile flaw growth is safe from tensile instability. Equations for evaluating tensile instability of flaws have recently been proposed for implementation into Appendix K by the Section Xi Working Group on Flaw Evaluation. The limit pressure equationf or ligament tensile instability of axial flaws that produced the best overall agreement with the results of tests on low-alloy steel pressure vesels and pipes contianing axial surface flaws, as well as on flat plates containing surface flaws, was proposed for implementation into Appendix K. Since there is a scarcity of representative experimental results for circumferential flaws in low allow steel vessels and pipes subjected to pure pressure or axial loading, comparison of limit pressure equations for circumferential flaws against experiments was not possible. A variant of the limit pressure equation for axial flaws was developed for variant of the limit pressure equation for axial flaws was developed for application to circumferential flaws, and was also proposed for implementation into Appendix K. The technical basis for the proposed ligament tensile instability equations is provided in this paper.
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