Plastic Collapse Stresses for Pipes With Inner and Outer Circumferential Cracks

Bending stresses at incipient plastic collapse for pipes with circumferential surface cracks are predicted by net-section stress approach. Appendix C-5320 of ASME B&PV Code Section XI provides an equation of bending stress at the plastic collapse, where the equation is applicable for both inner and outer surface cracks. That is, the collapse stresses for pipes with inner and outer surface cracks are the same, because of the pipe mean radius at the cracked section being entirely the same. Authors considered the separated pipe mean radii at the cracked ligament and at the uncracked ligament. Based on the balances of axial force and bending moment, equations of plastic collapse stresses for both inner and outer cracked pipes were developed. It is found that, when the crack angle and depth are the same, the collapse stress for inner cracked pipe is slightly higher than that calculated by the Appendix C equation, and the collapse stress for outer cracked pipe is slightly lower than that by the Appendix C equation, as can be expected. The collapse stresses derived from the three equations are almost the same in most instances. However, for less common case where the crack angle and depth are very large for thick wall pipes, the differences among the three collapse stresses become large. Code users pay attention to the margins of plastic collapse stresses for outer cracked pipes, when using Appendix C equation.