Elbows exhibit highly strained regions and are vulnerable to plastic collapse. It has been observed that available equations in literature for evaluation of plastic collapse load of a pipe bend have limited applicability and do not cover wide range of pipe bend radius ratios and bend angles which are used in power plant piping. Moreover, the elbow collapse load equation should approach to straight pipe collapse load as elbow bend radius increases or bend angle decreases. Generally, the available equations do not satisfy this asymptotic behaviour of an elbow. About 600 number of elastic plastic and geometric nonlinear finite element analyses of elbows having different geometric parameters bend radius, pipe radius, thickness and crack size (in case of cracked elbow), have been performed. For each of the elbow the in-plane plastic collapse moments have been evaluated from M-rotation curve by twice elastic slope (TES) method. Further two weakening factors were defined to quantify the degrees in plastic collapse moment, one due to elbow doubly curved geometry and second due to presence of crack. The geometric weakening factor was evaluated by comparing elbow plastic collapse moment with corresponding straight pipe collapse load while the crack weakening factor was evaluated by comparing cracked elbow plastic collapse moment with corresponding un-cracked elbow plastic collapse moment. The simple equation were developed for these weakening functions which can be used with pipe plastic collapse load equation for prediction of plastic collapse load of elbows.
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