THE LIMIT LOAD CALCULATIONS FOR PIPE BEND WITH AXIAL PART-THROUGH DEFECT

Pipe bend is very complicated element for the structural integrity assessment. Up to day there is no conventionally adopted technique for limit load calculation of pipe bend even without any defect. The problem is that at application of outer bending moment the pipe bend cross section ovalizes and the process of deformation can be described only with accounting for the geometrical nonlinearity. The paper deal with limit load calculation for pipe bend with axial part-through defect for particular case when circumferential stresses originated both from inner pressure and outer bending moment dominate over axial stresses from the moment and axial force. Two extreme cases are considered at start. First one is the action of the inner pressure only. The "Institute for Problems of Strength limit load model" (IPS model) can be applied here without any restrictions. The second case is consideration of circumferential bending stresses which have appeared due to ovalization from the outer bending moment. The model of the transmission of stresses from the defected region to the undamaged regions is suggested and the resulting formula for the stress concentration (or strength reduction) coefficient is obtained. At last the simultaneous action of both loadings is considered. As result the analytical formula for the reference stress calculation which is similar in appearance to that of API 579 for accounting for membrane stress as well as bending stress is suggested. The only difference is that strength reduction coefficients are considered for both the membrane stresses from inner pressure and bending stress from ovalization. This differs from API 579 approach where the influence of the defects length on the bending stresses is not taken into account.