There are numerous cases where a local thin area has found near a nozzle attachment in cylindrical pressure vessels. Since the stress concentration due to a nozzle hole under internal pressure significantly affect to the stress state of the vessel near the nozzle, it must be taken into account in its plastic collapse assessment. In this paper, a new plastic collapse assessment procedure for cylindrical vessels with a local thin area near a nozzle attachment under internal pressure was proposed. This proposal utilizes the existing reference stresses to estimate the plastic collapse load under three-axial condition derived in accordance with the Tresca theory. The distribution of stress concentrations in vessel wall due to a nozzle attachment depends on its reinforcement condition for the nozzle hole, so the stress state should be computed by linear FEA in advance. The stress concentration factors are provided with normalizing of generated stresses in vessels near a nozzle attachment for the similar type of dimensions, orientations and projections of the nozzle attachment by the computed stresses with already-proposed equations for no reinforcement conditions.

The plastic collapse loads predicted by the proposed reference stress are ascertained with the results of experiments and non-linear FEA.

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