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Plastic collapse analysis of square rings subjected to in-plane crushing between two opposed indenters is presented. Formulae for the plastic collapse load and the resisting load of the collapsed ring are derived. The plastic collapse load is determined from the stability analysis of the vertical members, treating them as slender columns with initial imperfection and elastic end-constraints and subjected to axial loads. The resisting load of the collapsed ring is found by assuming plastic hinges at mid-points of each of the arms of the ring and a constant “co-efficient of friction” between the tools and the ring surface. Results of experimental observations on the load-tool movement relationship of 3 in. × 3 in. square rings are also given and are shown in good agreement with theoretical predictions, if Tresca's criterion of yield is used.

Post-collapse cross-sectional flattening of a thick pipe at the centre of a plastic hinge formed during excessive bending due to unrestrained pipe whip is analysed with an aim to ascertain the extent of flow choking and consequent reduction of blowdown force. Based on the experimentally observed similarity between the plastic collapse mode of the critical pipe section during bending and plastic deformation of a ring under lateral compression, the effect of excessive bending on pipe section flattening is simulated using some well known analytical models for section collapse. A theoretical relationship between post-collapse bend curvature and section flattening is proposed for relatively thick pipes made of strain hardening material. The calculations made using the above relationship are found to compare well with those observed experimentally.