Experimental investigation of failure in thin rings subjected to uniform external pressure

Abstract

The object of this thesis was to investigate experimentally the strain distributions and ultimate failures in thin circular and non-circular aluminum rings subjected to uniform external pressure.

A test apparatus was designed to permit the application of hydraulic pressure to the outer circumference of a ring, placed between two sheets of Plexiglas. The small clearance between the edges of the ring and the Plexiglas was sealed by a continuous rubber gasket of slightly greater width fitted around the outer circumference of the ring. Except for friction at the gasket -Plexiglas interface, the ring was free of restraint.

Nine rings of aluminum alloy, 61S-T6, were tested to collapse; the variable in this series was out -of -roundness. Correlation between measured and predicted strains was obtained in five of the six rings in which the assumptions of the prediction were valid. A somewhat arbitrary strain distribution was observed in the two most circular rings. The four rings of moderate out -of -roundness collapsed very near predictions based upon a criteria related to the maximum stress level in the outer fibers while the three rings of relatively large out -of -roundness failed at pressures somewhat higher than predicted. The two most circular rings collapsed near a computed pressure when the minimum rather than average section thickness was used in the calculation. Collapse pressures were predicted most accurately when the stress used in conjunction with the failure criteria was defined by the point on the stress strain curve at which marked non-linearity occurred.

An analysis of the results indicates that strain distributions in an out -of -round ring loaded by uniform external pressure can be predicted with satisfactory accuracy provided the assumptions made in the prediction are satisfied; namely, that the thickness to diameter ratio be the order of 0,0285 and that the initial configuration assumed is actually predominate. A failure criteria based upon the stress level in the outer fibers vd.ll, when the assumptions above are fulfilled, predict collapse quite accurately in rings of out -of -roundness to thickness ratio between 0.10 and 6,30. At larger values of out -of -roundness the criteria is conservative but not over cautious.

It is recommended that the experimentation be extended to include a quantitative investigation of the effects of deviations from a basic two lobe out -of-roundness such as might be obtained in a practical situation. Such an investigation should be conducted using rings of constant thickness in order to avoid many of the unnecessary complications encountered in the analysis of data obtained in this thesis.