AN EXPERIMENTAL INVESTIGATION OF THE DEGRADATION AND BUCKLING OF CIRCULAR TUBES UNDER CYCLIC BENDING AND EXTERNAL-PRESSURE

Cyclic bending of tubes into the plastic range of the material leads to a progressive accumulation of ovalization of the tube cross-section. Persistent cycling leads to local catastrophic buckling of the tube. This paper presents an experimental study of the problem. The main objective of the study was to establish the effect of the cyclic bending history and of the external pressure on the rate of accumulation of ovalization and on the onset of instability. The cyclic loading histories examined include curvature symmetric bending, bending about a mean value of curvature and moment-controlled bending about a mean value of moment. The rate at which ovalization accumulates in curvature-controlled cyclic bending was found not to be significantly affected by a mean curvature in the cycles. Moment-controlled bending about a mean moment leads to ratcheting in curvature as well as in ovalization. External pressure accelerates the accumulation of ovalization and leads to buckling in fewer cycles than in the corresponding pure bending cases. This was found to be true for all bending histories considered. Significant similarities were observed between the response and onset of instability in the monotonic bending case and all cyclic bending cases. For a group of aluminum tubes instability was found to occur when the ovalization of the cross-section reached a critical value. This critical value was found to be relatively independent of the bending history followed.