ON THE OVALIZATION OF INELASTIC TUBES UNDER CYCLIC BENDING (STRUCTURAL, PLASTICITY, EXPERIMENTAL TECHNIQUES).

摘要

In many structural applications, tubular structures such as offshore structures, nuclear reactor components and others are subjected to cyclic bending loads. Under extreme loading conditions, the structures can be cyclically bent into the plastic range. Bending of thin-walled tubes leads to ovalization of their cross sections. This ovalization is responsible for limit load instability characteristic of the observed response. This study deals with the problem of predicting the response and assessing the stability of thin-walled tubes cyclically bent into the plastic range. Aluminum, carbon alloy steel and stainless steel tubes with diameter to thickness ratios ranging from 19 to 45 were cyclically bent in a specially designed pure bending testing facility. Loading histories involving curvature-symmetric cycling were performed. The moment, curvature and ovalization of the tubes were carefully measured. It was found that the ovalization accumulates after each cycle and the growth of ovalization eventually leads to instability. Complementary experiments were also conducted to establish the cyclic stress-strain relations of the tube material.; The analysis to predict the deformation of the tube cross section was based on the principle of virtual work in its incremental form. A detailed survey was carried out on the existing cyclic plasticity models suitable for the prediction of material behavior under complex loading paths. Predictions using Mroz and Two-surface models with different hardening rules revealed sensitivity of the growth of ovalization to the chosen model and, in particular, to the hardening rule adopted. A new two-surface incremental model using a J(,2)-type flow rule and a new hardening rule has also been developed. The new model was implemented in the analysis and shown to yield reasonable prediction for the cycle-dependent growth of ovalization.