Abstract:The bi-layered tubing components provide an alternative solution to make the best use of corrosion-resistant alloys and low-alloy steels. The numerical simulation and hydro bending experiments were carried out to analyze the wrinkling behavior of carbon steel/Al-alloy bi-layered tubes with different thickness ratios and internal pressures. Two types of instabilities, namely the bifurcation instability of inner tube and the limit load instability of outer tube are noticed and examined. It is indicated that the onsets of the wrinkling of inner and outer tube are delayed with increasing the thickness ratio. The bending capacity and the stability of bi-layered tube are remarkably improved as the thickness ratio increases. The optimized range of the thickness ratio is determined by numerical simulation. It is shown that separation between two layers occurs with a lower level of internal pressure, which causes the bifurcation instability of inner tube. However, the stability of inner tube is evidently enhanced with increasing the internal pressure, resulting in larger improvements of bending limit and moment capacity. The numerical predictions are verified by the hydro bending experiments with different internal pressures. Through the analysis, the selection of the internal pressure and outer tube thickness, and the mechanisms of increasing stabilities of the inner and outer tubes are clarified. The knowledge can be transferred to other bi-layered pipes with different materials and dimensions.

Key words: bi-layered tube; bending; internal pressure; wrinkling; bifurcation instability; limit load instability