Simulation of bending and hydroforming : constitutive modeling and failure prediction

Abstract: In recent years, the automotive industry has shown increased interest in reducing weight and improving environmental effects of cars. This has lead to an increased use of lightweight parts. The exchange to lighter materials is however not a simple task since all demands on driving performance, crashworthiness, driving economy etc should still be fulfilled. One attractive concept is the spaceframe, which uses extruded aluminium profiles. The profiles are formed by bending and/or hydroforming and are connected by different types of joints. By using hydroforming several advantages can be found compared to traditional forming methods, including better tolerances, a decreased number of parts and an increased range of forming options.The aim of this project is to study the effect from the forming and assembling processes on the crashworthiness of the spaceframe. In this work, the focus is on the bending and hydroforming processes of extruded aluminium tubes. The objective is to accurately and efficiently predict the final product properties by the use of Finite Element (FE) simulations.The present study accentuates the need for improved anisotropic constitutive models. It is shown that a material model, which accurately describes the anisotropic behaviour of aluminium tubes, can be obtained by using simple and robust experiments.Since the hydroforming often is preceded by a bending operation this yields a demand on history dependent failure criteria. A selected number of known failure indicators are studied, including Forming Limit Diagram (FLD), Forming Limit Stress Diagram (FLSD), bulk forming criteria and the Continuum Damage Approach (CDM). If an accurate constitutive model is used, a strain path independent failure indicator is shown to be an efficient tool to determine the remaining formability of hydroformed parts.