Complex Stress-Strain Relations of Tubular Materials Studied With a Flexible Hydroforming System

Extruded aluminum alloy tubes present strong anisotropic properties, making its deformation very sensitive to stress states. To better characterize a proper constitutive relationship, the use of experimental stress-strain response with biaxial stress states would be more suitable than that of a common uniaxial test. To achieve this purpose, an attractive testing method and homologous experimental system (i.e., hydro-bulging system) were developed. Different proportional and non-proportional loading conditions could be performed with this experimental system. Hydro-bulging tests under three typical proportional loading paths were done. A fourth-order hardening model proposed by the authors was used to reproduce these experimental stress-strain relations and compared them with the results predicted using a power law relationship. Regression results predicted by the fourth-order hardening model had a good consistency with the distribution of experimental data. Deviations of the fourth-order hardening model are much less than that obtained by a power law. The maximum deviation introduced by the power function is at least 2.6 times greater than that introduced by the fourth-order function. Therefore, the use of a new regression model to treat experimental data would improve the predicting accuracy of a related constitutive relation efficiently and further ensure the predicting accuracy regarding a hydroforming simulation.