| The primary intent of this work was to investigate the effect of end-conditions on hydrofomability and failure pheromone of aluminum extruded tubes. To further extend the concept of pressure vessels into tube hydroforming applications an analytical solution based on thick-walled pressurized cylinders subjected to axial loading was also developed.; In the first stage, free-bulge hydroforming experiments were conducted to investigate the biaxial behavior of extruded aluminum tubes. A special fixture, capable of providing only internal pressure and free sliding of tube-ends, was successfully designed to conduct this experiment. A finite element model was constructed to simulate the free-end hydroforming process and to study the influence of friction between the die walls and the tube, tube material properties and tube anisotropy on tube's hydroformability. The analysis also revealed that the material hardening coefficient had the most significant influence on the formability characteristics during hydroforming.; In the next step, tube hydroforming experiments were conducted to develop the forming limit diagram of AA6082-T4 by utilizing three types of end-conditions: (i) free-end; (ii) pinched-end or fixed-end and (iii) forced-end. It was found that free-end hydroforming gives the lowest forming limits followed by pinched-end and forced-end hydroforming. Finite element simulation of the tube including extrusion weld subjected to various end-conditions revealed that stress concentration in the tube cross section follows the same trend as those observed in the experiments.; To better understand the stress distribution development during hydroforming of thick-walled tubes, a generalized solution for small plastic deformation of thick-walled cylinders subjected to internal pressure and proportional axial loading was developed.; The solution was shown to reduce to the well-known Lame's elastic solution and Nadai's general plane strain solution under appropriate assumptions. The influence of proportionality factor (ratio of axial strain to hoop strain) and hardening exponent on the induced strain, deformation fields and thickness reduction was systematically investigated.; Finally, Optimization methods along with finite element simulations were utilized to determine the optimum loading paths for closed-die and T-joint tube hydroforming processes. The objective was to produce a part with minimum thickness variation while keeping the maximum effective stress below the material's ultimate stress during the forming process. In the closed-die hydroforming, the intent was also to conform the tube to the die shape whereas in the T-joint design, a high T-branch height was sought. It is shown that utilization of optimized loading paths yields a better conformance of the part to the die shape or leads to a higher bulge height. (Abstract shortened by UMI.)... |
