Mechanical Behavior Of Sheet Metal In Viscous Pressure Bulging

The forming application of high strength and lightweight alloys are wider and wider in modern industry. The way to promote the quality of these materials becomes one of the research focuses. Viscous Pressure Forming (VPF) is one of the soft-die forming processes for sheet metal proposed in recent years. The high viscous, flowable and semi-solid soft-die (so-called viscous medium) can promote the dimensional accuracy, surface quality and thickness uniformity of parts. VPF has a good application prospect in precision plastic forming. Research on the influence of viscous medium on sheet metal formability possesses guiding significance for reasonable using of viscous medium and optimization of processing scheme. In this dissertation, optical measuring methods for experimental research, theoretical analysis and Finite Element Analysis (FEA) are combined to analyze the influence of viscous medium on both stable and instable deformation behavior of specimens in Viscous Pressure Bulging (VPB). The influence of tangential adhesive stress on material flow, shape, stress and strain distributions and evolution of weak region of the specimen is obtained. The way how viscous medium promotes uniformity and forming limit of sheet metals is explained. The obtained results would help deeper understanding the interaction of viscous medium and sheet blanks and possess guiding significance for VPF applications.Firstly, the tension of specimens loaded by tangential adhesive stress is researched by viscous adhesive tensile test. Photoplastic method is used for qualitative and quantitative analysis of the specimens'strain distributions. Relation of stress and strain is also obtained. Deformations of viscous adhesive tensile specimens agree well with that of uniaxial tensile specimens. The surface loaded tangential adhesive stress can promote the continuity and stability of deformation. Strain localization of specimen is postponed as a result.Projection speckle digital correlation method is applied in the 3D shape measurement of viscous pressure bulge specimen. The influence of viscous medium and loading speed on specimens'shape, strain and stress distributions during the whole deformation process is analyzed. The tangential adhesive stress increases with viscosity of viscous medium and loading speed. The deformation of VPB specimen is promoted by viscous medium at positions away from the center of the specimen. However, viscus medium would restrain the deformation at the apex of the dome. The strain gradient along the specimen's radius is decreased. Curvature radius on the apex of dome is measured by PSCM to calculate flow stress. Flow stress increases with lower viscous medium viscosity or larger loading speed.Electronic speckle pattern interferometry is applied in the real-timely strain rate measurement of VPB. Strain localization and diffuse necking and localized necking of VPB specimen are analyzed though strain rate distribution. The evolution of weaken region, including defect emergence, diffuse necking, groove expanding and the onset of localized necking are observed directly and real-timely. The effects of viscous medium properties on the deformation of specimen from stable stage to failure process are discussed. Critical strains on diffuse necking and localized necking are determined with higher accuracy. Experimental results show that visocus medium can distinctly delays the evolution of weaken region and the onset of diffuse necking. The critical strain of diffuse necking is increased. So the forming limit is improved.Secondly, the effect of viscous medium on the instable deformation of VPB specimen is analyzed by finite element method (FEM). Plasticity with damage modle is used as material model to simulate the deformation of weaken region and predict necking. Each instable states including emergence of flaw, diffuse necking, localized necking and broken are simulated successfully by the proposed FEM model. Results such as weaken region's location, shape, and evolution agree well with the results measured by ESPI. This model indicates a new way for quality prediction and forming method optimization of VPF.A theoretical model of VPB is proposed to consider the effect of tangential adhesive stress on specimens'shape, stress and strain distribution. Variety of specimen's shape and strain distribution are explained. The location of largest strain is predicted. The results provide theoretical foundation for usage of vicous medium.