Study of inhomogeneous deformation of stretch-forming sheet metal using digital image processing technique

Constitutive equations are developed so as to be able to predict the deformation and fracture response of materials. It is our task to develop useful relations applicable under a wide range of conditions and for many materials, containing a relatively small number of material parameters, which can be measured in simple tests. This work deals with developing such relations primarily to study the inhomogeneous deformation of stretch-forming sheet metals. The main objective of this research was to demonstrate the feasibility of upgrading old and tedious approaches of obtaining the material parameters to a computerized process in an affordable, fast and technically efficient manner. The approach of synthesizing experimental results with the theoretical predictions is followed to identify the parameters which enter the constitutive description of materials. The instability, localization and post-localization deformation phenomena are examined using a digital image processing (DIP) technique for computation of strain field. Computer interfacing was carried out and a powerful and an efficient package of software was developed for monitoring and managing the DIP system and for analyzing material flow in uniaxial tension stretching. The role of microstructure is also examined by optical and scanning electron microscopy.; The surface gridding technique coupled with some kind of photography is widely employed to analyze metal forming processes. Although researchers have developed computerized procedures to simplify data reduction process, the gridding technique along with ordinary photography is still laborious, which logically led to the use of DIP. A planarly isotropic 70/30 cartridge brass and a highly textured 2090 Al-Li alloy are chosen to consider the effect of anisotropy and texture on flow localization.; With the introduction of DIP for the measurement of strain, the large deformations close to the fracture point are successfully monitored. The phenomena of necking, localization and post-localization deformation were characterized since the system could capture the nonuniformly developing strain field at a rate of at least a thirtieth of a second.; An analytical material model is developed making use of vertex type plasticity and tricomponent yield surface representing a full plane stress state. The model was used to predict yield stress and critical strains at failure. Analytical results were compared with the experimental findings. The investigations proved that the experimental setup is an efficient and accurate tool for studying basic material behavior as well as for determining the important parameters controlling the formability of materials.