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A finite element and experimental study of plastic compression for metal forming Houlston, Robin


Metal-forming involves the deformation of metals for the purposes of manufacturing a product. An understanding of the effect of process variables on metal forming operations is of fundamental importance to the engineer. However, little information is available regarding the effect of strain hardening and strain rate sensitive material properties, dynamic loading, surface friction and specimen dimensions on the way components deform during the forging process. In this work the plane strain compression of an initially rectangular specimen between flat, parallel and rigid platens is selected for investigation as being representative of a basic die forging operation. This configuration allows the complete deformation history of quasi-statically or dynamically deformed specimens to be recorded photographically and the effect of process variables identified. A finite element model is developed for this case. The code accounts for large strains, nonlinear material properties, inertia effects and surface friction on all boundaries. The results of dynamic compression tests on plasticine and quasi-static compression tests on aluminum are compared to the finite element code predictions. They give good correspondence over a large range of strain history. Further studies conducted with the finite element model identify many of the fundamental characteristics of the forging operation. It is shown that rigid-perfectly plastic material tends to deform along lines of intense shear and can be approximated by certain upper bound solutions. The normal interface stress distribution for this case is very different from the classical friction hills that may sometimes be assumed. A more normal type of friction hill stress distribution is obtained with strain hardening and strain rate sensitive materials. The lines of intense shear become wider and give more homogeneous deformation for this case. With dynamic loading inhomogeneous deformation occurs as the energy of impact is rapidly diffused throughout the specimen. It is possible that an inverse friction hill develops on the lower platen and a friction hill on the top platen.

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