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A workability test procedure has been developed using cylindrical upset tests and bend tests. Variation of the upset specimen aspect ratio (height-to-diameter ratio) and of lubrication of the compression dies produces a variety of stress and strain combinations at the free surface of the cylinders. Bend tests extend the range of available stress and strain states, and are also useful in cases where cylindrical upset test specimens are impractical. In either test, measurements of grid deformations at the surface permit calculation of the surface strains and, with the aid of plasticity equations, surface stresses.
Results show that the surface tensile and compressive strains at fracture fit a straight line relationship having a slope of −½ and an intercept along the tensile strain axis. The intercept depends on material, composition, structure, and surface condition. Such results have been found for a variety of ferrous and nonferrous alloys, as well as sintered powder metals at cold and hot working temperatures.
The fracture-strain relationship determined by workability tests can be used for workability analysis of bulk-forming processes. Progressive strain relationships in critical regions of the workpiece of interest are determined through plasticity analysis or through measurements on model materials. These relationships are compared with the fracture strain relationship to determine the likelihood of defect formation and to suggest alterations of the process for improved workability. This approach has been used for analysis of cracking in various modes of flow generic to forging, such as formation of hubs, flanges, rims, and gear teeth; it has been also applied to specific forged parts.
metalworking, workability, forging, ductility, deformation, fracture properties, bend tests, torsion tests, tension tests, formability
Professor of Metallurgical and Materials Engineering and professor of Mechanical Engineering, University of Pittsburgh, Pittsburgh, Pa