Material Plastic Properties Defects And The Formability Of Sheet Metal

摘要

A main thecnological concern of sheet metal forming is the analysis of the influence of material imperfections on its formability. The imperfections can be defined as local variations in thickness and in plastic properties which can affect the plastic flow and, therefore, influence the neck formation phenomena. From the mocrostructural point of view this could be due to the variations in grain size and orientation, inclusions and second phase contents, porosity, as well as residual cold work, originating from the melting, solidification and rolling processes. As consequence of these imperfections within the material, the thickness, strain hardening, strain rate sensitivity and strength coefficients and initial prestrain may exhibit local variations and thereby initiation of the neck and limit strains be influenced. Using the concept of strain gradient at the neck, the limiting strains are investigated with respect to the material parameters mentioned above. It is discussed the role of these defects on the development of the local neck and the forming limit diagram FLD. The predicted results are compared with Marciniak Theory and with the experimental results obtained by Azrin.