Criteria for prediction of plastic instabilities for hot working processes (part I: theoretical review)

Abstract

Hot working processes often induce high levels of strain at high strain rates, and impose very complex multi-axial modes of solicitation. These processes are essentially limited by the apparition and development of plastic instabilities. These may be the direct cause of rapid crack propagation, which lead to a possible final rupture. The complexity of strain modes and the simultaneous intervention of several parameters have led many researchers to develop various criteria, with different approaches, to predict the occurrence of defects and to optimize the process control parameters. The aim of the present paper is to summarize the general characteristics of some instability criteria, widely used in the literature, for the prediction of plastic instabilities during hot working. It was considered appropriate to divide the work into two parts. Part I presents the phenomenological criteria for the prediction of plastic instabilities, based on descriptive observation of the microscopic phenomena of the strain (strain hardening and strain rate sensitivity), and discusses the continuum criteria based on the principle of maximum rate of entropy production of irreversible thermodynamics applied to continuum mechanics of large plastic flow. Also, this part provides a bibliographical discussion among several authors with regard to the physical foundations of the dynamic materials model. In Part II of the work, a comparative study has been carried out to characterize the flow instability during a hot working process of a medium carbon micro-alloyed steel using phenomenological and continuum criteria.

Keywords:

• hot working
• plastic instability criteria
• dynamic material model
• flow localization

Acknowledgement

One of the authors, Anas Al Omar, would like to thank the CICYT (Spain) for their financing through project DPI2007-63665.