THE EFFECT OF DISCONTINUOUS STRAIN PATH ON THE FORMING LIMIT DIAGRAM FLD IN SHEET MATERIAL

Abstract

ABSTRACT It has long been recognised that the Keeler and Goodwin's forming limit diagram FLD is an useful analytic tool commonly employed in assessing the formability of sheet materials. Normally, the forming limit diagrams obtained are assumed to be under constant strain ratio or in another word, under linear strain path condition. Whereas in actual processing, such as complex forming or redrawing, the deformation will no longer follow a simple linear strain path and in most cases, a complex or even a discontinuous strain path will prevail. In view of this complication, this paper intends to address some of the uncertainty created when complex or discontinuous strain path are involved. Prediction of discontinuous limit strains based on the general framework of the Marciniak-Kuczynski model were obtained and compared with the experimental results. It can seen that with strain histories where one of the principal strain increment is negative, the subsequent limit strains obtained from both theory and experiment are higher or lower than the ones obtained under linear strain path depending on whether the subsequent deformation is biaxial tension or tension-compression.