Prediction of Failure in Incremental Forming Using Stress Based Forming Limit Diagram

Abstract

Forming Limit Diagrams are used to predict a failure in sheet metal forming but a main problem for strain based forming limit diagrams is inability to predict failures accurately in various sheet forming operations where there is a change in strain path and mode of deformation whereas the forming limit stress diagram FLSD can reveal formability limit of material more precisely. Strain based criterion is valid only when the strain path is linear throughout the whole deformation process but mostly the strain path is never linear in incremental sheet forming processes whereas stress based plot does not varies like strain based Forming Limit Diagram. Therefore the Stress based forming limit criterion is not depend on strain path.This is expected to resolve many problems associated with strain based FLDs. So stress based criterion be used for predicting forming limits in forming operations where strain ratio does not remain constant. There are several situations in forming operations where stress based FLD can be applied to predict failure correctly like incremental forming, hydroforming, multistage forming, deep drawing of pockets etc. So the different yield criterion is used to transform strain based FLD into stress based FLD. To establish strain based fracture forming line, two different shapes namely straight groove & cone have been used. From strain based FLD, stress based FLD is prepared using Hills quadratic criterion.