CRITERIA OF THE ONSET OF INSTABLE DEFORMATION OF AN ORTHOTROPIC SHEET-METAL

Based upon the principle of minimal strain energy and the general flow rules corresponding with the yield function, the criterion of the ons et of instable deformation of an orthotropic sheet metal in the tensil e deformation process is analyzed in the present paper. In this analys is, all binds of those existing postulations, such as the presence of an initial defect in the material and a branch of its yield surface, a re abandoned. It is shown from the analysis that for an infinitely sma ll element of the orthotropic sheet metal, generally, when its load-be aring ability in the direction of maximal principal strain increment r eaches the limit, it begins to lose its deforming instability. In a fe w of strain fields, the instable deformation occurs rather later till the following condition arrives. The stated condition is that the inst antaneous direction in which the element's load-bearing ability just r eaches the limit at that time is moving to the orientation just perpen dicular to the direction of zero strain increment. These criteria are first revealed by this paper.