Evaluation of the Mean Plastic Strain Ratio rm of Metallic Sheets Prestrained Along Different Biaxial Strain Paths

Abstract

Abstract ASTM standards are widely used to determine the plastic strain ratio r to characterize the normal anisotropy and the planar anisotropy of sheet metal. The values determined by mechanical testing are quoted at a strain of 15%. These are used for comparing two techniques of measuring the plastic strain ratio or the anisotropy of two materials. The present work reports the variation in the plastic strain ratio of prestrained material. The r values of biaxially prestrained steel sheet have been investigated using the ASTM as well as the magnetostrictive methods of measurement. An error analysis on the mechanical measurements has been performed to examine the agreement between the different definitions of the plastic strain ratio (rr, rint, rt, and rmx1) and the accuracy (based on coefficient of variation), in view of the need to measure the plastic strain ratio at low strain levels. Various means to ensure accuracy in measurement are described. Plastic strain ratio of sheet deformed to different biaxial prestrains has also been characterized using the magnetostrictive method (Modul r test). It is found that the difference between the r values determined using the ASTM and the Modul r methods changes with strain and strain path. The nominal difference between the two methods was found to be comparatively consistent over strain paths ranging from plane strain to equibiaxial tension. The Modul r values for prestrains along these strain paths investigated were found to be relatively consistent and hence usable. For prestrain along the strain path β = −0.45 (negative minor strain regime), the nominal difference was considerable. In view of the above, it is not appropriate to measure rapidly the r value of the material in the negative minor strain regime, in particular using magnetostrictive means, and to correct it using the “systematic error” between the two techniques. This restricts the usability of the Modul r test in measuring the plastic strain ratio of deformed sheet metal, particularly after a prestrain in the negative minor strain region.