The yield stress of a tension specimen machined from a 2-m wide by 0.6-cm thick copper sheet is 145 mpa. the sheet is rolled further with an applied tensile force in the plane of the sheet of 0.22 mn. what roll pressure is needed to just cause yielding? you may ignore friction between the metal and the rolls. for rolling, the deformation occurs by plane-strain in which there is no increase in the width of the sheet. you may also assume that for plastic flow the poisson's ratio v 0.5, and the copper obeys von mises criterion