STRAIN DISTRIBUTION IN COPPER TENSILE SPECIMENS PRESTRAINED IN ROLLING

Sequences of orthogonal rolling-tension experiments were performed on polycrystalline copper sheets. The effect of strain path change on sub sequent yield and flow behavior has been investigated. Optical microsc opy and transmission electron microscopy (TEM) were used to clarify th e physical mechanisms occurring during the second deformation. The obs erved increase in yield stress in reloading was related to the change of slip systems corresponding to the glide of dislocations with a Burg ers vector, which had not been active during prestrain. The transient observed in the work-hardening behavior after the path change correspo nds to the appearance of disorganization in the dislocation microstruc ture. It was shown that no special feature of slip behavior inside the grains can be related to the nonhomogeneous surface deformation obser ved at the beginning of reloading. Also, the plastic instability of pr estrained samples corresponding to the maximum load in tension does no t seem to be directly controlled by the developed local substructure. The nonuniform deformation observed in reloading was studied using a s implified macroscopic two-zone model. It takes into account the presen ce of geometrical defects in the samples and considers the importance of the mechanical behavior. The macroscopic results, concerning the de lay of starting deformation in some regions, are explained by the mode l, which allows formulation of an analytical condition necessary for d eformation to spread through the length of the sample before necking t akes place.