DEVELOPMENT OF MICROBANDS IN MILD-STEEL DURING CROSS LOADING

Mild steel specimens are submitted to a complex strain path (tension-s hear sequence). The stress decrease recorded at the beginning of the s econd path is associated at the grain scale with a localization of the deformation in microbands. These microbands are associated with a sin gle crystallographic slip system and carry the main part of the impose d strain. The prediction of the active slip systems during both the pr estrain and the second path with the Taylor and static models leads to a necessary condition for the development of microbands: the slip sys tem having the maximum Schmid factor must have been latent during the prestrain. At high strain level, the material resumes strain hardening and the dislocation structure is then composed of sheets parallel and /or perpendicular to the shear direction. The origin of the flow local ization and the transition between the two types of structures, i.e. m icrobands and dislocation sheets, are then discussed.