The Use of the DIC Method to Involve the Strain Instability Occurred in an Undergoing High Shear during the ECAE Process

Abstract

Equal-channel angular extrusion (ЕСАЕ) is one of the most efficient methods of severe plastic deformation of materials used to modify texture and microstructure without reducing sample cross-section. The application of single ECAE pass to polypropylene (PP) was meticulously investigated at room temperature using a 90° die-angle tooling. Depending on extrusion conditions, PP displayed various types of plastic flow. For ram velocities beyond 4.5 mm/min, severe shear bands consisting of successive translucent and opaque bands were observed, accompanied on the top surface by more or less pronounced periodic waves. Although the application of a back-pressure significantly reduced the wave and shear-banding phenomena, slightly inhomogeneous shear deformation was still observed. Shear bands were only suppressed by decreasing extrusion velocity. Application of backpressure and/or reducing ram velocity resulted in uniform texturing along the extruded sample. Mechanical properties changes of the extruded samples due to back-pressure and extrusion velocity effects were analyzed via uniaxial tensile tests. The tensile samples displayed multiple strain localizations in shear banded materials whereas quite homogeneous deformation appeared for non-banded ones. Digital image correlation technique suitable for large deformation was used for determining the full-field strain of the tensile samples in relation to tensile strain and ECAE conditions.