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Deformation behaviour and mechanical properties of polypropylene processed by equal channel angular extrusion: Effects of back-pressure and extrusion velocity

type de publication      article dans une revue internationale avec comité de lecture
date de publication 2009
auteur(s) Boulahia R.; Gloaguen Jean-Michel; Zaïri Fahmi; Naït Abdelaziz Moussa; Seguela Roland; Boukharouba T.; Lefebvre Jean-Marc
journal (abréviation) Polymer (Polymer)
volume (numéro) 50 (23)
  
pages 5508 – 5517
résumé Severe plastic deformation by equal channel angular extrusion (ECAE) is an ingenious deformation process 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. The ECAE-induced deformation behaviour was examined in relation to the load versus ram-displacement curves. 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. The strain-induced crystalline microstructure was investigated by X-ray scattering. Shear-banded samples exhibited a strong texturing of the (hk0) planes along the shear direction in the translucent bands whereas perfect crystalline isotropy appeared in the opaque bands. Application of back-pressure and/or reducing ram velocity resulted in uniform texturing along the extruded sample. Yet, texturing changed from single shear to twin-like shear orientation about the shear direction. 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. These effects were connected with the crystalline texturing. The results also revealed significant increase in the strain-hardening after ECAE. 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.
mots clés equal channel angular extrusion; back-pressure; extrusion velocity; polypropylene
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