Damage mechanisms of plasticized cellulose acetate under tensile deformation studied by Ultra Small X Ray Scattering Didier LONG

Amphithéatre Pierre Glorieux - CERLA

Damage mechanisms of plasticized cellulose acetate under tensile deformation studied by Ultra Small X Ray Scattering
Didier LONG
Laboratoire "Polymères et Matériaux Avancés" Unité Mixte de Recherche CNRS/Solvay We consider the microscopic mechanisms of damaging in plasticized cellulose acetate under tensile stresses, how they appear and develop during the course of deformation until failure. We observe by using Scanning Transmission Electron Microscopy the presence of cavities and the coexistence of homogeneous and fibrillar crazes.  Ultra Small Angle X-ray scattering experiments allows for describing the onset of damaging and the growth of crazes and for measuring the volume fractions of damages as well as their shapes and sizes at different stages during deformation. We propose that damages are initiated by the nucleation of cavities in the vicinity of pre-existing impurities. We then show that their initial growth after nucleation is blocked at a size of order 100 nm by the strain hardening in their immediate vicinity where deformation and stress are amplified. Increasing the stress further leads at some critical stress value to a new growth regime for a small fraction of crazes. We attribute this growth mechanism to homogeneous nucleation of new cavities just ahead of them. Failure of our samples is due to the propagation of a small number of crazes in a late stage of deformation and not to the accumulation of crazes and a coalescence process. The volume fractions of damage remain very low, of order of 10-4 at failure. The strain hardening behavior appears to be key for preventing an early failure of the material.