Damage Mechanisms of Plasticized Cellulose Acetate under Tensile Deformation Studied by Ultrasmall-Angle X-ray Scattering

We consider the microscopic mechanisms of damaging in plasticized cellulose acetate under tensile stress. We show how they appear and develop during the course of deformation until failure. By using scanning transmission electron microscopy, we observe the presence of cavities and the coexistence of homogeneous and fibrillar crazes. Ultrasmall-angle X-ray scattering experiments allow 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 strain hardening in their immediate vicinity where deformation and stress are amplified. Increasing the stress further leads to a new growth regime for a small fraction of crazes. Ultimate failure is due to the propagation of this small number of crazes and not to the accumulation of crazes and a coalescence process. We propose that this growth process is the consequence of homogeneous nucleation of new cavities just in front of existing ones. The volume fractions of damage remain very low, of order of 10(-4) at failure. The strain hardening behavior appears to be the key for preventing an early failure of the material and conferring high ductility to the material.