Creep of Solids - Micromechanisms of Deformation of Semicrystalline Thermoplastics and Their Long Term Behaviour

Abstract

A brief review is given of the relative importance of parameters determining the long time behaviour of thermoplastics (chain topology and mobility, sample morphology, physical and chemical aging, creep deformation). An example is then given, namely results from a study of the response of polyoxymethylene (POM) to static and dynamic load conditions at different temperatures. Unstable crack propagation occured in POM at K_app ≤ 1.67 MPa m^½, under static conditions at 100°C, corresponding to the ductile-brittle transition observed in the un-notched tensile specimens. The slow crack growth behaviour was described by a two stage mechanism involving interlamellar void formation and the appearence of a wedge shaped fibrillar deformation zone ahead of the crack tip (first stage) followed by stable crack propagation via breakdown of the fibrils (second stage). A strong increase in the time to failure with the number average molecular weight M_n was found, which has been accounted for in terms of disentanglement of the molecules in the fibrils.