Fibre Failure due to Thermal Residual Stresses in Model Polymer Based Composites

Abstract

Thermal stresses in a fibre reinforced composite are determined by the interaction between the solidifying matrix and the fibre. In composites with a semicrystalline matrix solidification is associated with crystallisation. In semicrystalline thermoplastic matrix composites transcrystallisation often takes place. Transcrystallisation occurs when spherulities are heterogeneously nucleated on the fibre surface. As the spherulities impinge they grow radially from the fibre forming the transcrystalline interlayer.

For a brittle fibre in a polymer matrix, the resulting residual stresses may exceed the compressive strength of the fibre. The fragmentation of the fibre is dependent on the load transfer from the fibre to the matrix and the transcrystalline interlayer plays a dominant role in controlling the level of thermal residual stresses.

Polarised Raman microspectroscopy has been used to determine crystal orientation within the transcrystalline interlayer of a carbon/polypropylene matrix composite. From the Raman scattering on samples subjected to varied thermal history a model of the conformational states within the microstructure of the polypropylene was proposed. This model is comprised of three phases: a crystalline phase, an isomeric defect phase and an amorphous melt like phase. Effective properties of the polymer can be predicted from the derived local response in terms of the parameters characterising the crystalline microstructure.

Having established a valid model for the thermal stress evolution, it is possible to determine the thermal stress distribution after cooling of the composite. In semicrystalline matrix systems cooling rate was found to have a pronounced effect on thermal stresses due to the rate dependence of the crystallisation as well as stress relaxation in the matrix. Theoretical predictions were compared to experimental results of thermal stresses and a close agreement was obtained.