A Micro Raman Investigation of Viscoelasticity in Short Fibre Reinforced Polymer Matrix Composites

The purpose of the present Ph.D. project is to investigate the load transfer mechanisms between the fibre and matrix and the stress/strain fields in and around single fibres in short fibre reinforced viscoelastic polymer matrix composites subjected to various loading histories. The materials considered are high modulus carbon fibres embedded in a polypropylene matrix. The polypropylene matrix displays nonlinear viscoelasticity and its constitutive behaviour is modelled using the Schapery model. The investigation of the load transfer mechanisms and the local stress/strain field is based on experimental work conducted on model composites consisting of one or a few fibres embedded in the polymer matrix. The fibre strains are measured in situ during loading, using micro Raman spectroscopy. Different loading histories are applied to the test specimens; Creep loading, mechanical conditioning and subsequent creep, creep loading and subsequent recovery, creep loading at an elevated temperature, creep loading of specimens with misaligned fibres and creep loading of specimens with interacting fibres. Experiments have shown two different load transfer mechanisms. The first which is of a friction-like nature where the load of the fibre is governed by the coefficient of friction and the initial radial pressure on the fibre, stemming from manufacture. The second load transfer mechanism is due to improved adhesion between the fibre and matrix obtained by grafting the polypropylene with maleic-anhydride. The experimental results for the two load transfer cases are subsequently used to determine the interfacial shear strength. When the load transfer is friction-like the interfacial shear strength is rather low, whereas the interfacial shear strength is governed by the matrix shear yield strength when the load transfer is due to adhesion between the fibre and matrix. In order to investigate the nonlinear stress/strain field due to the two load transfer mechanisms, two qualitative approaches for analysing the single fibre composite - subjected to creep loading conditions - have been proposed and is used along with nonlinear finite element models. The two models are based on the integration of point forces along the fibre boundary and the major difference of the two approaches is that the first uses experimental inputs, whereas the second is purely theoretical. The two models are plane models and are capable of taking misalignment of the fibres, with respect to the loading axis, into account. In the two models as well as in the finite element models thermal stress/strain from manufacture is taken into account. It is shown that the initial residual stress in the specimen from manufacture may have a beneficial influence on the loading transferred to the fibres, i.e. the residual stresses may prohibit tensile fibre fractures. Selected experimental results are compared to calculated results. Overall creep behaviour of short fibre reinforced composites is predicted using the Mori - Tanaka mean field approach where the fibres are modelled as volumes containing equavalent eigenstrains. These eigenstrains are related to strain through the Eshelby tensor. In the present work the Mori - Tanaka mean field approach has been extended to incorporate both the effect of a weakened interface through a modified Eshelby tensor and the effect of different fibre orientations through a statistical fibre orientation function. The degree of weakening of the interface is governed by an interfacial parameter based upon the interfacial shear strength obtained from experiments. Different fibre orientations are considered in terms of stiffness and creep properties.