A novel failure criterion for matrix dominant failure within fibre reinforced polymer composites
thesisposted on 02.03.2017 by Chowdhury, Nayeem Tawqir
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
This thesis investigates the failure behaviour of the matrix constituent found in composite materials. The investigation involves performing several experiments under both biaxial and uniaxial loading and subsequent finite element analyses of the experiment results. The results were used to establish a suitable matrix failure criterion. The need to understand matrix failure in advanced composites is of particular importance to the aerospace industry, where these materials are used in structural components which are optimised to be light weight yet strong. Experiments under biaxial tension required the designing of a suitable biaxial test fixture and a specimen design which is able to successfully demonstrate failure free of stress concentrators. In order to prevent any premature failure due to load interactions between two loading axes, removal of the cut-out corners found in cruciform specimens was very important. Reducing the central gauge region of the specimen was found to be the best way to induce a higher stress state within a predefined region of the specimen. The specimen design proposed focussed on testing material systems which are anisotropic in behaviour yet fail in the transverse direction. The biaxial tension experiments on a carbon-epoxy composite (EP 280 Prepreg) was accompanied with biaxial tension experiments on a neat resin epoxy material (EP 280 neat resin) and uniaxial off-axis tension experiments on the same material. Despite the differences in the experiments, the failure observed across them was predominantly the same where matrix tensile failure occurred. A fourth set of experiments were performed on uniaxial off-axis compression specimens to induce a compressive matrix failure mode. Over these four sets of experiments, the matrix failure envelope was able to be fully defined. Continuum modelling techniques were implemented through the use of representative volume elements to separately analyse the stress-strain states on the fibre and the matrix. The separation technique allowed the failure stress-strain states on the matrix to be examined in detail from which a failure criterion was established. Matrix failure was found to exhibit two modes of failure induced by the type of deformation (dilatational and distortional). The established failure criterion was similar to that proposed by Onset theory where a truncated region defined by the First Stress/Strain Invariant best captured matrix tensile failure (dilatational), whilst the Drucker-Prager failure criterion best described compressive and shear modes of failure (distortional). The study was concluded through a set of experiments performed under four-point bending which validated the hybrid failure criterion to accurately predict matrix failure in composites.