Deformation and Failure Behaviour of Self-reinforced Polypropylene and Self-reinforced Polypropylene/Steel Fibre Metal Laminate

Abstract

This thesis discusses material behaviours related to the failure of woven self-reinforced polypropylene (SRPP) and a fibre metal laminate (FML) based on SRPP and mild steel. Single polymer composites such as SRPP are relatively new, as compared to the traditional fibre-reinforced composites, and while there have been studies that investigate the optimal manufacturing methods, there are relatively few that look at the failure behaviour of SRPP in depth. On top of this, SRPP exhibits behaviours that are quite different from most traditional fibre-reinforced polymers due to its unique construction. As a result, there is also limited knowledge of the behaviour of SRPP-based FMLs, except under impact. A better understanding of these materials will be valuable, given that they have a lot of potential uses in many applications, especially for those that can benefit from recyclability, high impact resistance and weight reduction. For this reason, failure, and related deformation behaviours of SRPP and SRPP/steel FML, were investigated.

The material behaviours were studied using different reinforcing directions under uniaxial tension, and different specimen geometries under combined in-plane biaxial and out-of-plane bending deformations, for both the SRPP and FML. In addition, specimens of different thicknesses were studied for SRPP under uniaxial tension, which behave differently and give important insights into the failure mechanisms. Analyses were carried out using a combination of microscopy and surface strain analysis, which can effectively be used to study various damage types in SRPP and their relation to the failure of SRPP and FML.

In this research, various types of damage and related mechanisms were uncovered, some of which have not been previously reported in the literature. One of the most important aspects was that the critical damage mechanisms that cause failure in the materials were identified for different material and loading conditions. This includes a particular type of matrix damage in SRPP which was found to cause an unusually high strain concentration and, as a result, lead to material failure in some cases. It was found that failure from such damage can be suppressed in some specimens which exhibit mechanisms that can impede damage growth.

It was found that the process of damage development, and how this relates to the failure behaviour, can depend on one or more of the following factors, some of which are related: damage type, presence (or lack) of toughening mechanisms, mode of crack propagation, reinforcing direction, weave geometry, sensitivity to local damage, loading condition, material thickness, and consolidation quality. Some of these factors can influence the failure behaviour to the point that the same type of specimen, subjected to the same loading condition, can fail from different regions of the material, under different failure mechanisms. In the course of the analysis, it was found that, in many cases, the surface strains captured during the deformation process can indicate where, and under which conditions, failure occurred.

The findings from this research highlighted the importance of understanding the exact mechanisms behind the failure of SRPP, SRPP-based FML and similar materials, since they can vary significantly depending on numerous factors. It is anticipated that the findings from this study will lay the groundwork for future research on developing failure criteria for such material systems for the benefit of researchers and designers using composite and hybrid materials.