Fibre-matrix interfaces in thermoplastic composites: A meso-level approach

Abstract

A strong pressure towards efficiency improvements in the aviation sector is given by stringent environmental reduction goals and by the growing and volatile fuel prices. Such pressure is even stronger considering the steady growth of air traffic forecasted for the next 20 years. Structural mass reduction is one of the ways to achieve such efficiency improvements. High performance thermoplastic composites are possible candidates for primary aircraft structures: compared to metal, they have higher specific properties and corrosion resistance, compared to thermosetting composites, they are tougher and permit lighter assemblies. The application of thermoplastic composites in aircraft structures has been, so far, mostly limited to interiors, secondary and semi-structural parts. A more extensive use of such materials, for example in large primary aircraft structures, requires an increment of the performance to cost ratio of the products, in particular through the introduction of new materials on the market. The development of new fibre-matrix combinations is however not sufficient: good fibre-matrix interface properties are crucial for obtaining good mechanical and durability properties of the materials. The knowledge on carbon fibre-high performance thermoplastic matrix adhesion in the public domain is very limited, being most published studies focused on thermosetting composites and glass fibre sizings for engineering thermoplastic composites. The development of optimised pretreatments and sizings for thermoplastic matrices requires a deeper study and understanding of the fibre-matrix adhesion mechanisms and properties in the existing systems. Carbon fibre reinforced PPS composites are relatively widely used in aircraft structures and their application in large primary aircraft structures is promoted. Little research work has however been published about the fibre-matrix interface mechanisms in carbon fibre PPS composites and no work has been reported about the effect of the interface on the mechanical behaviour of the composites. Furthermore, published composite-level studies about the fibre-matrix adhesion are mostly limited to unidirectional and cross-ply composites, leaving the field of woven fabric reinforced composites almost untouched. The research work was therefore aimed at gaining a basic characterisation of the fibre-matrix interfacial mechanisms in carbon fibre PPS composites and at characterising in-depth the effect of the interface on the mechanical behaviour of woven composites. The chemical and morphological characteristics of the carbon fibres were studied. The extra surface treatment carried out by TenCate on the fibres provided by the manufacturer was found to at least partially remove the original sizing and consequently to enhance the fibre surface area. Possible mechanisms taking place at the fibre-matrix interfaces in PPS composites were discussed. In composites containing the original fibres, a sizing interlayer, or an interphasial area in which diffusion of the sizing has taken place, is present. In Cetex PPS commercial composites, a stronger mechanical interlocking mechanism is established. In order to obtain an in-depth characterisation of the woven thermoplastic composite behaviour, static mechanical properties, damage development and failure modes were studied. Interlaminar shear, flexural, interlaminar fracture toughness, tensile, and in-plane shear properties of the composites were discussed and coupled with microscopy analysis of the failed samples. Higher interlaminar fracture toughness and cohesive failure were obtained in composites with relatively strong interface, adhesive failure in those with poor interface. Interlaminar shear, flexural and in-plane shear strengths and in-plane shear modulus were found to be notably affected by the fibre-matrix interface. In all testing conditions, the damage evolution and sample failure were enormously affected by the interfacial characteristics: delamination was found to be the prevalent failure mode in composites with poor interface, whereas brittle failures were obtained in those with strong interface. Non-contact measurement of the strain fields in in-plane shear and tensile testing was introduced as a qualitative assessment methodology for the fibre-matrix: a uniform distribution of strain was obtained in samples with relatively good interface, contrary to those with poor interface. In in-plane shear tests, the strain fields provided a visual representation of the stress-transfer ability of the interface whereas in tensile tests, the study of the strain fields allowed early identification of damage and understanding of the failure mode. An outlook on the durability of the composites was also given through in-plane shear testing, damage development and local strain studies of composites after exposure to high temperature water, Chapter 8. Exposure to water was found to anticipate, but not alter, the failure mechanisms observed in in-plane shear testing of dry samples.