Fibre-dominated properties

Fibre choice is complex where cost often outweighs performance. For industrial composites, E glass therefore dominates. In aerospace, where cost can be spread over a longer life, often with savings in maintenance and service, carbon and other high-performance fibres are selected. For example, the high-modulus carbon fibres provide the highest dimensional stability of satellite dishes, where performance is critical. 

For pitch precursors, the refinement and spinning processes lead to larger crystals with better alignment to the fibre axis and hence a higher modulus. 

With respect to the effect of environments, carbon fibres can be largely considered to be inert since they are stable to higher temperatures than the resin matrices can withstand. At temperatures above 300 °C, the fibres begin to degrade in oxidising atmospheres, but most polymers have a lower maximum service temperature. The advanced polyimides and PMR (polymerisation of monomer reactants) systems can survive temperatures up to 450 °C, but these are usually in short-term applications [27]. It is also unlikely that composites will be in contact with damaging solvents such as concentrated oxidising acids such as sulphuric and/or nitric acids. 

The fibres are not resistant to the temperatures encountered in metal and ceramic matrix composite melt processing, and protective coatings or low-temperature manufacturing routes are needed. 

Aerospace composite structures can also employ high-performance polymeric fibres such as the aramids Kevlar (DuPont) or Twaron (Teijin Twaron), poly(p-phenyl-ene-2,6-benzobisoxazole) (PBO) (Toyobo) and high-modulus polyethylene (PE) (Dynema, Certran and Spectra). Generally ultra-high-molecular-weight PE (UHMWPE) can be considered to be inert to most environments except that the service temperature is limited to 130 °C. 

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With all the simplicity of the OA method it produces reasonable predictions for the fibre-dominated properties and low crimp, when deviations from the iso-strain assumption play a minor role. For off-axis properties, or for cases when transverse parts of the reinforcement (e.g. weft yams for warp-direction loading) play an important role, or in the presence of high-crimp yams (e.g. 3D reinforcements with significant fraction of binder), the iso-strain assumption is not valid any more, and quality of the OA predictions is decreased. There are approaches that combine iso-strain and iso-stress formulations [3], but these approaches leave open the choice of the combination mles open, which makes their predictive abilities limited. 

It should be noted that the composites damage and failure models discussed here are based on an assumption of rate-independent behaviour, and materials properties used for validation studies are based on quasi-static tests. This is currently standard practice in impact simulations of composite structures, which is an important assumption in the work. Reasons for this are the lack of international dynamic test standards for measuring rate-dependent composites properties, so established test procedures are missing. Dynamic failure models for composites are not well understood nor implemented in current commercial FE codes. In justification it should be pointed out that carbon fibres are highly elastic, thus fibre-dominated properties show no... 

The importance of stress amplitude during fatigue is quite marked for this type of reinforcement (combination glass fabric). The difference between matrix dominated and fibre dominated properties is determined by the presence of a knee point in a static load/deflection test (Figure 1.5(c)). The average loss in strength per decade of cycles approaches 10%, which is typical of glass-reinforced plastics. 

Glass fibres dominate this field either as long continuous fibres (several centimetres long), which are hand-laid with the thermoset precursors, e.g., phenolics, epoxy, polyester, styrenics, and finally cured (often called fibre glass reinforcement plastic or polymer (FRP)). With thermoplastic polymers, e.g., PP, short fibres (less than 1 mm) are used. During processing with an extruder, these short fibres orient in the extrusion/draw direction giving anisotropic behaviour (properties perpendicular to the fibre direction are weaker). 

Most structural composites are highly anisotropic, consisting of unidirectional materials where the mechanical properties in the longitudinal (or 0°) direction are fibre dominated, while in the transverse (or 90°) direction they are matrix dominated. 

While composite materials owe their unique balance of properties to the combination of matrix and fibres, it is the fibre system that is primarily responsible for strength and stiffness. However, the fibre dominates the field in terms of volume, properties and design versatility. 

This chapter is concerned with the short-term mechanical properties — moduli and strengths — of glass, aramid and carbon fibres in a thermosetting resin matrix. A little information on reinforced thermoplastic matrix systems is also included. The data mainly refer to the room temperature properties of 55-65 v/o fibre, unidirectional, systems. The effects of the variation in fibre volume loading, method of test and instantaneous and long term exposure to temperature are briefly mentioned. Longitudinal properties tend to be fibre dominated, and so are compressive properties to some extent for glass and carbon fibres. The anisotropy of unidirectional materials is noticeable. 

Figure 10.15 Tensile fatigue of different resin types and blends showing fibre and matrix dominated properties (Echtermeyer/SAMPE).

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