Fibre-reinforced composites factors

One of the key factors which make plastics attractive for engineering applications is the possibility of property enhancement through fibre reinforcement. Composites produced in this way have enabled plastics to become acceptable in, for example, the demanding aerospace and automobile industries. Currently in the USA these industries utilise over 1(X),000 tonnes of reinforced plastics out of a total consumption of over one million tonnes. 

Polymer materials are frequently used under stress loadings and these may be concentrated at certain parts of the structure. Thermal stresses may be induced by non-uniform heating or by differential expansion coefficients the latter may be an important factor in the degradation of fibre-reinforced composites in the radiation environment of space. 

George et al. [27] studied stress relaxation behaviour of pineapple fibre-reinforced polyethylene composites. They found stress relaxation to be decreased with an increase of fibre content due to better reinforcing effect It is also reported by George et al. [28] that properties of fibre-reinforced composites depend on many factors like fibre-matrix adhesion, volume fraction of fibre, fibre aspect ratio, fibre orientation as well as stress transfer efficiency of the interface. Luo and Netravah [29] found an increase in the mechanical properties of green composites prepared from PALFs and poly(hydroxybutyrate-co-valerate) resin (a biodegradable polymer) with the fibres in the longitudinal direction. However, the researchers reported a negative effect of the fibres on the properties in the transverse direction. 

Polymer-matrix fibre-reinforced composites offer an extremely high stiffness weight ratio to the engineer, and this factor, in combination with resistance to corrosion and relatively low looUng costs, has led to extensive use in stressed applications. 

Figure 6.7 Loss factor versus temperature plots of glass fibre-reinforced composites based on epoxies of different functionalities as matrices...

Therefore, in SiC fibre-reinforced sialon composites, thermal treatment of fibres, thermal mismatch and chemical reactions between the fibre and the sialon matrix are significant factors affecting the interfacial bonding in these composites. The sintering additive plays an important role in controlling the nature of the interface and requires careful selection. 

In-plane alignment of the fibres Due to the very nature of the technique used for processing the NW composites, inplane alignment of the NWs is a realistic possibility. From the Krenchel theory of short-fibre reinforcements [20], the orientation and length effects can be incorporated using an efficiency factor to evaluate E,... 

In principle, there are as many combinations of fibre and matrix available for textile-reinforced composites as there are available for the general class of composite materials. In addition to a wide choice of materials, there is the added factor of the manufacturing route to consider, since a valued feature of composite materials is the ability to manufacture the article at the same time as the material itself is being processed. This feature of composite materials contrasts with the other classes of engineering materials (metals, ceramics, polymers), where it is usual for the material to be produced first (e.g. steel sheet) followed by the forming of the desired shape. 

Almost as critical in commercial practice as the effects of reinforcement on proi>erties are the effects of reinforcement on the cost of the material and on its processing. The perceived effect of material cost depends on whether the decisive factor is cost per unit mass or cost per unit volume. Since the additive normally has a density considerably different from that of the po er matrix, the density of the composite differs from that of the polymer. Consider the fibre-reinforced polymer shown schematically in Figure 6.4. A mass m of composite occupies a volume u. It contains a mass of fibres occupying a volume and a mass of matrix occupying a... 

All of the different fibres used in composites have different properties and so affect the properties of the composite in different ways. The mechanical properties of most reinforcing fibres are considerably higher than those of unreinforced resin systems. The mechanical properties of the fibre/resin composites are therefore dominated by the contribution of the fibre to the composite. The four main factors that govern the fibre s contribution are (i) the basic mechanical properties of the fibre itself (ii) the surface interaction of fibre and resin (the interface ) (iii) the amount of fibre in the composite ( fibre volume fraction ) and (iv) the orientation of the fibres in the composite. 

Fibre-reinforced polymer composites (FRP) are lightweight and very durable, but their initial cost is higher than that of steel. However, these materials compare well on the basis of their lower construction and long-term maintenance costs. FRP have not realised their potential in construction applications. Contributing factors include the perceived expense and a lack of confidence on the part of clients and designers. 

The stiffness of polymer-based composite systems depends on numerous factors such as the stiffness of constituents, the volume fraction of each component, and the size, shape and orientation of reinforcements. As a whole there are three distinct types of polymer composites continuous fibre-reinforced polymer composites, short fibre-reinforced polymer composites, and polymer nanocomposites. Theoretical models based on micromechanical models are well developed and provide an adequate representation of composite stiffness. These micromechanical models are formulated based on assumptions of continuum mechanics. However, for nanocomposite materials, with fillers of size approximately 1 nm compared to the typical carbon fibre diameter of 50 tm, the rules and requirements for continuum... 

The parameter is a measure of fibre reinforcement of the composite materials that depends on several factors, such as fibre geometry and fibre arrangement. The exact elasticity calculations predict the moduli to rise faster with increasing fibre volume fraction above 0.7 as compared to the Halpin-Tsai equation [25]. The following empirical expressions were suggested by others [26] ... 

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