Mechanical behaviour of fibre reinforced composites

In composites, different materials are combined to exploit favourable properties of each. That such combinations may be attractive was already shown in section 6.4.4 for particle strengthening of metals and in section 7.5 for dispersion-strengthened ceramics. 

It is rather difficult to exactly define the meaning of the word composite . In the broadest sense, one might consider every material as a composite that comprises two physically distinct phases. However, using this definition would imply that almost every technically used material is a composite, for example almost all steels or precipitation hardened alloys, rendering the definition practically useless. Composites used today are characterised by the following properties  

To use these properties as definition is, however, problematic and, furthermore, not future-proof, for further developments (e. g., in nanotechnology) will surely lead to new composites which do not posses some of these properties. 

In this chapter, the focus is on fibre reinforced composites, or fibre composites, for short, in which the particles of the second phase are long fibres, surrounded by a matrix of the other component. One example is glass-fibre re- 

Fibres are frequently used as strengthening component because the load transfer from the matrix is especially effective if the strengthening phase is elongated in the loading direction. We will discuss this in some detail later on. Furthermore, fibres may be advantageous because they are rather thin with a diameter between 1 pm and 25 pm. Defects in the fibre are thus rather small. 

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A survey of the open literature indicated that the information available on the oxidation behaviour of continuous fibre-reinforced TMCs is extremely limited, although much work has been carried out to characterise the performance of these composite systems under severe mechanical loadings and temperature variations. Studies on the mechanical behaviours of fibre-reinforced TMCs clearly demonstrated that temperature and atmosphere had quite strong influences on the mechanical response of the material systems. Tests at elevated temperatures and in oxygen-containing atmospheres normally... 

Fibrous composites are a relatively new technology and there are too many material and structural variables to discuss fully all the details of their mechanical behaviour. Therefore, the present chapter will concentrate on certain categories of fibre reinforced composites, all with polymer matrix systems. 

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. 

Several recent papers have maintained interest in crystalline deformation processes. Shinozaki and Groves suggest that a theory proposed for fibre reinforced composites by Kelly and Davies is helpful in describing the tensile yield behaviour of oriented polypropylene. Three distinct failure mechanisms are proposed, as depicted in Fig. 8. 

The difference in thermal expansion between matrix and fibre reinforcement is an important parameter influencing the mechanical behaviour of the composites because it determines the residual stress distribution after fabrication [ 105]. In glass matrix composites, these residual stresses can be measured by photoelastic techniques [127]. 

Fibre orientation is another important parameter that influences the mechanical behaviour of short-fibre composites. This is because the fibres in such composites are rarely oriented in a single direction, which is necessary to obtain the maximum reinforcement effects. During the processing of short-fibre composites, a continuous and... 

Akil, H.M., Santulli, C., Sarasini, E, Tirillo, J., and Valente, T. (2014) Environmental effects on the mechanical behaviour of pultruded jute/glass fibre-reinforced polyester hybrid composites. Compos. ScL Technol., 94, 62—70. 

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