CONTENTS 3 Short fibre composites

Like other composite systems, the properties of short-fibre composites are strongly determined by the fibre concentration. The variation of the composite properties with fibre content can be predicted using the rale of mixtures, which involves the extrapolation of both matrix and fibre properties to a fibre volume fraction of 0 and 1. The following criteria must be taken into account ... 

The influence of external variables, i.e. testing and environmental conditions such as rate of loading, temperature and moisture, on fracture mechanisms and hence on fracture toughness in short fibre polymer composites is investigated as a function of fibre orientation with respect to the fracture plane. Unidirectionally oriented materials with different polymer matrix (polyamide 6.6 and polyoxymethylene) and different glass fibre content (15wt% and 30wt%) are examined. 

Short fibre polymer composites are being increasingly used as engineering materials because they provide mechanical properties superior to neat polymers and can be processed easily by the same fabrication methods, e.g. injection moulding. The mechanical properties of these materials are dependent on a complex combination of several internal variables, such as type of matrix, fibre-matrix interface, fibre content, fibre dimensions, fibre orientation, and external... 

FIGURE 16. Thermal conductivity at 50 °C versus fibre volume content and versus density of short fibre reinforced C/C-SiC composites... 

The mix compositions with fibres are therefore characterized by higher cement content and by higher values of fine/coarse aggregate ratio to increase the amount of the cement paste. These modifications are called reproportioning some recommendations are given by Nataraja et al. (2005) in view of maintaining the workability by the trial-and-error method. It has been proved that with appropriate mixture proportions and efficient superplasticizer it is possible to obtain SCC with uniformly distributed short fibres (Ferrara et al. 2007). 

In fibre reinforced concretes, the fibre volume is much lower (<2% by volume) and the fibres act as secondary reinforcement, mainly for the purpose of crack control. The production of such reinforced concretes is carried out by conventional means. Higher contents of fibres can be incorporated by relatively simple mixing technologies, but using advanced matrix formulations which are based on sophisticated control of the rheology and microstructure of the mix. Such formulations combine dispersants and fillers (e.g. DSP, RPC and DUCTAL [4-6]). The dense microstructure in these composites, as well as their improved rheology can enable the incorporation and uniform dispersion of 2-6% by volume of short fibres, which can provide effective reinforcement. 

The modulus of elasticity of a pitch carbon composite produced with short fibres tended to decrease slightly with an increase in fibre content. This is because... 

In order to use models to estimate composite properties, it is necessary to know the properties of the fibres, which vary widely depending on the source, age, separating techniques, moisture content, speed of testing, history of the fibre, etc. The properties of the individual fibres are therefore very difficult to measure. Moreover, in a natural fibre-polymer composite, the lignocellulosic phase is present in a wide range of diameters and lengths, some in the form of short filaments and others in forms that seem closer to the individual fibre. 

A. Arbelaiz, B. Fernandez, J. A. Ramos, R. Retegi, R. Llano-Ponte, and I. Mondragon, Mechanical properties of short flax fibre bundle/polypropylene composites Influence of matrix/fibre modification, fibre content, water uptake and recycling. Compos. Sci. Technol. 65, 1582-1592 (2005). 

Lin, T., Jia, D., Wang, M., He, R, Liang, D., 2009. Effects of fibre content on mechanical properties and fracture behaviour of short carbon fibre reinforced geopolymer matrix composites. Bulletin of Materials Science, 32(1), pp. 77-81. 

In this application, the fibre volume content must exceed the critical volume in order to obtain both strengthening and toughening. Composites with this level of fibre content can be produced in a number of ways. The early work on such materials involved hand lay-up of continuous fibrillated polypropylene mats [10,26], However, since then a number of processes suitable for industrial applications have been developed using short, discontinuous fibres ... 

Polyethylene fibres have been evaluated using either short, dispersed fibres mixed with concrete at volumes up to about 4% [84], or a continuous network of fibrillated fibres (polyethylene in pulp form) to produce a composite with about 10% by volume of fibres. In pulp form, they are intended for use as asbestos replacement. The effect of short, discontinuous fibres (load-deflection of concrete is shown in Figure 10.20. At a volume content of 2% the fibres led to a marked post-cracking load-bearing capacity, while at 4% the maximum load in the post-cracking range exceeded the first crack stress. Thus, these fibres seem to be very effective for crack control. The fibrillated continuous polyethylene fibres appeared to be more effective than similar polypropylene fibres, probably because of the higher elastic modulus of the polyethylene. 

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