Composites short-fibre

Although continuous filament composites are of considerable commercial importance, their fabrication is a rather complex process and a cheaper, though mechanically inferior product is obtained by mixing short lengths of fibre with a thermoplastic polymer. 

A prime requirement is good adhesion between fibre and matrix, a condition that will be dependent on features such as chemical bonding and surface cleanliness, as well as mechanical factors. The ratio of surface area to fibre volume should be as high as possible. Considering cylinders of length I and radius r  

Expressed in terms of the aspect ratio a = j2r the above expression becomes 

It can be seen, therefore, that for optimum adhesion the aspect ratio should be either very small, where becomes very large, corresponding to flat platelets (minerals such as talc 

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Reinforcing fibres have diameters varying from 7 /im to 1(X) /im. They may be continuous or in the form of chopped strands (lengths 3 mm-50 mm). When chopped strands are used, the length to diameter ratio is called the Aspect Ratio. The properties of a short-fibre composite are very dependent on the aspect ratio - the greater the aspect ratio the greater will be the strength and stiffness of the composite. 

Hull also proposed that the shear modulus and Poisson s Ratio for a random short fibre composite could be approximated by... 

Fukuda, H. and Chou, T. W A probabilistic theory of the strength of short-fibre composites with variable fibre length and orientation,. /. Mater. Sci., 17, 1003 (1982). 

Similar considerations of symmetry apply in other systems, for example nematic liquid crystals and aligned short fibre composites have symmetry D h, smectic A liquid crystals D , while in copolymers and certain fibre composites examples of hexagonal symmetry may be found and translational symmetry may also be present, which is not found in petrology. 

Intuitively, however, the RVE for an amorphous polymer may be expected to be no more than 10 nanometres or so. In an aligned fibre composite with continuous fibres we need to take a sample sufficiently large as to contain perhaps 30 to 50 fibres in a lateral dimension and in this case the coefficient of variation of stress as between fibres would enable a measure to be decided. In a random array of fibres however (chopped strand mat) the RVE must be of the order of a fibre length at least as is also the case for an aligned short fibre composite. 

Analyses of short fibre composites in two dimensions have also been made by various means, usually numerical. The overlapping array is commonly chosen, illustrated in Fig. 9. 

Fig. 9. Overlapping array used in analyses of short fibre composites...

Short fibre composites, fibre orientation, fracture toughness, fracture mechanisms, fibre pullout, fibre debonding, critical fibre angle. 

Fara, S. and Pavan, A. Fracture toughness in short fibre composites analysis of fracture mechanisms in relation to fibre orientation in unidirectional materials to be submitted. 

Figure 3. Effect of short-fibre composite strengthening demonstrated by the ratios of minimum creep rate (alloy) to minimum creep rate (composite).

A solid structural substance produced by a combination of two or more materials that retain their identities. Typically, one of the materials combined is the strengthening agent, the other being the matrix (a thermoset or a thermoplastic resin). The word composite is also used for systems that are reinforced (reinforced where cumulative properties are superior to the individual components) by addition of certain solid particles (i.e., short fibre composites - long fibre composites - continuous fibre composites). 

Short fibre composites, where the fibre are cut into short length, the reinforcing fibre have length-to-diameter atio between 10 and 250 to mixing with resin. 

The application of Mol is straightforward if the reinforcing particles/fibres are short, slender, straight and can be effectively approximated as ellipsoids. For textile composites, with well-organised structure (versus randomness in short fibre composites) and consist of crimped yams (versus straight short fibres), a mechanically equivalent... 

The linear elastic stress analysis of a short fibre composite by the shear lag theory is due originally to H. L. Cox. The real composite (see Figure 6.32(a)) is assumed to deform in the vicinity of any particular fibre as if it were the model tem shown in Figure 6.32(b). In the model, the fibre (of diameter d) is surrounded 1 a cylinder of matrix of radius R, embedded in a homogeneous block of material which deforms as the composite as a... 

For each of the following short-fibre composite materials, tested in tension parallel to the fibres, predict the tensUe modulus. (Percentages given are fibre volume firactions.)... 

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 condition for maximum reinforcement, that is the condition ensuring maximum stress transfer to the fibres, before the composite fails, is to have a length higher than the critical length (Fig. 19.5). If the fibre aspect ratio is lower than its critical value, the fibres are not loaded to their maximum stress value. A specificity of cellulose fibres is their flexibility compared to glass fibres which allows a desirable fibre aspect ratio to be maintained after processing, which is around 100 or 200 for high performance short-fibre composites. 

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... 

The effect of AAPP treatment on the properties of namral fibre reinforced cellulose acetate butyrate (CAB) composites was then further studied [66]. At a fibre loading fraction of 30 wt%, the storage modulus of the short fibre composites improved by as much as 370% (Fig. 6.4). This is due to the enhanced interfacial adhesion between the fibres and the matrix as a result of AAPP treatment (as measured by single fibre pull-out test) [79], In addition to this, the increment in the mechanical glass transition temperature and a reduction in the height of tan 5 showed better fibre-matrix bonding and fibre-in-matrix distribution. It is evident that AAPP treatment is able to improve the fibre-matrix interface, resulting in composites with improved mechanical properties. 

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