Fibre-reinforced plastic composite mechanical properties

Man-made composites fall into three broad classes, depending on whether the main part of the composite, the matrix, is a polymer, a metal or a ceramic. Often, but not always, the composite combines materials from two classes, as in glass-fibre-reinforced plastics. However, the most widely used composite material, concrete, is a ceramic -ceramic composite. The most important classes of artificial composite are described below. The mechanical properties of composites are discussed in Section 10.4. Biological composites are very varied and will not be considered here. 

Hull [21] discusses an expression by Nielsen and Chen [22] for the calculation of the modulus of laminae containing randomly oriented fibres. The issues involved in computing the mechanical properties of short, random fibre composites, and the mechanics of fibre reinforced plastics are also discussed. 

An further alternative approach being developed worldwide is to replace the steel completely by fibre-reinforced plastics (FRP), which consist of continuous fibres as carbon, glass or aramid, set in a suitable resin to form a composite rod or grid. These materials have high tensile strength, low density and are non-magnetic they can be used both for new structures and for repair of existing ones. The mechanical properties of FRP are determined by the amount and type of fibre, while the durability will be a function of both the resin and the fibre. 

Various methods have been developed for the production of GRC components. These methods have mostly been adapted from the glass fibre reinforced plastics industry, with proper modifications to adjust for the special nature of the cementitious matrix. To obtain a product of an adequate quality the mix composition should be carefully controlled, to be compatible with the production process, while at the same time providing the needed physical and mechanical properties in the hardened composite. Thus, the properties of GRC composites vary over a wide range, and are a function of a complex combination of the production process and the mix composition. A detailed discussion of the design and production of GRC components is beyond the scope of this chapter. These topics are covered in various publications and guidelines [31-37] and only some essential points will be discussed in Section 8.7 and Chapter 14. 

Reinforced plastic composite parts are manufactured using reinforcing materials— usually fibres of glass, plastics or carbon and thermosetting polymer resin. Often mechanical failme occurs at the brittle polymer resin due to the resin content s relatively poor resistance against compression, impact, fracture and delamination. In order to increase physical and mechanical properties of the resin system for the fibre reinforced plastic parts, oftrai toughaung particles are used to toughen the resin system. 

The mechanical properties of plastics materials may often be considerably enhanced by embedding fibrous materials in the polymer matrix. Whilst such techniques have been applied to thermoplastics the greatest developents have taken place with the thermosetting plastics. The most common reinforcing materials are glass and cotton fibres but many other materials ranging from paper to carbon fibre are used. The fibres normally have moduli of elasticity substantially greater than shown by the resin so that under tensile stress much of the load is borne by the fibre. The modulus of the composite is intermediate to that of the fibre and that of the resin. 

In this respect, (thermoset) plastics composites with discontinuous fibre products are already mostly used in the car body applications, where polyester/E-glass is predominating (mostly because of polyesters, economy, ease of processability and reasonable mechanical properties provided), followed by use of phenolics (when fire retardance is required, in friction linings and engine compartments), and epoxies. Replacement by carbon or aramid fibre reinforcements can reduce body mass by 40% (compared to steel) and with more added strength, but the cost is unfavourable at the moment, as mentioned previously [12, 13]. 

About 30% of all polymers produced each year are used in the civil engineering and building industries(23). Nevertheless, structural plastics such as fibre reinforced composites have so far received little attention by civil and structural engineers, despite some of their obvious advantages such as lightness, handleability and corrosion resistance. This may be due to reservations on credibility grounds or fire resistance properties, as well as to uncertainty on how to design structures with them. Whilst their mechanical properties are in fact fairly well understood, there are a number of... 

In the case of unidirectional (or longitudinal) fibre reinforced composites, the stress is transferred from the matrix to the fibre by shear. When stressed in tension, both the fibre and the matrix elongate equally according to the principle of combined action [14]. Hence, the mechanical properties of the composite can be evaluated on the basis of the properties of the individual constituents. For a given elongation of the composite, both constituents, fibre and matrix, may be in elastic deformation the fibre may be in elastic deformation whereas the matrix may be in plastic deformation, or both the fibre and the matrix may be in plastic deformation (Fig. 19.3). 

Reinforced Plastic—A plastic with fibres of relatively high stiffness or very high strength embedded in the composition. This improves the mechanical properties over those of the base resin. 

All polymer materials used in reinforced plastics display some viscoelastic or time-dependent properties. The origins of creep in composites stem from the behaviour of polymers under load together with local stress redistributions between fibre and matrix as a function of time. There is little creep at normal temperatures in the reinforcing fibres. The origin of the creep mechanisms is related to the nature and levels of internal bonding forces between the chains of the polymer, which are influenced by temperature and moisture. 

Sanadi AR, Caulfield DF, Jacobsaon RE, Rowell RM (1995) Renewable agricultural fibres as reinforcing fillers in plastics mechanical properties of kenaf fibre-polypropylene composites. Ind Eng Chem Res 34 1889-18%... 

Polyimide characterised by thermal and thermal-oxidative stability at elevated temperatures, chemical resistance and good mechanical properties is relatively new in the family of polymer foams [3]. In some cases, depending on the nses, additional reinforcement can be inclnded. Examples are fibre reinforced foams and syntactic foams which are composites containing hollow glass, ceramic or plastic micro-spheres dispersed throughout the polymer matrix. 

Chemical compounds which contain reactive groups such as the methanol group (-CH2OH) as in methanolamine compounds are able to form stable, covalent bonds with cellulose fibres. This treatment decreases the moisture pick-up and increases the wet strength of reinforced plastics. Isocyanates are also suitable to modify the chemical structure via its reaction with the OH groups of cellulose. The mechanical properties of composites reinforced with wood-fibre and PVC or PS can be improved by an isocyanate treatment of those cellulose fibres or the polymer matrix. The improvement of the properties of the composites can be explained by the reduction in the number of OH groups responsible for moisture uptake and consequently the increase in the hydrophobicity of the fibre s surface... 

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