Fibre chopped

GFRP (chopped fibre, moulding grade) =37 =30 OK Creep above 60°C... 

Galculate the upper and lower values for the modulus of the composite material, and plot them, together with the data, as a function of Vf. Which set of values most nearly describes the results Why How does the modulus of a random chopped-fibre composite differ from those of an aligned continuous-fibre composite ... 

For many applications (e.g. body pressings), it is inconvenient to use continuous fibres. It is a remarkable feature of these materials that chopped fibre composites (convenient for moulding operations) are nearly as strong as those with continuous fibres, provided the fibre length exceeds a critical value. 

Consider the peak stress that can be carried by a chopped-fibre composite which has a matrix with a yield strength in shear of d (d = jd ). Figure 25.4 shows that the axial force transmitted to a fibre of diameter d over a little segment 8x of its length is... 

This is more than one-half of the strength of the continuous-fibre material (eqn. 25.3). Or it is if all the fibres are aligned along the loading direction. That, of course, will not be true in a chopped-fibre composite. In a car body, for instance, the fibres are randomly oriented in the plane of the panel. Then only a fraction of them - about - are aligned so that much tensile force is transferred to them, and the contributions of the fibres to the stiffness and strength are correspondingly reduced. 

Short Chopped Fibres These are fibres with lengths up to about 6 mm. The fibre (L/D) ratio is typically about 800. They are more expensive than milled fibres but provide better strength and stiffness enhancement. 

Long Chopped Fibres These are chopped fibres with lengths up to 50 mm. They are used mainly in the manufacture of SMC and DMC (see Section 4.10.2). 

Chopped Strand Mat This consists of strands of long chopped fibres deposited randomly in the form of a mat. The strands are held together by a resinous binder. 

The price of the fibrous reinforcements (Figure 2.14) depends on the nature of the fibres and the form of the reinforcement continuous or chopped fibres, mats, rovings, fabrics or unidirectional. 

Mats chopped fibres (50 mm) are held together with a binder to form a sheet. 

Mats chopped fibres (50 mm) are held together with a binder to form a sheet. Continuous filament mats sheets of felt of continuous filaments with a binder. Stratipregs or prepregs rovings impregnated with a resin. 

The fibres generally used for polymer reinforcement are glass, carbon and less often PTFE or metals. For use in thermosets, glass or carbon fibre may be used in the form of a pressed mat, or bundles of fibres may be laid in a preferred orientation. This enables anisotropic properties to be obtained where this is beneficial. The alternative is to disperse chopped fibres randomly in the polymer matrix. This gives... 

Typical systems are two-part polyurethane systems that may produce rigid, rubbery, foamed or filled products. Typical fillers include chopped fibres and mineral fillers. Composite systems may also be produced by having pre-placed reinforcements in the mould, in which case the technique is known as reinforced reactive injection moulding (RRIM). 

Many different fibre/matrix combinations have been proposed some are still in their infancy, some have already reached commercial maturity. Systems based on discontinuous reinforcements (whiskers, chopped fibres) are usually developed to improve selected properties of the matrix (creep resistance, toughness, stiffness) or to extend its temperature range sometimes it happens that improvements are sought to the detriment of other characteristics, first of all ductility. 

Reinforcements are available in various physical forms - filament bundles, short chopped fibres, woven fabrics, three-dimensional fabrics, resin-impregnated filaments, and so on. There is often a surface treatment applied, which fulfils several purposes, such as to protect against abrasion and promote adhesion to the matrix. 

Spray moulding Chopped fibres and catalysed thermosetting resin sprayed simultaneously onto open mould and cured at ambient temperature As for hand lay-up. More rapid but short fibres provide less reinforcement... 

A mechanized development of hand lay-up is spray moulding, in which the gelcoat, resin and chopped fibres are applied by spraygun rather than by hand. The process is much quicker than hand lay-up, but the capital costs are no longer rock bottom. The fiow characteristics and hence the styrene content of the resins are different. The fibres cannot be very long and the mechanical properties of the products will reflect this. 

Commingled fibre technology involves supplying the matrix in filament form and mingling it with the reinforcing fibres (Fig. 2.4(b)). The two materials can even be cowoven in the same strands. Thus the material is available in the form of continuous roving, chopped fibres, fabrics, etc. Excellent drape qualities can be obtained with fabrics. The technique can be used for pultrusion, but not for filament winding. 

This is similar to the filament winding process, but the mandrel is made of a continuous steel band. The vessel is wound on the mandrel, at the end of which the steel band is fed up the centre of the vessel to the beginning, in order to recommence. Frequently, sand and chopped fibres are also dropped on to the mandrel to increase the thickness and thus stiffen the vessel. 

Fibre production began in 1972 at 100000 Ib/annum with plans to increase as the market develops. Large price reductions for the fibre were timed by Du Pont to coincide with the 1972 Farnborough air show bringing epoxy pre-preg fabric for instance down to 20-30 per lb, and projected costs are expected to be lower than for carbon and boron fibres. It has been reported that the fibre will be availaWe as collimated tape, chopped fibre, yams, rovings and as woven fabric. ... 

Ref. DaimlerChrysler Research and Technology / Domier LSI-C/C-SiC (chopped fibres)... 

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