Stiff fibres

This gives us an upper estimate for the modulus of our fibre-reinforced composite. The modulus cannot be greater than this, since the strain in the stiff fibres can never be greater than that in the matrix. 

These values hold for the case that the fibres are oriented in the stress direction. In reality the orientation of the fibres is at random in three dimensions we should, therefore, not only consider Ep but also the much smaller Et, which, even for very stiff fibres, is not higher than about Eo(l + 1,5-isotropic composite with chaotically arranged fibres, is ... 

Polypropylene homopolymer (PP) is a widely used thermoplastic material, despite its brittle behaviour at either low temperature or high loading rates. Improvement in the fi acture toughness of PP can be achieved by either modifying the crystalline structure, or addition of a second phase material [16], The toughening effect and mechanisms of different second phase materials such as stiff fibres, soft rubbery inclusions (EPR, EPDM), and some mineral fillers have been analysed. Recent developments concern the effect of hybrid system consisting of rigid and rubbery inclusions. 

The word reinforcement will refer in this book exclusively to strong, stiff fibres. They can be made of glass, aramid (e.g. KevlaT (DuPont)) or high molecular weight polyethylene (e.g. Dyneema (DSM)), carbon/graphite, polyamide (nylon), jute, and so on. The fibres can be long, virtually continuous or short (e.g. 1mm). 

Thermosetting polymers are viscoelastic at ambient temperature and unreinforced are susceptible to creep. Fibre reinforcements are generally elastic and creep resistant but some types may show the phenomena of stress rupture if exposed to high stresses in adverse environments. Composites reinforced with unidirectional stiff fibres and stressed in the fibre direction can be highly creep resistant as the fibre attracts a high proportion of the load in line with its stiffness relative to the polymer matrix. The behaviour of composites under static loading is a function of many material variables polymer type... 

The method is confined to strong stiff fibres embedded in a soft matrix and thus encompassing the fibre/resin composites considered in this code. 

Figure 11.22b shows tan 5 versus temperature plots of neat resin (cured) and composites cmitaining UT and AT fibres. It has been reported by other researchers that incorporatiOTi of stiff fibres reduced the tan 8 peak by restricting the movement of polymer molecules [36,38]. From Fig. 11.22b it can be observed that the height of the tan 8 peaks of the composites lie far below that of the neat resin. It can also be noted that the peak height of the UT and AT fibre composites are almost same, indicating the same damping capabilities of the composites. Similar results have been reported... 

Therefore, the first and the most important problem is the fibre-matrix adhesion. The role of the matrix in a fibre-reinforced composite is to transfer the load to the stiff fibres through shear stresses at the interface. This process requires a good bond between the polymeric matrix and the fibres. Poor adhesion at the interface means that the full capabilities of the composite cannot be exploited and leaves it vulnerable to environmental attacks that may weaken it, thus reducing its life span. Insufficient adhesion between hydrophobic polymers and hydrophilic fibres result in poor mechanical properties of the natural fibre-reinforced polymer composites. Pre-treatments of the natural fibre can clean the fibre surface. 

Fibre compositions represent a class of composite material that combines low weight with good mechanical properties." Usually, a strong and stiff fibre, such as glass or carbon, is incorporated in a polymer matrix. 

The composite, then, is seen to consist of very strong and stiff fibres embedded in a matrix, which, because of the presence of stress-concentrating cracks, has a much lower strength than the fibres. The matrix serves to maintain fibre position and orientation, to transmit shear forces, to protect the fibre snrface and to transfer loads to the reinforcement. For this reason, fibre-matrix adhesion is essential. 

Stepped-lap joints with glass fibre fabrics and brittle resins are easily made as each layer can be identified and peeled back. With tougher resins and stiff fibres, such as carbon-fibre fabrics and aramids, peeling back the layers is difficult, so scarf joints are more often specified in recent Structural Repair Manuals (SRMs). 

Consider a short length of fibre aligned with the tensile stress direction. The stiff fibre will tend to restrain the deformation of the matrix, and so a shear stress will be set up in the matrix at its interface with the fibre, which will be a maximum at the ends of the fibre and a minimum in the middle (Figure 8.3(a)). This shear stress then transmits a tensile stress to the fibre, but as the fibre-matrix bond ceases at the fibre ends there can be no load transmitted from the matrix at each fibre extremity. The tensile stress is thus zero at each end of the fibre and rises to an intermediate maximum or plateau over a critical length /q/2 (Figure 8.3(b)). For effective reinforcement the fibre length must be greater than the critical value /q, otherwise the stress will be less than the maximum possible. 

In brittle matrices the reinforcement may be either in the form of ductile fibres to increase toughness or of deformable particles and pores to block the crack propagation. Stiff fibres or hard particles are introduced into ductile matrices to increase their strength. Short chopped fibres are used in many cases, but also long continuous fibres and different types of nets and fabrics. The polymer structure introduced to a porous stiff matrix of hardened concrete may be also considered as its reinforcement. 

New experimental evidence [54] indicates that the C-termini of Nephila clavipes major ampullate spidroins, which are present in the high molecular weight fractions of both the proteins derived from the secretions of the glands and the spun thread, are involved in the formation of disulfide bridges. However, it is rather unlikely that such covalent cross-linking has a strong impact on the material properties as the C-termini of the minor ampullate spidroins that form a very stiff fibre do not contain cysteines [50]. 

Stefanie Kwolek and Paul Morgan, research chemists at DuPont, reported that solutions of poly(phenylene terephthalamide) could be spun to superstrong and stiff fibres. They showed that the solutions possessed liquid-crystalline order. The fibres were later commercialized under the name Kevlar . 

S. Tokito, P. Smith, and A. J. Heeger, Highly conductive and stiff fibres of poly(2,5-dimethoxy-p-phenylenevinylene) prepared from soluble precursor polymer. Polymer 52 464 (1991). 

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