Composites long-fibre

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). 

Certain types of laminate are susceptible to creep, as discussed in Chapter 1. The use of 0° plies greatly reduces the creep rate and the problem is not usually severe with long fibre composites, although laminates consisting entirely of 45° plies can show a scissor-closing motion, that is, a reduction in the angle to 43° or thereabouts if held under a sustained load with a sufficiently ductile matrix. 

A Plumtree and G Shen, Prediction of fatigue damage development in imidirec-tional long fibre composites , Polym Polym Compos 1994 2 83-90. 

Two alternative modes of fracture have been defined, both involving shear on the crack plane. In mode II, shear is parallel to rhe crack propagation direction. In mode III, shear is normal to the propagation direction. These types of fracture are important in long-fibre composites and in adhesives, because of anisotropy. They are of minor importance in thermoplastics. Cracks in isotropic materials tend to turn in a direction normal to the tensile stress, giving mode I fracture, whatever the initial orientation of the crack plane. 

Polymers vary widely in their processing characteristics, and it is important to ensure that the design developed can be produced economically and efficient from the chosen material. The most obvious differences in processing behaviour are between thermoplastics and long-fibre composites, but fiiere are also significant variations within each class of materials. To take an extreme example, PTFE has such a high melt viscosity that it cannot be injection-moulded, but has to be formed from powder by... 

PE has good low temperature (and electrical) properties, but it is not applicable as an impregnation material or as a matrix for long fibre composites. It might be applicable to short fibre composites, for example, in connection with whiskers. Such a material could have a high mechanical strength and some toughness even at helium temperatures. 

P. J. Hoggs and S. Turner, The mechanical testing of long-fibre composites Harmonization and standardization in U. K. , Report of the Department of Trade and Industry, United Kingdom, January 1988. 

Mieck, K. P., Reussmann, T., and Hauspurg, C. (2000). Correlations for the fracture work and falling weight impact properties of thermoplastic natural/long fibre composites. Materialwissen-schcift und Werkstoffteclmik 31, 169-174. 

Long-fibre composites are mainly used with hand lay-up systems using polyester or epoxides as the matrix in which glass fibres are suspended. It is possible in long-fibre composites to calculate the strength and stiffness more accurately using conventional composite theory. For example, in a unidirectional continuous fibre composite, the equation for the total stress, in the composite can be shown to be ... 

One limitation of epoxide moulding compositions is their short shelf life (typically 1-3 months), which necessitates strict stock control. The compounds may be compression, transfer or injection moulded, although compression moulding is preferred for long-fibre grades. 

El and E2 refer to the longitudinal and transverse moduli for aligned fibre composites of the type shown in (Fig. 3.29). These values can be determined experimentally or using specifically formulated empirical equations. However, if the fibres are relatively long then equation (3.5) and (3.13) may be used. These give results which are sufficiently accurate for most practical purposes. 

The Z-direction is perpendicular to the page. For simplicity the material is assumed to be isotropic, ie same properties in all directions. However, in some cases for plastics and almost always for fibre composites, the properties will be anisotropic. Thus E and v will have different values in the x, y and z direction. Also, it should also be remembered that only at short times can E and v be assumed to be constants. They will both change with time and so for long-term loading, appropriate values should be used. 

This book covers what are broadly called plastics materials. Composites based on long fibres and thermosetting resins are mentioned but not studied specifically. Elastomers, coatings, textiles and adhesives are generally excluded but attention is drawn to a companion volume on the life prediction of rubbers [1]. 

Thermoplastic composites, GMT. According to The European Alliance for SMC, the production of glass mat reinforced thermoplastics (GMT) and long fibre reinforced thermoplastics (LET) was up about 9% in 2002, demonstrating bright future prospects. Some examples are ... 

Processing of LFRT or LFT - long fibre (reinforced) thermoplastics injection, composite insert moulding, compression moulding, extrusion-compression. 

It is well known that the Young s modulus of a composite can be calculated by the rule of mixtures for long-fibre reinforced material. In the case of whiskers, the rule of mixture is also applied to estimate the change of modulus (conventionally, reinforcements are added to improve the stiffness of a material, though for ceramic matrix composites this is not always the primary concern). 

Large aspect ratio Because the reinforcements are nanosized, the large aspect ratio (length/diameter) could make the composite behave like a continuous fibre composite. This scenario is unlikely as the aspect ratios of the NWs used are "- 100-200, which are lower than the value of "-T000 or so for a transition between short-fibre reinforced composite behaviour and the long-fibre reinforced one [18]. The specimen dimensions are also much larger than the NW dimensions. The anomalous increase from the relatively smaller sized specimens is ruled out. 

Hou (1986) presented a resin-flow model for long-fibre-reinforced-composite prepreg lamination processing. The model includes the following chemorheological model ... 

As it is commonly known and applied for a long time, the properties of paste can be improved by reinforcement with the fibres. For example, the production of asbestos-cement materials started 80 years ago. The fiber reinforcement technology has been developed extensively for the last 20 years and the other fibres has been taken into account, first of all the steel fibres, but also the carbon, glass, resin, polypropylene and cellulose fibres as well. The fibres reinforcement gives the pos-sibihty to enhance the flexural and tension strength, as well as the impact resistance. The fibre composite modifies the properties of concrete by control cracking and the mode of failure by means of post—cracking ductility. 

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