Unidirectionally aligned fibres

Example 3.7 A thin unidirectional Kevlar fibre/epoxy composite has the properties listed below. If the fibres are aligned at 6 to the x-axis, show how... 

It is worth pointing out that carbon fibre itself has anisotropic thermal expansion properties, and therefore this mismatch between the carbon fibres and the a-sialon matrix should be considered in both the radial and axial directions when carbon fibres are unidirectionally aligned in the composite. The thermal stress caused by thermal expansion differences between the carbon fibres and the matrix in the radial (cr) and axial (oa) directions can be estimated from the formulae (Chawla, 1993 Kerans and Parthasarathy, 1991) ... 

When the fibres are all aligned in one direction, we say that a fibrous composite is unidirectional, to distinguish it from bidirectional materials and those with randomly aligned fibres. The word interface refers to the boundary region where the resin and fibres (or resin and filler particles) are in immediate contact and where they must adhere to each other if load transfer is to take place. Without adhesion, there is no reinforcement, other than that provided by frictional forces. Adhesion between fibres and matrix is sometimes easy to achieve, and sometimes it needs special effort, such as by surface treating the fibre surfaces. 

The first realised kind of fibre-reinforced composites was in form of preforms where fibres, unidirectionally aligned or weaved or processed in one or another way, were impregnated by epoxide, phenol formaldehyde or imide resins. As mechanical strength of materials in fibrous form is usually one order of magnitude higher than... 

Example 3.6 A unidirectional composite consisting of carbon fibres in a PEEK matrix has the fibres aligned at 25° to the loading axis. If the fibres and matrix have the properties indicated below, calculate Ex, Ey, Gxy, Vxy, and Vyx. 

Consider a unidirectional sheet of material with the fibres aligned in the x-direction and subjected to a stress, If the sheet has thickness, h, as shown in Fig. 3.15 and we consider unit width, then the normal axial force Nx is given by... 

Most carbon fibre reinforced plastics (CFRP) used and investigated to date are produced from preimpregnated continuous carbon fibre prepregs. Polymers reinforced with aligned short carbon fibres have certain advantages as materials for structural components, because they can easily be formed into complicated shapes with satisfactory mechanical properties. Woven fabrics produced from carbon fibres find increasing application in the aerospace and many other industries, because they are easy to handle, they have the ability to conform to complicated shapes and the in-plane properties are more isotropic than those of equivalent unidirectional materials. 

For a material such as a unidirectional lamina that contains all of its fibres aligned in one direction, the conditions of transverse anisotropy apply. The stress/strain relationship can be written as (Eckold 1994, Herakovich 1998) ... 

Materials made from unidirectional fibre tows laid in parallel to each other or held at precise predetermined orientations should allow volume fractions of fibre to exceed 50% within plies and highly aligned properties to be generated. Materials with no crimp are beneficial to creep behaviour. 

As a conclusion, it must be noticed that, in fact, all the tests performed on laminates are recommended only when the level of adhesion established between fibre and matrix is already known. The main problem involved in these types of tests is to control precisely the degree of alignment of fibres in the given direction. For example, a recent work [5] has shown that the mechanical responses of unidirectional laminates are very sensitive to the misorientation of fibres, even when this latter does not exceed 1 to 2%. Obviously, bridges or entanglements between fibres tend to increase greatly the measured mechanical properties and, thus, caution should be exercised when interpreting the results. 

An alternative approach to building up components in which the fibres are aligned in various directions, which is often favoured with carbon fibres, is to produce a pre-impregnated material, known as a prepreg. Unidirectional fibres are spread out thinly and impregnated with a thermosetting resin which is partially cured or B-staged. The sheet or roll material (which is a fraction of a millimetre in thickness)... 

Blaker JJ, Lee KY, Walters M, Drouet M, Bismarck A (2014) Aligned unidirectional PLA/bacterlal cellulose nanocomposite fibre reinforced PDLLA composites. React Funct Polym 85 185-192... 

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