Fibre load transfer

The sheath mass is typically a third of the rope mass. When the rope is under load, the sheath extends axially the angle between the 45° strands decreases, so the sheath diameter decreases, compressing the core. This encourages fibre-to-fibre load transfer, important when some fibres have failed. If a section of sheath is removed from a rope, it stretches easily. The braided sheath has relatively small air gaps, making the ingress of dirt relatively difficult (see Section 11.3). 

Amirbayat, J. and Hearle, J.W.S., Properties of unit composites as determined by the properties of the interface. Part 1. Mechanism of matrix-fibre load transfer. Fiber Sci. Technol., 2, 123-141 (1969). 

Fig. 25.4. Load transfer from the matrix to the fibre causes the tensile stress in the fibre to rise to peak in the middle. If the peak exceeds the fracture strength of the fibre, it breaks.

It is evident from Fig. 3.29 that there is a minimum fibre length which will permit the fibre to achieve its full load-carrying potential. The minimum fibre length in which the maximum fibre stress, ((rf)maxt be achieved is called the load transfer length, The value of may be determined from a simple force balance... 

Example 3.17 Short carbon fibres with a diameter of 10 fim are to be used to reinforce nylon 66. If the design stress for the composite is 300 MN/m and the following data is available on the fibres and nylon, calculate the load transfer length for the fibres and also the critical fibre length. The volume fraction of the fibres is to be 0.3. 

In the macrocomposite model it is assumed that the load transfer between the rod and the matrix is brought about by shear stresses in the matrix-fibre interface [35]. When the interfacial shear stress exceeds a critical value r0, the rod debonds from the matrix and the composite fails under tension. The important parameters in this model are the aspect ratio of the rod, the ratio between the shear modulus of the matrix and the tensile modulus of the rod, the volume fraction of rods, and the critical shear stress. As the chains are assumed to have an infinite tensile strength, the tensile fracture of the fibres is not caused by the breaking of the chains, but only by exceeding a critical shear stress. Furthermore, it should be realised that the theory is approximate, because the stress transfer across the chain ends and the stress concentrations are neglected. These effects will be unimportant for an aspect ratio of the rod Lld> 10 [35]. 

Systems reinforced with continuous fibres are developed to take advantage of the reinforcement high stiffness and strength the matrix protects the fibres fit>m environmental attack, participates in load transfer and distribution, determines shear and compression strengths and contributes to the composite toughness. All the systems share the need for reliable, reproducible consolidation processes whose conditions of temperature and pressure do not cause deleterious reactions between fibres and matrices. 

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 same trends can stiU be observed in fibre reinforced plastics, although the reinforcement will moderate the changes in mechanical properties. However, there are other possibilities, such as glass fibre-resin debonding, caused by water absorption from aqueous liquids. This can mean a reduction in translucency. Load transfer between fibres is also less effective. Carbon fibres, in contrast, are unaffected by water below 1000°C. 

The local load transfer from the fibre into the matrix can lead to an overstressing of the matrix or a fibre/matrix debonding. After that a crack propagation can occur, but not as smoothly as in metals [14],... 

Fibre to matrix adhesion plays a very important role in the reinforcement of composites with short fibres. During loading, loads are not applied directly to the fibres but to the matrix. It is necessary to have an effective load transfer from the matrix to the fibres for the ensuing composites to have good mechanical properties. This requires good interaction as well as adhesion between the fibres and the matrix, that is strong and efficient fibre-matrix interface. 

The effective fibre fraction that determines the structural reinforcing effect in any in-plane direction is usually not more than 10% by volume and the use of discontinuous fibres leads to a greater dependency on the resin for load transfer. Lower long term strength properties compared with those of continuous fibres may result. 

Mechanisms of creep in FRP materials are related to the progressive changes in the internal balance of forces within the materials resulting from the behaviour of the fibre, adhesion and load transfer at the resin—fibre interface, and from the deformation characteristics of the matrix. Thus any factors which either directly or indirectly cause changes to any of these key areas will affect the creep process. 

Great attention has been recently devoted to the development of fibre reinforced composites of high mechanical performance. Obviously, the final performances of a composite material are directly related to the properties of both basic constituents the fibre and the matrix. The matrix gives materials its cohesion, while the fibres support most of the mechanical stresses. However, it is also well-known that the mechanism of load transfer at the fibre-matrix interface plays a major role in the mechanical and physical performances of composites and, consequently, the fibre-matrix interface can actually be considered as the third constituent of such materials. 

At the optimum fibre loading, there is maximum wetting of the fibre and effective stress transfer at the fibre/matrix interface occurred. The decrease in tensile properties at higher loadings may be due to improper wetting and adhesion between fibre and matrix resulting in inefficient stress transfer. 

This is the well-known rule-of-mixture which describes a rather idealised situation and can predict the modulus only for continuous fibre-reinforced composites where there is sufficient stress transfer from the matrix to the fibre. However, short fibres are usually much shorter than the specimen length. For short fibres we must consider the matrix-fibre stress transfer. When the matrix is under stress, the maximum stress transferred to the fibre is described by the interfacial stress transfer (t). The stress transfer depends on the fibre length (/), so that at some critical length, /, the stress transferred is large enough to break the fibre. The stress transferred to the fibre builds up to its maximum value (o that which causes breakage) over a distance 1 from the end of the fibre. This means that the long fibres carry load more efficiently than short fibres. 

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