Orientation, of fibres

Fig. 11.10 SEM photographs showing (a) longitudinal and (b) transverse orientation of fibres in the composite X30...

Orientation of fibres can be studied by RRIM by contact microradiography. In Fig. 6.12 the polymer matrix shows a typical random distribution. Each individual fibre can be readily seen to be surrounded by a coating of polymer, and exists in a straight needle-like form. Also some fibres are present as undispersed bundles observed by darker circles which represent vertically aligned fibres. 

Fibres are used mainly in the form of short or long chopped filaments/strands. mats made of random chopped strands, or woven fabrics of varying density. Woven and non-woven fabrics can be used to improve. surface qualities such as appearance, impact resistance, abrasion, and chemical resistance. To improve distribution and orientation of fibres in a three-dimensional moulding, there has been considerable development of preforming techniques, and machine-made three-dimensional arrangements of fibre, which offer better drape in a mould. When more than one fibre is used, the composite is termed a hybrid. 

Usually, the fibres are not unidirectional oriented in composite materials. Fibres orientation varies in function of the pieces thickness, processing method, and mould geometry. Accepting a random orientation of fibres, or a known and measured orientation, the laminates theory was appealed in order to describe the behaviour of this type of composites. The method allows the determination of the weakest layers, which will be firstly fractured [1285]. The Figure 3.456 illustrates a good agreement between the theory and practice, considering, for comparison, the data furnished by P.E. Chen [1285] and J.K. Lees [1286]. 

Figure 6.8 shows idealizations which are not realized exactly in practice. For example, no commercial process produces truly random orientation of fibres in three dimensions. Fibres are always acted on by forces due to viscous flow during forming. 

The efficiency of fibre reinforcement is dependent very much upon distribution and orientation of fibres in the matrix. The fibres form an internal structure and various kinds of these structures are described in Section 6.6. 

The orientation of fibres is an important factor in the design of a composite material strncture. Strong anisotropic effects may be created by fibres and there have been several attempts both to determine the influence of fibre orientation on mechanical properties and to optimize it. Much of the research was concentrated on advanced composites with ductile matrices and the objective function for design or optimization was the composite strength. These composites were extensively applied not just in aircraft and rockets, but also in the construction of cars the review of these investigations is published by Ashby and Jones (2005). 

The problem of the orientation of fibres in cement matrices was considered first by Morton (1979) where it was shown that the work of a fracture may be considerably increased when fibres are not aligned with the direction of the principal tensile strain. These results have been based on previously published experimental works by Hing and Groves (1972), Harris et al. (1972) and Morton and Groves (1974). They have shown that the work of fracture, calculated as the amount of work of external load absorbed by the element, is the most important magnitude to be considered in the design of brittle matrix composites. This approach was developed in a proposal of formulae for energy calculation (Brandt 1982, 1984), and later in the solution of a... 

The orientation of fibres is certainly an important factor influencing the fatigue strength, but experimental confirmations were published only from the tests of advanced composites with ductile matrices. Similar tests for cement-based composites are not yet available because in most applications, fibres are randomly distributed. 

Carefully consider the orientation of fibres to improve weld line strength. 

S. (1998) Composite of short coir fibres and natural mbber effect of chemical modification loading and orientation of fibre. Polymer, 39 (6-7), 1483-1491. 

Figure 2 shows the rose diagram of Pl(6) for both series of specimen (measurements were made on a mean number of eight pictures per block) Partial orientation of fibres is made clear in this diagram. 

Some chosen and new problems connected with the determination of the impact resistance of steel fibre reinforced concrete are presented. They are based on the experiments carried out by the author and other investigators. The problems discussed concern the relationship between the impact resistance of steel fibre reinforced concrete with 2D and ID orientation of fibres, the relationship between the energy absorbed during the static flexure tests and the impact resistance of this material and the effect of the impact velocity as well as the strain-rate on this resistance. 

Figure Impact resistance of specimens with ID and 2D orientation of fibres /U and Numbers in the table indicate the number of tests.

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