Tensile fibre

In this section, the effects of clearance on the damage response of the C1 C1 C1 double-lap joint (i.e. a control case) and the C3 C3 C1 double-lap joint (i.e. a worst case scenario in the sense that all the load is initially carried by one bolt) are investigated. The progression of damage in the joints is shown at applied load levels of 10,30 and 50 kN for the four different failure modes considered, i.e. tensile matrix failure compressive matrix failure tensile fibre failure and compressive fibre failure. These load levels were chosen so that the damage could be tracked fiom initiafion up to the point of extensive damage. 

Figure8.4 Variation of the peak position of the 1610cm" Raman band with tensile fibre strain for the five different aramid fibres (after [38])...

Polymerisation of a diol with a dicarboxybe acid is exemplified by the production of a polyester from ethylene glycol and terephthabc acid either by direct esterification or by a catalysed ester-interchange reaction. The resulting polyester (Terylene) is used for the manufacture of fibres and fabrics, and has high tensile strength and resibency its structure is probably ... 

There are less exotic ways of increasing the strength of cement and concrete. One is to impregnate it with a polymer, which fills the pores and increases the fracture toughness a little. Another is by fibre reinforcement (Chapter 25). Steel-reinforced concrete is a sort of fibre-reinforced composite the reinforcement carries tensile loads and, if prestressed, keeps the concrete in compression. Cement can be reinforced with fine steel wire, or with glass fibres. But these refinements, though simple, greatly increase the cost and mean that they are only viable in special applications. Plain Portland cement is probably the world s cheapest and most successful material. 

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.

This is more than one-half of the strength of the continuous-fibre material (eqn. 25.3). Or it is if all the fibres are aligned along the loading direction. That, of course, will not be true in a chopped-fibre composite. In a car body, for instance, the fibres are randomly oriented in the plane of the panel. Then only a fraction of them - about - are aligned so that much tensile force is transferred to them, and the contributions of the fibres to the stiffness and strength are correspondingly reduced. 

A unidirectional fibre composite consists of 60% by volume of continuous type-1 carbon fibres in a matrix of epoxy. Find the maximum tensile strength of the composite. You may assume that the matrix yields in tension at a stress of 40 MPa. 

Its = tensile strength parallel to fibres d] = fracture strength of fibres d = yield strength of matrix. 

Compared with nylon 66 fibres, the polyurethane fibres (known as Perlon U) have a tensile strength at the higher end of the range quoted for nylon 66, they are less prone to discolouration in air, are more resistant to acid conditions and they have a lower moisture absorption. On the debit side they are less easy to dye, are hard, wiry and harsh to handle and have too low a softening point for many applications. They are currently of little importance but have found some use in bristles, filler cloths, sieves and a few other miscellaneous applications. 

Spandex fibres, because of their higher modulus, tensile strength and resistance to oxidation, as well as their ability to be produced at finer deniers, have made severe inroads into the natural rubber latex thread market. They have also enabled lighter weight garments to be produced. Staple fibre blends with non-elastic fibres have also been introduced. 

As shown in Fig. 3.4 stress-strain tests on uniaxially aligned fibre composites show that their behaviour lies somewhere between that of the fibres and that of the matrix. In regard to the strength of the composite, Ocu, the rule of mixtures has to be modified to relate to the matrix stress, o at the fracture strain of the fibres rather than the ultimate tensile strength, o u for the matrix. 

Example 3.4 For the PEEK/carbon fibre composite referred to in Example 3.2 calculate the values of V j and Vent if it is known that the ultimate tensile strength of PEEK is 62 MN/m. ... 

In order to understand the effect of discontinuous fibres in a polymer matrix it is important to understand the reinforcing mechanism of fibres. Fibres exert their effect by restraining the deformation of the matrix as shown in Fig. 3.28. The external loading applied through the matrix is transferred to the fibres by shear at the fibre/matrix interface. The resultant stress distributions in the fibre and matrix are complex. In short fibres the tensile stress increases from zero at the ends to a value ([Pg.226]

The maximum value of it will occur when (tensile strength of the fibre, Ofu, and this is defined as the critical fibre length, Ic... 

If the fibres are aligned at 15° to the jc-direction, calculate what tensile value of Ox will cause failure according to (i) the Maximum Stress Criterion (ii) the Maximum Strain Criterion and (iii) the Tsai-Hill Criterion. The thickness of the composite is 1 mm. 

Solving this gives X = 169. Hence a stress of Ox = 169 MN/m would cause failure. It is more difficult with the Tsai-Hill criterion to identify the nature of the failure ie tensile, compression or shear. Also, it is generally found that for fibre angles in the regions 5°-15 and 40 -90 , the Tsai-Hill criterion predictions are very close to the other predictions. For angles between 15° and 40° the Tsai-Hill tends to predict more conservative (lower) stresses to cause failure. 

A reinforced plastic sheet is to be made from a matrix with a tensile strength of 60 MN/m and continuous glass fibres with a modulus of 76 GN/m. If the resin ratio by volume is 70% and the modular ratio of the composite is 25, estimate the tensile strength and modulus of the composite. 

If the matrix in 3.7 was reinforced with the same volume fraction of glass but in the form of randomly oriented glass fibres rather than continuous filaments, what would be the tensile strength of the composite. The fibres are 15 mm long, have an aspect ratio of 1000 and produce a reinforcement efficiency of 0.25. The fibre strength is 2 GN/m and the shear strength of the interface is 4 MN/m". 

The mechanical properties of plastics materials may often be considerably enhanced by embedding fibrous materials in the polymer matrix. Whilst such techniques have been applied to thermoplastics the greatest developents have taken place with the thermosetting plastics. The most common reinforcing materials are glass and cotton fibres but many other materials ranging from paper to carbon fibre are used. The fibres normally have moduli of elasticity substantially greater than shown by the resin so that under tensile stress much of the load is borne by the fibre. The modulus of the composite is intermediate to that of the fibre and that of the resin. 

For fibres and filaments such orientation is desirable, but for solid objects where impact strength is often more important than tensile strength such orientation is usually unwelcome. It can also have further unwanted effects. This arises from the fact that oriented molecules are basically unstable and will at the first opportunity try to coil up. Thus on heating samples up to temperatures near severe distortion can occur leading to warped mouldings. 

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