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lution concluded that this test method is valid for measuring the shear modulus of laminated composites provided that the length/width ratio is at least 10. An additional necessary condition is that the effective Poisson's ratio of the laminate with respect to the specimen edges should be less than unity. In the case of laminates having a high effective Poisson's ration the shear strength distribution is very irregular and leads to extensively low values of shear strength [11].

Figure 2.13: Two rail shear test fixture [5].

The figure above shows the dis-assembled diagram of a two-rail shear test fixture which is used to calculate the in-plane shear stress of composites. There are three primary weaknesses for this method [11].

Sliding of the specimen in the rails for the laminate with high shear strength may lead to untimely bearing failures.

Interlamination debonding might occur in the specimen due to drilling in the specimen to produce multiple holes.

Uniform shear stress is not developed within the specimen, and the stress concentration is induced in the regions where the rails are clamped on to the specimen.

The project is dealing with the testing of composites (Glass Reinforce Plastic) using the two rail shear test fixture. ASTM D4255 [] is the teat standard based on this test method [11].

2.4 复合材料的应用-2.4 Applications of Composites

Composite materials have a massive range of applications. The vast application of composite material is due to their flexibility to different situations and relative ease of combination with other materials. The subjects of weight and corrosion resistance are common to almost every industry. Composites offer a wide range of properties; high strength to weight ratio and corrosion resistance is some of those properties. These properties are helpful for the enhancement of component performance in any industry. Main applications of composites are explained below in detail [9].

2.4.1 Aerospace

The main applications of composites in aerospace industry include aircraft, spacecraft, satellites, space telescopes, the space shuttles, the space station, missiles, booster rockets and helicopters. The main reasons for the use of composites in aircraft are due to their favourable properties like specific stiffness and specific strength, design tailorability and fatigue resistance. For space applications dimensional stability is a major consideration. Depending upon the type of loading experienced by the individual component, different composite materials are used for its manufacture [8].

Much of the early motivation to the development of glass reinforced plastic was derived from the needs of military aircraft during the Second World War [7]. Military aircraft were the first to use GRP composites in significant quantities. The first applications were in radomes and then in the secondary structures and internal components [9]. There was an urgent requirement for a structural material which was also transparent to radar, with the introduction of airborne radar [7]. One of the most significant requirements for a randome is that the electrical thickness shall be closely controlled and shall match the wavelength of the radar equipment which with it is to be used. In general there are two main methods which satisfy these requirements; they are sin