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produced by force couples, the iosipescu shear test achieves a state of pure shear stress at the mid-length of the specimen. A state of constant shear force is induced through the middle section of the test specimen. The induced moments cancel at the mid-length of the test specimen and this moment will produce a pure shear loading at that location [12].

Figure 2.11: Iosipescu shear test Fixture [12].

The schematic representation of the Iosipescu testing fixture and the specimen is shown in the above diagram.

2.3.4 Rail shear test method

In this method the in-plane shear properties is found using two/three pairs of loading rails. The specimen id clamped between the rails to introduce the shear force in the specimen [5]. The rail shear test is described in ASTM (American Society for Testing and Materials) standard D4255 [22].

For this purpose, in 1957, a four-rail shear test was developed by the US Forest Products Laboratory. In early 1960s this test fixture was further modified by the Douglas Fir Plywood Association. The resulting test fixture was a two rail shear test fixture. As a result of the development of this new test fixture, it was possible to produce pure shear and to induce failure along the test specimen's weakest shear plane. This also helped to reduce the cost and complexity of the four rail shear test. The two most commonly used shear test fixtures are two rail shear test fixture and three rail shear test fixture [6].

2.3.5 The Standard Three Rail Shear Test Method

The standard three rail shear test method was described fully in ASTM standard D4255/D4255m [22]. In this method the test fixture consists of three rails clamping a specimen. The two outside rails, the top and the base plate together form a frame. The frame rests on the test machine. The load is transmitted to the specimen by the sliding movement of the middle (third) rail through a slot in the top plate. The in-plane shear is generated in both sides of the specimen while the compressive load is applied through the sliding rail by the test machine [13].

The specimen is gripped by the three rails. During the test, with respect to the two outer rails, the central rail has a vertical motion. This movement can be either up or down. As a result a state of shear stress is induced in the specimen [14]. The main benefits of three rail shear test method are a better approximation of pure shear can be obtained since the compressive load is applied to the clamped edges and it is possible to calculate two independent shear stress/strain responses from one specimen.

The three rail shear test fixture is shown in the diagram below.

Figure 2.12: Three rails shear test fixture [5].

2.3.6 The Standard Two Rail Shear Test Method

In the mid-1960s, to accommodate thin plates made from composites, two rail test fixture was modified. The new fixture consists of two rails that would be bolted onto both sides of a composite specimen. This new test, reported by Boller at the U.S Department of Agriculture was capable of measuring shear properties in multidirectional composite laminates.

In 1983, this two rail shear test was slightly modified and standardized by ASTM under the designation of ASTM D4255 [22]. An extensive theoretical stress analysis of the two rail specimen using a Fourier series so