1.1 This International Standard specifies a method for determining the in-plane shear modulus (G12) of fibre-reinforced plastic composites using a standard plate specimen. When applied to isotropic materials, the shear modulus measured is independent of direction. 1.2 The method is used to determine the shear modulus of the test specimens but not to determine the shear strength. It applies to a plate supported on two points on one diagonal and loaded on the other diagonal by the simultaneous movement of two loading points attached to a cross-beam. 1.3 The method is suitable for use with fibre-reinforced plastic composites with both thermoset and thermoplastic matrices. Due to the shear deformation being applied under flexural conditions, for laminated materials with different fibre formats and/or different orientations, the layers of material must be well distributed across the section so that it is approximately "homogeneous" in the through-thickness direction. The principal material axes, if present, must be orientated normal to the plate edges (see 3.8). NOTE This method can be applied to unreinforced polymers and other materials (e.g. metals, ceramics and metal- or ceramic-matrix composites). For material fabricated using unidirectional plies, the shear modulus obtained using a multidirectional specimen (i.e. 0°/90°/±45°) is not the same as that obtained for unidirectional or cross-ply (0 °/90°) material. 1.4 The method is performed using specimens which may be moulded to the chosen dimensions, machined from test plates or machined from flat areas of products. 1.5 The method specifies preferred dimensions for the specimen. Tests which are carried out on specimens of other dimensions, or on specimens which are prepared under different conditions, may produce results which are not comparable. Other factors, such as the speed of testing and the conditioning of the specimens, can influence the results. Consequently, when comparative data are required, these factors must be carefully controlled and recorded. NOTE The stress-strain response in shear is very non-linear at higher strain levels. This test method determines the modulus within a low strain region and is not applicable to higher strains.