Method for Determining the Shear Moduli of Composite Materials from Experiments in the Three-Point Transverse Bending

In order to reduce material costs, a simple and economical method without the use of strain gages was chosen, which is not widespread in practical implementations due to the high spread of the characteristic determined and its instability. It is based on the three-point transverse bending of specimens and measuring the deflection under the load applied. In this case, the shear modulus is calculated in two different ways. The tests were carried out on isotropic and on three types of composite materials with different reinforcement schemes. The control of the results obtained was carried out by testing similar samples in tension and bending by other proven methods. The main factors preventing the obtaining of stable and reliable values of shear moduli were established, one of which is the neglect of angular displacements of cross sections of the sample during calculation of the desired characteristics. It is shown that both the ways give very underestimated values of shear modulus for both isotropic and composite materials, with a very significant spread, which does not allow one to use the method considered in the presented form without corrections. It was established, that deflection under the load is determined by the rotation angles of sample cross sections and by the span, and its value is equal to the product of the maximal cross-sectional rotation angle and one third of the span length. It is shown that, taking into account the identified factors and some adjustment, this method is suitable for determining the shear modulus by the first or second way for both isotropic and composite materials.