The method of three-point bending in testing thin high-strength reinforced plastics at large deflections

A method is presented for determining the flexural strength of unidirectional composites from three-point bending tests at large deflections. An analytical model is proposed for calculating the flexural stress in testing thin bars in the case of large deflections. The model takes into account the changes in the support reactions at bar ends and in the span of the bar caused by its deflection. In the model offered, the influence of transverse shear and the friction at supports are neglected. The problem is solved in elliptic integrals of the first and second kind. The results obtained are compared with experimental tension data. The method elaborated for calculating flexural stresses has an obvious advantage over the conventional engineering procedure, because the calculation accuracy of the stresses increases considerably in the case of large deflections.