Features of Nonlinear In-Plane Shear Deformation of a Unidirectional and Orthogonally Reinforced Polymer Sheets of Composite Materials

A comparative analysis of typical stress-strain diagrams obtained for in-plain shear of 25 unidirectional and cross-ply reinforced polymer matrix composites under quasistatic loading was carried out. Three of them were tested within the framework of this study, and the experimental data on other materials were taken from the literature. The analysis of the generalized shear-strength curves showed that most of the tested materials exhibit a similar deformation pattern depending on their initial shear modulus: a linear section is observed at the beginning of loading, whereas further increase in the load decreases the slope of the curve, reaching the minimum at the failure point. For three parameters (end point of the linear part, maximum reduced deviation of the diagram, tangent shear modulus at the failure point) characterizing the individual features of the presented stress-strain diagrams, approximating dependences on the values of the reduced initial shear modulus are obtained. At the characteristic points of the deformation diagrams, boundary conditions are determined that can be used to find the parameters of the approximating functions. A condition is proposed for determination of the end point of the linear section on the experimental stress-strain curve, according to which the maximum deviation between the experimental and calculated (according to Hooke’s law) values of the shear stress in this section is no more than 1%, thus ensuring rather high accuracy of approximation in the linear section of the diagram. It is recommended to use the results of this study when developing universal and approximating functions relatively simple in structure that take into account the characteristic properties of the experimental curves of deformation of polymer composite materials under in-plane shear of the sheet. The minimum set of experimental data is required to determine the parameters of these functions.