A New Inverse Method for the Determination of Mechanical Properties of an Angle-Ply Composite Laminate by the CLPT

Utilization of fiber-reinforced composite laminates to their full potential requires consideration of angle-ply laminates in structural design. This category of laminates, in comparison with orthotropic laminates, imposes an additional degree of challenge, due to a lack of material principal axes, in determination of elastic laminate effective properties if the same has to be done experimentally. Consequentially, there is a strong inclination to resort to the usage of “CLPT” (Classical Laminated Plate Theory) for theoretically estimating the linear elastic mechanical properties including the cross-correlation coefficients coupling normal and shear effects. As an angle-ply laminate is architecturally comprised of layers of biased orthotropic laminas (based on unidirectional or woven bidirectional fibers), an essential prerequisite for the application of CLPT is an a-priori knowledge of elastic mechanical properties of a constituent lamina. It is natural to expect that the properties of such a lamina are obtained experimentally as the effects of factors such as manufacturing process cannot be captured if an idealized theoretical approach such as “rule of mixtures” is employed in estimating the properties of a single lamina. However, determining the mechanical properties of a lamina directly using uniaxial tensile and shear tests in a UTM (Universal Testing Machine) cannot also be done in a standard fashion due to its extremely low thickness. In order to overcome the difficulties stated, a new inverse method is being proposed using testing of uniaxial coupon specimens extracted from two distinct laminates, an orthotropic and an angle-ply laminate with the same number of plies and approximate thickness, and CLPT leading to a set of four nonlinear simultaneous algebraic equations which are then solved to obtain the four in-plane properties (viz. E₁, E₂, ν₁₂ and G₁₂) of a single lamina. The suggested methodology can be considered as time- and cost-efficient as only uniaxial responses from coupon specimens are needed without the necessity of measuring lateral strains which usually call for additional strain gages. The efficacy of the current method has been demonstrated with examples.