Tension–compression Biaxial Test with ±45° Symmetric Angle-Ply Laminates for Determining the Pure Shear Response in Principal Material Directions of a Lamina

The tension–compression biaxial test with cruciform specimens using ±45° symmetric angle-ply laminates is proved to be suitable for determining the in-plane pure shear response of a lamina in principal material directions. The full stress–strain evolution presents a linear stage followed by an inelastic region with high shear deformations. At the linear stage a pure and homogeneous shear strain state is measured by Digital Image Correlation (DIC). Afterwards, the DIC registers the initiation of a shear band in the measurement region at 45° from the loaded directions that leads to the final failure plane. It is verified that the arms of the specimen, which are uniaxially loaded, do not develop pseudo-ductile effects during testing that could affect the central response. For comparison purposes, the ability to obtain an accurate shear elastic response from uniaxial loading with ±45° angle-ply laminates is investigated. Based on the elastic strain energy involved in the linear stage of the testing process, it is concluded that both uniaxial tensile and compressive tests could overestimate the shear stiffness. Moreover, if the material has higher tensile than compressive elastic properties, the uniaxial tensile tests should provide a slightly better approximation of the shear elastic modulus than uniaxial compressive tests.