Measuring constitutive shear behavior of orthotropic composites and evaluation of the modified Iosipescu test

The modified Iosipescu test uses a variable notch opening angle θ, depending on the material anisotropy and orientation, to accomplish even stress- and strain-fields in the test region. Constitutive shear properties can thus be measured more accurately, more completely, and with fewer sources of error. Here more materials and properties were studied, and a 3D optical strain- and displacement-monitoring system was used to evaluate and improve both the modified test and the novel fixture used here. It performed substantially better than the commonly used Wyoming fixture in terms of producing constant through specimen thickness conditions and preventing undesired specimen twisting. Additionally, loading–unloading behavior of two high-performance, essentially unidirectional, fiber composites was investigated to assess stiffness reduction, damage, localization, and irreversible strains. In the present of such, well controlled testing conditions are even more important.