9. Testing the 2-3 Shear Strength of Unidirectional Composite

In this study, the objective was to measure the “out-of-plane” 2-3 shear strength of unidirectional composite, working within constraints in supplied material geometry. Unidirectional carbon/epoxy composite material was tested using a sandwich-type beam specimen under 3-point bending with a low span-to-thickness ratio to achieve failure under 2-3 shear. Specimens were carefully designed to deliberately cause shear failure near the midplane, avoiding other possible failure mechanisms. A photoelastic coating and post-mortem microscopy were used to verify failures. Results were compared with a simple analytical description of failure and found to have good agreement. Notably, this approach was able to accommodate the limitations of the supplied material (thin sheets) while still providing an accurate means of obtaining the F₂₃ shear strength of the material. The results also imply the possibility of testing the transverse tensile strength (F_2t) in lieu of performing a shear test, which is far simpler, and inferring the out-of-plane shear strength F₂₃.