Why do wood and adhesively bonded joints require different shear tests than untreated steel castings?

The primary reason wood and adhesively bonded joints require different shear tests than untreated steel castings lies in their internal structure. While an untreated steel casting is a homogeneous material, wood and adhesively bonded joints are considered anisotropic materials.1

These anisotropic materials possess preferential planes of weakness, meaning they exhibit different levels of resistance to stress depending on the plane in which the force is applied. Because their deformation response is not uniform, they cannot be evaluated using the same testing methods as homogeneous materials. Consequently, engineers must utilize specific shear tests that account for these directional variations in shear strength to accurately assess the material's integrity.12