Case study
Shear in tension
Shear testing in tension (also known as 'lap shear') is commonly used to test adhesives and involves the pulling apart of a material specimen by applying tensile force from opposite sides in a parallel plane. In this way, the material is stressed in a sliding motion. Lap shear testing is typically used where an adhesive bond joins flat surfaces.
The required surfaces may be single, double or multiple. In the case of a single lap shear test, one part is pulled across another at a constant rate. Alternatively, it can be double-shear, where a bonded section is pulled out from between the sections on either side.
Lap joint strength is a function of the materials used, including their absorbency to adhesive, relative elastic properties, and the cohesive properties of the bonding material. The majority of standard test methods are usually tensile, but materials like wood are often tested under compression.
Being able to analyse test result characteristics in fine detail is important where shear failure may occur in either the bonding, the bonded layers or both (adhesion and cohesion). Results for lap shear tests are usually expressed as the applied load divided by the area of the bonded overlap (MPa or N/mm²).