Shear is considered to be the force which causes two contiguous parts of one body to slide relative to each other in a parallel direction to their plane of contact. The strength is a stress which is required to fracture or yield the material in the plane of material cross- section. Simply put, this means either punching or chopping a section from a larger body and the strength necessary to accomplish the task. For example, cutting paper with scissors is considered a shear method to trim or remove. Common materials which get tested in shear are adhesives, layered composites, and rigid substances.
There are a number of common applications which get affected by a material’s shear strength. A few important factors are the choice of tooling, product quality, and ease of processing. For example, trying to cut a piece out of a steel sheet with a plastic cutter will not show any results, but at the same time using a diamond blade to cut a piece of paper could be overkill. While those are extreme cases, they point out how misunderstanding requirements to shear materials can affect the results, and it is important to know the material’s shear properties.
Depending on the type of material shear testing could be important. Isotropic materials are those which have consistent properties in all directions – most common metals being isotropic until they are cold worked, which is when they turn anisotropic. Materials which are anisotropic (cold worked metal, composites, wood) have varying properties. Testing is important in anisotropic materials, as there is a complicated relationship between shear stress and tensile stress. For example, the strength that a composite rod has is higher along its length than it is in a transverse direction. In a similar fashion, the properties of a fastener which is cold-rolled- can be very different to the properties of fastener which is hot-rolled and has the same tensile strength.
Additionally, shear characteristics can be important too when you have to characterize the structural integrity of a bond which is between two surfaces – as the bond could be an adhesive bond, a friction joint, or a weld. Regardless of the type, failure of the bond is mainly dependent on the shear strength that it has – which is something that can be determined experimentally by a test.
Despite the fact that the direction of forces differs between shear and tensile stress, the two have a relationship. When you pull in one direction it will cause a tensile stress in that direction, but the same axial force causes it to be stressed in shear in different directions. When it comes to isotropic materials, planes which are of maximum shear stress are 45 degree oriented from planes which have maximum compressive or tensile stress. As a result, components will fracture under the torsional forces, which are imparting shear stress. Another example of the relationship between shear and tensile stress is that a component which is subjected only to tensile forces can fail due to the excess shear stress of the material shear strength.