A shear test is designed to apply stress to a test sample so that it experiences a sliding failure along a plane that is parallel to the forces applied. Generally, shear forces cause one surface of a material to move in one direction and the other surface to move in the opposite direction so that the material is stressed in a sliding motion. Shear tests differ from tension and compression tests in that the forces applied are parallel to the two contact surface, whereas, in tension and compression they are perpendicular to the contact surfaces.

Purpose of shear testing:

The most common use of a shear test is to determine the shear strength, which is the maximum shear stress that the material can withstand before failure occurs, of a material. This is a very important design characteristic of many types of fasteners such as bolts and screws. For example, when a bolt is used to secure two plates together it will experience a shear forces if the plates themselves experience any forces parallel to their plane that attempt to separate them. If the small fastener fails in shear it may lead to a chain of failures that could lead to the entire destruction of a much larger structure.

Types of shear tests:

Generally there are two common types of shear tests. The first requires that the sample be setup in a modified three point flexure or four point bend fixture. The purpose of this test is to load the sample so that it experiences double shear or so that the sample has two locations where the forces are applied. Each end of the sample is anchored and the force is applied over the middle of the sample in an attempt to remove the midsection so that both ends are left behind. The second test requires that the sample have tapered ends that are each placed into grip fixtures that have been offset from the vertical axis of the sample. The sample is then pulled so that the opposite faces are pulled in opposing directions.

Types of materials tested in shear:

Three materials are commonly tested in shear: rigid substrates, adhesives, and layered composites. Rigid substrates may be composed of metals, plastics, ceramics, composites or woods but usually come in a solid form and are used as building materials or fasteners. Adhesives are used to bond two materials together and experience shear stress when the materials are pulled in opposing directions in an attempt to slide them apart. Layered composites experience shear stress in a similar manner to adhesives as the shear forces are applied to the glue or laminate used to hold the layers together.

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