Compression, Bearing, and Shear Properties

Compressive Properties

In theory, the compression test is simply the opposite of the tension test with respect to the direction of loading.  In compression testing the sample is squeezed while the load and the displacement are recorded.  Compression tests result in mechanical properties that include the compressive yield stress, compressive ultimate stress, and compressive modulus of elasticity.

Compressive yield stress is measured in a manner identical to that done for tensile yield strength.  When testing metals, it is defined as the stress corresponding to 0.002 in./in. plastic strain.  For plastics, the compressive yield stress is measured at the point of permanent yield on the stress-strain curve. Moduli are generally greater in compression for most of the commonly used structural materials.

Ultimate compressive strength is the stress required to rupture a specimen. This value is much harder to determine for a compression test than it is for a tensile test since many material do not exhibit rapid fracture in compression.  Materials such as most plastics that do not rupture can have their results reported as the compressive strength at a specific deformation such as 1%, 5%, or 10% of the sample's original height.

For some materials, such as concrete, the compressive strength is the most important material property that engineers use when designing and building a structure.  Compressive strength is also commonly used to determine whether a concrete mixture meets the requirements of the job specifications. 

Bearing Properties

Bearing properties are used when designing mechanically fastened joints. The purpose of a bearing test is to determine the the deformation of a hole as a function of the applied bearing stress.  The test specimen is basically a piece of sheet or plate with a carefully prepared hole some standard distance from the edge. Edge-to-hole diameter ratios of 1.5 and 2.0 are common. A hardened pin is inserted through the hole and an axial load applied to the specimen and the pin. The bearing stress is computed by dividing the load applied to the pin, which bears against the edge of the hole, by the bearing area (the product of the pin diameter and the sheet or plate thickness). Bearing yield and ultimate stresses are obtained from bearing tests. BYS is computed from a bearing stress deformation curve by drawing a line parallel to the initial slope at an offset of 0.02 times the pin diameter. BUS is the maximum stress withstood by a bearing specimen.

Shear Properties

A shearing stress acts parallel to the stress plane, whereas a tensile or compressive stress acts normal to the stress plane. Shear properties are primarily used in the design of mechanically fastened components, webs, and torsion members, and other components subject to parallel, opposing loads. Shear properties are dependent on the type of shear test and their is a variety of different standard shear tests that can be performed including the single-shear test, double-shear test, blanking-shear test, torsion-shear test and others. The shear modulus of elasticity is considered a basic shear property. Other properties, such as the proportional limit stress and shear ultimate stress, cannot be treated as basic shear properties because of “form factor” effects.