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Mechanical Properties and Tests - A to Z

Bearing area. Area through which a bearing load is transmitted. Diameter of bearing hole multiplied by thickness of specimen is area used to compute bearing stress.

Bearing load. Compressive load transmitted to a structural member through area of contact (bearing area).

Bearing stiffness.Slope of the tangent at any point on the stress-strain diagram plotted from data obtained in test for bearing strength. (ASTM D-953, plastics and ASTM E-238, metals). Gives an indication of the behavior of materials subjected to edge-wise loads such as applied through mechanical fasteners. Strain used to determine bearing stiffness is defined as % deformation of bearing hole through which the load is applied.

Bearing strength. Measure of maximum usable bearing stress that can be developed in a material. Equal to the stress that corresponds to the point on the bearing stress-strain diagram where the slope of the curve equals the stress divided by a strain of 4%. A standard procedure for determining bearing strength is given in ASTM D-953 (plastics) and ASTM E-238 (metals). While it is known that materials with higher compressive and tensile strengths have higher bearing strengths, there is no widely accepted method for estimating bearing strength from compression or tensile properties.

Bearing stress. earing load applied to a material divided by original bearing area. In the bearing strength test (ASTM D-593 for plastics, ASTM E-238 for metals) a rectangular specimen is loaded in tension or compression by a pin or rod passing through a bearing hole. Bearing stress and bearing strain are recorded as bearing load is increased and are plotted to form a stress strain diagram. Maximum bearing stress is equal to load at rupture divided by original bearing area.

Bend test. Method for measuring ductility of certain materials. There are no standardized terms for reporting bend test results for broad classes of materials; rather, terms associated with bend tests apply to specific forms or types of materials. For example, materials specifications sometimes require that a specimen be bent to a specified inside diameter (ASTM A-360, steel products). Results of bend test of welds are given as fiber elongation (ASTM E-16). And results of tests of fiberboard are reported by a description of the failure or photographs. (ASTM D-1037).

Bending strength. Alternate term for flexural strength. It is most commonly used to describe flexure properties of cast iron and wood products. Bond strength. Stress (tensile load divided by area of bond) required to rupture a bond formed by an adhesive between two metal blocks. (ASTM D-952).

Breaking load. Load which causes fracture in a tension, compression, flexure or torsion test. In tension tests of textiles and yarns, breaking load also is called breaking strength. In tensile tests of thin sheet materials or materials in form of small diameter wire it is difficult to distinguish between breaking load and the maximum load developed so the latter is considered the breaking load.

Breaking strength. Tensile load or force required to rupture textiles (e.g., fibers, yarn) or leather. It is analogous to breaking load in a tension test. Ordinarily, breaking strength is reported as lb or lb/in. of width for sheet specimens.

Brinell hardness number (BHN). Measure of the indentation hardness of metals, calculated from the diameter of the permanent impression made by a ball indentor of a specified size pressed into the material by a specified force. BHN increases with increasing indentation hardness. A standard test for determining BHN is given in ASTM E-10.

Conversion tables that relate BHN to diamond pyramid hardness, Rockwell hardness and Rockwell superficial hardness are published in ASTM E-140. BHN test is particularly useful where deep penetration is required to avoid surface effects or where a large impression is required to avoid errors due to in homogeneity of material. It is not used for thin sections or very hard materials.

Brittle fracture. Failure or rupture of a material with little or no plastic flow or deformation of a metal's crystal lattice. Usually this type of failure is associated with impact loads. However, many materials at low temperatures also show brittle fracture failures under static loads. Two common methods for determining resistance to brittle fracture are the Izod and Charpy impact tests. (ASTM E-23).

Brittleness temperature. Temperature at which plastics and elastomers exhibit brittle failure under impact conditions specified in ASTM D 746. Brittleness temperature often is used as part of specifications for plastics or elastomers, but is not considered an accurate measure of materials lowest use temperature. A test for determining brittleness temperature of plastic film is given in ASTM D-1790.

Bulk modulus of elasticity. Ratio of stress to change in volume of a material subjected to axial loading. Related to modulus of elasticity (E) and Poisson's ration (r) by the following equation: K=Er/3(1-2r).

Bursting strength. Measure of ability of materials in various forms to withstand hydrostatic pressure. For round rigid plastic tubing it usually is reported as internal fluid pressure required to produce rupture (ASTM D-1180). For coated fabrics it is reported as force required to rupture a diaphragm divided by its area. (ASTM D-751).