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Hardness. Measure of a material’s resistance to localized plastic deformation. Most hardness tests involve indentation, but hardness may be reported as resistance to scratching (file test), or rebound of a projectile bounced off the material (scleroscope hardness). Some common measures of indentation hardness are Brinell hardness number, Rock well hardness number, ASTM hardness number, diamond pyramid hardness number, durometer hardness, Knoop harness and Pfund hardness number. A table relating various type of hardness values of metals is given in ASTM E-140. Hardness often is a good indication of tensile and wear properties of a material.

Heat distortion point. Temperature at which a standard plastic test bar deflects 0.010 in. under a maximum fiber stress of 66 or 264 psi. (ASTM D-648). An alternate term is deflection temperature.

Heat distortion temperature. An alternate term for deflection temperature.

Hooke’s law. Stress is directly proportional to strain. Hooke’s law assumes perfectly elastic behavior. It does not take into account plastic or dynamic loss properties.

Hoop stress. Circumferential stress in a cylinder subjected to internal hydrostatic pressure. For thin wall cylinders it can be calculated by Barlow’s formula: S = PD/2t where S is hoop stress in psi; P, applied pressure in psi; D, cylinder o.d. in in.; and t, wall thickness in in. Barlow’s formula does not hold for thick-wall cylinders where stress varies across wall thickness.

Hot hardness. Measure of hardness at elevated temperature. Often it is determined by heating a specimen, removing it from the oven and testing it with standard hardness testers. However, this is not a true indication of hardness at temperature because the surface cools quickly after removal from the oven and surface properties are critical in hardness testing. Several methods and apparatus for hot hardness testing are described in “Property Measurements at High Temperatures,” W. D. Kingrey, John Wiley & Sons, Inc.