measurement in engineering of the capacity of metal, wood, concrete, and other materials to withstand stress and strain. Stress is the internal force exerted by one part of an elastic body upon the adjoining part, and strain is the deformation or change in dimension occasioned by stress. When a body is subjected to pull, it is said to be under tension, or tensional stress, and when it is being pushed, i.e., is supporting a weight, it is under compression, or compressive stress. Shear, or shearing stress, results when a force tends to make part of the body or one side of a plane slide past the other. Torsion, or torsional stress, occurs when external forces tend to twist a body around an axis. Materials are considered to be elastic in relation to an applied stress if the strain disappears after the force is removed. The elastic limit is the maximum stress a material can sustain and still return to its original form. According to Hooke's law, the stress created in an elastic material is proportional to strain, within the elastic limit (see elasticity). In calculating the dimensions of materials required for specific application, the engineer uses working stresses that are ultimate strengths, or elastic limits, divided by a quantity called factor of safety. In laboratories materials are frequently "tested to destruction." They are deliberately overloaded with the particular force that acts against the property or strength to be measured. Changes in form are measured to the millionth of an inch. Static tests are conducted to determine a material's elastic limit, ductility, hardness, reaction to temperature change, and other qualities. Dynamic tests are those in which the material is exposed to a combination of expected operating circumstances including impact (e.g., a shell against a steel tank), vibration, cyclic stress, fluctuating loads, and fatigue. Polarized light, X rays, ultrasonic waves, and microscopic examination are some of the means of testing materials.

See H. E. Parker, Simplified Mechanics and Strength of Materials (rev. ed. 1961); S. Timoshenko and D. H. Young, Elements of Strength of Materials (5th ed. 1968); M. G. Bassin, Statics and Strength of Materials (4th ed. 1988).

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