STRENGTH OF MATERIALS. A term which may be defined as the resistance which materials offer against deformation. Every load produces in the structure an internal resistance which balances it, and this inter nal resistance is called a stress. Three kinds of direct stress may be produced by load, viz. a stress tending to pull apart, or tensile stress; a stress tending to push together, or compressive stress; and a stress tending to slide on parallel planes, or shearing stress. Complex stresses such as flexure and torsion are capable of being resolved into tension, compression, and shearing stresses. A unit stress is the stress per unit of area; the unit of area is the square inch in English-speaking countries and the square centimeter in countries where the metric system of measures prevails. In all eases of direct stress the total stress is supposed to be uniformly distributed over the area of the cross-section. When a tensile or compressive force is applied to a bar of metal it elongates or is shortened, and up to a certain limit the elongation or short ening is proportional to the load. Beyond this limit the elongation or shortening increases more rapidly than the load. The unit stress at which the deformations begin to increase in a faster ratio than the stress is called the elastic limit, or less commonly the elastic strength. When the
unit stress in a bar is less than the elastic limit, the bar returns, when the stress is removed, to its original dimensions. When, however, the unit stress is greater than the elastic limit, the bar does not fully return to its original dimensions, but a permanent distortion remains. Therefore, if a material is strained beyond its elastic limit, a permanent injury results to its elastic prop erties; and for this reason it is the universal practice in designing engineering structures to make certain that the unit stresses never exceed the elastic limit of the material. When a ma terial is under a stress exceeding its elastic limit it is usually in an unsafe condition. If the stress be increased the deformation rapidly increases until finally the material ruptures.
The unit stress which occurs just before rup ture takes place is called the ultimate strength of the material. The ultimate strengths of ma terials are from two to four times as great as their elastic limits. The strength of materials is determined by straining a piece of the ma terial to rupture and observing the elastic limit, ultimate strength, and other cord Mate phenom ena.