Strainand Stress

Strain and Stress. Stress is an action between two bodies. If a spring be held in a stretched position by a body, there is a pull of the spring on the body, and an equal and opposite pull of the body on the spring. As long as things remain in that condition this mutual pull produces no effect, and is known as a stress; but if one end of the spring be released or the object removed, motion at once ensues. This motion is the result of a certain force, and the stress is numerically equal to this force, which is also numerically equal to either the action of, or reaction on, the spring. The dlrection of this force is, of comse, in the line of the action and reaction, but it may be in either direction, according to the circumstances which allow it to act. This force is sometimes known by the misleading name of "total stress," but stress is actually the force divided by the area over which it acts; this quotient (force per unit area,) is sometunes also badly named the "intensity of the stress." If a spring be attached to a point at one end and allowed to hang with, say, a piece of metal at the other, the stress will be simply the weight of that piece of metal divided by the sectional area of the spring, and will be the same if we remove the piece of metal and fix tbat end of the spring in the same position. The action of such a stress causes the body to be altered in length or in some other way - it produces, in fact, a deformation, and this deformation is properly known as strain. In the case of the stretched spring the strain is shown as increased length, and is measured as the ratio of the increase in the length to the original unchanged length. The ratio stress/strain, or, in the case of the stretching of a line or a spring,

force per unit area/elongation per unit length

is known as Young's modulus of elasticity. A body may be distorted by a shearing stress, and such a distortion is called a shearing strain. Let A D und B C be two parallel layers or a body in its original position, and let the body be distorted so that B C assumes the position B' C', all intermediate parallel layers taking up corresponding sheared positions. If B P be drawn perpendicular to these parallel layers the angle through which it moves is B P B', and the shearing strain is measured by the tangent of this angle, or B B'/B P. The shearing strain is of importance in the case of liquids, and is one of the factors entering into the measurement of viscosity (q.v.).