Repeated experiments with materials in testing machines and in practice have proved that there are certain laws which always hold true. These laws may be enumerated as follows :
I. When a body is subjected to a small stress a small strain is produced. When the stress is removed the body springs back to its original shape.
II. Within certain limits the change of shape, or strain, is directly proportional to the stress producing it. This is the same as saying that when a tensile force is gradually applied to a bar it elongates the bar and that up to a certain limit this elongation is proportional to the force.
For example, if we take a bar of wrought iron 1 sq. in. in section and subject it to a tension of 5000 lbs., it will be found to elongate .02 in.; if a tension of 10,000 lbs. be applied the elongation will be .04 in.; if a tension 15,000 lbs. be applied it will be .06 in.; for a tension of 20,000 lbs., .08 in., and for a tension 25,000 lbs., 10 in. When, however, the next 5000 lbs. is added, making a total stress of 30,000 lbs., it will be found that the total elongation is .14 in., which shows that the elongation is increasing more rapidly than the stress.
The point at which the elongation begins to increase more rapidly than the stress is called the elastic limit.
III. When the stress is sufficiently great a strain is produced which is partly permanent; that is, the body does not spring back entirely to its original shape when the stress is removed. This lasting part of the strain is called a set, and when a body is strained sufficiently to give it a permanent set it is said to be strained beyond its elastic limit.
IV. When a still greater stress is applied to a body after the elastic limit is reached, the strain rapidly increases and the body is finally ruptured or broken. Many materials, such as iron and steel, after the elastic limit is reached, act very much like molasses candy. When pulled they stretch and draw down thinner and thinner until finally they break apart. The machine designer must remember that the stress should never exceed the elastic limit of the material, because when a bar is thus stressed it is very unsafe and is likely to break.
V. A force acting suddenly, such as a sledge hammer blow, is called a shock and causes greater injury than the same force gradually applied, because of the velocity or speed of the blow and the effect of its sudden application. Familiar examples of steel subjected to repeated stresses and shock are found in the piston and connecting rods of a locomotive. These parts are always made heavier than would be necessary if they were to remain stationary.