Ultimate strength and the unit of ultimate elongation are closely related. The ultimate elongation is a strain produced in a unit of length by a stress equal to the ultimate strength of the material. In other words, the elongation of a test piece 1 in. long just at the point of rupture is its ultimate elongation. A rule for finding the ultimate elongation is: Divide the total elongation of the piece at rupture by the original length.

In making tests of materials, it is often well to record the percentage of elongation. This is nothing more than the ultimate elongation expressed in per cent.

Suppose we find that a 50-in. rod elongates 1/2 in. under a certain load. The unit of ultimate elongation will then be 1/2 50 or 1/100 in. The per cent of elongation will be 1/100 in. expressed in per cent. If the ultimate elongation is expressed as a decimal the same rule will hold; that is, simply multiply the decimal by 100 and call the answer per cent. For example, if an ultimate elongation figures .025 in., multiplying this by 100 will give us 2.5%.

In figuring the ultimate elongation of a test piece broken in the machine, it does not matter what the sectional area of the piece is. All we need to know is the increase in length over the original length. This increase should be divided by the original length. The quotient will be the ultimate elongation of the tested piece.