Tensile Strain

Tensile strain ε is the extension of a sample divided by its original length: ε = x/L, where x is the extension and L is the original length.Strain is a ratio of two lengths, so it has no units. It is sometimes given as a percentage by multiplying by 100.Using strain rather than extension allows samples of different length to be compared. A long wire extends more than a short wire of the same material under the same stress, but the strain is the same.The strains in metal wires are usually very small. For example, a 2.0 m wire that extends by 1.0 mm has a strain of 0.0010 ÷ 2.0 = 5.0 × 10^-4, or 0.05%.Because the extension is so small, it must be measured accurately, for example using a vernier scale or a marker against a ruler. Using a long wire gives a larger extension, which reduces the percentage uncertainty.