ABSTRACT
Strain is a relative measure of deformation. A relative measure of deformation is needed to estimate the stresses and the state of material. For example, a 1-mm change in the length of a 2-mm-long steel rod is a large deformation and will cause permanent deformation, but a 1-mm change in the length of a 2-m-long rod is a small deformation and is likely to keep the rod in an elastic state. The relative deformation can be expressed in a number of ways. Accordingly, there are different strain measures. If the deformation is very small, the longitudinal engineering strain can be expressed as the change in length divided by the original length. This definition would produce inconsistent results for a large deformation case. For example, if a rod is compressed to reduce its length by 50%, its longitudinal engineering strain will be −0.5. If the same rod is stretched to bring it to its original state, its strain will be 1. The sum of these strains is not 0, which is inconsistent with physical understanding.
