Entry by Emily Redston, AP 225, Fall 2011
The classic definition of strain is
where <math>l_0</math> is the original length before any load is applied, and <math>l_i</math> is the instantaneous length. <math>\Delta l</math> is the deformation elongation, or change in length at some instant, as referenced to the original length. This strain is typically referred to as "engineering" strain, and it a unitless quantity. Oftentimes strain is expressed as a percentage, so the strain value is simply multiplied by 100.