 Engineering strain

Engineering strain is the change in a measured segment's length divided by that segment's initial length:It is dimensionless. A segment initially $50\mathrm{mm}$ long that extends by $0.5\mathrm{mm}$ has $e=0.01$, or one percent. The initial gauge length must match the instrument's measured extension; actuator motion can include deformation of grips and the test frame.

For a uniform stretch ratio $\lambda =L/L_0$, engineering strain is $\lambda -1$. It agrees with the small axial strain to leading order, while the logarithmic length change is $\log \lambda$. Engineering strains over successive finite stretches do not simply add: two extensions of ten percent multiply length by $1.1^2$, giving a total of twenty-one percent. This makes the choice of reference length essential when comparing measurements taken over different loading stages.