Engineering strain
= Engineering strain
{title2=$\varepsilon=\Delta L/L_0$}
<Engineering strain> is the change in a measured segment's length divided by that segment's initial length:
$$
e=\frac{L-L_0}{L_0}.
$$
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.