I've been somewhat confused over the concepts of true and engineering strain, and I just want to see if I am understanding this correctly. Let us denote true and engineering strain as $\epsilon_t$ and $\epsilon_e$.
I know that $\epsilon_t = \ln(1+\epsilon_e)$.
For small deformations, we have the following relationship between strain and displacement:
$\epsilon = 0.5[\nabla u + (\nabla u)^T]$
This relationship is for the engineering strain, and not the true strain right?
And I also understand that strain gauges measure the engineering strain.
If all of this is true, then what is the point of true strain?