Principal strain rates
= Principal strain rates
{title2=$a,b,c$}
<Principal strain rates> are the <eigenvalues> $a,b,c$ of the symmetric <rate-of-strain tensor>. Positive values are local extension and negative values local compression. In <incompressible flow> $a+b+c=0$, implying $a^3+b^3+c^3=3abc$. Their <eigenvectors> describe the principal deformation directions.