The principal values (eigenvalues) ε 1 , ε 2 , ε 3 {\displaystyle \textstyle \varepsilon _{1},\varepsilon _{2},\varepsilon _{3}} of a strain tensor ε {\displaystyle \textstyle {\boldsymbol {\varepsilon }}} are called the principal strains.
If the corresponding principal directions (eigenvectors) are n 1 , n 2 , n 3 {\displaystyle \textstyle \mathbf {n} _{1},\mathbf {n} _{2},\mathbf {n} _{3}} , then
is called the spectral decomposition of ε {\displaystyle \textstyle {\boldsymbol {\varepsilon }}} .
Introduction to Elasticity
of a strain tensor 
are called the principal strains.
, then
.
