Let ( n 1 , n 2 , n 3 ) be the directions of the three principal strains. An octahedral plane is one whose normal makes equal angles with the three principal directions. The engineering shear strain on an octahedral plane is called the octahedral shear strain and is given by γ γ o c t = 2 3 ( ε ε 1 − − ε ε 2 ) 2 + ( ε ε 2 − − ε ε 3 ) 2 + ( ε ε 3 − − ε ε 1 ) 2 where ε ε 1 , ε ε 2 , ε ε 3 are the principal strains.