Principal Strains:
If the state of stress is given in terms of the principal stress components s1, s2 and s3, the equivalent strains components might be calculates as,
e 1= (s 1 )/E - (v sy2) /E - (v s3 )/E
e 2= (s 2 )/E - (v s1 ) /E - (v s3 )/E
e 3= (s 3 )/E - (v s1) /E - (v s2 )/E
Alternately, we may rewrite as,
Using abbreviations, [e] for
, [s] for
and [C] for = 
[e] = [C ] [s]
Eq. shall be useful in writing compact expressions & simplified derivations.