equivalent strain

Dear reaserchers

Here is my question in developing the eqivalent strain via in-plane stress problems. please let me find your answers.

I did an experimental test on a 6 mm thickness plate. Principal strains (ε₁,ε₂) was developed from recorded strains via Rosset strain gauges . To develop the equivalent strains I need ε_{1 ,} ε_{2 ,} ε₃  in the following eqation:

dε_e=(√2/3)(dε₁²+dε₂²+dε₃²)^1/2

As there are not recorded any ε₃ ,thus let me know that how i can calculate the eqivalent strains?

regard

Far