The coefficients F 12 , F 13 , F 23 can be determined using equibiaxial tests. If the failure strengths in equibiaxial tension are σ σ 1 = σ σ 2 = σ σ b 12 , σ σ 1 = σ σ 3 = σ σ b 13 , σ σ 2 = σ σ 3 = σ σ b 23 then