3.5 STRAIN ENERGY IN TERMS OF PRINCIPAL STRESSES
Let the major and minor principal stresses in a strained material be σ1 and σ2 and of the same sign. Then, the principal strains are given as
Strain energy per unit volume due to σ1,
Total strain energy per unit volume U = U1 + U2
In the case of 3D principal stress (with same sign) condition, ...