$\sigma = E\epsilon \label{3.1.1}$
where $$E$$ is the Young’s (elasticity) modulus. All types of steels, independent on the yield stress have approximately the same Young modulus $$E = 2.$$ GPa. The corresponding value for aluminum alloys is $$E = 0.80$$ GPa. What actually is $$\sigma$$ and $$\epsilon$$ in the above equation? We are saying the “uni-axial” state but such a state does not exist simultaneously for stresses and strains. One dimensional stress state produces three-dimensional strain state and vice versa.