This equation computes the x component of the strain, `epsilon_x` in the three dimensional plane stress (`sigma_z` = 0) case of Hooke's Law.
The reciprocal term of the equation is 1/E where E is the modulus of elasticity (Young's Modulus) .
`nu` is Poisson's ratio = - (lateral strain / longitudinal strain)
`sigma_x`, `sigma_y` are the x and y components respectively of the plane stress