Consider an elastic beam of thickness a bent to a radius of curvature R, such that R >>a, the bending moment M is given by
M=EI/R
where E is Young’s modulus for the material of the beam and I is the second moment of area of the cross-section of the beam about an axis which is normal to the plane of bending and passes through the neutral plane. For a beam of thickness a and width b, the expression for I is
I = ba3/12.