Assumptions of the Bernoulli-Euler theory of bending:

  1. The beam is bent by pure terminal couples so that shear and axial forces are not present.
  2. The deformations are small, so that stresses other than longitudinal normal stresses are negligible.
  3. The relation between longitudinal stress and strain is the same in flexure as in tension or compression.
  4. Originally plane cross-sections remain plane.