1. The neutral axis is an axis in the cross section of a beam (a member resisting bending) or shaft along which there are no longitudinal stresses or strains. == Theory == If the section is symmetric, isotropic and is not curved before a bend occurs, then the neutral axis is at the geometric centroid of a beam or shaft. All fibers on one side of the neutral axis are in a state of tension, while those on the opposite side are in compression. Since the beam is undergoing uniform bending, a plane on the beam remains plane. That is: γ x y = γ z x = τ x y = τ x z = 0 {\displaystyle \gamma _{xy}=\gamma _{zx}=\tau _{xy}=\tau _{xz}=0}... from wikipedia.org