When analyzing the deformation of a symmetric prismatic member subjected to bending by equal and opposite couples, it becomes clear that as the member bends, the originally straight lines on its wider faces curve into circular arcs, with a constant radius centered at a point known as Point C. This phenomenon helps to understand the stress and strain distribution within the member more clearly.

When the member is segmented into tiny cubic elements, it is observed that the primary stress experienced within the member is normal stress, leading to uniaxial stress conditions at any point. This arrangement reveals the existence of a neutral surface, where both the strain and stress longitudinal components are zero. This surface runs parallel to the upper and lower faces of the member, and the distance from the neutral surface to point C is ρ

To explore the deformation of this member, consider an arc at a distance y from the neutral surface. The deformation is the difference in lengths from point C between the arc at y (L') and the neutral surface arc (L). Dividing the deformation δ = L' - L by the length of the neutral arc shows that the longitudinal normal strain varies linearly with the distance from the neutral surface. By applying Hooke's Law, which relates stress and strain in elastic materials, the stress can be determined at any point based on its distance from the neutral surface.