横断面の変形

When uniaxial stress is applied, the material elongates or contracts along the stress direction. Simultaneously, deformation occurs perpendicular to the applied stress, governed by Poisson's ratio. This causes expansion or contraction in the transverse directions, which varies depending on whether the deformation is vertical or horizontal relative to the neutral surface.

The neutral surface is where no longitudinal stress occurs. In the vertical transverse direction, expansions and contractions above and below the neutral surface compensate each other and vanish. However, in the horizontal transverse direction, these deformations do not cancel, causing horizontal lines to bend into circular arcs with varying radii of curvature.

Anticlastic curvature is the curvature of the transverse section of a material under bending, representing the reciprocal of the neutral surface's radius of curvature. It describes how the material bends in directions orthogonal to the primary bending direction. This curvature is inversely proportional to Poisson's ratio and reveals the material's complex three-dimensional deformation behavior.

The radius of curvature of the neutral surface is expressed as the ratio of the bending radius of curvature to Poisson's ratio. This radius corresponds to a circle whose center is located on the opposite side of the primary bending radius. Understanding this relationship is essential for analyzing deformations in a symmetric member in bending.

Horizontal lines bend into circular arcs because expansions and contractions vary across the material's thickness in the horizontal transverse direction. Unlike the vertical direction where deformations cancel at the neutral surface, horizontal deformations accumulate, forcing the lines to curve. The radius of these arcs depends on the material's Poisson's ratio and bending conditions.

Poisson's ratio measures the ratio of transverse strain to axial strain and governs how much the material deforms perpendicular to the applied stress. It directly determines the radius of curvature of the neutral surface and the magnitude of anticlastic curvature. Materials with different Poisson's ratios exhibit different transverse deformation patterns under identical bending conditions.

The radius of curvature due to primary bending (centered at point O) and the radius of curvature of the neutral surface (centered at point O') are located on opposite sides of the material. The neutral surface radius equals the bending radius divided by Poisson's ratio. This geometric relationship explains how the material curves in orthogonal directions simultaneously during bending.