A bending moment produces longitudinal normal stress across a beam cross-section. Under the assumptions of elementary beam theory,

$$\sigma=-\frac{My}{I},$$

where $M$ is the internal bending moment, $y$ is the signed distance from the neutral axis and $I$ is the area moment of inertia about that axis.

Neutral axis

At the neutral axis, $y=0$, so the bending stress is zero. Stress magnitude increases linearly with distance from that axis.

One side of the beam is in tension and the other in compression, with the roles reversing when the sign of the bending moment reverses.

Maximum bending stress

At the extreme fibre a distance $c$ from the neutral axis,

$$|\sigma_{\max}|=\frac{|M|c}{I}.$$

The ratio $I/c$ is the section modulus, a geometric measure directly related to bending strength.