Mechanics of Materials

Neutral Axis

The line in a bent beam’s cross-section where bending stress and strain are zero.

When a beam bends, fibres on one side shorten and on the other lengthen; between them is a surface that does neither, and its trace on the cross-section is the neutral axis. For elastic bending of a homogeneous beam it passes through the centroid of the section.

Bending stress grows linearly with distance from it, σ = My/I, so the extreme fibres carry the most. Transverse shear stress does the opposite: it is greatest at the neutral axis, where for a rectangle it reaches 1.5 times the average.

How much of the FE Civil exam is Mechanics of Materials?