When a rectangular beam is supported at both ends and loaded transversely the upper fibres are compressed and the lower extended, the stresses being greatest in the outer fibres, and proportionally less towards the middle of the depth, until a layer is reached where they both vanish. This may be shown experimentally by marking parallel vertical lines upon a beam of indiarubber as Fig. 1, and supporting it at the ends with a load on top, when the lines will be found closer together in the upper part of the beam and further apart in the lower, as Fig. 2, while at some intermediate depth their distances will be unaltered, as in line a b marking the neutral layer, or neutral
axis of the cross section, and showing that neither tension nor compression exists there. If A, B, C, D, Fig. 3, represent a cross section through the centre of a beam under transverse load, e f the maximum intensity of compression drawn to scale, and g h the maximum intensity of tension, then when these stresses are produced, the neutral axis will pass through the intersection k of lines eh, fg, and when ef and gh are equal this will also be the centre of gravity of the beam. When the stress and strain are proportional to each other, and equal in tension and compression, the horizontal lines will show by their length the intensity of the tensile and compressive stress respectively in the various layers.
Professor Henry Adams