Neutral plane

[1] The location of the neutral plane can be an important factor in monocoque structures and pressure vessels.

If the structure is a membrane supported by strength ribs, then placing the skin along the neutral surface avoids either compression or tension forces upon it.

The US Fleet submarines of World War II had a hull section that was not quite circular, causing the nodal circle to separate from the neutral plane, giving rise to additional stresses.

This gave no resultant bending moment on the frames and so allowed a lighter and more efficient structure.

This was inconvenient, as it was impossible to measure directly between the two marks, but only with an offset trammel down the wells.

An evenly loaded beam, bending (sagging) under load. The neutral plane is shown by the dotted line.
British bronze yard of 1855. The yard is defined by the distance between two lines scribed on the bottom surface of holes at each ends of the bar, on the neutral plane. [ 3 ]
Two grey metal bars, one on top of the other, laying on a red table. The cross section of the bars is like an X with a central surface in the middle and with the tops and bottoms of the X flattened.
Computer generated image of platinum-iridium standard metre bars. The neutral plane of the bar is designed to coincide with the bottom surface of the central web connecting the two sides.