Neutral fiber
In strength theory, a neutral fiber , also known as the zero line , is the fiber or layer of a cross-section of a beam whose length does not change when twisted or bent . There, the stress does not cause any tensile or compressive stress . It runs in straight bars made of isotropic materials through the geometric center of gravity of the cross-sectional area.
In the fiber furthest away from the neutral fiber in the vertical direction, the tension is maximum.