中性面（Neutral surface）
承受弯曲的长构件中，纤维应力近似为零的一个假想纵向曲面。（见 Young 第 10 页。）对于横截面相对于载荷作用方向对称的构件，中性面位于上、下两个实体表面之间的中间高度处。
Neutral surface
An imaginary longitudinal surface of approximately zero fiber stress in a long member subject to bending. (See Young p. 10.) For a member whose cross-section is symmetric with respect to the direction of the applied load, the neutral surface will be mid-height between the top and bottom physical surfaces.