When a structural element like a beam is subjected to a bending moment, stresses are developed within the material to resist this bending. These stresses vary across the cross-section of the beam. The outermost fibers experience the maximum stress, while the neutral axis experiences zero stress.

Understanding Stress Caused by Bending Moment

A bending moment causes one part of the beam's cross-section to compress and the other part to stretch. For example, in a simply supported beam with a downward load, the top fibers are in compression and the bottom fibers are in tension. The stress developed due to this internal resistance to bending is specifically termed flexural stress.

Let's analyze the given options:

• Shear stress: Shear stress occurs when forces are parallel to the surface or cross-section, causing layers to slide relative to each other. While bending often comes with shear forces (varying along the beam's length), the stress directly caused by the bending moment itself is not shear stress.
• Compressive stress: Compressive stress is caused by forces that push materials together, shortening the object. Bending does induce compressive stress on one side of the neutral axis, but this is only one component of the stress state caused by bending. Flexural stress is the overarching term for the stress distribution due to bending.
• Flexural stress: Flexural stress (also known as bending stress) is the internal stress generated in a structural element when it is subjected to a bending moment. This stress is tensile on one side of the neutral axis and compressive on the other, varying linearly with distance from the neutral axis.
• Tensile stress: Tensile stress is caused by forces that pull materials apart, elongating the object. Bending does induce tensile stress on the opposite side of the neutral axis from compression, but like compressive stress, it's a component of the overall flexural stress state.

Therefore, the specific term used to describe the stresses resulting from a bending moment is flexural stress.

Flexural Stress Distribution

The magnitude of flexural stress ($\sigma_b$) at any point in the cross-section is given by the formula:

$\sigma_b = \frac{M \cdot y}{I}$

Where:

• $M$ is the bending moment at the section.
• $y$ is the distance from the neutral axis to the point where stress is being calculated.
• $I$ is the moment of inertia of the cross-section about the neutral axis.

This formula shows that the stress is zero at the neutral axis (where $y=0$) and maximum at the points furthest from the neutral axis (where $y$ is maximum, corresponding to the top and bottom surfaces in a typical beam). These maximum stresses are either tensile or compressive, depending on which side of the neutral axis they occur and the direction of the bending moment.

Revision Table: Types of Stresses

Additional Information on Bending Moment and Stress

Understanding bending moment and the resulting flexural stress is fundamental in structural analysis and design. The ability of a beam to resist bending depends on its cross-sectional shape (which determines the moment of inertia, $I$) and the material properties (specifically, the material's yield strength or ultimate tensile/compressive strength).

Structural engineers calculate the maximum bending moment in a beam under expected loads and then design the beam's cross-section such that the maximum flexural stress does not exceed the material's allowable stress limits, ensuring the structure's safety and preventing failure.