What is characterized by a line where no stress occurs in a beam?

The neutral axis is a key concept in the analysis of beams under bending. When a beam is subjected to bending moments, it experiences both compression and tension within its cross-section. The neutral axis is defined as the line along the length of the beam where there is no longitudinal stress—neither compression nor tension is present.

In a bending scenario, the material above the neutral axis experiences compressive forces, while the material below it undergoes tensile forces. This differentiation is crucial in understanding how beams respond to loads and how they deform. The area around the neutral axis is where the stresses transition from tension to compression.

This concept is fundamental for engineers when designing structural elements, as it helps to determine how much material is needed in different regions of the beam for safety and efficiency. The other options refer to different sections or types of forces in the beam, but only the neutral axis is characterized by the absence of stress.