Neutral axis behavior: at the layer passing through the centroid (neutral axis) of a loaded beam section, the fiber is in which state?

Introduction / Context:
When a beam bends under transverse loading, the top fibers and bottom fibers usually experience opposite normal stresses (compression versus tension). Between them is a unique surface—the neutral surface—whose intersection with the cross-section is the neutral axis.

Given Data / Assumptions:

• Prismatic beam, linearly elastic behavior.
• Plane sections remain plane (Bernoulli–Euler assumption).
• Symmetric bending without significant axial force.

Concept / Approach:
The bending stress varies linearly with distance y from the neutral axis: sigma = M * y / I. At y = 0 (neutral axis), sigma = 0. Therefore, at the neutral axis, the fiber carries neither tension nor compression in bending.

Step-by-Step Solution:

Verification / Alternative check:
Stress diagrams for standard cases (simply supported beam under point or uniform load) always show zero normal stress at the neutral axis.

Why Other Options Are Wrong:
Tension/Compression occurs away from the neutral axis at extreme fibers.“Shear only” is not correct as a general statement; while transverse shear is maximum at or near the neutral axis in many sections, the question asks about normal stress state due to bending.

Common Pitfalls:
Confusing normal bending stress with transverse shear stress distribution; the neutral axis refers to zero normal bending stress, not necessarily zero shear stress.

Final Answer:

neither in tension nor in compression