Defines neutral axis and nature of stresses above and below neutral axis.

 The neutral axis is an imaginary line or axis within a cross-sectional shape that experiences neither tensile nor compressive stresses during bending. In other words, it is the axis about which a bent or curved beam is free to rotate without any deformation. The concept of the neutral axis is fundamental to understanding the distribution of stresses in a beam under bending loads.

When a beam is subjected to bending, the material experiences both tension and compression. The distribution of these stresses varies across the cross-section of the beam, and the neutral axis is the location where the stress is zero.

The nature of stresses above and below the neutral axis can be described as follows:

1. Above the Neutral Axis:

   • Compressive Stresses: The material above the neutral axis is typically under compression. As you move farther away from the neutral axis toward the outer fibers of the beam, the compressive stresses increase. The maximum compressive stress occurs at the extreme outer fiber of the cross-section.
   • Concave Side of the Beam: In a simple bending scenario, the side of the beam that is in compression is referred to as the "concave" side. This is because, during bending, the beam takes on a curved shape, and the compression occurs on the inner side of the curve.

2. Below the Neutral Axis:

   • Tensile Stresses: The material below the neutral axis is typically under tension. As you move farther away from the neutral axis toward the outer fibers of the beam, the tensile stresses increase. The maximum tensile stress occurs at the extreme outer fiber of the cross-section.
   • Convex Side of the Beam: The side of the beam that is in tension is referred to as the "convex" side. This is because, during bending, the outer side of the curve experiences tension.

In summary, the neutral axis is a key reference line within the cross-section of a beam undergoing bending. Above the neutral axis, the material experiences compressive stresses, and below the neutral axis, it experiences tensile stresses. Understanding the distribution of these stresses is essential for designing and analyzing structural elements subjected to bending loads. Engineers use this knowledge to select materials and dimensions that can withstand the expected stresses without failure.