What is the part of the material that is neither stretched nor compressed called when bending metal?

The correct answer is known as the neutral line. When bending metal, the behavior of the material is characterized by different regions experiencing tension and compression. The neutral line is the specific area in the cross-section of the material where there is no stress; it neither stretches nor compresses as the metal bends. This concept is fundamental in understanding how materials behave under bending loads, as it helps predict how the metal will react during the bending process.

In practice, when a metal piece is bent, the outer fibers experience tensile stress (stretching), while the inner fibers experience compressive stress (squeezing). The neutral line is located at a point that divides these two zones, and any deformation occurs away from this line. Understanding where the neutral line is situated helps engineers and technicians make informed decisions about material selection, design, and stress analysis in sheet metal structures.

The other options, while related concepts, do not accurately represent the specific part of the material that experiences no strain during bending. The neutral region refers generally to the area in proximity to the neutral line, while the centerline and balance line do not directly convey the concept of zero stress in the bending context.