What is referred to when two originally perpendicular line segments change angle?

The correct concept in this situation is shear strain. Shear strain is a measure of how much a material deforms when an applied force causes it to change shape without changing volume. When two originally perpendicular line segments change angle due to applied forces, it indicates that the material is experiencing shear deformation.

In this scenario, as forces act parallel to the surfaces of the material, they cause the angles between the segments to distort rather than elongate or compress them, demonstrating how shear strain affects the relationship between shapes and angles in the material. This concept is crucial in the study of materials and structures to understand how they will behave under various loads and conditions.

The other options—bending moment, torsional strain, and axial strain—represent different kinds of deformations or forces. Bending moment pertains to the internal moment that causes an object to bend, torsional strain is related to twisting, and axial strain concerns changes in length along the axis of a material under axial loading. None of these terms quite capture the specific change of angle observed between normally perpendicular line segments.