What describes the change in length relative to the original length when a material is stressed?

The term that describes the change in length relative to the original length when a material is stressed is strain. Strain is a dimensionless quantity representing the deformation of a material as a response to applied stress. It is calculated as the change in length divided by the original length, allowing for a standardized measure of deformation regardless of the size of the material being analyzed. This makes strain crucial in understanding how materials behave under different loading conditions.

In the context of mechanical engineering and materials science, strain provides insights into the material's performance, indicating how it will react under various forces or pressures. Understanding strain is essential for designing components that can withstand specific loads without failing.

The other terms listed have specific meanings that relate to other aspects of material behavior. The elastic limit refers to the maximum stress that a material can withstand without permanent deformation, tensile stress refers to the internal resistance of a material to being pulled apart, and the factor of safety is a design criterion that ensures structures can withstand unexpected loads or failures. Thus, while all terms are important in the study of materials and engineering, strain specifically addresses the relative change in length as materials are subjected to stress.