What term is used for the ratio of stress to strain below the proportional limit?

The term used for the ratio of stress to strain below the proportional limit is known as Young's modulus. This property is a measure of the stiffness of a material and describes how much it will deform under a given stress within the elastic limit, where the material returns to its original shape upon unloading. Young's modulus is specifically applicable in the context of uniaxial tensile stress and is defined mathematically as the slope of the stress-strain curve in the elastic region.

Understanding Young's modulus is crucial for engineers, as it provides insight into the behavior of materials under load. It is particularly important in applications where materials are subjected to tension or compression, as it helps in selecting appropriate materials for specific structural applications based on their stiffness and ability to withstand deformations.