Which term describes the maximum stress level where stress is no longer proportional to strain?

The maximum stress level where stress ceases to be directly proportional to strain is referred to as the yield point. At this point, a material begins to deform plastically, meaning any increase in stress will not result in a proportional increase in strain.

Understanding the relationship between stress and strain is crucial in materials science and engineering. Initially, materials exhibit elastic behavior, where they return to their original shape upon removal of the load. However, once the yield point is reached, the material undergoes permanent deformation.

The term "proportional limit" refers to the maximum stress at which the material will still behave elastically, and any stress beyond this limit leads to nonlinear behavior where the strain is no longer proportional to the stress. However, the yield point is the more widely accepted term used specifically in defining the point of transition from elastic to plastic behavior.

In contrast, the tensile limit and ultimate strength denote points beyond the yield point where the material can undergo significant deformation before failure, but do not specifically identify the transition in the proportionality of stress to strain as clearly as the yield point does.