NANTeL Mechanical Engineering Certification Practice Test 2026 - Free Mechanical Engineering Practice Questions and Study Guide

The stress level that causes a small permanent deformation is the yield strength. Up to yield, a metal deforms elastically, meaning it returns to its original shape when the load is removed. Once the applied stress reaches the yield strength, the material begins to deform plastically, so some permanent elongation remains even after unloading. In many metals, a standard way to define this point is the 0.2% offset yield, which gives a reproducible measure of when plastic flow starts. This is different from tensile strength, which is the maximum stress before fracture, from proof load, which is a testing load, and from shear strength, which relates to resisting shear rather than the onset of permanent deformation in tension.