Which term is commonly used to define yield strength in a standard stress-strain test?

• A. The slope of the elastic region
• B. The stress at which plastic deformation begins (often defined by a 0.2% offset)
• C. The maximum stress
• D. The stress at fracture

Answer: (B)

Yield strength is the stress level at which a material begins to deform plastically in a standard stress-strain (tensile) test. Before this point, deformation is elastic and reversible; after it, permanent plastic deformation remains even when the load is removed. For many metals there isn’t a sharp, well-defined yield point, so engineers use a practical convention: the 0.2% offset method. A line parallel to the elastic portion of the curve is drawn but offset by 0.2% strain; the intersection with the actual curve defines the yield strength. This gives a consistent, reproducible criterion for when plastic deformation starts. The slope of the elastic region corresponds to Young's modulus (stiffness), the maximum stress is the ultimate tensile strength, and the stress at fracture is the fracture strength.