Yield strength is commonly defined using an offset corresponding to which strain value?

• A. Proportional limit
• B. Yield strength
• C. Tensile strength
• D. Fracture strength

Answer: (B)

Yield strength is defined using an offset criterion so we can pin down the onset of plastic deformation even when a sharp yield point isn’t visible. The standard approach uses a 0.2% offset, meaning we look for the stress needed to produce 0.2% permanent (plastic) strain. Graphically, you draw a line parallel to the initial elastic, linear portion of the stress–strain curve but shifted so that it intersects the strain axis at 0.2% (0.002) strain; where that offset line meets the actual curve gives the yield stress. This makes yield strength a practical and repeatable measure.

The other terms refer to different features on the curve. The proportional limit marks where linear elasticity ends, not where plastic deformation begins. Tensile strength (or ultimate tensile strength) is the maximum stress the material withstands before necking, and fracture strength is the stress at fracture. None of these use the 0.2% offset to define their values, which is why the offset-based yield definition is the standard for yield strength.