The yield stress is best described as:

• A. The stress at which a specific amount of strain (0.2%) occurs in the tensile test.
• B. The energy absorbed before fracture.
• C. The stress at which plastic deformation begins, often defined by a 0.2% offset.
• D. The modulus divided by density.

Answer: (C)

Yield stress is the stress level where material stops behaving purely elastically and begins to deform plastically. In a tensile test, the material follows a linear elastic path up to this point, then permanent, non-reversible deformation starts as dislocations move. Because many metals don’t have a sharp, clearly defined yield point, engineers commonly define yield strength using a 0.2% offset: draw a line parallel to the elastic portion of the stress-strain curve, offset by 0.2% strain, and the intersection with the actual curve gives the yield strength. This makes the yield value a practical and repeatable measure of when plastic deformation begins. The other options describe different concepts—toughness (energy absorbed before fracture) or specific modulus (modulus divided by density)—or refer to a stress at a small strain within the elastic region, which does not mark the onset of plastic deformation.