Yield Strength is defined as which of the following?

• A. The maximum stress that a material can withstand before failing
• B. The onset of plastic deformation and the end of the linear portion of the curve (offset of 0.2%)
• C. The stiffness of a material
• D. The ratio of stress to strain in plastic region

Answer: (B)

Yield strength is the stress level where plastic (permanent) deformation begins. In many materials there isn’t a sharp yield point, so engineers use a practical definition: the 0.2% offset yield strength. This means drawing a line parallel to the elastic (linear) portion of the stress–strain curve but offset by 0.2% strain, and taking the stress at which this line intersects the curve as the yield strength. That description matches the option that talks about the onset of plastic deformation and the 0.2% offset to define the yield value. The other statements refer to the ultimate tensile strength (maximum stress before fracture), the elastic modulus (stiffness), or a plastic region ratio, none of which define yield strength.