Which statement differentiates yield strength from ultimate tensile strength?

• A. Yield strength is the stress where plastic deformation begins (often defined by a 0.2% offset); UTS is the maximum stress reached before fracture.
• B. Yield strength is the maximum stress before fracture; UTS is the stress where plastic deformation begins.
• C. Yield strength is the elastic modulus; UTS is the hardness.
• D. Yield strength occurs at the onset of necking; UTS occurs after necking.

Answer: (D)

Yield strength is the stress at which plastic (permanent) deformation begins in a material, typically defined by an offset method (often 0.2%). It marks the elastic–plastic boundary, not the point where the material necks.

Ultimate tensile strength is the highest engineering stress the specimen reaches in a tensile test—essentially the maximum load before failure. In most ductile metals, necking starts around the UTS, and after that the engineering stress drops as the cross-section reduces toward fracture.

So the key difference is that yield strength signals the start of permanent deformation, while UTS represents the maximum load the material can carry before necking and eventual fracture.