Yield strength is the stress at which a material has undergone some arbitrarily chosen amount of deformation. Which option best matches this definition?

• A. The stress at which a material fractures.
• B. The stress at which a material yields a small amount of deformation.
• C. The stress at which a material has undergone some arbitrarily chosen amount of deformation.
• D. The maximum stress before failure.

Answer: (C)

Yield strength is the stress at the onset of plastic (permanent) deformation. Below this level, the material returns to its original length when unloaded; above it, permanent deformation accumulates. Because the exact yield point can be ambiguous for many materials, engineers define yield strength using a convention: a small, prescribed amount of permanent deformation is chosen, and the stress that produces that amount is labeled the yield strength. That’s why the statement describing the stress at which the material has undergone some arbitrarily chosen amount of deformation best matches the concept. The other options refer to different failure properties: fracture strength (fracture) and ultimate tensile strength (maximum stress before failure). The yield concept specifically centers on the onset of permanent deformation defined by a chosen deformation offset.