The yield point is the stress level at which a metal begins to deform plastically.

The yield point is the stress level at which a metal begins to deform plastically.

• A. Elastic
• B. Plastic
• C. Brittle
• D. Ultimate

Answer: (B)

The yield point is the stress level where permanent, plastic deformation begins. Up to this point, deformation is elastic, meaning the metal would return to its original shape if the load is removed. Once the yield point is reached, dislocations move and the material stretches plastically, so the deformation remains even after unloading. The term elastic doesn’t fit because it describes reversible deformation, not permanent change. Brittle refers to materials that fracture with little or no plastic deformation, which isn’t about where plasticity starts. Ultimate strength is the maximum load before necking and fracture, not the onset of plastic deformation. So the stress level at which permanent, plastic deformation begins is plastic.

The yield point is the stress level where permanent, plastic deformation begins. Up to this point, deformation is elastic, meaning the metal would return to its original shape if the load is removed. Once the yield point is reached, dislocations move and the material stretches plastically, so the deformation remains even after unloading. The term elastic doesn’t fit because it describes reversible deformation, not permanent change. Brittle refers to materials that fracture with little or no plastic deformation, which isn’t about where plasticity starts. Ultimate strength is the maximum load before necking and fracture, not the onset of plastic deformation. So the stress level at which permanent, plastic deformation begins is plastic.