Which description best matches plastic deformation on a stress–strain curve?

• A. Elastic Deformation
• B. Plastic Deformation
• C. Yield Strength
• D. Proportional Limit

Answer: (B)

Plastic deformation is permanent, non-reversible change in shape that remains after the load is removed. On a stress–strain curve, the elastic portion is linear and reversible—the material would snap back to its original shape if the load is released. Once the stress exceeds the yield strength, the material begins to deform plastically as dislocations move and the shape changes irreversibly. As loading continues, the curve can rise further due to work hardening, but the defining feature is the permanent strain that remains after unloading. This distinguishes plastic deformation from elastic deformation, where no permanent change remains, and from the limits that mark the end of linearity or the onset of plasticity.