Which term marks the onset of plastic deformation, where elasticity ends and deformation cannot be recovered?

• A. Yielding
• B. Proportional limit
• C. Tensile strength
• D. Fracture strength

Answer: (A)

The onset of plastic deformation is the point at which the material stops behaving elastically and some of the deformation becomes permanent. Up to this point, the deformation is recoverable when the load is removed, and the stress–strain response is linear (Hooke’s law). When yielding occurs, dislocations begin to move and permanent, irreversible strain remains after unloading. In practice this boundary is defined as the yield strength, often taken at a small specified plastic strain (commonly 0.2% offset).

Proportional limit marks the end of linear elasticity but isn’t always the same as the start of permanent deformation. Tensile strength is the maximum stress the material can withstand before necking, and fracture strength is the stress at fracture—neither describes where plastic deformation begins. So the onset of plastic deformation corresponds to yielding.