What is the proportional limit?

• A. Point where plastic deformation starts
• B. End of the linear proportional relationship between stress and strain
• C. Maximum strength before necking
• D. Onset of fracture

Answer: (B)

In stress–strain behavior, the proportional limit is the end of the linear region where stress is proportional to strain according to Hooke’s law. Up to this point, deformation is elastic and fully recoverable when the load is removed. Beyond it, the curve becomes nonlinear and plastic effects begin to appear. This point is not the onset of plastic deformation itself (that occurs at yielding) nor the maximum strength or fracture; it simply marks the boundary of the linear elastic relationship.