Elastic limit and strain hardening of thin wires in torsion

A theory for the size effect in the strength of wires under torsion is reported and compared with data from thin copper wires. Critical thickness theory is solved rigorously and used to validate a useful approximation which is combined with slip-distance theory modified for a finite structure size. Experimental data with high accuracy around and above the elastic limit show excellent agreement with the theory. The results strongly imply that the physical principle is the constraint that size, whether grain size or structure size, puts on allowed dislocation curvature.