Torsion testing – plastic deformation to high strains and strain rates

A new method of calculating stress–strain curves from torsion measurements is described. Contrary to the usual calculation of stress and strain at the surface of the specimen – where the material properties are distorted by microcracks, notch effects, etc. – stress and strain are determined at a ‘critical radius’ inside the specimen. For this purpose an initial approximation for the flow curve of the type σ_f ∼ ΦⁿΦ^m is improved by calculating a ‘correction function’ from the test results. This calculation is based on a Taylor series expansion which converges more strongly if tubular specimens are used instead of solid ones. Tubular specimens are therefore used if the flow curve appears to deviate strongly from the exponential law. The new method of test evaluation makes use of the fact that contrary to tensile and upsetting tests, in torsion tests the measured curve gives an almost undistorted linear projection of the flow curve. In fact the test evaluation consists only of ‘calibrating’ the measured torque and angle of twist in terms of yield stress or equivalent strain, respectively.

MST/94