Chapter 10: The Torsion Test

A key distinction is drawn between specimen geometries; while solid circular bars are common, thin-walled tubular specimens are often preferred because they minimize the stress gradient across the cross-section, ensuring a nearly uniform shear stress distribution,. The chapter details the mathematical derivation of stress and strain in torsion, explaining that in the elastic range, shear stress varies linearly from zero at the center to a maximum at the surface. It further explores the complexities of calculating stress during plastic flow, where the linear relationship ceases to apply, requiring differential analysis of the torque-twist diagram to determine the ultimate torsional shearing strength,. Significant attention is given to failure analysis, distinguishing between ductile fractures, which occur along planes of maximum shear perpendicular to the longitudinal axis, and brittle fractures, which form helically at a 45-degree angle due to maximum tensile stresses. The text also compares the torsion test to the tension test, highlighting that torsion allows for the fundamental study of plasticity and flow curves to large strains without the complications of necking or barreling, although yielding is harder to detect in solid bars. Finally, the chapter covers hot torsion testing as a method to simulate metalworking processes like rolling, allowing researchers to study flow properties, strain rate sensitivity, and microstructure evolution under high-temperature conditions.