According to the graphical relationship of torque and degree of rotation, we can notice that
the torsion specimen deformed elastically and then plastically similar to the case of the tension tested
specimen. The initial stage of elastic behavior shows a linear relationship of torque and degree of
rotation with its slope representing the shear modulus of the modulus of rigidity, G. The stress at the
proportional limit is frequently determined at 0.04 rad.m-1 of the gauge length. Beyond the
proportional limit, specimen deformed in a plastic manner and the relationship between the torque and
the degree of rotation is no longer linear. However the determination of the proportional limit carried
out using a torsional specimen of a thin-wall tube type will provides a more accurate value in
comparison to that obtained from a cylindrical rod type specimen. Since the stresses vary across the
section of the specimen from the center toward the peripheral of the specimen as mentioned previously, the reduced effect of stress distribution in the thin-walled specimen is therefore beneficial
for the calculation of the stress. Within the elastic range of deformation, the shear stress can be
calculated according to equation 1