Calculation of the amplitude dependence of internal friction from measurements of torsional damping

In some materials the internal friction increases with the amplitude of the applied stress or strain. The damping of a specimen in torsion is then an average value over the specimen. A method is presented by which the dependence of internal friction on stress amplitude may be calculated from the average values measured in torsion. The method has general applicability but is developed for the particular case of the torsion of a flat parallelepipedal specimen. The results of the analysis for a circular cylindrical specimen are also given. The method is compared with earlier analyses and demonstrated by reference to results obtained on a specimen of annealed aluminium.