An Extension of a Theory of Mechanical Damping Due to Dislocations

Abstract

The theory of damping due to dislocations developed by Granato and Lücke concerning a model of a dislocation line pinned by major (unbreakable) and minor (breakable) pins, the latter distributed at random along its length, is extended in two ways: the hysteretic breakaway loss is calculated for higher amplitudes; and the frequency dependent or dynamic loss is shown to be, also, amplitude dependent. The presence in a crystal of two dislocation systems of different average loop lengths is considered as an explanation of the discrepancy between some of the experimental results and the G and L theory. The configuration of the combined dynamic and breakaway loss for the long loop system for various L_N/L_c ratios and L_N values is described, where L_N, L_c are the average loop lengths between major and minor pins, respectively. An interesting feature is the prediction of a low amplitude maximum in the decrement. The theory does not extend to the high frequency region and like the G and L theory is unreliable for L_N/L_c less than 5.