A theory of mechanical damping based on the model developed by Granato and LUcke was extended to small numbers of pinning points per network length and to streoses high enough to produce complete breakaway. The detailed response of a dislocation array to an applied stress was calculated using computer techniques in the low-frequency (kilocycle) range and both the viscous and hysteretic damping were computed. The amplitude dependence of the viscous and hysteretic damping was computed as a function of loop-length distribution, distribution of dislocation orientations, pinning point density, and stress distribution. The computation techniques allowed the,e calculations to be made without the usual mathematical approximations which are shown to result in a severe limitation on the previous analytic expressions. The results were applied to a discussion of the use of the damping theory to obtain parameters which describe the dislocation array and dislocation behavior. The time dependence of dislocation damping which results from diffusion of pins along the dislocation and in the lattice is discussed.

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The behavior of pinning points at dislocations during internal friction measurements is discussed. Diffusion of pins along the dislocation prior to breakaway of the dislocation from the pinning points and in 1 he lattice subsequent to breakaway is qualitatively considered. Both types of motion lead to time dependence of the internal friction and the variation of this time dependence with temperature and strain amplitude is discussed. It is shown that a study of these effects can lead to an understanding of the con tribu tion of the amplitude-dependent viscous damping and of the amplitude-dependent hysteretic damping to the total specimen damping. An experimental technique is described which allows a separation of these two contributions. This technique is applied to Al and AI-Mg alloys at high and low temperatures and the existence of these two damping contributions is demonstrated.