Dislocation damping in copper single crystals
Barnett, David Morton
Roberts, John M.
Master of Science
Hysteresis loops were observed in 99.999 % pure copper single crystals under unidirectional cyclic stressing using a capacitance gage microstrain measuring technique. These loops; which represent an energy loss presumably due to dislocation damping; were measured at low frequencies as a function of stress amplitude; prestrain, and temperature. The prestrain region extended from 10-6 in/in to 10-2 in/in, and the temperature range covered was 135°K to 300°K. The decrement, or energy loss per total energy input, was found to decrease markedly with prestrain. For prestrains greater than 1 % only small decrements (less than 0.03) could be observed below stress amplitudes of 250 psi. No temperature dependence of the decrement was noted in the temperature range concerned. The microyield point, or the stress level at which an open hysteresis loop was first observed, was found to increase with prestrain, but was also temperature independent. Room temperature stress-strain curves indicated a work-hardening effect at strains less than 1%. Experimental checks revealed that the crystals tested exhibited only a limited easy glide region. The early onset of Stage II hardening is explained as a specimen size effect. The microyield point is associated with the stress necessary to operate a Frank-Read source. The prestrain dependence of the decrement and of the stress to create an open loop are thought to be due to the shortening of the average dislocation network loop length in the Stage II region. A theory describing the temperature independence of the decrement is currently unavailable.