Changes of the internal friction in copper after cold working Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/00000298s

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  • Investigations of the effect of cold working on the internal friction of copper have become numerous in the last few years due to the discovery that part of the internal friction is due to the motion of dislocations in the metal. Most of the investigations so far have been done at frequencies of about 150 kilocycles per second. At these frequencies the internal friction has been found to be amplitude dependent and a function of the amount of cold work performed on the metal. This experiment was performed at a frequency of 54 cycles per second by vibrating the reed transversely in a vacuated symmetrical transducer at room temperature. The reed was annealed by placing it in a vacuum and passing electric current through it so that it was heated to a dull red. This was repeated and the reed was then left in the vacuum until cool. The cold working was done by placing compressive loads of 2, 4, 6, 8 and 10 tons on two reeds of the same size. The pressure on each reed would then increase in steps of 1 ton effective load on the total surface area of each reed. It was found that contrary to some theories, the internal friction of the reed was still amplitude dependent and a function of the amount of cold work. The internal friction decreased upon additional cold work after the annealing. As the amount of cold work was increased, the internal friction passed through a minimum and then sharply increased. It is proposed that the thermal currents present in the metal at this frequency remove the free dislocations before the stress starts them oscillating under the influence of the stress. Once the dislocation has been removed it may become a bound dislocation and as such will not enter into the internal friction. Cold working serves to increase the number of free dislocations. The thermal currents simply remove these and as a result there is a decrease in the internal friction. The final increase is believed due to the passing of the yield point. Once this point was passed the resulting disorder and flattening of the grain structure would lead to a rapid increase in the internal friction.
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