The purpose of this paper is to propose new experiments in the field of the plastic deformation of solids. A qualitatively satisfactory theory is discussed and is then used to suggest worth-while experiments. This theory assumes that plastic deformation is a result of the formation and motion of certain crystalline imperfections called dislocations. The nature and behavior of these imperfections are discussed. The origin of these imperfections and the changes produced in them during deformation are considered. Certain new experiments are then proposed. It is suggested that all of these experiments be done using single crystals in an attempt to simplify the interpretation of the experiments: (a) It is suggested that the internal friction, the electrical resistance, and the rate of creep all be measured on the same spectroscopically pure metal single crystal. These three measurements should be made at various temperatures and should be done successively on crystals of various crystallographic orientations. (b) It is also suggested that stress-strain curves be obtained on fine single-crystal wires. In this connection it has been shown in glass fibers that the stress concentrations at the cracks can be reduced in very thin fibers. (c) It is proposed that the electrical resistance and the internal friction of an ordered alloy be measured while the single-crystal specimen is subjected to a very small plastic deformation. (d) An attempt will be made to suggest an experiment which would enable the observer to “see” the dislocations.

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