The plastic deformation of iron and the formation of Neumann lines

The object of the investigation described in the present paper was to ascertain the manner in which wide variations in speed affect the mechanism of deformation in plastic metals. Iron was selected as the first metal for experiment, mainly because it is known that rapid deformation produced by shock is accompanied in this metal by special features, known as Neumann lines or lamellæ. When a piece of nearly pure iron, of suitable size and shape, with one face polished and etched, is subsequently subjected to plastic deformation at a moderate rate, the crystal surfaces, when examined under the microscope after deformation, show the well-known appearance of slip bands (1). The present experiments were undertaken in the first instance to ascertain whether the character, number and appearance of such slip bands would be appreciably affected by varying the rate of deformation over a wide range. For the sake of convenience, deformation by compression has been employed, the metal being used in the form of small rectangular prisms, measuring in some instances 0·44 inch by 0·44 inch in section by 0·7 inch in height, and 0·3 inch by 0·3 inch in section and 0·5 inch in height. These were prepared with considerable care, particularly in regard to their length, and the amount of deformation applied to them was measured in every case. In the majority of experiments these small prisms were placed in a compression testing machine, and were surrounded by a hardened steel ring of such size as to limit the deformation exactly to the desired amount. A similar guard-ring was employed in connection with deformation by a blow from a falling weight. The actual amount of deformation could be varied, while still employing the same ring, by inserting small flat strips of hardened steel as packing-pieces under the specimen of iron.