An experimental investigation of the time delay for initiation of plastic deformation in an annealed low carbon steel is described. The delay time is determined as a function of the applied stress at temperatures of -75°F, 73°F, 150°F, and 250°F. The stress is applied in a continuous manner within a period of about 7 millisec, and the stress is maintained substantially constant thereafter, until plastic deformation begins. At any given temperature, the relation between the logarithm of the delay time and the stress is found to consist of two straight lines which join together at a point in the delay time and the stress plane. One is a line of constant stress, and the other is a line along which the logarithm of the delay time decreases linearly as the stress is increased above the constant stress line. The experimental results are compared with a dislocation theory of yielding in low carbon steel which has been given by A.H. Cottrell and B. A. Bilby (21)*.It is shown that this theory, in its present form, does not describe the experimental results adequately. Two suggestions are made for modification and addition to the theory which might lead to satisfactory agreement with the experimental observations.
*The figures appearing in the parentheses refer to the references listed at the end of this thesis.
