The phenomena occurring at the upper and lower yield point in mild steel is not yet fully understood. This paper is an attempt to study some of these phenomena and to correlate them with the facts already known about the first yielding of this unique material.

In order to investigate the rate of loading on the yielding of mild steel a machine was designed in which the amount of increase in load per unit time could be held constant regardless of the rate of strain. Results of tests are presented in which the rate of loading for separate tests was changed by a maximum ratio of 1 to 21,000. The effect of rate of loading was studied on four different shapes of test specimens.

Factors affecting the initial yielding of mild steel and their influence on the shape of the stress-strain diagram are discussed. These factors include the effect of the speed of deformation and of stress concentration upon the stress at which yielding takes place. The manner in which localized yielding affects the stress distribution in the bar and changes the actual strain rate is also described. The constant-load-rate type of test such as presented here is compared with the ordinary constantstrain-rate type of test. Various shapes of stress-strain diagrams that have been obtained in practice are explained in terms of the factors previously mentioned.

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