The Effect of Grain Size on the Yielding Behavior of Very Low Carbon Steel.

Author: Russell, Thomas Lee

Year: 1958

Degree: Dissertation (Ph.D.)

Advisors: Wood, David Shotwell; Clark, Donald S.

Committee Member: Unknown, Unknown

Option: Mechanical Engineering; Physics

DOI: 10.7907/77XQ-5M30

Abstract

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This thesis presents the results of an experimental investigation of the effect of grain size on the yielding behavior of very low-carbon steel. Steel specimens of five grain sizes were produced for testing. The average grain diameter of these specimens ranged from 1.1 x 10(-3) in. to 8.1 x 10(-3) in. Two types of tensile tests were performed, one by the application of very low loading rates and the other by the rapid application of a constant stress. Both types of tests were performed at temperatures of 72°F and -320°F, while a few rapid loading tests were performed at a temperature of -109°F.

Both the static upper and lower yield points are observed to increase linearly with [...], where d equals the average grain diameter. Delay time and pre-yield micro strain for specimens that exhibited a yield point were determined from the rapid loading tests conducted at room temperature. Delay time decreases with increasing grain diameter for a given stress.

The experimental results are compared with several dislocation models for yielding. These models are shown to be incapable of describing the experimental results of this investigation. A more detailed model, based on Cottrell's yielding mechanism, is described.

Plastic deformation by twinning occurred in both static and rapid loading tests performed at a temperature of -320°F. A critical stress for twinning was observed. This stress is independent of stress rate in the range from [...] to [...]. The critical twinning stress increases linearly with [...]. The existence of a critical stress for twinning is shown to be compatible with Cottrell and Bilby's theory for the formation of twin bands.

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