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The statistical nature of the fatigue properties of SAE 4340 gun tubes was investigated as a part of a systematic study of the mechanical properties of forging steels. Both high and low quality forgings (quality determined by transverse tensile reduction of area) of the same tensile strength level were studied. Specimens were taken in the longitudinal and transverse directions. Procedures were used to permit statistical analysis of the variability of the endurance limit and life to failure.
The variability of the fatigue properties of even high quality forgings was found to be greater than is generally estimated from the results of conventional non-statistical testing programs. This was especially true of the endurance limit. The variability of properties in the transverse direction of the low quality forgings was significantly greater than the variability of properties in the longitudinal direction of low quality forgings or in either direction of high quality forgings. The variability was greatest at stresses near the endurance limit. The variation in life to failure resulting from the variation in several experimental factors, that is, testing machine, eccentricity of rotation, stress relief, fracture criterion, stress calculation, and position of specimen in the forging, was shown to be insignificant. It was concluded that the variability in fatigue properties is an inherent characteristic of this property in this material and cannot be ignored if significant answers are to be obtained.
Ransom, J. T.
Engineering Research Laboratory, Engineering Department, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.
Mehl, R. F.
Director of Metals Research Laboratory, and Head of the Department of Metallurgical Engineering, Carnegie Institute of Technology, Pittsburgh, Pa.