A Study of Single and Polycrystalline Ingot Iron Under Repeated Stress

Citation

McClintock, Frank Ambrose (1950) A Study of Single and Polycrystalline Ingot Iron Under Repeated Stress. Bachelor's thesis, California Institute of Technology. doi:10.7907/A49D-6315. https://resolver.caltech.edu/CaltechETD:etd-03272007-130713

Abstract

Specimens of single and polycrystalline ingot iron were tested in a rotary bending fatigue machine and examined microscopically in order to check previous observations of slip and to study the nature of crack formation and growth. As has been reported, slip takes place on surfaces of high shear stress parallel to a <111> direction in the crystal lattice. In contrast to recent work, the slip was found to occur on wavy surfaces rather than pairs of crystallographic planes. Fatigue cracks were found to follow surfaces of high shear stress containing a <111> direction. The conclusion that fatigue failures are primarily shear failures is strengthened by a statistical explanation of the mechanism by which a microscopic shear failure can lead to the apparent tension failure commonly observed in rotary bending tests on polycrystalline iron. Microscopic observation of the polycrystalline specimens revealed that cracks originate at the grain boundaries before 30,000 cycles in a specimen which would very probably run for at least 106,000 cycles.

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