SEDL / STP / STP519-EB / STP38026S

An Automatic Flash Photomicrographic System for Fatigue Crack Initiation Studies

Papirno, R
Research mechanical engineer and mechanical engineer, Theoretical and Applied Mechanics Research Laboratory, Army Materials and Mechanics Research Center, Watertown, Mass.

Parker, BS
Research mechanical engineer and mechanical engineer, Theoretical and Applied Mechanics Research Laboratory, Army Materials and Mechanics Research Center, Watertown, Mass.

Pages: 11    Published: Jan 1971

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Abstract

An automatic system has been developed to take flash photomicrographs of the roots of notches in flat doubly-notched specimens during low cycle fatigue tests to determine the cyclic time of crack initiation. The photomicrographs are taken on 35-mm film periodically during the test at preprogrammed cyclic intervals without interrupting the cyclic loading procedure, until a well-defined crack has formed and extends across the full thickness of the specimen at the notch root. In making the data analysis, the developed negatives are projected on a screen at a magnification of approximately 100, and the final well-defined crack is traced backwards in “time”, frame by frame, to its initiation. A crack is judged to have just initiated when it is visible in one frame and not evident in the frame just preceding. Cracks with a surface length of 0.001 in. (0.025 mm) and an opening of 0.0001 in. (0.0025 mm) are easily visible in the projected images.