Fatigue Crack Initiation Studies Using Flash Photomicrography: Part I. Experimental Equipment and Procedure.
ARMY MATERIALS AND MECHANICS RESEARCH CENTER WATERTOWN MASS
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An automatic system was 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 periodically during the test usually at 50- or 100-cycle intervals until a well-defined crack has formed at the notch root and extends across the full thickness of the specimen. For analyzing the film record, each frame of the developed negative is projected on a screen starting at the last frame on the film, and the well-defined crack is traced backward in time, frame by frame to its initiation. A crack is judged to have initiated where it is visible in one frame and not evident on the frame just preceding. Cracks with a surface length on the specimen of approximately 0.001 inch and an opening of 0.0001 inch are easily visible in the projected images. In the report, the system is described in detail. Author
- Metallurgy and Metallography
- Test Facilities, Equipment and Methods