An Automated Photomicroscopic System for Monitoring the Growth of Small Fatigue Cracks

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Abstract
An automated photomicroscopic system for monitoring the growth of small fatigue cracks was developed. Using a specimen that localized crack initiation within a small region, a 35 mm camera mounted on a reflected light microscope was employed to record the growth of small fatigue surface cracks of approximate lengths ranging from 25 μ to 2 mm. A microcomputer was used to control the testing, operate the camera, and record pertinent data. Upon completion of the test, photographs of the small cracks were projected onto a computer digitizing tablet in order to obtain digital crack length data. The precision of the resulting crack length measurements was approximately 1 μm for cracks on the order of 25 μm in length. In order to utilize the available crack growth data fully, a modified incremental polynomial method for reduction of data to the form of da/dN versus ΔK was developed. In general, the photomicroscopic system provided an efficient, cost effective method for monitoring the growth of small, naturally initiated fatigue cracks.

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