SYMPOSIA PAPER Published: 01 January 1986

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


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.

Author Information

Larsen, JM
Materials Laboratory, Air Force Wright Aeronautical Laboratories, AFWAL/MLLN, Wright-Patterson Air Force Base, OH
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Developed by Committee: E08
Pages: 226–238
DOI: 10.1520/STP17398S
ISBN-EB: 978-0-8031-4972-4
ISBN-13: 978-0-8031-0472-3