SYMPOSIA PAPER Published: 01 January 1988
STP23282S

Crack Opening Displacement Measurements on Small Cracks in Fatigue

Source

A computerized interferometric displacement gage was used to monitor the crack mouth opening behavior of naturally initiated small fatigue cracks in the high-strength alloy Ti6Al-2Sn-4Zr-6Mo. Complete load versus crack opening displacement curves were obtained for naturally initiated semicircular cracks of surface length as small as 50 µm. For a crack of this size, the maximum crack mouth opening displacement was less than 0.5 µm, and the precision of the individual displacement measurements was approximately 0.01 μm. An excellent correlation between crack mouth opening compliance and surface crack length was demonstrated, and the data were shown to agree with an analytical expression for part-through surface cracks. A matrix of experiments was performed for a range of loading conditions, and crack growth data were plotted versus the nominal ΔK and ΔKeff, which was defined using crack closure measurements. Experimental measurements obtained for a small crack indicated that closure often develops discontinuously as the crack initially extends. This behavior was speculated to result from the formation of individual asperities on the fracture surface. In general, the interferometric method proved to be very effective and demonstrated a unique capability to monitor the behavior of very small fatigue cracks.

Author Information

Larsen, JM
Materials Laboratory Air Force Wright Aeronautical Laboratories, AFWAL/MLLN, Wright-Patterson Air Force Base, OH
Jira, JR
Materials Laboratory Air Force Wright Aeronautical Laboratories, AFWAL/MLLN, Wright-Patterson Air Force Base, OH
Weerasooriya, T
University of Dayton Research Institute, Dayton, OH
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Details
Developed by Committee: E08
Pages: 896–912
DOI: 10.1520/STP23282S
ISBN-EB: 978-0-8031-5036-2
ISBN-13: 978-0-8031-0949-0