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R-curves for surface cracks in Ti-6A1-4V titanium alloy were generated for the purpose of studying the resistance of the alloy to their subcritical growth under a monotonically increasing load typical of a proof test. Another purpose was to evaluate Ehret's compliance calibration for measuring the instantaneous size of a surface crack using the unloading slope of a load-versus-clip-gage-opening curve. Another purpose was to study “leak-before-burst” during sustained load and during subcritical crack growth under rising load.
Clips were mechanically attached to either side of the surface crack on the face of the specimen. A clip-gage extensometer was used to plot load-versus-clip-gage opening. Slopes were taken periodically by unloading about 10 percent so that subcritical crack growth could be distinguished from crack-tip plastic deformation. The tests were run in accordance with the ASTM Recommended Practice for R-Curve Determination (E 561-78T) as much as possible.
R-curve, surface crack, titanium, stress-intensity factor, fracture toughness
Member, technical staff, TRW, Defense and Space Systems Group, Redondo Beach, Calif.
Professor, School of Engineering and Applied Science, University of California at Los Angeles, Los Angeles, Calif.