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Fatigue Crack Growth Behavior of Small and Large Cracks in Titanium Alloys and Intermetallics

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This report is a compilation of results of research performed on small fatigue cracks in titanium alloys and titanium aluminide intermetallics. The principal theme underlying this investigation is the measurement of surface crack lengths and aspect ratios during the growth of three-dimensional small surface cracks in fatigue using a laser interferometric and photomicroscopic system at the Materials Behavior Branch, Materials Directorate, Wright Laboratory. It has been shown that such measurements could be made accurately on a number of candidate alloys systems comprising titanium alloys and newly developed titanium aluminide intermetallics. Fatigue crack growth rates could be accurately calculated and were correlated to data obtained on large cracks in the corresponding materials. Specific test programs, which were designed to accomplish this task and the corresponding results of the study, are categorically discussed in the following. Measurements of shapes of three dimensional surface cracks continuously during fatigue crack growth were made in a near-alpha titanium alloy, Ti-6Al-2Sn-4Zr-6Mo. Crack aspect ratio measurements are demonstrated for cracks growing from electro-discharge-machined EDM notches of different geometries shallow or deep. The experimentally determined aspect ratio variations during crack growth are shown to be in good agreement with the expected variations in aspect ratio. The fatigue crack growth rates of surface cracks, after incorporating the variations in aspect ratio in the calculations, agreed with the large-crack growth data. Fatigue Crack Growth, Small Crack Behavior, TiAl Fatigue Cracks, TiAl Intermetallics Fatigue.

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Final rept. 21 Jan 1991-29 Jan 1992



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Approved for public release; distribution is unlimited.

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