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Fractographic Evaluation of Fracture Specimens

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Final technical rept. 15 Mar-31 Oct 1975

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This fractographic study of 2219-T851 aluminum and Ti 6Al-4V titanium was undertaken to investigate crack growth interaction effects in greater detail than was possible macroscopically. The specimens examined had been tested during the Crack Growth Analysis for Arbitrary Spectrum Loading Program, Air Force Contract No. F33615-72-C-1744. The transmission electron microscope was used for most measurements obtained. It was concluded that there is a one-for-one relationship between striations and loading cycles for the approximate striation size range from 1 to 50 micro inch. It was shown that striation spacing data, obtained along the centerline and edges of the fracture faces, correlated closely with macroscopic data obtained from the specimen surfaces. The plane strain plastic zone diameter described the overload affected crack length for specimens tested under plane strain conditions. The existence of delayed retardation was neither confirmed nor denied by this study, but a theory using plasticity and internal growth was proposed to explain delayed retardation as a surface phenomenon. The ratio of the closure level after one overload cycle to the stabilized closure level for the overload, decreased with increasing overload ratio. This ratio was larger for titanium than for aluminum at a given overload ratio. A local increase in striation spacing just prior to an overload was credited to stretching of the existing fracture surface by the overload. The stretch bands produced by the first few consecutive overload cycles were found to be much greater than could be calculated.

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  • Metallurgy and Metallography

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