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The Fatigue of Powder Metallurgy Alloys.
Annual scientific rept. 1 Dec 80-30 Nov 81,
CONNECTICUT UNIV STORRS DEPT OF METALLURGY
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Fatigue crack growth in the powder metallurgy alloys X7090-T6 and X7091-T7E69 has been studied at two R ratios 0.05 and 0.5 in air. Additional tests in a 3-12 NaCl environment have also been made. Particular emphasis has been placed on the study of the crack closure process on the near-threshold region. Comparisons are made with the fatigue crack growth characteristics of an ingot metallurgy product, 7075-T76. In all alloys high closure levels are observed at R equal 0.05 in the near threshold region, an indication that Mode II is operative. At R equal 0.5 the closure effect is much less pronounced except at the threshold itself, an indication that Mode I is dominant. These indications of the operative crack growth modes are supported by fractographic examination. In the NaCl environment it was found that the threshold level is higher than in air of closure. Further, a unique relationship between the rate of crack growth and the range of the stress intensity factors was not observed as the result of corrosion product build-up behind the crack tip. Author
APPROVED FOR PUBLIC RELEASE