Crack closure in the near-crack-tip region has been considered to be an important contribution to the development of a crack-growth threshold for rnacroscopic cracks. Recent analytical work, however. has suggested that closure well back of the tip may be the controlling factor. In order to check on this possibility, material has been machined away far behind the crack tip in order to eliminate long-range closure. Removal of this material did not eliminate the threshold, but did lower the threshold level by approx. lo"/,, for tests conducted on 6.3 mm-thick X7090-T6 powder metallurgy aluminum alloy.
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