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5. New Techniques and Phenomena

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In our continuing efforts to investigate methods to characterize the state of fatigue nondestructively and to apply these observations to lifetime predictions, we have initiated acoustic surface wave studies using Al 2024-T351. A smooth bar specimen was designed which can be tested in tension-compression fatigue. Acoustic wedge transducers were mounted on the specimen to sample fatigue induced changes within the gauge section. Data on acoustic harmonic generation were taken as a function of applied external load at any point of the sample's fatigue life. An optical microscope was attached to the load frame to monitor microcrack formation in the gauge section. Significant changes of the second harmonic have been observed as a function of stress and fatigue. First results indicate a decrease of the second harmonic as a function of stress. At about 50% of the fatigue life microcracks initiated at surface intermetallics and a substantial increase of the second harmonic amplitude in the vicinity of zero external applied load was observed.



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5 p.