Fatigue-life prediction by an order statistics treatment of acoustic-emission signals |
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Authors: | Joseph Baram |
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Institution: | (1) Ben-Gurion University, P.O. Box 653, Beer-Sheva, Israel |
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Abstract: | In fatigue, both the crack-propagation rates and the cumulative acoustic-emission activity are known to be related to the
applied stress-intensity range. By considering the energy balance during crack propagation and the relation of strain energy
release to the acoustic-emission characteristics, a formal relation between acoustic emission amplitudes and initial fatigue-crack-propagation
rates has been derived. Continuous monitoring of acoustic emission during low cycle (tension-tension) fatigue tests has been
conducted on aluminum 2024-T3 and 7075-T6 alloys, until fracture. Initial crack sizes and orientations in the fatigue specimens
were randomly distributed. Every few hundred cycles, the acoustic signal having the highest peak amplitude was recorded as
the extreme acoustic-emission event for the elapsed period. The extreme peak amplitudes, related to extreme crack-propagation
rates, were shown, by an order statistics treatment, to be extremally distributed.
Statistical, nondeterministic, approach to fatigue considers that only extreme crack-propagation rates are vital to fatigue
lives. Knowledge of the distribution function of propagation rates is therefore essential in design for fatigue. Such knowledge
can now be obtained in a nondestructive manner, during service in real time, by analyzing the distribution of amplitudes of
acoustic-emission signals emitted during cyclic stressing. The statistical treatment enables the prediction of the number
of cycles left until failure. Predictions performeda posteriori, based on results gained early in each fatigue test, were in good agreement with actual fatigue lives. The amplitude distribution
analysis of the acoustic signals emitted during fatigue tests has been proven to be a feasible nondestructive method for predicting
fatigue life. |
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