ACOUSTO-ULTRASONIC MEASUREMENTS TO MONITOR DAMAGE DURING FATIGUE OF COMPOSITES.
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An acousto-ultrasonic technique was used to monitor damage initiation and development in (0, 90//2)//s, (0, plus or minus 45)//s, and (0, 90, plus or minus 45)//s graphite epoxy laminates. The laminates were either loaded in quasi-static tension or were fatigued with tension-tension loading cycles. At a number of intermediate stages (strain levels for quasi-static tests and load cycles for the fatigue tests), the loading was halted and the acousto-ultrasonic measurements were made on the specimens. Several quantitative parameters, which include moments and ratios of moments of the frequency spectrum, were found to correlate extremely well with the stiffness change and the stress-strain behavior of the specimens.