In this experiment, the method of phase spectral analysis was used to monitor ultrasonic moduli changes in a two part epoxy adhesive bonded between steel adherends during cure as well as during cyclic loading. Ultrasonic signals were generated using a piezoelectric transducer operated in the pulse-echo mode. These signals were digitized using a high-speed transient digitizer (100 MHz sampling rate) and stored for post-test analysis in the memory of a minicomputer. Based on this information, ultrasonic attenuation and phase velocity measurements (as a function of frequency) were obtained for both longitudinal and shear waves throughout the cure cycle. This technique was also used to monitor the cure of adhesives with different mix ratios in order to evaluate the utility of this approach for quality control purposes. The potential use of the method to study fatigue damage development was also considered.

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