Use of Marginal Oscillator for Acoustic Monitoring of Curing of Epoxy Resin

The attenuation and velocity of ultrasound in epoxy resins have been shown to provide a reliable method of determining the degree of cure of the resin. Most authors have used broadband techniques for measuring velocity of a resin. While broadband measurements inherently provide more information than narrowband, difficulties in achieving sufficient bandwidth to clearly identify echoes of interest has led to the application of digital filters to separate the individual echoes. In this work, a measurement system is described which uses the resin as a narrowband acoustic resonator which is placed in the feedback loop of an amplifier. Using gain control, the system is allowed to marginally oscillate. The frequency of oscillation depends on the velocity of sound in the resin, while the gain necessary to maintain stable oscillations depends on the attenuation of the resin. The use of a narrowband resonant technique results in a measurement system which is accurate, stable, and may be produced at reasonable cost