Non-Destructive Evaluation of the Curing of Resin and Prepreg Using an Acoustic Waveguide Sensor

This paper will discuss a new technique that is currently being developed to monitor, in-situ, the cure of thermosetting materials. An acoustic waveguide is embedded within the thermosetting material as it cures. Monitoring the attenuation and transit time of ultrasound transmitted through the waveguide provides information on the curing process. Subsequently, the waveguide can be used to monitor impact damage and externally imposed stresses. Relative advantages of the technique include the ability to interrogate large volumes of the material, compatibility with host material and the potential for lifetime monitoring of material properties. Experiments with the cure of 3501-6 epoxy resin and AS4/3501-6 graphite-epoxy prepreg will be described in this paper. The waveguide, which is a solid rod, is embedded within the curing material which is either in an oven or a press. External to the curing environment, at both ends of the waveguide are bonded pieaoceramic sensors which are used to transmit and receive ultrasound. The change in the peak amplitude and transit time of the signal through the waveguide are measured. We have found these parameters to be very sensitive to changes in the viscosity and rigidity of the surrounding medium which take place during cure. Experiments with two different waveguides, a polymer-glass composite and a metal waveguide, indicate a strong attenuation of the signal with increasing viscosity and a strong minimum in amplitude at or near gelation. After gelation, the signal attenuation decreases as the rigidity of the polymer increases with cure in the solid state. Preliminary results indicate that a rapid increase in transit time (slower acoustic wave velocity) accompanies the liquid-to-solid transition (gelation)