The piezoelectric implant method compared to classical ultrasonic spectroscopy: Application to the monitoring of polymer curing

Previous studies have described an original method enabling the in situ real time monitoring of a composite polymer structure by means of an inserted piezoelectric sensor. An analytical one-dimensional model has demonstrated that the electrical impedance of such an inserted element depends in particular on the acoustical parameters of the surrounding media. Then, by solving the inverse problem by means of a nonlinear optimization algorithm, it is possible to determine these acoustical parameters characteristic of the materials in contact with the inserted piezoelectric plate. This technique has been applied to the monitoring of the cure of gel-forming and glass-forming polymers such as polyurethanes and epoxies and compared to classical ultrasonic spectroscopy method. Thus it is possible to detect chemical evolution, transitions, and damage phenomena in such systems, provided that the material is homogeneous in the ultrasonic field or in the area surrounding the piezoelectric element. Particular attention has been focused on a polyimide (PMR-15) which is used as a matrix for composites in high-temperature applications. [Work supported by SNECMA/HISPANO-SUIZA.]