Time-domain-reflectometry cure monitoring

This, talk describes a Time-Domain-Reflectometry (TDR) system for monitoring the cure, of advanced polymer matrix composites. A rapid voltage pulse is applied to an inexpensive capacitance sensor, and the returning reflection analyzed quantitatively for viscosity and percent cure. The returning reflection is either Fourier-transformed to a microwave rotation spectrum for detailed laboratory analysis, or tracked in the direct time domain for robust factory-grade cure monitoring. The advantages of this approach are: 1) an absolute tracking mechanism based on a known polar-group species rather than stray ionic contaminants, 2) a small sensor size due to increased sensor efficiency at microwave frequencies, 3) a microwave measurement technique which is suitably robust for factory application, and 4) the potential use of lines remaining after cure for in-service strain and damage detection. Results are presented in the Hexcel 8552 and 3501-6 cure systems, both under controlled laboratory conditions and in factory autoclave use. The system provides an alternative between high-frequency fiber-optic methods and low-frequency dielectric methods, combining optical-style precision and miniaturization with electrode simplicity and robustness. The system is important in overcoming quality control problems and improving uniformity in composite manufacture, particularly in critical structures, and in increasing production speed.