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TDR Composite Cure Monitoring

MSI Time-Domain-Reflectometry (TDR) Composite Cure Monitoring [1-4] provides a new approach to cure monitoring of advanced polymer composites. Using a high-speed pulse and inexpensive microwave sensor, our TDR system follows rotation of a polar resin molecule, providing real-time information on viscosity and percent cure. Operating in the microwave frequency range, TDR Cure Monitoring provides an alternative between high-frequency fiber-optic methods and low-frequency dielectric methods, combining optical-style precision and miniaturization with electrode-based simplicity and robustness.

Dipole Rotation Approach

TDR Cure Monitoring provides a signal from small, embedded electrode sensors. The location of significant cure events are identified, such as viscosity minimum, gel point, and end of cure. The information is used in designing process control and optimization for active autoclave control. The TDR system is straightforward to integrate and provides a new high level of performance compared to conventional dielectric sensing.

TDR Cure Monitoring relies exclusively on rotating polar molecules called dipoles. Rotating dipoles are a better indicator of absolute cure, since their chemistry and concentration is well-known. The dipole spectrum provides a unique signature, detailing viscosity, percent cure, and degree of sensor contact. Results are quantitative and reproducible, and not based on poorly-characterized conducting ions.

Rotating dipoles have an intrinsic molecular dynamics independent of sensor configuration. Conducting ions have no internal dynamics other than charging effects at the electrode. Rotating dipoles thus provide molecular-level information which can be used to deduce individual cure parameters.

Frequency-Domain Cure Monitoring

In Frequency-Domain Cure Monitoring, the reflected pulse is transformed to the frequency domain and displayed as a dipole relaxation spectrum. Results are displayed as a complex storage and loss modulus, similar to mechanical rheological measurement. Changes occurring in loss modulus for Hexcel 8552 during 125C isothermal cure are seen in the sequence below.

The frequency of the dipole loss peak is related to viscosity through the Debye Model of Viscous Rotation. The specific frequency-viscosity relationship is initially calibrated using simultaneous TDR and rheometry measurement; subsequent runs then use this calibration to predict viscosity based on loss peak frequency. Typical results for Hexcel 8552 are shown below.

Time-Domain Cure Monitoring

The same features are seen in the direct reflected pulse, from which the frequency data is derived. From Fourier analysis, high frequency signals in the frequency domain are concentrated at short times in the time domain; low frequency signals in the frequency domain are concentrated at long times in the time domain. A shift in reflected pulse amplitude from long to short times in the time domain thus mirrors a shift in transition frequency from low to high frequencies in the frequency domain, and vice-versa. This forms the basis for time-domain TDR cure monitoring.

When the reflected pulse amplitude at 1 ns is plotted as a function of cure time it qualitatively mirrors the viscosity data. It also tracks during the entire cure cycle, even when the loss peak frequency is below the field of view. The 1 ns amplitude thus provides a “window” on a high-frequency component of the changing dipole spectrum (~1GHz) and rises and falls as the spectrum slides in and out of this window. The measurement provides a simple and robust measurement of high-frequency dynamic response.

The reflected pulse amplitude at other delays provides additional information. The amplitude at 50 ns monitors a low frequency component in the changing dipole spectrum (~20 MHz) such that the bulk of the spectrum must pass through this range on its way from low to high frequencies. As it passes through the signal reverses, since the spectrum is now moving away, rather than toward, the field of view. This reversal provides a unique signature, at both the beginning and end of viscosity profile, which can be used to interpret viscosity at points other than the viscosity minimum.

Planar Capacitance Sensors

TDR Cure Monitoring has a clear advantage because of the simplicity of the sensing transducers. Electrode sensors are versatile, inexpensive, and tolerant of hostile environments. They can be unobtrusively embedded in the process material and left in place after use. Unlike mechanical transducers they cover wide ranges in frequency with a single geometry.

TDR cure sensors are small because of the high operating frequency. A small low-capacitance electrode becomes an efficient capacitance sensor when operating at TDR frequencies of 100 MHz and above. A small sensor size is critical in minimizing intrusion into the process material.

TDR planar strip sensors keep sensor intrusion to a minimum. Available in thickness' of 0.25 mm and 0.125 mm, TDR strip sensors can be custom- engineered to a variety of lengths and widths. The sensing area is typically 5 mm wide, with a 3 mm transmission path leading to the sensor. The sensor can either be trimmed to these dimensions, or used as part of a wider strip.

TDR Cure Monitoring has an additional advantage. Because the traveling pulse is localized, the line leading to the sensor can also be used for time-of-flight fault testing when cure is complete. This can be useful in detecting flaws and damage in the finished material in-service.

Technical References

Additional information on TDR Composite Cure Monitoring can be found at:

  1. High-Performance Composites Magazine, September 2006 http://www.compositesworld.com/articles/monitoring-the-cure-itself

  2. N.E. Hager III, R.C. Domszy, "Time-Domain-Reflectometry Cure Monitoring", SAMPE Proceedings Long Beach CA, 2001 May 6-10, p. 2252. [downloadable pdf format:, pubs/msi_composite_sampe_2001.pdf

  3. N.E. Hager III, R.C. Domszy, "Time-Domain-Reflectometry Cure Monitoring", American Helicopter Society - Affordable Composite Structures Proceedings, Briegeport CT, Oct 1998.

  4. N.E. Hager III, R.C. Domszy, U. S. Patent 5,872,447 (1999).

  5. S. Carrozzino, G. Levita, P. Rolla, E. Tombari, Poymer Engr. & Sci. 30, 366 (1990).

  6. A. Liva, G. Levita, P. A. Rolla, J. Appl. Polymer Science, 50, 1583 (1993).

  7. E. Marad, K. R. Baker, J. D. Graybeal, Macromolecules, 25, 2243 (1992).

 

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