NDE THERMAL MAPPING USING THERMOCHROMIC SENSOR FILMS

Mapping thermal exposure of components over large areas without the added complexity of wired thermocouples is a step forward in an industry where parts are increasing in size and complexity. We have developed polymer films that use thermochromic probe molecules to more easily determine the thermal exposure over large areas. Following a first-order kinetic reaction model, the films show dramatic color and fluorescence changes related to the time and temperature of a thermal exposure. Sensor molecules have been developed which transition from a colorless, non-fluorescent state to a colorful, highly fluorescent state when heated. These molecules are dispersed in removable polymer films and can be attached to components to act as large area sensors. Our group has created a sensor system spanning three temperature ranges: low temperature (50⁰C-100⁰C), medium temperature (100⁰C-150⁰C), and high temperature (150⁰C-200⁰C). By combining two or more of these films, we can determine the time-temperature exposure based on quantitative data collected through spectroscopy. If either an estimated time or temperature is known for the exposure, it is possible to determine the other variable based on a single film. Upon thermal exposure, the fluorescent molecules within the film are activated and cause a color change and fluorescence which can be evaluated qualitatively by visual inspection and measured quantitatively using spectroscopy. In our work, we studied both absorbance of the films and fluorescence intensities under ultraviolet light. Based on our data, we were able to determine the activation energies of the probe molecules. Theoretical look-up charts generated by determining activation energies of the probe molecules allow quick and efficient determinations of time-temperature exposure. There are many potential uses of these thermal exposure mapping films, including detection of incipient thermal damage of composite materials for efficient part servicing, ensuring appropriate curing conditions for composite repairs, and validation of thermal models.  © 2019 by Ever J. Barbero. Published by CAMX - The Composites and Advanced Materials Expo.