Composite heat damage measurement using the handheld Agilent 4100 ExoScan FTIR

Significance of the Topic

Composite materials are essential in aerospace and other high performance applications due to their high strength to weight ratio and reduced maintenance requirements. However, the polymer resin matrix in these materials is prone to oxidative degradation under heat or ultraviolet exposure. Early detection of incipient thermal damage is critical to prevent mechanical failure and to guide maintenance decisions without resorting to destructive disassembly.

Objectives and Study Overview

This application note demonstrates the use of a handheld Fourier transform infrared spectrometer for non-destructive, in situ measurement of heat damage in large composite parts. The aims are to showcase rapid field-deployable analysis, establish a quantitative calibration linking spectral changes to exposure temperature, and validate the approach on components affected by engine fires.

Methodology and Instrumentation

The technique employs diffuse external reflectance mid-infrared spectroscopy using the Agilent 4100 ExoScan FTIR system. Key features include:

• External reflectance interface at a 45-degree angle for composites.
• Spectral range from 4000 to 650 cm-1 with up to 4 cm-1 resolution.
• Measurement times of approximately 20 to 25 seconds at 8 cm-1 resolution.
• Software with Administrator mode for method development and Technician mode for routine inspections with preset limits.

Key Results and Discussion

Spectral analysis of heat treated composite panels revealed a pronounced increase in ester and perester absorbance near 1700 cm-1 and other fingerprint region changes associated with epoxy oxidation. A partial least squares regression model correlated first derivative spectra to heat treatment temperatures ranging from 350 to 500 F. Cross validation produced a correlation coefficient of 0.93 and a standard error of approximately 10 F using a single loading vector. Field measurements on aircraft parts damaged by engine fire provided spatially resolved oxidation levels, correlating higher carbonyl band intensities with areas adjacent to metal supports.

Benefits and Practical Applications of the Method

• Non-destructive, in situ assessment of large composite structures without disassembly.
• Rapid detection of sub-visible oxidative damage to predict strength loss.
• User-friendly software with pass/fail limits to guide maintenance personnel.
• Targeted repair or replacement based on localized damage mapping.

Future Trends and Potential Applications

• Development of chemometric models for ultraviolet-induced degradation.
• Integration with automated or robotic inspection platforms.
• Extension to a broader range of composite resin systems.
• Combining FTIR with other non-destructive techniques for comprehensive health monitoring.

Conclusion

The handheld FTIR approach provides a robust, field-deployable method for quantifying thermal oxidation in composite materials. Its rapid analysis and quantitative output support informed maintenance decisions, enhancing safety and reducing downtime for large aerospace components.

References

1. Oak Ridge National Laboratory. Composite Heat Damage Spectroscopic Analysis. 1990.
2. Great Lakes Composites Consortium & Navy Center of Excellence for Composites Manufacturing Technology. Diffuse Reflectance Mid Infrared Spectroscopy for Incipient Heat Damage. 1994.
3. Seelenbinder J. Composite Heat Damage Measurement Using the Handheld Agilent 4100 ExoScan FTIR. Agilent Technologies Application Note. 2008–2011.