Composite heat damage measurement using the handheld Agilent 4100 ExoScan FTIR

Importance of the Topic

The detection and quantification of heat-induced damage in carbon-epoxy composites are critical for maintaining structural integrity in high-performance applications such as aerospace. Early oxidation of the resin matrix compromises mechanical strength before cracks or delamination appear. Implementing a rapid, non-destructive, in situ analytical technique enhances safety, reduces maintenance costs and downtime, and supports regulatory compliance.

Aims and Overview of the Study

This application note evaluates the capabilities of the handheld Agilent 4100 ExoScan FTIR for non-destructive measurement of thermal oxidation in large composite parts. Key objectives include:

• Demonstrate field-deployable FTIR analysis on intact components without disassembly.
• Establish quantitative calibration to correlate spectral changes to exposure temperature.
• Showcase real-world measurements on aircraft composites damaged by overheating or fire.

Methodology and Instrumentation

The 4100 ExoScan FTIR uses diffuse external reflectance at a 45° incidence to collect mid-infrared spectra (4000–650 cm⁻¹) with up to 4 cm⁻¹ resolution. Customized optics optimize sample contact and signal strength. Two operator levels are provided:

• Administrator: method development and calibration via software.
• Technician: simplified data acquisition with color-coded pass/marginal/fail results.

Calibration employed partial least squares (PLS) regression on first-derivative, mean-centered spectra (Savitzky–Golay smoothing) to correlate oxidation bands (notably carbonyl peaks near 1700 cm⁻¹) to treatment temperatures.

Main Results and Discussion

Spectral analysis of composite panels (e.g., 977-3, 5250-4, BMS 8-212) revealed progressive increases in ester/perester carbonyl absorption with heat exposure. A PLS model using a single loading vector achieved a cross-validated standard error of ~10 °F and R² = 0.93 for predicted versus actual treatment temperatures. Field measurements on an aircraft part exposed to an engine fire identified the highest oxidation adjacent to metallic support structures, demonstrating spatial mapping capability.

Benefits and Practical Applications of the Method

• In situ, handheld FTIR eliminates the need for sample removal and destructive preparation.
• Rapid analysis (~20–25 s per spectrum) accelerates maintenance workflows.
• Color-coded software output simplifies decision-making for engineers and technicians.
• Quantitative temperature prediction aids in assessing repair versus replacement strategies.

Future Trends and Potential Applications

Advances in portable spectrometer design, such as the Agilent 4200 FlexScan FTIR (detachable optics and electronics), expand access to tight or confined measurement locations. Further integration of chemometric models could enable real-time monitoring of ultraviolet degradation or combined environmental effects. Implementation in other industries—wind energy, automotive composites or civil infrastructure—can broaden non-destructive diagnostics of polymer-based materials.

Conclusion

The Agilent 4100 ExoScan FTIR provides a robust, user-friendly approach for assessing early heat damage in composite materials directly in the field. High spectral quality, rapid measurement, and quantitative chemometric analysis support informed maintenance decisions, ultimately improving safety and reducing lifecycle costs.