Composite thermal damage – correlation of short beam shear data with FTIR spectroscopy

Significance of the Topic

The use of carbon fiber epoxy composites in aerospace and other high-performance applications is rapidly growing. Assessing the effects of environmental stresses—thermal cycles, chemicals, UV exposure—on composite integrity is critical for safety and durability. Portable, non-destructive molecular analysis enables on-site condition monitoring before mechanical failure manifests, reducing downtime and improving maintenance decision making.

Study Objectives and Overview

This study aimed to evaluate whether a handheld Fourier transform infrared (FTIR) analyzer can detect and quantify thermal damage in an epoxy/carbon composite and whether those spectral changes correlate with interlaminar shear strength loss measured by a short beam shear (SBS) test. Composite coupons were exposed to a range of elevated temperatures, then analyzed spectroscopically and mechanically.

Methodology and Instrumentation

The experimental workflow included:

• Composite Preparation: Cytec 977-3/IM7 epoxy/carbon composite coupons fabricated by the Center for Composite Materials (CCM).
• Thermal Exposure: Coupons were heated for 15 minutes at temperatures from 350°F to 550°F.
• Handheld FTIR Analysis: The Agilent 4100 ExoScan FTIR measured spectra from multiple surface spots at 8 cm⁻¹ resolution (approx. 30 seconds each).
• Mechanical Testing: Short beam shear strength was measured on seven replicates per temperature to determine relative SBS values.
• Data Processing: Spectral data were preprocessed using a Savitzky–Golay first derivative and mean centering. Partial least squares (PLS) regression was applied, and cross-validation yielded a correlation coefficient of 0.95 between actual and predicted SBS strength.

Main Results and Discussion

Key findings include:

• FTIR Spectral Changes: Increasing thermal exposure intensified carbonyl bands at 1680 and 1720 cm⁻¹, evidencing epoxy oxidation. Additional fingerprint-region peaks indicated backbone degradation.
• Strength Degradation: SBS strength decreased progressively with higher temperatures, reflecting resin damage before visible delamination.
• Correlation Performance: The handheld FTIR predicted SBS loss with an average error of 1.89%, within the 3–8% standard deviation range of the mechanical test. A separate validation set across temperatures confirmed robust predictive capability.

Benefits and Practical Applications

Integrating handheld FTIR enables:

• Rapid, on-site screening of composite parts for thermal damage without sample removal.
• Early detection of molecular degradation before mechanical failure or delamination occurs.
• Optimized maintenance schedules and reduced inspection turnaround.

Future Trends and Opportunities

Advancements may include:

• Expanding non-destructive FTIR methods to detect chemical contamination, moisture ingress, and cure state in composites, metals, and ceramics.
• Integrating machine learning models for automated damage classification and life-cycle prediction.
• Miniaturized, higher-sensitivity sensors to probe deeper beneath surfaces and in confined geometries.

Conclusion

This study demonstrates that a handheld FTIR spectrometer can accurately detect thermal degradation in carbon fiber epoxy composites and reliably predict mechanical strength loss. The non-destructive, rapid analysis offers a valuable tool for field inspection, contributing to safer and more cost-effective composite maintenance.

Instrument Used

Agilent 4100 ExoScan handheld FTIR spectrometer, 8 cm⁻¹ resolution.