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

Importance of the Topic

Advanced carbon fiber epoxy composites are widely used in aerospace structures due to their high strength-to-weight ratio. Assessing environmental damage, such as thermal degradation, is critical to ensure long-term safety and performance. Conventional infrared analysis requires laboratory sampling, which can be time-consuming and destructive. A portable, non-destructive method capable of predicting bulk mechanical properties from surface measurements would greatly enhance in-field inspection and maintenance efficiency.

Objectives and Study Overview

This study aimed to demonstrate that a handheld Fourier transform infrared (FTIR) spectrometer can detect molecular changes in epoxy–carbon composites induced by thermal exposure and correlate these changes with interlaminar shear strength measured by short beam shear (SBS) testing. The work was conducted in collaboration with the University of Delaware Center for Composite Materials (CCM).

Methodology

Composite coupons manufactured from Cytec 977-3/IM7 epoxy/carbon were exposed for 15 minutes to temperatures from 350°F to 550°F. Surface spectra were recorded at 8 cm⁻¹ resolution using the handheld FTIR in approximately 30 seconds per spot. Seven replicates per temperature underwent SBS testing to determine relative interlaminar shear strength. Spectral data were pre-processed (Savitzky–Golay first derivative and mean centering) and correlated to SBS results using partial least squares (PLS) regression with cross-validation.

Instrumentation Used

• Agilent 4100 ExoScan handheld FTIR spectrometer
• Short beam shear test fixture for interlaminar shear strength measurement

Main Results and Discussion

Thermal exposure produced increasingly intense carbonyl absorption bands at 1680 cm⁻¹ and 1720 cm⁻¹, as well as changes in the fingerprint region, indicating progressive oxidation and backbone degradation of the epoxy resin. Measured SBS strength decreased with higher exposure temperatures, reflecting resin weakening prior to visible delamination. A PLS model yielded a cross-validated correlation coefficient of 0.95 between FTIR spectra and SBS strength. Validation on independent samples produced an average prediction error of 1.89%, within the 3–8% standard deviation of the SBS data.

Benefits and Practical Applications

By enabling rapid, non-destructive surface analysis that reliably predicts bulk composite strength loss, handheld FTIR facilitates in-field condition monitoring. This approach supports:

• Preventive maintenance and life-cycle management of aircraft components
• Quality control during manufacturing and repair operations
• On-site assessment without sample removal or downtime

Future Trends and Opportunities

Ongoing work will expand the application of handheld FTIR to detect damage in composites, metals, and ceramics, as well as surface contamination affecting bonding and coating processes. The Agilent 4200 FlexScan model offers a smaller optical head for tight access areas while maintaining the same analytical performance. Further integration with predictive analytics and digital maintenance platforms could enhance real-time decision support across multiple industries.

Conclusion

This study validates that a handheld FTIR spectrometer can non-destructively detect thermal damage in epoxy–carbon composites and accurately predict corresponding reductions in interlaminar shear strength. The high correlation and low prediction error demonstrate the viability of this approach for in-field composite health monitoring.

References

Rein A, Seelenbinder J. Composite thermal damage – correlation of short beam shear data with FTIR spectroscopy. Agilent Technologies Application Note. 2011; Publication Number 5990-7798EN.