Terahertz Spectroscopy for Nondestructive Evaluation of Composite Materials

Terahertz spectroscopy occupies the region of the electromagnetic spectrum between microwave and infrared frequencies (approximately 0.1–10 THz) and offers unique insights into the internal structure and composition of dielectric and polymer-based composites without contact or ionising radiation. By emitting sub-picosecond pulses and analysing their transmission or reflection, terahertz time-domain spectroscopy (THz-TDS) can resolve layer thicknesses, detect voids, delaminations, microcracks and moisture ingress, and measure refractive index and absorption coefficients. Unlike ultrasound or X-ray methods, terahertz waves penetrate many fibre-reinforced polymers and sandwich structures while providing material-specific spectral fingerprints. Imaging modalities—including transmission, reflection and tomographic reconstruction—enable rapid scanning of large areas or detailed mapping of critical regions. Applications span aerospace and automotive airframes, wind-turbine blades, civil-infrastructure components and advanced manufacturing quality control. Recent advances in signal processing, such as time-gated spectral analysis and sparsity-based deconvolution, have enhanced depth resolution and contrast, overcoming scattering and multilayer interference. Challenges remain in improving source power, detector sensitivity and system robustness for field deployment, but the technology is poised to complement existing nondestructive evaluation toolsets and to address growing demands for lightweight, durable composite structures in safety-critical industries.

Research from Nature Portfolio

Recent studies have demonstrated the power of time-gated terahertz spectral imaging to extract subsurface details in layered composites with subwavelength precision. By locking onto the electric-field statistics at sub-picosecond time delays and applying spectral kurtosis within defined temporal gates, researchers achieved over an order-of-magnitude improvement in defect contrast. This approach enabled the identification of micrometre-scale separations and inhomogeneities in polymer and wooden composite specimens, and it suggests a pathway towards automated layer discrimination in complex laminate assemblies. Foundational work further showed that coupling ultrafast terahertz pulses with advanced deconvolution algorithms can isolate reflections from closely spaced interfaces, thereby facilitating accurate thickness measurement and defect localisation in multi-layer composite systems.

Terahertz Spectroscopy for Nondestructive Evaluation of Composite Materials publication trend

The graph below shows the total number of articles in terahertz spectroscopy for nondestructive evaluation of composite materials across all publications each year (not limited to Nature Index journals).

Technical terms

Terahertz time-domain spectroscopy (THz-TDS): A technique generating and detecting sub-picosecond terahertz pulses to probe the time-resolved electric-field response of a material, yielding frequency-dependent absorption and refractive index.
Composite material: An engineered assembly of two or more distinct constituent materials (e.g. fibres and resin) designed to achieve superior mechanical or functional properties.
Delamination: The separation of adjacent layers within a laminated composite, often caused by manufacturing defects, impact or mechanical fatigue.
Reflectometric imaging: A non-contact method that collects reflected terahertz pulses to form cross-sectional or volumetric images of subsurface features.
Spectral contrast: Variation in a material’s frequency-dependent absorption or refractive index that enables differentiation between layers or defects.
Time-gated spectral kurtosis: A signal-processing metric applied within selected temporal windows to enhance detection of interfaces by maximising deviations from Gaussian behaviour in the spectral domain.

References

1. Non-Contact, Non-Destructive Testing in Various Industrial Sectors with Terahertz Technology. Sensors (2020).
2. Terahertz time-gated spectral imaging for content extraction through layered structures. Nature Communications (2016).
3. Nondestructive evaluation of aircraft composites using transmissive terahertz time domain spectroscopy.. Optics Express (2008).
4. Nondestructive Evaluation of Carbon Fiber Reinforced Polymer Composites Using Reflective Terahertz Imaging. Sensors (2016).
5. Terahertz Quality Inspection for Automotive and Aviation Industries. Journal of Infrared, Millimeter, and Terahertz Waves (2019).
6. Frequency-Wavelet Domain Deconvolution for terahertz reflection imaging and spectroscopy.. Optics Express (2010).