###### Abstract  
This study explores the application of low-power vibrothermography (LVT) for detecting barely visible impact damage (BVID) in carbon fibre-reinforced polymer (CFRP) laminates. Composite specimens with varying impact energies (2.5-20 J) were excited using a single piezoelectric transducer with a nominal centre frequency of 28 kHz, operated at a fixed excitation frequency of 28 kHz. Thermal data were captured using an infrared camera. To enhance defect visibility and suppress background noise, the raw thermal sequences were processed using principal component analysis (PCA) and robust principal component analysis (RPCA). In LVT, RPCA and PCA provided comparable signal-to-noise ratios (SNR), with no consistent advantage for either method across all cases. In contrast, for pulsed thermography (PT) data, RPCA consistently resulted in higher SNR values, except for one sample. The LVT results were further validated by comparison with PT and phased array ultrasonic testing (PAUT) data to confirm the location and shape of detected damage. These findings demonstrate that LVT, when combined with PCA or RPCA, offers a reliable method for identifying BVID and can support safer, more efficient structural health monitoring of composite materials. b ac ac  
BVID [14]. Active thermography has emerged as one of the promising NDT approaches to tackle this problem [15]. Active thermography requires external excitation sources such as optical radiation (halogen lamp and laser), electromagnetic stimulation (induced eddy currents and microwaves), and mechanical ultrasonic waves to induce localised heating within the material [15; 16; 17]. These excitation sources generate thermal contrast between defective and non-defective areas, which an infrared camera can capture [17]. Among the various active thermography techniques, vibrothermography stands out for its ability to use vibration and/or ultrasonic excitation to generate heat at damage sites. This process is coupled with monitoring the sample using an infrared camera to assess the structural health of the material [18; 19]. As explained by [PERSON] et al. [20], heat is produced at cracks and defects primarily through mechanisms such as friction, localized plastic deformation, and viscoelastic effects. Recent work by [PERSON] et al. [21] demonstrated that optimizing the excitation frequency and sweep duration in low-power vibrothermography, using a laser Doppler vibrometer and a variable-frequency PZT transducer, can significantly improve the detection of BVID in CFRP by enhancing thermal contrast at defect sites. While their approach provides valuable insight into the role of excitation parameters and advanced instrumentation, it requires more complex equipment and procedures that may limit practicality for routine inspections. In contrast, this study employs a fixed-frequency (28 kHz) transducer for low-power vibrothermography (LVT), representing a more accessible and industry-relevant experimental setup. A key feature of this work is the systematic comparison of principal component analysis (PCA) and robust principal component analysis (RPCA) as image processing tools to enhance defect detection in thermal images. By evaluating both techniques through quantitative signal-to-noise ratio (SNR) analysis, we provide new insights into their relative performance for identifying subtle damage. This research also explores the effect of sample positioning by comparing detection results when CFRP specimens are placed in upper and lower positions within the experimental setup. Understanding how the orientation of the sample affects detection outcomes offers valuable practical guidance for future inspections. Moreover, this study goes further by directly comparing the effectiveness of low-power vibrothermography with PT and PAUT for BVID detection and quantification. To the best of our knowledge, this is the first work to validate LVT findings using both PT and PAUT, allowing for a more comprehensive and reliable evaluation of each method's capabilities.