## 3 Materials and Methods
### Experiment Set-Up  
This study aimed to detect BVID in CFRP laminates using LVT. The experimental design was inspired by recent research on identifying LDR frequencies through thermal responses. In particular, [PERSON] et al. [PERSON] et al. (2018) showed how repeated frequency sweeps could be used to extract periodic thermal patterns caused by defect activation, while [PERSON] et al. [PERSON] et al. (2018) developed a thermal spectroscopy approach that relies on upward and downward frequency sweeps, combined with signal derivative analysis, to estimate LDR frequencies while correcting for thermal latency.  
In this work, instead of using one broadband transducer, we used commercially available narrowband transducers with a centre frequency of 28 kHz (Hesentec HS-4 SS-3528). The transducer was attached to the surface of the CFRP sample using couplant and excited one at a time using a function generator (TTI TG550) and a voltage amplifier (Piezo System EPA-104). Because automated sweeping was not possible, we manually adjusted the excitation frequency in small steps and monitored the surface temperature response live with an infrared camera.  
To address the limited sample size (150 mm \(\times\) 100 mm) and the relatively large diameter of the ultrasonic transducer, a dual-sample setup was employed. One CFRP specimen (the \"lower sample\") was clamped vertically in a bench vice with a protective cork layer to prevent surface damage, while a second specimen (the \"upper sample\") was placed horizontally on top. The transducer was positioned such that half of its contact surface was bonded to the vertical sample and the other half to the horizontal sample, as shown in Figure 2. This configuration prevented the transducer from overlapping a defect area on a single sample, allowed simultaneous excitation of both specimens and enabled direct comparison of their thermal responses under identical excitation conditions. A lightweight clamp ensured consistent contact without obstructing the thermal camera's view.  
For all CFRP specimens, a 28 kHz piezoelectric transducer was used as the sole excitation source, chosen based on preliminary tests which showed strong thermal contrast in a 1.6 mm-thick sample impacted at 12 J. However, since each sample has a different LDR frequency, the raw thermal response varied, some samples showed clear defect signatures, while others did not.  
Thermal measurements were captured using a long-wave infrared (LWIR) camera (FLIR T560) placed approximately 1 metre from the sample surface. To minimise vibration artefacts and ensure a stable thermal signal, the recording procedure was standardised: the infrared camera was activated first and recorded for five seconds before ultrasonic excitation began. The signal generator then excited the specimen for five seconds, after which the camera continued recording until a total of 25 s had elapsed. Only frames captured between 5 and 15 s were used for data processing, discarding the first and last ten seconds to improve signal clarity and reduce computational time. Figure 2 illustrates the arrangement of the samples during LVT testing.  
To improve the visibility of BVID in the thermal image sequences, we applied two processing techniques: PCA and RPCA. These methods help reduce background noise and enhance localised heating signals that may indicate defects.