## 4 Result
### Effect of Sample Positioning on Detection  
Initially, the upper sample in the dual-sample arrangement was intended solely to support the transducer placement. However, experimental results revealed that the upper sample, positioned horizontally above the transducer, often exhibited superior defect detection performance compared to the lower (vertically clamped) sample. This section evaluates the effect of sample positioning on thermal image-based detection across all specimens.  
Ten CFRP samples were tested, five thin (1.6 mm) and five thick (3.8 mm), each subjected to various impact energies ranging from 2.5 J to 20 J. For each sample, thermal data were processed using both PCA and RPCA. From each method, the first five principal components (PC1 to PC5) were extracted for evaluation. This resulted in a total of 100 processed images for the upper-sample configuration and another 100 for the lower-sample configuration.  
As shown in Figures 5 and 6, the defect was only detectable when the sample was positioned in the upper configuration. Notably, in this 2.5 J thin sample, the BVID was only revealed in Principal Component 3, indicating that the thermal response was too subtle to be captured in the dominant PC1 or PC2. This highlights the importance of evaluating multiple components, especially in low-energy impact cases where defect signals are weak. In contrast, the same defect remained undetectable in the lower configuration regardless of the processing method or principal component.  
A similar result was observed in a thick CFRP sample subjected to an 8 J impact, where the defect was not clearly detected when the sample was positioned below another material (see Figures 7 and 8). In contrast, the defect became visible in PC3 when the same sample was positioned above another material. This consistency across multiple samples suggests that the sample position significantly influences defect detectability in vibrothermographic testing.  
In contrast, samples with higher impact energies or more prominent damage, such as the thin 12 J specimen, exhibited clear defect visibility in both configurations. However, even in these cases, the upper configuration often resulted in marginally higher contrast or more defined defect shapes. (See Figures 9 and 10).  
Figure 4: Comparison of defect visibility in dual-sample configuration: (**Left**) defects in both the upper and lower samples are visible; (**Right**) only the lower sample defect is visible.  
Figure 5: RPCA and PCA applied to a thin CFRP sample with a 2.5 J BVID, positioned above another material (upper configuration).  
Figure 6: RPCA and PCA applied to a thin CFRP sample with a 2.5 J BVID, positioned below another material (lower configuration).  
Figure 7: RPCA and PCA applied to a thick CFRP sample with an 8 J BVID, positioned above another material (upper sample).  
Figure 8: RPCA and PCA applied to a thick CFRP sample with an 8 J BVID, positioned below another material (lower sample).  
Figure 10: RPCA and PCA applied to a thick CFRP sample with a 12 J BVID, positioned below another material (lower sample).  
Figure 9: RPCA and PCA applied to a thick CFRP sample with an 12 J BVID, positioned above another material (upper sample).