# Delamination Detection in Bimetallic Composite Using Laser Ultrasonic Bulk Waves
## 4 Results and Discussion
### Interaction of Laser Ultrasonic Bulk Waves with a Delamination  
In order to study the interaction of laser ultrasonic bulk waves with delamination defect, a two-dimensional finite element model was established. Figure 6 shows the distribution of the total velocity field simulated in models without and with delamination defect at 750 ns, where colors represent the amplitude of the vibration velocity of the particle. In Figure 6, through the analysis of wave velocity and propagation, five types of waveforms, including the L, S, 2L, 2L\({}_{\text{b}}\), and LS, are clearly identified. In Figure 6a, when the laser ultrasonic longitudinal waves (L) arrived at the composite interface, part of the energy was reflected, generating reflected longitudinal waves (2L) and mode conversion  
Figure 4: Reflection, refraction, and waveform conversion of ultrasonic wave oblique incident on the interface: (**a**) Oblique incidence of longitudinal waves; (**b**) oblique incidence of transverse wave.  
Figure 5: Laser ultrasonic reflected waves in a model without defect (PRD = 1.5 mm).  
waves (LS). The other part of the longitudinal wave energy entered into the copper layer through the interface, continued to propagate forward, and reflected on the bottom surface, generating reflected longitudinal waves (2L\({}_{\text{b}}\)). In Figure 6b, the laser ultrasonic longitudinal waves (L) could not pass through the delamination defect, the longitudinal waves (2L\({}_{\text{b}}\)) reflected from the bottom surface disappeared, and the energy of reflected longitudinal waves (2L) and mode conversion waves (LS) increased significantly, because most of the energy of P-wave was reflected by interface separation defects.  
The out-of-plane displacement curves of nodes 1 mm, 2 mm, 3 mm, and 4 mm away from the excitation point were extracted from the models with and without defects, as shown in Figure 7. In the time domain, the arrival time of 2L and 2L\({}_{\text{b}}\) was close to the arrival time of Rayleigh wave, and it was difficult to distinguish them from aliasing. The displacements of LS-SL, 2S, 2S\({}_{\text{b}}\) in a good model are higher than those in a model with interface separation, so they are salient features that can characterize delamination defects. The relative position of the detection point and the excitation point has obvious influence on the detection result, so choosing the appropriate PRD can improve the detection ability of defects.  
To study the effect of the PRD on laser ultrasonic waveforms, the displacements of LS-SL, 2S, and 2S\({}_{\text{b}}\) are extracted from the models with and without defects, as shown in Figure 8a-c. The displacements of the three modes of ultrasonic waves at the non-defective position were greater than that of the defective position. In Figure 8a, the increase of PRD had little influence on the displacement of 2L, but a peak appeared when PRD was 4.5 mm, which is due to the aliasing between 2L and Rayleigh waves. In Figure 8b,c, displacements of 2S and 2S\({}_{\text{b}}\) in the models with and without defects increased with the increase of PRD. In Figure 8d, the displacement differences of LS-SL, 2S and 2S\({}_{\text{b}}\) in the models without and with defects increased first and then decreased with the increase of PRD. When the displacement differences of LS-SL, 2S, and 2S\({}_{\text{b}}\) reached the maximum, the PRDs were 0.5 mm, 1 mm, and 2 mm, respectively. When the PRD is too small, the detection efficiency is low, when the PRD is too large, the resolution is low. Considering the detection efficiency and resolution, the PRD was set to 2 mm.  
Figure 6: The total velocity field simulated in Cu/Al bimetallic composites with and without delamination defects at 0.75 \(\upmu\)s: (**a**) Without defect; (**b**) delamination defect width of 6 mm.