## 2 Related Theory  
### TTU Method Using Water Injection  
In the TTU method, ultrasound is generated by one sensor and received by the other sensor while water is injected through the squirters on both sides, as shown in Figure 1; the water is used as a couplant and it must be continuously injected. The sound pressure of the received ultrasound varies, depending on the acoustic impedance or attenuation/scattering. The defects are detected by scanning the area that is to be inspected and representing the sound pressure change of the scanned area on a two-dimensional (2D) plane. The sound pressure at the receiver varies, depending on the acoustic impedance, attenuation, and scattering. The acoustic impedance results in a change in the sound pressure of ultrasound, which is reflected or transmitted at an interface, because it propagates from one medium to another or to a defect. The reflection and transmission coefficients of ultrasound at an interface are determined by Equations (1) and (2) at normal incidence.  
\[R_{1\to 2}=\frac{P_{r}}{P_{i}}=\frac{Z_{2}-Z_{1}}{Z_{1}+Z_{2}} \tag{1}\]\[T_{1\to 2}=\frac{P_{t}}{P_{i}}=\frac{2Z_{2}}{Z_{1}+Z_{2}}=1+R_{1\to 2} \tag{2}\]  
where \(R_{1\to 2}\) is the reflection coefficient at the interface for propagation in two media, \(T_{1\to 2}\) is the transmission coefficient at the interface, and \(Z\) is the acoustic impedance, which is expressed as the product of the density and the speed of sound in the material.  
As shown in Figure 2, when ultrasound propagates into another medium, the sound pressure received by the opposite side varies depending on the impedance of the medium. If the magnitude of this sound pressure is represented on a 2D plane, it can be expressed with different contrasts, as shown on the right. When the obtained ultrasonic images are evaluated, intact and defective parts exhibit different sound pressures, making it possible to determine the presence of a defect.  
Figure 3 shows the case of delamination, which is a defect that frequently occurs in composite materials. The delaminated part causes total reflection (100%) of ultrasound due to the air layer formed. The parts of the structure that receive ultrasound and those that do not (as a result of total reflection) clearly show differences in the transmitted sound pressure. Delamination can be detected by finding such differences in sound pressure in ultrasonic images.  
Figure 1: Through-transmission ultrasonic method with squirter.  
Figure 2: Ultrasonic image of a material with different impedances.  
A defect that frequently occurs in composite materials along with delamination is a void, as shown in Figure 4. Ultrasonic scattering occurs on the surface of a void, resulting in a difference in sound pressure between an intact part without a void and a defective part with a void. The defects can be detected using this difference in the sound pressure.  
The ultrasound sound pressure changes because of the acoustic impedance change and attenuation/scattering in a material. Defects are also detected through the ultrasonic sound pressure images obtained as a result of such changes.  
The pre-developed multi-joint robotic ultrasonic inspection system performs inspections by arranging the transmitter and receiver sensors in a straight line. If their coordinates are misaligned due to deformation of the mechanism, a spurious sound pressure change occurs because the two sensors are no longer arranged in a straight line. In this case, the reliability of the inspection is degraded because the sound pressure change occurs even in an intact part that is not a defect. To address this problem, it is necessary to measure the degree of misalignment caused by the deformation of the mechanism and to correct the misalignment.  
Figure 4: Change in ultrasonic sound pressure due to a void.  
Figure 3: Ultrasonic image obtained from a delamination layer.