## 3 Experiment System and Measurement
### Measurement of Ultrasonic Testing  
Figure 6a shows the ultrasonic inspection system set up in this study. We also designed a defect evaluation system that moves in six axes to develop inspection algorithms for various surface defects. In each direction, the X, Y, and Z axes were constructed symmetrically from left to right. The inspection areas of the X, Y, and Z axes were about 300 mm, 420 mm, and 200 mm, respectively. The system allowed us to adjust the angle of the rotating module by 0.1\({}^{\circ}\) up to 240\({}^{\circ}\). Using the system, we produced Rayleigh waves by introducing ultrasonic waves at a theoretical Rayleigh angle. The error of this theoretical angle was minimized by slightly changing the angle to confirm the angle with the largest signal reflected from the surface. The tests were performed by applying the pitch-catch technique using the ultrasonic inspection system. The transmitting and receiving transducers were fixed to maintain the Rayleigh angle, and the receiving transducer scanned the defect while moving in 0.5 mm increments toward the defect. The tests were conducted in the same way as those in the FEM simulation setup. Figure 6b shows the setup and the measurement method used to generate the Rayleigh waves. The transmitting transducer was fixed at the location of \(-5\) mm. Both the transmitting transducer and receiving transducer were almost touching in the immersion tank. Then, the receiving transducer was moved to the right in 0.5 mm increments to measure the peak-to-peak amplitude. The optimal angle of incidence (\(\theta\)) was set to 29.1\({}^{\circ}\) according to [PERSON]'s law, and the size and frequency of the ultrasonic transducer were set to 6.35 mm and 10 MHz, respectively. The Rayleigh angles of the ultrasonic waves were changed depending on the difference in the ultrasonic impedance of the specimens, the properties of the materials, and the presence or absence of  
Figure 4: Samples with artificial defects. (**a**) Overall samples (**b**) An example with Circum crack.  
Figure 5: Overview of sample types with micro-sized defects; (**a**) Non-crack, (**b**) Axial crack, (**c**) Circum crack, and (**d**) Shear crack.  
surface treatment. Here, experiments on ultrasonic waves were performed with specimens used in commercial vehicles, and the reproducibility of the ultrasonic testing results was shown to be very high. This study shows a correlation between the types of defects and the transmission/reception locations of the transducers. In this system, the use of an algorithm could allow for the sizing and evaluation of defect signals. If the optimal inspection location is selected based on the research results and defects are analyzed through the selected inspection location, it is expected that a system can be established to determine the number of defects.  
Figure 7 shows the typical peak-to-peak amplitudes used to evaluate the characteristics after machining defects were fabricated on the surfaces of the piston rods. Here red dotted boxes mean mainly amount of amplitudes. They are examples of measuring the amplitude by measuring each type of defect on the piston rods. The figure shows the A-scan signals of the maximum peak points for \"Non-crack\", \"Circum\", \"Axial\", and \"Shear\" defect samples with a defect depth of 700 \(\upmu\)m. Figure 7a shows the peak-to-peak amplitude of the sample with no defects, and Figure 7b-d shows peak-to-peak amplitudes of the different defect types. According to the values in Figure 7, the amplitude is largest when there are  
Figure 6: Manufactured UT system used to inspect Rayleigh waves in the immersion tank. (**a**) Ultrasonic inspection system; (**b**) Ultrasonic setup and measurement.  
no defects, while the other three defect types show slight differences. By measuring the peak-to-peak amplitude, we can implement a Rayleigh wave profile to evaluate the defect behavior characteristics.