# A Titanium Alloy Defect Detection Method Based on Optical-Acoustic Image Fusion
## 2 Principle
### Acoustic Detection of Internal Defects  
Ultrasonic is a kind of mechanical wave that transmits in an elastic medium with a frequency of more than 20 kHz. It can be classified into shear waves, longitudinal waves, and surface waves, according to the relationship between the particle's vibration direction and the transmitting direction of the ultrasonic. Because of good directivity and high energy, ultrasonic is widely used in various fields of NDT. The principle of acoustic detection is closely related to the transmitting characteristics of acoustic waves in the medium. When defects occur inside titanium alloys, parameters such as material elastic modulus and density will change, resulting in interfaces with different acoustic impedances. In addition, the acoustic waves will reflect when encountering these interfaces. During the ultrasonic NDT process via the reflection method, a probe is used to emit ultrasonic waves toward the titanium alloy plate, and the transmitting description is shown in Equation (3). \[|f(t)|=Ae^{-\frac{(t-t_{signal})^{2}}{2/f_{inter}}} \tag{3}\]  
In this equation, \(|f(t)|\) is the modulus value of the acoustic signal, while \(A\) is the amplitude of the acoustic signal. \(t\) is the transmitting time of the ultrasonic wave. When  
Figure 2: Ti-YOLO structure. internal defects are detected, \(t_{signal}\) is the occurring moment of the signal peak value, while \(f_{center}\) is the working center frequency of the ultrasonic probe. Only the initial wave and the bottom wave appear when there are no defects in the detecting object. On the contrary, three kinds of reflecting waves will occur including the initial wave, bottom wave, and defect wave in existing internal defects. The principle of ultrasonic NDT via the reflection method is shown in Figure 3.  
With the data obtained from ultrasonic NDT, ultrasonic imaging work can be carried out. Ultrasonic imaging is a technology that combines ultrasonic NDT with a set of manual or automatic scanning devices to display the inner defects of the object via an image. When carrying out reflection NDT, the detection acoustic beam can only detect a small area. By using a manual or automatic scanning device for the movement and then reconstructing the image according to the positions of each point and the defect signal, the defect distribution image of the entire workpiece can be obtained. Nowadays, there are three common ultrasonic imaging methods, such as the A-scan, B-scan and C-scan, as shown in Figure 4.  
In Figure 5, the result of the A-scan is displayed in the form of a waveform diagram. The horizontal axis represents the transmitting time of the acoustic wave, and the vertical axis represents the amplitude of the acoustic wave. The scanning result records the characteristics of the signal at a detection point. The result of the B-scan is an image of the cross-section perpendicular to the surface of the detecting object. The horizontal axis of the image represents the scanning trajectory of the probe on the detecting object, and the vertical axis represents the propagation time of the acoustic wave. The result of the C-scan is a top view of the defects of the detecting object being inspected. The horizontal axis represents the scanning distance along the X-axis, and the vertical axis represents the scanning distance along the Y-axis. Although an A-scan is simple and easy to use, its result is not intuitive and lacks in information. For the ultrasonic NDT of the titanium alloy, a combination of A-scans and C-scans can be adapted to display the defect information of the detecting object visually. Moreover, the frequency of the probe is closely related to the imaging quality. Generally speaking, the higher the probe working frequency is, the higher the imaging sensitivity and quality will be. Figure 4: Ultrasonic detection imaging methods in common use. Figure 3: Principle of reflection NDT.