Lamb Wave Phased Array Ultrasonic Inspection Of Titanium Alloy Sheet

Titanium alloy is widely used in aerospace, shipbuilding, nuclear industry and other fields because of its good heat resistance, corrosion resistance, high strength performance, and the application of titanium alloy thin plate (thickness of 0.5-6.0 mm) is gradually increasing. An offshore engineering with structural components using 4 mm thick TC4 titanium alloy sheet, subject to corrosion and other harsh service environment, while titanium process performance is poor, in the thin plate processing process is prone to absorb impurities or produce scratches, pores, interlayers and other defects, so the need to develop an effective testing process to ensure the quality of titanium alloy sheet parts.
At present, titanium alloy thin plate defect detection means usually use C scanning imaging, but for large-size plate scanning speed is slow, low efficiency. lamb wave is an emerging ultrasonic inspection method of plate, lamb wave is the excitation of acoustic wave wavelength and plate thickness for the same order of magnitude, the longitudinal and transverse waves synthesised by the special form of sound waves can be achieved in the medium to achieve long-distance propagation, its transverse and longitudinal waves in the plate for many times. Interference and superposition of the vibration of the special characteristics of the Lamb wave defect detection technology in the plate flaw detection is very good, is the focus of researchers and applications of the direction of attention.
Lamb wave propagation at different angles of incidence through the superposition of transverse and longitudinal waves to form a variety of modes, different specifications of the plate detection needs to choose the appropriate mode, the echo signal is affected by the nature of the defects and Lamb wave modes, so the first need to explore the detection of the object's phase velocity, group velocity and the frequency of the frequency thickness product of the relationship between the detection of the selected modes. Although the Lamb wave detection method can achieve good results, but the modal selection and extraction is more complex, which limits the efficiency of the actual detection work. In this paper, the phased array detection technology is used as a research tool to form multimodal Lamb waves for defect detection of titanium alloy sheet by exciting the incident waves at different angles, and to develop a convenient Lamb wave detection process suitable for the application of actual engineering sheet inspection.
For different inspection objects, the dispersion characteristics of the Lamb wave need to be analysed to select the appropriate modes for inspection. After the ultrasonic longitudinal wave enters the workpiece through oblique incidence, it is continuously reflected at the boundary of the thin plate, resulting in the conversion and superposition of transverse and longitudinal waves, which is repeated many times to form the Lamb wave covering the entire thin plate, and when it encounters interfaces, hole defects, etc., different defect echoes will be generated.
At any frequency, there are at least two modes of Lamb wave in the plate, with the increase of frequency, the number of modes of Lamb wave also increases, and the propagation speed of each mode also changes. In order to reduce the detrimental effect of the multimodal characteristics of Lamb waves on defect detection, a single mode of Lamb is usually excited by technical means, which is difficult to implement in practical engineering applications.