Research on Lamb wave detection process of titanium alloy thin plate

Lamb wave has dispersion and multi-mode characteristics, by analysing the group velocity dispersion curve, the mode and plate wave vibration morphology change with different excitation angles, and the phased array probe realizes the variable angle acoustic beam incidence by adjusting the excitation array element delay, which then excites different modes of plate wave.
The inspection object defect is Φ2 through-hole, when detecting from any direction can ensure that the received defect echo is the same, control the angle of acoustic beam incidence as the only variable, the equipment selected ISONIC 2009DUET phased-array ultrasonic detector, the probe specification for the 2.25 MHz 16 × 0.5.
The amplitude of the echo signal at a distance of L=50 mm from the end of the plate was used as the reference value, and the incidence angle was increased from 10° to 75° in 5° steps to scan for through-hole defects. When the angle of incidence is 40° and 55°, the identification distance of defects in 4 mm thick titanium plates is larger, which is conducive to the identification and detection of defects. When the angle of incidence is 25°, 35° and 40°, the gain is at a lower level under the same wave height state, which ensures a good signal-to-noise ratio. In summary, a probe with an incidence angle of about 40° (e.g. K1 oblique probe) can be used to detect thin titanium alloys. The mixed modes are different for different incidence angles.
Lamb wave propagation process there is attenuation is not proportional to the distance, or not monotonically change with the distance, observe the trend of the wave amplitude with the incident distance in Figure 6, due to the longitudinal and transverse waves interfere with each other, most of the incident angle does not conform to the conventional ultrasound propagation trend, there is no obvious wave peaks and valleys of morphology, or there are more than one wave peaks phenomenon in the process of moving, only in 40 °, 45 °, 50 ° incidence angle The change of wave amplitude with distance is in accordance with the law of acoustic wave propagation. Combined with the signal-to-noise ratio and the law of sound wave propagation, the results obtained by using the 40° probe to detect the titanium alloy sheet are more reliable and effective.