The difference between transverse waves and longitudinal waves in ultrasonic testing: Longitudinal wave small angle flaw detection

　　Ultrasonic waves are sound waves with a frequency higher than that audible to the human ear (20Hz to 20KHz). Ultrasonic waves are waves, so they obey the laws of wave transmission during transmission. For example, ultrasonic waves travel in a straight line in a material; they are reflected at the interface of two different materials; and their propagation speed obeys the law of wave transmission: ν=λf (ν is the wave speed, λ is the wavelength, and f is the wave frequency).

　　The faster the wave travels in the dielectric material, the harder the dielectric material is; conversely, the softer the dielectric material is. The hardness of the dielectric material actually reflects the strength of the material. Therefore, the higher the material strength, the faster the wave speed should be; the lower the material strength, the slower the wave speed should be. In this way, knowing the wave speed means knowing the material strength. Some detection equipment uses the principle of ultrasonic reflection to perform detection.

　　Difference Between Transverse Waves and Longitudinal Waves

　　Waves are the propagation of vibrations, which propagate through a medium. In the same homogeneous medium, the propagation of vibrations is uniform linear motion, which is characterized by the wave velocity V. Transverse waves and longitudinal waves are two types of waves. Waves are the propagation of vibrations, which propagate through a medium. Transverse waves are also called "concave and convex waves". Transverse waves are waves in which the vibration direction of the particles is perpendicular to the propagation direction of the wave. Longitudinal waves are waves in which the vibration direction of the particles is parallel to the propagation direction of the wave.

　　However, ultrasonic waves are still sound waves and are propagated in the longitudinal wave mode. The propagation of ultrasonic waves requires a medium and ultrasonic waves cannot propagate in a vacuum.

　　The difference between transverse waves and longitudinal waves in ultrasonic testing

　　Longitudinal wave flaw detection

　　Shear wave flaw detection

　　Surface wave flaw detection

　　Longitudinal wave small angle flaw detection

　　During the ultrasonic testing process, the emission and reception of ultrasonic waves are achieved through the probe.

　　The commonly used shear wave probe makes the longitudinal wave obliquely incident on the interface and realizes shear wave flaw detection through waveform conversion. When the incident angle is between the first and second critical angles, the longitudinal wave is totally reflected and only the shear wave is refracted in the second medium.

　　The shear wave probe is relatively more accurate, and when the shape of the object being detected is irregular, longitudinal wave small angle flaw detection is more often used. When the workpiece being detected is limited by the geometric shape and the probe movement range is restricted, such as detecting axial transverse defects at the end of shafts, pins, bolts, etc., or the workpiece material is austenitic stainless steel and other coarse grains or the thickness of the workpiece is large and the ultrasonic attenuation is very serious, it is usually considered to use a small angle longitudinal wave probe for detection.

　　For example, most of the flaw detection of shaft forgings uses longitudinal wave straight probes. However, when the shaft is too long or has multiple shaft sections with different diameters, there will be dead zones that cannot be detected by the sound beam scan, and a small-angle longitudinal wave straight probe should be used for axial detection.