Wave Types and Wave Velocity Measurements
Types of Ultrasound
According to the difference between the vibration direction of the medium particle and the wave propagation direction when the wave propagates, the wave can be divided into longitudinal wave, transverse wave, surface wave and plate wave.
1. Longitudinal wave L
The wave in which the vibration direction of the particle in the medium is parallel to the propagation direction of the wave is called a longitudinal wave, denoted by L.
When the medium particles are subjected to alternating tension and compression stress, corresponding stretching deformation occurs between the particles, thus forming longitudinal waves; any medium that can bear tensile or compressive stress can propagate longitudinal waves. Solid media can withstand extensional or compressive stress; liquids and gases cannot withstand tensile stress, but can withstand compressive stress to produce volume changes. So solids, liquids, and gases can all propagate longitudinal waves. The sound velocity of longitudinal waves in steel is generally 5960m/s. Longitudinal waves are generally used for flaw detection of steel plates and forgings.
2. Shear wave S(T)
The wave in which the vibration direction of the particle in the medium is perpendicular to the propagation direction of the wave is called a transverse wave, denoted by S or T.
When the medium particle is subjected to alternating shear stress, shear deformation is generated, thereby forming a shear wave; only solid media can withstand shear stress, and liquid and gas media cannot withstand shear stress, so shear waves can only propagate in solid media , cannot propagate in liquid and gaseous media. The sound velocity of shear waves in steel is generally 3230m/s. Shear waves are generally used in flaw detection of weld seams and steel pipes.
3. Surface Wave R
When the surface of the medium is subjected to alternating stress, a wave propagating along the surface of the medium is generated, which is called a surface wave and is often represented by R. Also known as Rayleigh waves.
When the surface wave propagates on the surface of the medium, the particles on the surface of the medium move in an ellipse, the long axis of the ellipse is perpendicular to the direction of wave propagation, and the short axis is parallel to the direction of wave propagation; the elliptical motion can be regarded as the synthesis of longitudinal vibration and transverse vibration, that is, the Synthesis, so surface waves can only propagate in solid media, not in liquid and gas media.
The energy of the surface wave weakens rapidly as the depth increases. When the propagation depth exceeds twice the wavelength, the amplitude of the particle is already very small. Therefore, it is generally believed that the surface wave flaw detection can only find defects within the depth of twice the wavelength from the surface of the workpiece. Surface waves are generally used in steel pipe flaw detection.
4. Board wave
A wave propagating in a thin plate whose thickness is equal to the wavelength is called a plate wave. According to the different vibration directions of particles, plate waves can be divided into SH waves and Lamb waves. Plate waves are generally used for flaw detection of thin plates and thin-walled steel pipes.
