Sound wave is a kind of longitudinal wave that can be felt by the human ear. Its frequency range is 16Hz~2KHz. When the frequency of sound waves is lower than 16 Hz, it is called infrasound wave, and when it is higher than 2 kHz, it is called ultrasonic wave. Generally, sound waves are having a frequency in the range of 2 kHz to 25 MHz called ultrasonic waves. It is a mechanical vibration wave excited by a mechanical vibration source in an elastic medium. Its essence is to transmit vibration energy in the form of stress waves. The necessary condition is to have a vibration source and an elastic medium capable of transmitting mechanical vibration (actually including Almost all gases, liquids and solids) penetrate into the interior of the object and can travel through the object. Using various propagation characteristics of piezoelectric ceramic transducers in an object, such as reflection and refraction, diffraction and scattering, attenuation, resonance, and speed of sound, it is possible to detect the size, surface and internal defects, tissue changes, etc. many objects, and thus is an application. The most extensive and important non-destructive testing technology has ultrasonic testing technology. For example, for medical ultrasound diagnosis (such as B-ultrasound), sonar in oceanography, fish detection, seabed topography, ocean sounding, geological structure detection, defect detection on industrial materials and products, hardness measurement, thickness measurement, microstructure evaluation, concrete component inspection, moisture measurement of piezceramic , gas medium property analysis, density measurement, etc.
Ultrasound has the following characteristics: 1) Ultrasonic waves can be effectively propagated in media such as gases, liquids, solids, and solid solutions. 2) Ultrasonic waves can transmit very strong energy. 3) Ultrasound produces reflection, interference, superposition and resonance. 4) When the ultrasonic wave propagates in the liquid medium, reaching a certain level of sound power can produce a strong impact on the interface of the object in the liquid (based on "cavitation phenomenon") - thus leading to the "power ultrasonic application" technology -- For example, "ultrasonic cleaning", "ultrasonic drilling", "ultrasonic deburring" (collectively referred to as "ultrasonic processing"), and the like.It can also be used for "ultrasonic welding" of materials such as plastics by the vibration of high-power ultrasonic waves.
Ultrasonic Testing (UT), which is applied in industrial non-destructive testing technology, is the fastest growing and most widely used non-destructive testing technology in NDT technology, and it plays a very important role. The method used to generate and receive ultrasonic waves in the ultrasonic testing technology which mainly utilizes the piezoelectric effect of crystals, that is, pizoelectric ceramics disc crystals (such as quartz crystal, barium titanate, and piezoelectric ceramics such as lead zirconate titanate). When deformation occurs under the action, there will be an electric phenomenon, that is, its charge distribution will change (positive piezoelectric effect). Conversely, when a charge is applied to the piezoelectric crystal, the piezoelectric ceramic crystal will be strained, that is, elastically deformed. (inverse piezoelectric effect). Therefore, an ultrasonic transducer (probe) is fabricated by using a piezoelectric crystal, and a high-frequency electric pulse is input thereto, and the probe generates ultrasonic waves at the same frequency to be emitted into the object to be inspected, and when is receiving the ultrasonic wave, the probe generates the same frequency. The high frequency electrical signal is used to detect the display. In addition to the use of the piezoelectric effect, in some cases, the magnetostrictive effect (the phenomenon that the strong magnetic material is deformed during magnetization, which can be used as a vibration source or for strain measurement), and the use of electrodynamic methods (for example, Electromagnetic-acoustic or eddy-sound methods described later in this chapter.
When the ultrasonic wave propagates in the elastic medium, depending on the relationship between the vibration pattern of the fulcrum of the medium and the direction of propagation of the ultrasonic wave, the ultrasonic wave can be divided into the following types
(1) Longitudional wave (L wave, also called compression wave, sparse wave) - The characteristic of longitudinal wave is that the vibration direction of the particle of the sound medium is the same as the direction of propagation of the ultrasonic wave (see the figure on the right)
(2) Shear Wave (referred to as S wave, also known as transverse wave, referred to as T wave, also known as shear wave or shear wave) - The characteristic of transverse wave is the vibration direction of the particle of the sound medium and the propagation direction of the ultrasonic wave. and the relationship between the vibration plane of the image point and the direction of propagation of the ultrasonic wave is further divided into a vertically polarized transverse wave (SV wave, which is the most commonly used transverse wave in industrial ultrasonic testing) and horizontally polarized transverse wave (SH wave, also known as Love Wave-le Libo is actually the vibration mode of seismic waves).
One end of the sensor rod in the longitudinal wave probe is fixed with a large mass rigid body, and the other end is inlaid with a diamond. When the indenter is not in contact with the test piece (left a), the indenter is in a free state. After the longitudinal vibration is formed, the fixed end of the sensor rod is the wave node of the vibration, and the head end becomes the antinode of the vibration due to the largest amplitude, so the length of the rod is equal to 1/4 of the vibration wavelength, and the frequency is that the sensor is at the resonant frequency in the free state. When the end of the sensor is completely clamped by the test piece and the large-mass rigid body, it is ideal that both ends of the sensor rod will become vibration wave nodes, and the length of the rod is equal to the vibration wavelength is 1/2, and the resonance frequency at this time is equal to twice the initial frequency when the indenter end is in the free state.
When the piezo electric ceramic is pressed onto the test piece, it is generally between the above . Under the fixed load, for the test piece with the same elastic modulus, if the hardness of the test piece is lower, The larger the contact area between the indenter and its surface, the greater the degree of clamping of the indenter end of the sensor rod, so that the vibration amplitude of the end is smaller, and the corresponding vibrational antinode point moves toward the fixed end of the rod. Therefore, the smaller the vibration wavelength, the higher is the resonance frequency of the rod. The hardness of the test piece can be determined by measuring the change in the resonant frequency of the sensor rod. The elastic modulus of the test piece will also affect the contact area, that is, the change of the resonant frequency of the sensor bar. Therefore, the ultrasonic hardness test method is a comparative measurement method, and it is necessary to eliminate the influence by using a test piece having the same elastic modulus and the test piece as a calibration test piece. In the probe there is a sensor rod with magnetostrictive effect, one end welded to a steel cylinder, the cylinder is much larger than the sensor, the other end is set with 136 diamond pyramid indenter, the excitation coil is around on the sensor rod, a piezoelectric crystal piece is fixed near the junction of the sensor rod and the cylinder.
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