Non-destructive testing technology and its application (2)

The sensor rod acts as a mechanical resonator and is inserted into the feedback circuit of the excitation amplifier. Under the action of the excitation coil, the sensor rod generates longitudinal ultrasonic vibration. The signal is detected by the piezoelectric wafer and is positively fed back to the input end of the excitation amplifier. It constitutes a self-excited oscillator whose oscillation frequency is the resonant frequency of the sensor rod, which is reflecting the hardness of the test piece. A signal is output from the driver amplifier and fed into the pulse circuit to form a repetition frequency, which is a square wave pulse of 1/2 of the above oscillation frequency, which is amplified by a pulse power amplifier to activate the discriminator. In the discriminator, the frequency change is reflecting the different hardness is converted into a change in the direct current, and then indicated by a direct current micro-ampere meter directly scaled by the hardness unit. After the hardness scale has been previously calibrated with the standard test block, the hardness value of the piezoelectric rings piezoelectric transducers can be read directly from the indicator.

As the ultrasonic hardness tester, the charging device is also used to directly charge the battery pack by 220V alternating current, and the voltage regulator is used to eliminate the influence of the voltage drop of the battery pack on the stability of the indication during the working process. According to the current development of electronic technology, the ultrasonic hardness tester should be digital, thereby further improving the accuracy, stability and reliability of the measurement. Ultrasonic testing technology is applied in a variety of ways, and is constantly exploring and developing new application methods and exploring new application areas, such as the now developed ultrasonic spectrum analysis method, which is based on the spectral characteristics of ultrasonic reflected echoes. to examine the microstructure of the evaluation material, to assess the shape, type and nature of the defect, as well as to assess the quality of the glued joint. In addition, there are ultrasonic tomography scanning technology, in particular, it should be pointed out that with the rapid development of computer technology, the digital processing, analysis and display of ultrasonic detection signals provide more space for the application and expansion of ultrasonic detection technology, and have great potential for development.

(3) Surface wave - Surface waves applied in industrial ultrasonic testing mainly refer to rayleigh waves (Ray waves), which are transmitted along the surface of the medium, while the particles of the sound-transmitting medium vibrate along an elliptical path. As shown on the left, the effective penetration depth of the rayleigh wave on the medium is only one wavelength range. Therefore, it can only be used to check the defects on the surface of the medium. It cannot penetrate the inside of the medium like the longitudinal wave and the transverse wave, so that it can be inspected. Defects inside the media. In addition, the horizontally polarized transverse wave (SH wave, also known as Love Wave) is also a surface wave propagating along the surface layer, which is actually the vibration mode of the seismic wave, but it has not yet been practically applied in industrial ultrasonic testing.

(4) Lamb Wave - This is a guided wave that is generated by superposition of longitudinal and transverse waves and is enclosed in a specific finite space at a specific frequency. In industrial ultrasonic testing, the Lamb wave is mainly used to detect a thin metal plate having a thickness equivalent to that of a wavelength, and is therefore also called a plate wave (P wave). When the Lamb wave is transmitted in the thin plate, the lower surface layer of the thin plate vibrates along the elliptical path, and the particle in the middle layer of the thin plate will vibrate in the form of a longitudinal wave component or a transverse wave component, thereby forming a full-plate vibration, which is a prominent feature of the Lamb wave detection. According to the vibration of the middle layer of the thin plate, it is a longitudinal wave component or a transverse wave component, and can be divided into two modes: S mode (symmetric type) and A mode (asymmetric type). Lamb waves can also be excited in thin rods and thin-walled tubes, which are called twisted waves, expanded waves, and the like.

In addition to the four main application waveforms described above, Head wave and longitudional wave (also known as creeping longitudinal waves) have been developed, especially in the latter. Subsurface transfer, suitable for detecting near-surface defects in the case of detecting particularly rough surfaces or stainless steel surfacing layers on the surface. The propagation velocity of piezoelectric ceramics ring in the medium (related to the medium, wave type, etc.), the vibration frequency f (the number of complete vibrations per unit time, one Hertz-Hz per second) and the wavelength λ of the ultrasonic waves (ultrasonic completion) .The distance transmitted by one full vibration has the following relationship: C = λ · f should pay attention to different propagation speeds in different media and different ultrasonic modes. Ultrasonic waves have short wavelengths, travel along a straight line (in many cases, geometric and acoustic relations can be applied for analysis), good directivity,which can propagate in solids, and can be wave-transformed. Their propagation characteristics include reflection and refraction, diffraction. With a variety of changes such as scattering, attenuation, resonance, sound velocity, etc., it is widely used, including metal, non-metal, forgings, castings, welded parts, profiles, bonded structures and composites, fasteners and so on. The advantages of ultrasonic testing are strong penetrating power, light equipment, low detection cost, high detection efficiency, instant detection of test results (real-time detection), automatic detection and permanent recording, and greater hazard in defect detection. The crack-like defects are particularly sensitive and so on. The disadvantage of ultrasonic testing is that the coupling medium is usually required to allow sound energy to penetrate into the object to be inspected, and a reference evaluation standard is required, in particular, the displaying detection result is not intuitive, and thus the technical level of the operator is required to be high, it is small, thin or complex shapes, as well as workpiece inspection of coarse-grained materials, etc., still have some difficulties. The application of the ultrasonic propagation characteristics as a clue is described below.