Principle introduction
Non-destructive testing methods for testing internal defects of materials using the effects of differences in acoustic properties of materials and their defects on ultrasonic propagation. Ultrasonic pulse reflection methods for observing the reflection of acoustic pulses in materials are widely used, as well as penetration methods for observing changes in the amplitude of incident acoustic waves after passing through the material. Commonly used frequencies are between 0.5 and 5 MHz.
The commonly used inspection instrument is a type A display pulse reflection type ultrasonic flaw detector. According to the presence or absence of the reflected signal on the instrument oscilloscope, the time interval of the reflected signal and the incident signal, and the height of the reflected signal, the presence or absence of the reflective surface, its position and relative size can be determined. The basic structure and principle of the instrument are shown in Figure 1.
Ultrasonic flaw detection structure and schematic Ultrasonic waves have multiple wave patterns when propagating in the medium. The most commonly used ones are longitudinal waves, transverse waves, surface waves and plate waves. Longitudinal waves can be used to detect defects such as inclusions, cracks, shrinkage, white spots, delamination, etc. in metal ingots, billets, plate, large forgings and relatively simple shapes; Circumferential and axial cracks, scratches, stomata in the weld, slag inclusions, cracks, incomplete penetration, etc.; surface waves can be used to detect surface defects on simple shapes; plate waves can be used to detect thin plates defect.
The B-type and C-type flaw detectors developed on the basis of the A-type flaw detector can obtain the signals of the reflective surfaces in different directions, and can also combine the B-type and C-type displays to obtain the three-dimensional display of the internal reflective surface of the material. .
Each of the above types of flaw detectors uses a pulsed electrical signal to excite the piezoelectric transducer to emit ultrasonic waves, but an eddy current transducer can also be used to inspect the conductive material. The transducer's transducing process is carried out on the surface of the flawed part without the need to contact the material or the coupling agent, and the surface roughness and metal materials with temperatures up to 500 ° C can be inspected. .
Ultrasonic waves propagate in the material, and the intensity is attenuated due to absorption, scattering, etc., so that the attenuation in the alloy material smelted in a vacuum consumable furnace is measured, and it is possible to unreasonably understand the uniformity of the material structure.
The main advantages of pulse reflection ultrasonic method compared with other non-destructive testing methods are:
It has strong penetrating ability and high detection depth up to several meters. It can be found that the reflector with the air gap reflection capacity of about a few tenths of a millimeter is more accurate in determining the orientation, size, shape and nature of the internal reflector. The defect inspection result can be provided immediately from the side to be inspected. The main disadvantages of the equipment are:
Careful operation by experienced personnel is difficult to check for rough, irregular, small, thin or heterogeneous materials. It is still difficult to accurately and quantitatively characterize the defects found.
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