Classification, function and selection principles of probes used in ultrasonic flaw detection
1. Classification of ultrasonic probes:
In ultrasonic flaw detection, different types of probes are needed due to the different shapes, materials, flaw detection purposes, and flaw detection conditions of the tested workpieces. Ultrasonic probes can be classified according to different induction methods, generally there are the following types.
1) According to the wave type generated in the detected workpiece, it can be divided into longitudinal wave probe, transverse wave probe, plate wave (Lamb wave) probe, creeping wave probe and surface wave probe.
2) According to the direction of incident sound beam, it can be divided into straight probe and oblique probe.
3) According to the coupling mode of the probe and the surface of the tested workpiece, it can be divided into contact probe and liquid immersion probe.
4) According to the material of the piezoelectric wafer in the probe, it can be divided into ordinary piezoelectric wafer probe and composite piezoelectric wafer probe.
5) According to the number of piezoelectric wafers in the probe, it can be divided into single crystal probe, double crystal probe and polycrystalline probe.
6) According to whether the ultrasonic sound beam can be focused, it is divided into focused probe and non-focused probe.
7) According to the ultrasonic frequency spectrum, it can be divided into broadband and narrowband probes.
8) According to the curvature of the matching detection workpiece, it can be divided into flat probe and curved probe.
9) Special probe. In addition to general probes, there are some probes under special conditions and for special purposes.
2. The role of common typical probes
1) Longitudinal wave probes are usually called straight probes, which are mainly used to detect defects parallel to the detection surface, such as plate, casting, forging, etc.
2) Shear wave oblique probes use shear wave detection. The incident angle is between the first critical angle and the second critical angle and the refracted wave is pure shear wave. It is mainly used to detect defects perpendicular or at a certain angle to the detection surface. Used for the inspection of welds, pipes and forgings.
3) The longitudinal wave oblique probe is a probe whose incident angle is smaller than the first critical angle. The purpose is to use small-angle longitudinal waves for defect inspection, or when the transverse wave attenuation is too large, use the characteristics of strong longitudinal wave penetration to perform longitudinal wave oblique incident inspection. When using, pay attention to the presence of transverse wave interference in the specimen.
4) Creeping wave probe. Since the angle of a creeping wave is between 75º～83º, which is almost perpendicular to the thickness direction of the workpiece to be inspected, and it is close to 90º with the vertical crack in the workpiece, it has good detection sensitivity for vertical cracks, and it has good detection sensitivity for the workpiece. The surface roughness requirement is not high, and it is suitable for surface and near-surface crack detection.
5) The incident angle of the surface wave (Rayleigh wave) probe should be near the critical angle where the Rayleigh wave is generated, usually slightly larger than the second critical angle. Since the energy of the surface wave is concentrated within 2 wavelengths below the surface, the sensitivity of the inspection of surface cracks is extremely high, and the inspection is mainly for surface or near-surface defects.
6) Dual crystal probe. The dual crystal probe has two piezoelectric wafers, one is used to transmit ultrasonic waves, and the other is used to receive ultrasonic waves. According to the difference of incident angle αL, it is divided into longitudinal wave dual crystal straight probe and transverse wave dual crystal oblique probe. The dual crystal probe has the following advantages: high sensitivity, less clutter, small blind area, small length of the near field in the workpiece, and adjustable detection range. The dual crystal probe is mainly used for detecting near-surface defects.
3. The selection principle of the probe in ultrasonic flaw detection
There are many types of ultrasonic probes with different performances. Therefore, according to the shape of the ultrasonic flaw detection object, the ultrasonic attenuation and technical requirements, a reasonable selection of the probe is the basis for ensuring the correct and reliable flaw detection results. The choice of ultrasonic probe is mainly reflected in: probe type, probe frequency, probe chip size and probe angle, etc.
3.1 Probe type
Generally, the form of the probe should be selected according to the shape of the workpiece and the location and direction of the defect that may occur, and try to make the ultrasonic beam axis perpendicular to the defect. For details, please refer to the function part of the above-mentioned common typical probes.
3.2 Probe frequency
The ultrasonic flaw detection frequency is between 0.5 and 15MHz, and the selection range is relatively large. Generally, the following factors should be considered when selecting the frequency.
1) Due to the diffraction of ultrasonic waves, the sensitivity of ultrasonic flaw detection is about one-half wavelength. In the same material, the ultrasonic wave velocity is constant, so increasing the frequency will shorten the ultrasonic wavelength and improve the detection sensitivity, which is helpful for finding smaller defects.
2) The frequency is high, the pulse width is small, and the resolution is high, which is beneficial to distinguish adjacent defects and improve the resolution.
3) It can be seen from the diffusion formula that if the frequency is high, the ultrasonic length is short, the half-diffusion angle is small, the sound beam has good directivity, and the ultrasonic energy is concentrated, which is conducive to finding and locating defects, with high quantitative accuracy.
4) From the formula for the length of the near field zone, it is known that the high frequency, the length of the ultrasonic wave, and the large length of the near field zone are unfavorable for flaw detection.
5) It can be seen from the attenuation and absorption formula that the attenuation of ultrasonic waves increases sharply with the increase of ultrasonic frequency and medium grain size.
Through the above analysis, it can be seen that the frequency has a greater influence during ultrasonic flaw detection, the frequency is high, the flaw detection sensitivity and resolution are high, and the beam directivity is good, which is beneficial to flaw detection. However, the high frequency, long near-field area, and large medium attenuation are not good for flaw detection. Therefore, when selecting the probe frequency, comprehensive consideration should be given to comprehensive analysis of various factors and reasonable selection. Generally speaking, on the premise of satisfying the requirements of flaw detection sensitivity, the probe with lower frequency should be selected as much as possible; for forgings, rolled parts and welded parts with finer grains, higher frequency probes are generally selected, usually 2.5-5.0 MHz. For castings with coarse grains, austenitic steel and other workpieces, a soft low-frequency probe should be used, usually 0.5 ~ 2.5MHz, otherwise if the frequency is too high, the ultrasonic energy will be seriously attenuated.
3.3 Probe chip size
The shape of the probe wafer is generally round and square. The chip size of the probe has a certain influence on the results of ultrasonic flaw detection. The following factors should be considered when selecting
1) Half-diffusion angle. From the diffusion angle formula, it can be seen that the increase in the size of the wafer, the decrease in the half-diffusion angle, the good beam directivity, and the concentrated ultrasonic energy are beneficial to flaw detection.
2) Flaw detection near field area. From the formula for the length of the near-field zone, it can be seen that the increase in the size of the wafer and the increase in the length of the near-field zone are not good for flaw detection.
3) The chip size is large, the radiated ultrasonic energy is strong, and the scanning range of the probe's non-diffusion area is large, and the ability to find long-distance defects is enhanced.
For workpieces with a large flaw detection area, in order to improve the efficiency of flaw detection, a large chip probe should be used; when detecting a workpiece with a large thickness, a large chip probe should be used to effectively find long-distance defects; for small workpieces, in order to improve the location of defects For quantitative accuracy, small-chip probes should be used; for workpieces with uneven surfaces and large curvatures, small-chip probes should be used to reduce coupling loss.
3.4 Angle
In the inspection, the ultrasonic beam axis should be perpendicular to the defect as much as possible. Therefore, the choice of angle should be based on the type and position of the defect that may exist in the inspection object and the allowable detection conditions of the workpiece. Use the reflection and refraction laws and related geometric knowledge to select the appropriate angle. Probe. Taking the K value of the probe in transverse wave detection, for example, the refraction angle has a greater impact on the detection sensitivity, the direction of the sound beam axis, and the sound path of the primary wave (the distance from the incident point to the bottom reflection point). For the detection of steel workpieces with plexiglass oblique probes, when β=40°(K=0.84), the sound pressure reciprocating transmittance is the highest, that is, the detection sensitivity is the highest. It can be seen that the K value is large, the β value is large, and the sound path of the primary wave is large. Therefore, in actual detection, when the thickness of the workpiece is small, a larger K value should be selected to increase the sound path of the primary wave and avoid detection in the near-field area. When the thickness of the workpiece is large, a small K value should be selected to reduce the attenuation caused by the excessive sound path, and it is convenient to find the defects at the large depth. In the weld inspection, it is also necessary to ensure that the main sound beam can scan the entire weld section. For single-sided welding roots without penetration, the end angle reflection problem should be considered. K=0.7～1.5, because K<0.7 or K>1.5, the end angle reflectivity is very low, which is easy to cause missed inspection.
