Classification, function and selection principle of probes used in ultrasonic flaw detection
With the continuous emergence of new technology and the continuous updating of testing equipment, ultrasonic testing technology is one of the fastest developing and most widely used methods in nondestructive testing technology, which plays a very important role in nondestructive testing technology.
In the detection process, in addition to the ultrasonic testing instrument, the probe that transmits and receives the ultrasonic also plays a very important role. Therefore, the performance of the probe and the selection of the probe in the detection process will directly affect the accuracy and reliability of the detection results. The following focuses on the classification, function and selection principle of piezoelectric ultrasonic probe.
Classification, function and selection principle of ultrasonic probe
Table of Contents
Classification of ultrasonic probes
In ultrasonic flaw detection, due to the different shape, material, purpose and condition of the detected workpiece, different types of probes are needed. Ultrasonic probes can be classified according to different induction methods, generally as follows.
- 1) According to the wave pattern produced in the workpiece, it can be divided into P-wave probe, S-wave probe, plate wave (Lamb wave) probe, creeping wave probe and surface wave probe.
- 2) According to the direction of incident beam, it can be divided into straight probe and oblique probe.
- 3) According to the coupling mode between the probe and the workpiece surface, it can be divided into contact probe and liquid immersion probe.
- 4) According to the material of piezoelectric chip, the probe can be divided into ordinary piezoelectric chip probe and composite piezoelectric chip probe.
- 5) According to the number of piezoelectric chips in the probe, it can be divided into single crystal probe, double crystal probe and polycrystalline probe.
- 6) According to the focusing ability of ultrasonic beam, it can be divided into focused probe and unfocused probe.
- 7) According to the ultrasonic spectrum, it can be divided into broadband and narrowband probes.
- 8) According to the curvature of the workpiece, it can be divided into planar probe and curved probe.
- 9) Special probe. In addition to general probes, there are also some probes used under special conditions and for special purposes.
Functions of common typical probes
- 1) Longitudinal wave probe is usually called straight probe, which is mainly used to detect the defects parallel to the detection plane, such as plate, casting, forging, etc.
- 2) S-wave oblique probe is a kind of probe with incident angle between the first critical angle and the second critical angle, and the refraction wave is pure S-wave. It is mainly used to detect the defects perpendicular to or at a certain angle with the detection surface, and is widely used in the detection of welds, pipes and forgings.
- 3) The longitudinal wave oblique probe is a probe whose incidence angle is less than the first critical angle. The purpose is to use small angle P-wave for defect inspection, or in the case of excessive attenuation of S-wave, use the characteristics of strong penetration of P-wave for oblique incidence inspection, and pay attention to the interference of S-wave in the specimen.
- 4) Creeping wave probe. Because the angle of a creeping wave is 75 º～ eighty-three º It is almost perpendicular to the thickness direction of the inspected workpiece and nearly 90% to the vertical crack in the workpiece º， Therefore, it has a good detection sensitivity for vertical cracks, and does not require high surface roughness, so it is suitable for surface and near surface crack detection.
- 5) The incident angle of surface wave (Rayleigh wave) probe should be near the critical angle of Rayleigh wave, which is usually slightly larger than the second critical angle. Because the energy of surface wave is concentrated in two wavelengths below the surface, the sensitivity of surface crack detection is very high, mainly for surface or near surface defects.
- 6) Double crystal probe. The bicrystal probe has two piezoelectric chips, one is used to transmit ultrasonic wave, the other is used to receive ultrasonic wave α L is divided into P-wave double crystal straight probe and S-wave double crystal oblique probe. The double crystal probe has the following advantages: high sensitivity, less clutter, small blind area, small length of near-field area in the workpiece, adjustable detection range. The double crystal probe is mainly used to detect near surface defects.
Selection principle of 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 attenuation of the ultrasonic wave and the technical requirements, the reasonable selection of the probe is the basis to ensure the correctness and reliability of the flaw detection results. The selection of ultrasonic probe is mainly reflected in: probe type, probe frequency, probe chip size and probe angle.
Generally, the probe type is selected according to the shape of the workpiece and the position and direction of the defect, so as to make the axis of the ultrasonic beam perpendicular to the defect. For details, please refer to the above common typical probe function section.
The frequency of ultrasonic flaw detection is between 0.5 MHz and 15MHz, and the selection range is wide. Generally, the following factors should be considered when selecting the frequency.
- 1) Due to the diffraction of ultrasonic wave, the sensitivity of ultrasonic testing is about half of the wavelength. In the same material, the ultrasonic wave velocity is certain, so increasing the frequency can shorten the ultrasonic wave length and improve the detection sensitivity, which is conducive to finding smaller defects.
- 2) High frequency, small pulse width and high resolution are beneficial to distinguish the adjacent defects and improve the resolution.
- 3) According to the diffusion formula, if the frequency is high and the ultrasonic wave is long or short, the half diffusion angle is small, the directivity of the sound beam is good, and the ultrasonic energy is concentrated, which is beneficial to find and locate the defects with high quantitative accuracy.
- 4) According to the formula of near-field area length, high frequency, long and short ultrasonic wave and large near-field area length are unfavorable to flaw detection.
- 5) According to the attenuation and absorption formula, the attenuation of ultrasonic wave 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 great influence on the ultrasonic flaw detection, with high frequency, high detection sensitivity and resolution, and good beam directivity, which is beneficial to the flaw detection. However, high frequency, long near-field area and large medium attenuation are unfavorable to flaw detection. Therefore, when selecting the probe frequency, we should comprehensively consider and comprehensively analyze various factors and reasonably select the probe frequency.
Generally speaking, on the premise of meeting the requirements of flaw detection sensitivity, the probe with lower frequency should be selected as far as possible; For forging, rolling and welding parts with fine grain, the probe with higher frequency is generally selected, and 2.5-5.0mhz is commonly used. For castings with coarse grains and austenitic steel, it is better to select the probe with soft and low frequency, usually 0.5-2.5mhz. Otherwise, if the frequency is too high, it will cause serious attenuation of ultrasonic energy.
Probe chip size
The shape of the probe chip is generally round and square. The chip size of the probe has a certain influence on the ultrasonic flaw detection results
- 1) Half diffusion angle. According to the diffusion angle formula, with the increase of wafer size, the half diffusion angle decreases, the beam directivity is good, and the ultrasonic energy is concentrated, which is beneficial to the flaw detection.
- 2) Near field area of flaw detection. According to the formula of near-field region length, the increase of wafer size and near-field region length is unfavorable to flaw detection.
- 3) The chip size is large, the ultrasonic energy radiated is strong, the scanning range of the non diffusion area of the probe is large, and the ability to find long-distance defects is enhanced.
In order to improve the efficiency of flaw detection, the large wafer probe should be selected for the workpiece with large flaw detection area; In order to detect the defects in long distance effectively, the large wafer probe should be selected when detecting the workpiece with large thickness; For small workpieces, in order to improve the positioning and quantitative accuracy of defects, small wafer probe should be selected; In order to reduce the coupling loss, the small wafer probe should be selected for the workpiece with uneven surface and large curvature.
In the detection, the axis of ultrasonic beam should be perpendicular to the defect as far as possible, so the angle should be selected according to the possible defect type, position and the allowable flaw detection conditions of the workpiece in the detection object, using the law of reflection and refraction and the relevant geometric knowledge.
Taking the K value of the probe as an example, the refraction angle has a great influence on the detection sensitivity, the direction of the 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 workpiece with angle probe of plexiglass, β= forty °（ When k = 0.84), the reciprocating transmission of sound pressure is the highest, which means the detection sensitivity is the highest.
It can be seen that the value of K is large, β When the value is large, the sound path of primary wave is large. Therefore, in the actual detection, when the thickness of the workpiece is small, a larger K value should be selected in order to increase the sound path of the primary wave and avoid near-field detection. When the thickness of the workpiece is large, a smaller K value should be selected to reduce the attenuation caused by the excessive sound path, so as to find the defects with larger depth.
In the weld inspection, it is necessary to ensure that the main acoustic beam can scan the whole weld section. For the root of single-sided welding, the corner reflection should be considered, and K = 0.7 ~ 1.5, because K < 0.7 or k > 1.5, the corner reflection is very low, which is easy to cause missed inspection.
Ultrasonic probe is just like its eyes for ultrasonic testing. The probe has a great influence on the flaw detection results. As the saying goes, if a worker wants to be good at his work, he must first sharpen his tools. In the actual flaw detection process, he should carefully select according to the workpiece conditions, flaw detection conditions, defect conditions and implementation standards, so as to make the ultrasonic flaw detection as accurate, reliable and efficient as possible.
In addition, for the performance index of the probe and the combination performance index of the probe and instrument, the relevant standards such as JB / T10062 probe, JB / T9214a type ultrasonic detection system performance, GB / T18694 probe and sound field, EN12668-2 probe, EN12668-3 combination performance, ISO10375 probe and sound field, ASTM E1065 probe, etc. can be referred.
Source: Network Arrangement – China Flanges Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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