Classification, function, and selection principles of ultrasonic testing probes

2) Shear wave angled probes use shear waves for detection. They are probes with an incident angle between the first and second critical angles, and the refracted wave is a pure shear wave. They are mainly used to detect defects perpendicular or at a certain angle to the detection surface, and are widely used in the detection of welds, pipes, and forgings.

3) Longitudinal wave angled probes are probes with an incident angle less than the first critical angle. The purpose is to use small-angle longitudinal waves for defect inspection, or to use the strong penetration ability of longitudinal waves for oblique incidence inspection of longitudinal waves when the attenuation of shear waves is too large. When using, it is necessary to pay attention to the interference of shear waves that simultaneously exist in the specimen.

4) Creeping wave probes. Since the angle of a creeping wave is between 75º and 83º, it is almost perpendicular to the thickness direction of the inspected workpiece, and is close to 90º to the cracks in the vertical direction of the workpiece. Therefore, it has good detection sensitivity for vertical cracks, and the requirements for the surface roughness of the workpiece are not high, suitable for surface and near-surface crack detection.

5) Surface wave (Rayleigh wave) probes require an incident angle near the critical angle for generating Rayleigh waves, 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 to surface cracks is extremely high, mainly for surface or near-surface defect inspection.

6) Dual-crystal probes. Dual-crystal probes have two piezoelectric crystals, one for transmitting ultrasonic waves and the other for receiving ultrasonic waves. According to the different incident angles αL, they are divided into longitudinal wave dual-crystal straight probes and shear wave dual-crystal angled probes. Dual-crystal probes have the following advantages: high sensitivity, low noise, small blind zone, small near-field length in the workpiece, and adjustable detection range. Dual-crystal probes are mainly used for detecting near-surface defects.

There are many types of ultrasonic probes with different performances. Therefore, according to the shape of the ultrasonic flaw detection object, the attenuation of ultrasonic waves, and technical requirements, the reasonable selection of probes is the basis for ensuring the correctness and reliability of the flaw detection results. The selection of ultrasonic probes mainly involves: probe type, probe frequency, probe crystal size, and probe angle.

3.1 Probe Type

Generally, the type of probe is selected according to the shape of the workpiece and the location and direction of possible defects, so that the ultrasonic beam axis is as perpendicular to the defect as possible. Refer to the above section on the functions of common typical probes.

3.2 Probe Frequency

The frequency of ultrasonic flaw detection is between 0.5 and 15 MHz, with a wide selection range. 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 approximately half a wavelength. In the same material, the ultrasonic wave velocity is constant. Therefore, increasing the frequency shortens the ultrasonic wavelength, improves the flaw detection sensitivity, and is conducive to finding smaller defects.

2) High frequency, small pulse width, high resolution, which is conducive to distinguishing adjacent defects and improving resolution.

3) According to the diffusion formula, the higher the frequency, the shorter the ultrasonic wavelength, the smaller the semi-diffusion angle, the better the directivity of the sound beam, the more concentrated the ultrasonic energy, which is conducive to finding defects and improving the accuracy of defect location and quantification.

4) According to the near-field length formula, the higher the frequency, the shorter the ultrasonic wavelength, and the longer the near-field length, which is not conducive to flaw detection.

5) According to the attenuation and absorption formula, the attenuation of ultrasonic waves increases sharply with the increase of ultrasonic frequency and the grain size of the medium.

From the above analysis, it can be seen that the frequency has a great influence on ultrasonic flaw detection. High frequency improves flaw detection sensitivity and resolution, and the beam directivity is good, which is beneficial to flaw detection. However, high frequency results in a long near-field zone and large medium attenuation, which is not conducive to flaw detection. Therefore, when selecting the probe frequency, comprehensive consideration and comprehensive analysis of various factors should be made for reasonable selection.

Generally speaking, while meeting the requirements of flaw detection sensitivity, probes with lower frequencies should be selected as much as possible; for forgings, rolled materials, and welded parts with finer grains, higher-frequency probes are generally used, commonly 2.5-5.0MHz. For castings, austenitic steel, and other workpieces with coarser grains, soft, low-frequency probes should be used, commonly 0.5-2.5MHz; otherwise, if a frequency that is too high is selected, it will cause a serious attenuation of ultrasonic energy.

3.3 Probe Crystal Size

The shape of the probe crystal is generally round or square. The size of the probe crystal has a certain impact on the results of ultrasonic flaw detection. The following factors should be considered when selecting it:

1) Half-diffusion angle. As can be seen from the diffusion angle formula, increasing the crystal size reduces the half-diffusion angle, improves beam directivity, concentrates ultrasonic energy, and is beneficial for flaw detection.

2) Near-field area of flaw detection. As can be seen from the near-field length formula, increasing the crystal size increases the near-field length, which is not conducive to flaw detection.

3) With a larger crystal size, the radiated ultrasonic energy is stronger, the scanning range of the non-diffused area of the probe is larger, and the ability to detect distant defects is enhanced.

When inspecting workpieces with a large area, in order to improve the detection efficiency, a large crystal probe should be used; when inspecting workpieces with a large thickness, in order to effectively detect distant defects, a large crystal probe should be used; for small workpieces, in order to improve the accuracy of defect location and quantification, a small crystal probe should be used; for workpieces with uneven surfaces and large curvatures, in order to reduce coupling loss, a small crystal probe should be used.

3.4 Angle

During the inspection, the ultrasonic beam axis should be kept as perpendicular to the defect as possible. Therefore, the angle selection depends on the type, location of possible defects in the inspection object, and the flaw detection conditions allowed by the workpiece. Using the laws of reflection and refraction and relevant geometric knowledge, a probe with a suitable angle should be selected.

For example, in shear wave detection, the K value of the probe has a great influence on the detection sensitivity, the direction of the beam axis, and the acoustic path of the primary wave (the distance from the incident point to the bottom reflection point). For steel workpieces inspected with an organic glass angled probe, when β = 40° (K = 0.84), the sound pressure reciprocating transmission rate is the highest, that is, the detection sensitivity is the highest.

Therefore, a larger K value means a larger β value and a longer acoustic path of the primary wave. Therefore, in actual detection, when the workpiece is thin, a larger K value should be selected to increase the acoustic path of the primary wave and avoid near-field detection. When the workpiece is thick, a smaller K value should be selected to reduce the attenuation caused by the excessively long acoustic path, making it easier to find defects at greater depths.

In weld detection, it is also necessary to ensure that the main beam can scan the entire weld cross-section. For single-sided welds that are not fully welded at the root, the end angle reflection problem must also be considered, and K should be 0.7-1.5, because K