With the continuous emergence of new technologies and the ongoing advancement of inspection equipment, ultrasonic testing has become one of the fastest-growing and most widely applied nondestructive testing methods, occupying a critically important position within the field of NDT.

During the inspection process, in addition to the ultrasonic testing equipment, the transducer—responsible for both transmitting and receiving ultrasonic waves—plays a crucial role. Therefore, the performance of the transducer and the appropriateness of its selection during flaw detection directly affect the accuracy and reliability of the inspection results.

The following section focuses on the classification, functions, and selection criteria for piezoelectric ultrasonic transducers.

In ultrasonic flaw detection, different types of transducers must be used depending on the shape and material of the test piece, the inspection objective, and the inspection conditions.

Ultrasonic transducers can be classified in various ways depending on the classification criteria; the following are the common categories.

1. Based on the wave modes generated in the test workpiece, probes can be classified as longitudinal-wave probes, transverse-wave probes, plate-wave (Lamb-wave) probes, creep-wave probes, and surface-wave probes.
2. According to the direction of the incident sound beam, probes can be classified as straight probes and angled probes.
3. Based on the coupling method between the probe and the surface of the test workpiece, probes can be classified as contact probes and immersion probes.
4. Based on the material of the piezoelectric crystal in the probe, probes can be classified as conventional piezoelectric probes and composite piezoelectric probes.
5. Based on the number of piezoelectric elements in the probe, probes can be classified as single-element, dual-element, and multi-element probes.
6. Based on whether the ultrasonic beam is focused, probes are classified as focused probes and unfocused probes.
7. According to the ultrasonic frequency spectrum, probes can be classified as broadband or narrowband.
8. Based on the curvature of the workpiece being inspected, probes can be classified as flat probes and curved probes.
9. Special probes: In addition to standard probes, there are also probes designed for specific conditions and applications.

Functions of Common Typical Probes

1. Longitudinal wave probes are commonly referred to as straight probes and are primarily used to detect defects that are parallel to the inspection surface, such as in the inspection of plates, castings, and forgings.

2. A shear-wave angled probe is used for shear-wave inspection; it operates when the incident angle lies between the first and second critical angles and the refracted wave is a pure shear wave. It is primarily employed to detect defects that are either perpendicular to or at a certain angle relative to the inspection surface, and is widely used in the inspection of welds, pipes, and forgings.

3. A longitudinal shear probe is one whose angle of incidence is less than the first critical angle. Its purpose is to use low-angle longitudinal waves for flaw detection, or, when transverse-wave attenuation is excessive, to employ the strong penetration capability of longitudinal waves for oblique-incidence longitudinal-wave testing. When using such a probe, care must be taken to account for the concurrent presence of transverse-wave interference in the test piece.

4. Creep-wave probes, with a first-creep-wave angle ranging from 75° to 83°, are nearly perpendicular to the thickness direction of the test workpiece and form an angle close to 90° with cracks oriented in the thickness direction. Consequently, they offer excellent detection sensitivity for through-thickness cracks and impose relatively low requirements on surface roughness, making them well suited for detecting surface and near-surface cracks.

5. The incidence angle of the surface-wave (Rayleigh-wave) probe must be close to the critical angle for Rayleigh-wave generation, typically slightly greater than the second critical angle. Because the energy of surface waves is concentrated within two wavelengths below the surface, inspection sensitivity for surface cracks is extremely high, making this method particularly suitable for detecting surface or near-surface defects.

6. Dual-element transducers. A dual-element transducer comprises two piezoelectric crystals: one for transmitting ultrasonic waves and the other for receiving them. Depending on the angle of incidence αL, they are classified as longitudinal-wave dual-element straight probes and shear-wave dual-element angled probes.

Dual-element probes offer the following advantages: high sensitivity, low clutter, small blind zone, short near-field length in the test piece, and adjustable detection range. They are primarily used for detecting near-surface defects.

Probe Selection

There are many types of ultrasonic transducers, each with different performance characteristics. Therefore, selecting the appropriate transducer based on the geometry of the test object, the degree of ultrasonic attenuation, and the technical requirements is essential for ensuring accurate and reliable flaw detection results.

The selection of ultrasonic transducers primarily involves considerations such as transducer type, operating frequency, piezoelectric element size, and beam angle.

3.1 Probe Type

Typically, the probe type is selected based on the workpiece’s geometry and the locations and orientations where defects are likely to occur, with the goal of aligning the ultrasonic beam axis perpendicular to the defect. For specific guidance, refer to the sections on the typical functions of the commonly used probes mentioned above.

3.2 Probe Frequency

The ultrasonic testing frequency ranges from 0.5 to 15 MHz, providing a wide selection range. When selecting the frequency, the following factors should generally be considered:

1. Due to the diffraction of ultrasonic waves, the sensitivity of ultrasonic flaw detection is approximately one-half the wavelength. Since the velocity of ultrasonic waves is constant in a given material, increasing the frequency shortens the wavelength and enhances the detection sensitivity, thereby facilitating the identification of smaller defects.
2. High frequency, narrow pulse width, and high resolution facilitate the differentiation of adjacent defects, thereby enhancing resolution.
3. According to the diffraction formula, a higher frequency and shorter ultrasonic wavelength result in a smaller half-power beamwidth, improved beam directivity, and more concentrated ultrasonic energy, which facilitates defect detection, accurate defect localization, and high quantitative accuracy.

4. As indicated by the formula for near-field length, a higher frequency results in a shorter ultrasonic wavelength and a longer near-field length, which is detrimental to flaw detection.
5. As indicated by the attenuation and absorption formulas, ultrasonic attenuation increases sharply with increasing ultrasonic frequency and medium grain size.

As demonstrated by the preceding analysis, frequency has a significant impact on ultrasonic flaw detection: higher frequencies result in greater detection sensitivity and resolution, as well as improved beam directivity, all of which are advantageous for flaw detection.

However, a high frequency results in a long near-field zone and significant medium attenuation, which are detrimental to flaw detection. Therefore, when selecting the probe frequency, a comprehensive consideration of all relevant factors is necessary to make a rational choice.

In general, provided that the flaw detection sensitivity requirements are met, probes with the lowest possible frequency should be selected.

For forgings, rolled products, and welded components with fine grain sizes, probes with higher frequencies are generally selected, typically in the range of 2.5 to 5.0 MHz.

For castings with coarse grains and for workpieces made of austenitic steels, it is advisable to use probes with low frequency and soft characteristics, typically in the range of 0.5 to 2.5 MHz; otherwise, selecting a frequency that is too high will result in severe attenuation of ultrasonic energy.

3.3 Probe Chip Size

The transducer element is typically circular or square in shape, and the size of the transducer element has a certain influence on the results of ultrasonic flaw detection. When selecting a transducer, the following factors should be primarily considered:

1. The half-power beamwidth: As indicated by the beamwidth formula, increasing the transducer element size reduces the half-power beamwidth, resulting in better beam directivity, more concentrated ultrasonic energy, and improved flaw detection performance.

2. The near field in ultrasonic testing. As indicated by the formula for near-field length, an increase in transducer size leads to a longer near field, which is detrimental to flaw detection.
3. The chip is large in size, resulting in strong ultrasonic energy radiation; the unscattered zone of the transducer has a wide scanning range, thereby enhancing the ability to detect defects at long distances.
When inspecting workpieces with large inspection areas, a probe with a large piezoelectric element is recommended to improve inspection efficiency; when inspecting thick workpieces, a probe with a large piezoelectric element is also recommended to effectively detect defects at greater distances; for small workpieces, a probe with a small piezoelectric element is recommended to enhance the accuracy of defect location and quantification; and for workpieces with uneven surfaces and significant curvature, a probe with a small piezoelectric element is recommended to minimize coupling losses.

3.4 Angle

During inspection, the ultrasonic beam axis should be oriented as perpendicular as possible to the defect. Therefore, the selection of the probe angle is based on the types and locations of potential defects in the test object, as well as the inspection conditions permitted by the workpiece, and is determined by applying the laws of reflection and refraction together with relevant geometric principles.

For example, in shear-wave testing, the probe’s K-value—i.e., the refraction angle—has a significant impact on detection sensitivity, the direction of the sound-beam axis, and the one-way travel distance of the first echo (the distance from the entry point to the bottom-surface reflection point).

For inspecting steel workpieces with an acrylic oblique probe, the sound-pressure reciprocity transmittance—and thus the detection sensitivity—is maximized when β is approximately 40° (K = 0.84).

It can thus be seen that a larger K value corresponds to a larger β value and a longer acoustic path for the primary wave. Therefore, in practical inspections, when the workpiece thickness is relatively small, a larger K value should be selected to increase the acoustic path of the primary wave and avoid inspection in the near-field region.

When the workpiece thickness is relatively large, a smaller K-value should be selected to minimize attenuation caused by excessive sound path length, thereby facilitating the detection of defects at greater depths.

In weld inspection, it is also necessary to ensure that the main sound beam scans the entire cross-section of the weld.

For single-sided root lack-of-fusion welds, the issue of end-corner reflection must also be considered; the K value should be set within the range of 0.7 to 1.5, since when K is less than 0.7 or greater than 1.5, the end-corner reflectivity is very low, which can easily lead to missed defects.

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