Selection of ultrasonic detection frequency and probe

When the frequency is high, the wavelength is short, the sound beam is narrow, the divergence angle is small, and the energy is concentrated. Therefore, the ability to detect small defects is strong, the resolution is good, and the defect location is accurate. However, the scanning space is small, and only defects near the sound beam axis can be detected. In addition, high-frequency ultrasonic waves attenuate greatly in the material and have poor penetration ability.

When the frequency is low, the wavelength is long, the sound beam is wide, the divergence angle is large, and the energy is not concentrated. Therefore, the ability to detect small defects is poor, and the resolution is poor. However, the scanning space is large. In addition, low-frequency ultrasonic waves attenuate less in the material and have strong penetration ability.

In general contact testing, for materials with fine grains, considering factors such as detection range and resolution, a frequency of 2.5 to 5 MHz is suitable. For example, 2.5 MHz is used for large forgings and welds, and 2.5 MHz or 5 MHz is used for small forgings. The smallest defect that ultrasonic flaw detection can detect is generally around λ/2. When using a frequency of 2.5 MHz or 5 MHz, the smallest detectable defect is about 1 mm, which is sufficient for the defect size we usually require to detect.

For materials with coarse grains and strong ultrasonic scattering, when the frequency is high, linear echoes caused by grain boundaries will appear, making it impossible to judge the damage. Therefore, for such materials, 0.5-1 MHz is generally used. For materials such as cast iron and non-metals with strong acoustic attenuation, frequencies of even tens of kilohertz are used.

As for the 10 MHz frequency, it is only used when the inspected workpiece is relatively thin and a particularly high sensitivity to small defects is required. 10 MHz frequency crystals are thinner and easily damaged.

II. Probe Selection

Regarding the frequency of the probe, the principles described above can be followed.

When the probe crystal size is large, the emission energy is large, the divergence angle is small, the scanning space is large, the near-field length is long, and the ability to detect small defects at long distances is high.

For probes with circular crystals, crystals with diameters of 14-20 mm are generally used. For the detection of large workpieces, probes with large-diameter crystals should be used as much as possible. Probes with small-diameter crystals have a narrow sound beam within the near-field range, which is beneficial for defect location and is suitable for flaw detection of workpieces with smaller thicknesses.

The selection of the probe crystal size also needs to consider the workpiece detection surface and structure. For example, when checking the swallowtail root cracks of blades and using angled probes to check the axial keyway cracks with narrower shoulder sizes, probes with smaller crystal sizes often have to be used.

According to the needs of detection, the ultrasonic waves emitted by the probe are required to have an appropriate sound beam shape. The sound beam shape is related to the frequency and crystal size of the probe, so both must be considered comprehensively.

For defect location and quantification, the principle of probe selection is that the sound beam should be as narrow as possible, and the sound beam should be vertically incident on the defect. In some cases, it is necessary to make the energy more concentrated, and line-focused or point-focused probes can be used. In recent years, progress has been made in using focused probes to solve the qualitative and quantitative problems of defects.

In terms of detection sensitivity, if the attenuation of ultrasonic waves in the material is ignored, the near-field length determines how quickly the sensitivity decreases with increasing distance between the defect and the probe. The longer the near-field, the slower the sensitivity decreases. Therefore, to minimize the decrease in sensitivity with distance, a probe whose near-field length is not less than 1/3 of the maximum depth of the defect should be selected as much as possible. This condition often cannot be satisfied when detecting large workpieces. Because to increase the near-field length, one must increase the frequency and increase the crystal diameter. Too high a frequency results in high attenuation and is not suitable; although the near-field length increases with the square of the crystal diameter, most workpieces do not have a sufficiently large contact area, and the crystal diameter cannot be too large.

The choice of probe type also depends on the specific situation. The choice of a straight probe or an angled probe mainly depends on the location and orientation of the defect to be found. For example, when detecting forgings, because most defects are perpendicular to the forging direction, a straight probe is generally used to detect on the forging surface; also, when inspecting welds, because there is a weld reinforcement surface and most dangerous defects are perpendicular or approximately perpendicular to the detection surface, an angled probe is generally used for detection.

Some defects in special locations and shapes cannot be found using only a single probe (acting as both transmitter and receiver). For example, for near-surface defects, longitudinal wave detection often requires the use of a double probe (one transmitter and one receiver) or a double crystal probe. Also, for thin-walled tube weld detection, in order to concentrate acoustic energy and reduce clutter, a double crystal angled probe is also needed.

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