1) Due to the diffraction of ultrasonic waves, the sensitivity of ultrasonic flaw detection is about one-half wavelength. The speed of ultrasonic waves is constant within the same material, so increasing the frequency will shorten the ultrasonic wavelength and increase the sensitivity of flaw detection, which is beneficial to finding smaller defects.
2) High frequency, small pulse width, and high resolution are helpful for distinguishing adjacent defects and improving resolution.
3) From the diffusion formula, it can be seen that if the frequency is high and the ultrasonic wavelength is short, the half-diffusion angle is small, the sound beam directivity is good, and the ultrasonic energy is concentrated, which is beneficial to finding and locating defects, and the quantitative accuracy is high.
4) It can be seen from the formula of the length of the near field area that the frequency is high, the ultrasonic wavelength is short, and the length of the near field area is large, which is detrimental to flaw detection.
5) From the attenuation and absorption formulas, it can be seen that the attenuation of ultrasonic waves increases sharply with the increase of ultrasonic frequency and medium grain size.
From the above analysis, it can be seen that the frequency of the ultrasonic probe has a greater impact on ultrasonic flaw detection. High frequency, high flaw detection sensitivity and resolution, and good beam directivity are beneficial to flaw detection. However, the frequency is high, the near field area is long, and the medium attenuation is large, which is unfavorable for flaw detection. Therefore, when selecting the frequency of the ultrasonic probe, comprehensive considerations should be taken, all factors should be analyzed, and a reasonable selection should be made. Generally speaking, under the premise of flaw detection sensitivity requirements, a probe with a lower frequency should be selected as much as possible; for forgings, rolled parts and welded parts with fine grains, a probe with a higher frequency is generally used, and 2.5-5.0MHz is commonly used. . For workpieces such as castings and austenitic steels with relatively coarse grains, a soft and low-frequency probe should be used, usually 0.5-2.5MHz. Otherwise, if the frequency is too high, the ultrasonic energy will be severely attenuated.
3.3 Selection of ultrasonic flaw detector probe chip size
The shapes of ultrasonic probe chips are generally round and square. The chip size of the probe has a certain impact on the results of ultrasonic flaw detection. The following factors should be mainly considered when selecting
1) Half diffusion angle: According to the diffusion angle formula, as the chip size increases, the half diffusion angle decreases, the beam directivity is good, and the ultrasonic energy is concentrated, which is beneficial to flaw detection.
2) Flaw detection near field area: It can be seen from the near field area length formula that as the chip size increases, the length of the near field area increases, which is detrimental to flaw detection.
3) Large chip size: The radiated ultrasonic energy is strong, the scanning range of the undiffused area of the probe is large, and the ability to detect long-distance defects is enhanced.
When detecting workpieces with a large flaw area, a large chip probe should be used for flaw detection efficiency; when detecting workpieces with a large flaw thickness, a large chip probe should be used in order to find long-distance defects; for small workpieces, in order to locate and quantify defects accurately, a large chip probe should be used. Select a small element probe; for workpieces with uneven surfaces and large curvatures, in order to reduce coupling losses, a small element probe should be selected.
3.4 Selection of ultrasonic flaw detector probe angle
During inspection, the axis of the ultrasonic sound beam should be perpendicular to the defect as much as possible. Therefore, the angle selection should be based on the type and location of defects that may exist in the inspection object and the allowable flaw detection conditions of the workpiece. The law of reflection and refraction and related geometric knowledge should be used to select the appropriate angle. Probe. Taking the K value of the probe in shear wave detection as an example, the refraction angle has a great influence 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 detecting steel workpieces with organic glass angle probes, when β=40° (K=0.84), the sound pressure reciprocating transmittance is high, that is, the detection sensitivity is high. It can be seen from this that when 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 in order 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 smaller K value should be selected to reduce the attenuation caused by the excessive sound path and facilitate the discovery of defects at greater depths. In weld inspection, it is also necessary to ensure that the main sound beam can scan the entire weld cross section. For single-sided welding roots that are not penetrated, the problem of end-angle reflection must also be considered, and K=0.7~1.5. Because K<0.7 or K1.5, the end-angle reflectivity is very low, which can easily lead to missed inspections.
