There are many types of ultrasonic probes with different performances. Therefore, according to the shape of the ultrasonic testing object, the attenuation of the ultrasonic and the technical requirements, the reasonable selection of the probe is the basis to ensure the correct and reliable detection results. The selection of ultrasonic probe mainly includes: probe type, probe frequency, probe chip size and probe angle.
1 probe type
Generally, the form of probe is selected according to the shape of workpiece and the position and direction of possible defects, so as to make the axis of ultrasonic beam perpendicular to the defects. For details, please refer to the above common typical probe functions.
2 probe frequency
The frequency of ultrasonic detection is between 0.5MHz and 15MHz, and the selection range is large. Generally, the following factors should be considered when selecting frequency.
1) Because of the diffraction of ultrasonic, the sensitivity of ultrasonic detection is about half of the wavelength. In the same material, the velocity of ultrasonic wave is certain, so increasing the frequency, shortening the wavelength of ultrasonic wave and improving the sensitivity of flaw detection are conducive to the detection of smaller defects.
2) High frequency, small pulse width and high resolution are conducive to distinguish adjacent defects and improve resolution.
3) According to the diffusion formula, if the frequency is high and the ultrasonic wave is long and short, the half diffusion angle is small, the sound beam has good directivity and the ultrasonic energy is concentrated, which is conducive to the detection and location of defects, and the quantitative accuracy is high.
4) It can be seen from the formula of near-field length that high frequency, long and short ultrasonic wave and large near-field length are unfavorable for flaw detection.
5) According to the attenuation and absorption formula, the attenuation of ultrasonic increases rapidly with the increase of ultrasonic frequency and medium grain size.
Through the above analysis, it can be seen that the impact of frequency on ultrasonic flaw detection is large, with high frequency, high flaw detection sensitivity and resolution, and good beam directivity, which is beneficial to flaw detection. However, high frequency, long near-field area and large medium attenuation are unfavorable for flaw detection. Therefore, when choosing probe frequency, we should consider comprehensively, analyze all factors comprehensively and select reasonably. Generally speaking, on the premise of meeting the requirements of flaw detection sensitivity, the probe with lower frequency shall be selected as far as possible; for forgings, rolled parts and weldments with finer grain, the probe with higher frequency shall be selected, usually 2.5-5.0mhz. For castings, austenitic steel and other workpieces with coarse grains, soft and low frequency probe should be selected, usually 0.5-2.5mhz, otherwise, if the frequency is too high, it will cause serious attenuation of ultrasonic energy.
3 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. The following factors are mainly considered in the selection
1) Half diffusion angle. It can be seen from the diffusion angle formula that the increase of wafer size, the decrease of half diffusion angle, the good directivity of beam, and the concentration of ultrasonic energy are favorable for flaw detection.
2) Flaw detection near field area. From the formula of near-field length, it can be seen that the increase of wafer size and near-field length is unfavorable to flaw detection.
3) The chip size is large, the ultrasonic energy radiated is strong, and the scanning range of the non diffusion area of the probe is large, so the ability to detect long-distance defects is enhanced.
In order to improve the efficiency of flaw detection, the large wafer probe should be selected for workpieces with large flaw detection area; in order to effectively find defects in a long distance, the large wafer probe should be selected for workpieces with large flaw detection thickness; in order to improve the positioning and quantitative accuracy of defects, the small wafer probe should be selected for small workpieces; in order to reduce coupling loss for workpieces with uneven surface and large curvature Small chip probe should be selected.
In the detection, the axis of ultrasonic beam should be perpendicular to the defect as much as possible. Therefore, the selection of angle should be based on the type and position of the defect that may exist in the detection object and the allowable detection conditions of the workpiece, and the law of reflection and refraction as well as 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 sound beam axis, and the sound path of the primary wave (the distance from the incident point to the bottom reflection point). When β = 40 ° (k = 0.84), the acoustic pressure reciprocating transmissivity is the highest, that is to say, the detection sensitivity is the highest. It can be seen that the value of K is large, the value of β is large, and the sound path of primary wave is large. Therefore, in the actual detection, when the thickness of the workpiece is small, the larger K value should be selected, so as to increase the sound path of the primary wave and avoid near-field detection. When the thickness of the workpiece is large, the smaller K value should be selected to reduce the attenuation caused by the excessive sound path and facilitate the detection of defects in the deeper part. In weld inspection, it is also necessary to ensure that the main acoustic beam can scan the whole weld cross section. For incomplete penetration at the root of single side welding, the reflection of end angle should be considered. K = 0.7 ~ 1.5, because K < 0.7 or k > 1.5, the reflection of end angle is very low, which is easy to cause missed inspection.