Because of the different shape, material, flaw detection and testing condition of the workpiece, it is necessary to use different kinds of probes in ultrasonic flaw detection. Ultrasonic probes can be classified according to different inductive methods, and there are usually the following.
1 According to the wave type produced in the workpiece, it can be divided into longitudinal wave probes, transverse wave probes, plate waves (lamb waves) probes, crawling probes and surface wave probes.
2 According to the incident beam direction, can be divided into straight probes and oblique probes.
3 in accordance with the probe and the surface of the workpiece to the coupling mode, can be divided into contact probes and liquid immersion type probes.
4 in accordance with the probe in the piezoelectric chip materials, can be divided into ordinary piezoelectric wafer probes and composite piezoelectric wafer probes.
5 According to the number of piezoelectric chips in the probe, can be divided into single crystal probes, dual-crystal probes and polycrystalline probes.
6 in accordance with the focus of the ultrasonic beam can be divided into focus probes and non focus probes.
7 according to the ultrasonic spectrum, can be divided into broadband and narrow-band probes.
8 The curvature of the workpiece can be divided into planar probes and surface probes.
9 Special probes. In addition to general probes, there are a number of probes under special conditions and for special purposes.
The shape of the probe chip is generally circular and square. The chip size of the probe has certain influence on the ultrasonic flaw detection results, and the following factors are mainly considered when choosing
1) semi-diffused angle. By the formula of diffusion angle, the size of the wafer increases, the half diffusion angle decreases, the beam directivity is good, and the ultrasonic energy concentration is beneficial to the flaw detection.
2) Flaw detection near the field. By the formula of near field length, the size of wafer increases and the length of near field increases, which is disadvantageous to flaw detection.
3 the size of the wafer, the radiation of the strong ultrasonic energy, probe non-proliferation area of large sweep scope, found that the capacity of long distance defects enhanced.
In order to improve the efficiency of flaw detection, the inspection of the workpiece with large area it is advisable to select large chip probes; to detect the workpiece with large thickness, it is advisable to choose the large wafer probe in order to find the defect of long distance effectively. For small workpieces, in order to improve the quantitative accuracy of the positioning of defects, small wafer probes should be chosen; In order to reduce the coupling loss, it is advisable to select small wafer probes.
In the detection should be as far as possible to make the ultrasonic beam axis and defects perpendicular, so the choice of angle according to the detection object may exist in the type of defect, position and the allowable flaw detection conditions, the use of reflection, refraction law and related geometry knowledge, select the appropriate angle of the probe. In the transverse wave detection, the K value of the probe is taken as an example, the refraction angle has a great influence on the detection sensitivity, the direction of the beam axis and the distance of the first wave (Mosse to the base point). For the detection of steel workpiece by using plexiglass oblique probe, the β=40° (k=0.84) has the highest transmission rate, which is the highest detection sensitivity. It is known that the K value is big, the β value is big, the sound path of a wave is big. Therefore, when the workpiece thickness is small, the larger K value should be chosen in order to increase the sound path of a wave and avoid near field detection. When the workpiece thickness is large, the smaller K value should be chosen to reduce the attenuation caused by the excessive acoustic path, which is easy to find the defect in the larger depth. In weld inspection, it is also ensured that the main beam can scan the entire weld section. For a single side welding root is not welded, but also to consider the problem of end-angle reflection, should be k=0.7~1.5, because k<0.7 or K>1.5, the reflectivity of the end angle is very low, which is easy to be missed.