The phase angle difference in eddy current testing usually refers to the difference between the phase angle of the surface defect of the tested tube and the phase angle of the through-hole signal under a certain excitation frequency. The bigger the phase angle difference is, the stronger the detection system can distinguish the surface defects from the internal defects, and the stronger the ability to suppress the vibration interference in the inspection of the pipe. It can be seen that the magnitude of the phase angle difference reflects the performance of the detection system, which is the pursuit of most eddy current equipment manufacturers.
The main factors affecting the phase angle of eddy current testing are as follows:
1) Excitation Frequency of Flaw Detection
2) Performance of Phase Sensitive Detector in Flaw Detection Instrument
3) Performance of eddy current probe (coil)
In view of the above three points, we discuss them separately.
1) The excitation frequency of flaw detection: Because the excitation frequency of flaw detection is directly related to the penetration depth of eddy current in the tested material, the higher the frequency, the shallower the penetration (flaw detection) depth; conversely, the lower the frequency, the deeper the penetration (flaw detection) depth. In order to ensure the detection effect, the relevant national flaw detection standards have clear requirements on the selection of the excitation frequency of flaw detection, so it is difficult to improve the phase angle difference by changing the excitation frequency.
2) The performance of phase-sensitive detection in flaw detection instruments: There are many versions of "phase-sensitive detection" in our country at present. Through a lot of research and comparison, we find that a German company's "phase-sensitive detection" technology is particularly ideal. It uses "frequency doubling full-wave phase-sensitive detection" technology, and the phase angle difference obtained is obviously greater than that of other companies. Method. After years of intensive research in Shanghai Weiyuan Company, we have applied this technology to "UNIK 26800EX Intelligent Eddy Current Detector" and "UNIK 26860EX Computer Eddy Current Detector" and achieved good results.
3) The performance of eddy current probes (coils): At present, most of the through eddy current probes (coils) can meet the requirements when they are tested with small specifications (usually the inner diameter of the probe is less than 100 mm), but when the detection specifications are increased, the performance decreases sharply, showing only a straight line on the impedance plane. By measuring and analyzing the magnetic field inside the probe, we find that we usually only pay attention to lowering the zero potential of the large size eddy current probe, but neglect the "magnetic field balance" of its internal magnetic field distribution, resulting in small phase angle difference of the large size eddy current probe. In view of this result, the design, fabrication and fabrication of large-scale eddy current penetrating probe ensure the "magnetic field balance" of the probe, which has achieved quite good results.
Through the above discussion, we can see that it is feasible to improve the phase angle difference of eddy current flaw detection by improving 2) and 3) strips, thus improving the stability and reliability of eddy current flaw detection.