Research On Circumferential Sensing Method For Guided Wave Testing Of Pipes Based On Magnetostrictive Effects

The guided wave inspection technology based on the magnetostrictive effect is one of theresearch hotspot of nondestructive testing in recent years for its advantages of contactlessnessand high conversion efficiency. Compared with the traditional piezoelectric guided wavesensor which is easy to realize circumferential positioning, the magnetostrictive sensor withwhole bound coils to transmit and receive signals has no circumferential resolution because itgets totally circumferential information only, which limits the detection ability and application.Combining the numerical simulation with experiments, the circumferential sensing methodfor pipes detection based on the magnetostrictive effect is researched in this thesis.Firstly, a guided wave inspection method based on detecting the magnetic field in the airoutside the pipe is obtained, by studying the lift-off effect of the guided wave based on themagnetostrictive effect. The space surrounded by the magnetostrictive guided wave receivingsensor is divided to different regions, so that the change rule of the lift-off effect is analyzedboth theoretically and experimentally. The experiment shows that the variation of the airmagnetic field outside the pipe is equal to the sum of the variation of the magnetic field insideand in the pipe, which indicates the magnetic field caused by the magnetostrictive effect canspread to the air outside the pipe. According to the above phenomenon, a guided waveinspection method is obtained by detecting the variation of the air magnetic field outside thepipe and verified by the experiment, which provides basis for the development of the newreceiving sensor.Secondly, a magnetostrictive receiving sensor of guided wave with circumferentialresolution is developed based on studying the influence of structure parameters such as thecentral angle of receiving coil, the size of inside and outside ring, etc. The vibrationinformation of different circumferential positions in the pipe can be obtained through thecircumferential layout of the receiving coils along the region outside the pipe. Consequentlythe sensor can distinguish between the axisymmetric structures such as welds and flanges andthe non-axisymmetric structures such as defects, which is verified by experimental results.Finally, the study develops a method of positioning circumferential defect in the pipebased on the mode analysis of the simulation data and the experiment results, using acircumferential sensor consists of four independent right-angle parts as the receiving sensor.The angle of the defect center is got from the ratio of the peak value of the L(0,2) and theF(1,3) mode signals received by the four parts. By placing the center of any right-angle part of the receiving sensor on this center angle, the moments of the maximum and minimum valuesof the F(1,3) mode signal is got, then place the right-angle side is placed on the same angle,the circumferential extent of the defect is got from the absolute value of the signal amplitudeof F(1,3) mode in the former moments. Experiment results show that the circumferentialquantification can be achieved when the defect depth is greater than40percent of the wallthickness.