| Thin-walled small-diameter tubes are mainly used in petroleum equipment manufacturing,electric power construction engineering,chemical industry and other key industries of national economy and science and technology.Therefore,nondestructive testing of thin-walled small-diameter pipes is essentially a check on national security and people’s safety.Among the various detection technologies for thin-walled small-diameter tubes,ultrasonic guided wave detection technology has attracted more and more attention due to its unique advantages such as fast,long-range and low destructiveness.However,due to the high cost of ultrasonic guided wave detection and the multi-mode characteristics of ultrasonic guided wave,the accuracy of defect identification is not high in the detection process.At the same time,it is difficult for the traditional ultrasonic method to excite a low-order torsional mode (0,1)guided wave with detection advantages which is single mode and has no dispersion.This paper combines the ultrasonic guided wave detection technology with the design of electromagnetic acoustic transducer(EMAT),utilizing the advantages of low cost,no coupling,and flexible sensor design of electromagnetic ultrasound.The research has been carried out from the theoretical,sensor design,and testing,and certain results have been achieved.Based on the principle of electromagnetic ultrasonic detection,the paper analyzes the characteristics of ultrasonic waves that can be excited by probes with different coils and magnets,and designs a tile-shaped Nd Fe B magnet that can provide a bias magnetic field and an FPC flexible coil that generates a pulsed magnetic field.Tile-shaped magnets have a larger surface area on the tube surface than ordinary cuboid magnets,which greatly improves the detection signal.FPC flexible coils are more suitable for small-diameter pipes with curved surfaces,and makes the lifting distance between the coil in EMAT and the tested specimen smaller,which ensures the generation and reception of effective excitation signal.The hardware system of electromagnetic ultrasound detector is designed.The hardware part includes the control circuit with STC89C52 chip as the core and the transmitting circuit with IRF840 as the core.In order to make the received signals meet the requirements of data acquisition,a two-level controllable amplifier circuit with MAX4104EUK+T and AD603 AR chips as the core and an ADC driver circuit with OPA690 ID chip as the core were designed.In the data acquisition part,A/D conversion circuit with ADC0809 CCN chip as the core and RS-232 serial communication circuit are designed.The software part is based on Lab VIEW,and the display interface of the electromagnetic ultrasonic detector is designed.Its basic performance basically meets the needs of the electromagnetic ultrasonic detection system.The test verified that the designed sensor had the best signal amplitude at the excitation frequency of 136 k Hz,and the excitation frequency was used to detect the AL6063 aluminum tube and Q235 steel tube with artificial circumferential grooving,axial grooving and through-hole defects.After processing the test results,the actual sound velocity of the guided wave detected in the aluminum tube and steel tube was3.08mm/μs and 3.17mm/μs respectively,and the error with the corresponding theoretical value was 1.2% and 1.6%,respectively.The positioning errors of torsional guided waves for the three types of defects in aluminum pipes are not more than 2%,and the positioning errors in steel pipes are also less than 4%.In the pipeline defect detection of two kinds of materials,the axial defect detection presents an advantage.The comparison of the amplitude of the echo signal proves that reducing the coil resistance(connecting the coils in parallel)can effectively improve the detection accuracy.In comparison,the electromagnetic ultrasound torsional guided wave is better than the steel pipe in detecting defects in aluminum pipes. |
PDF Full Text Request