| The noncontact magnetostrictive guided wave is used to excite and receive the guided wave directly in the component by the magnetostrictive effect of the material,which is suitable for the detection of ferromagnetic materials.Compared with the traditional point-to-point non-destructive testing method,magnetostrictive guided waves can detect long distances with high efficiency.Such sensors do not need to move along the structure to detect internal and external defects.Therefore,it is of great significance to study the application of non-contact magnetostrictive guided wave in non-destructive testing and structural health monitoring.The low excitation efficiency of noncontact magnetostrictive guided waves is the main factor limiting their wider application,making the guided-wave signal have small amplitude,short propagation distance,and low defect resolution.Traditional non-destructive testing technology only detects defects.If the abnormal change of tension of steel wire ropes or steel belts cannot be detected in time,their service life and safe operation of systems using them will be seriously and adversely affected.The dead zone of guided-wave defect detection is produced near the transducer.This reduces the reliability of magnetostrictive guided wave detection.To enhance the defect-detection efficiency of magnetostrictive guided waves,magnetostrictive patch sensors are widely used as coupling agents to generate guided waves.However,it is necessary to remove the coating,anti-corrosive paint,and other materials outside the pipe surfaces before a magnetostrictive patch sensor can be used.This increases the difficulty of their use and limits their application scope.In this study,it improves the signal-to-noise ratio of the longitudinal guided waves,monitors the change of tension on the steel strip and realizes detection of steel-strip defects,overcomes the dead zone of guided-wave detection and realizes non-contact magnetostrictive torsional detection for small-diameter steel pipe.The aspects of theoretical research and engineering practice are:Noncontact magnetostrictive guided waves have low excitation efficiency and small amplitude.A method of determining the natural frequencies of pipes based on the use of white noise is proposed.The detected natural frequencies are used as the excitation frequency of the guided wave to improve the low excitation efficiency and small amplitude of the magnetostrictive guided waves.Firstly,the principle of different white noise generation is compared to determine the circuit structure of white noise generation.When the vibration is at the natural frequencies of the pipes,the vibration of the pipes can reach the maximum amplitude.White noise is used as the excitation signal of a guided wave transducer to measure the natural frequencies of pipe efficiently and accurately,so as to overcome the shortcomings of time-consuming and low-frequency resolution of the frequency-sweeping method in detecting the natural frequency of pipes.The natural frequency is used as the excitation frequency of the guided waves to improve the small amplitude of the magnetostrictive guided waves and detect pipe defects.A single guided wave sensor cannot realize abnormal tension monitoring and defect detection of steel strips.A method of measuring the natural frequencies of steel strips by loading white noise into a magnetostrictive longitudinal guided wave sensor is proposed to monitor the tension change of a steel strip.The longitudinal guided wave is used to detect the steel-strip defects.There is no effective way to realize abnormal tension monitoring and defect detection for steel wire rope or steel strips currently available in the market.Firstly,the structure of the steel belt guided wave transducer is determined by comparing the effects of different guided wave transducers through three-dimensional simulation analysis.White noise is used to measure the natural frequency of a steel strip to monitor the change of tension and determine the natural frequencies with the largest guided-wave amplitude.Using this approach,the excitation and detection of guided waves and defect detection can be realized,and the application range of magnetostrictive guided waves can be increased.A dead zone exists near the transducer in guided-wave detection that makes defect detection difficult.It is therefore proposed that eddy-current testing and magnetostrictive guided-wave testing be combined to realize the non-destructive testing of pipes without adding transducers.This can improve the reliability of guided-wave detection for pipes.In the guided wave detection,the dead zone of guided wave detection will be formed near the transducer,which makes it difficult to detect the defects in the dead zone.In order to realize the fusion of magnetostrictive guided wave and eddy current testing,the feasi bility of the fusion of the two testing methods is analyzed firstly.Then the relationship between the amplitude and phase of eddy current signal and defects is analyzed through eddy current theory analysis and three-dimensional electromagnetic simulation.The defects located at the far end of the pipe are detected using the guided-wave signal.The defects located in the guided-wave testing dead zone are detected using the eddy-current signal.The combination of eddy-current and guided-wave detection can increase the ability to detect defects without additional sensors.It is necessary to pre-treat the pipe surface and coat the coupling agent before the contact guided wave detection.A method of non-contact magnetostrictive torsional wave detection for small-diameter pipe is proposed to increase the application range of guided-wave detection.T(0,1)is the only guided wave that does not disperse in the full frequency range.But the guided wave transducer needs to use magnetostrictive patch to excite the guided wave.At the same time,the pipe surfaces must be pre-treated and coated with coupling agent before magnetostrictive patch sensor detection.Therefore,the difficulty and cost of guided-wave transducers are increased.Firstly,through three-dimensional simulation analysis and comparison of the performance of different structure transducers,a sensor with tile-shaped permanent magnet is used to realize a uniform circumferential magnetic field,and a solenoid coil is used to provide the axial dynamic magneti c field.Then,the structure of the sensor is optimized by the static excitation intensity to improve the amplitude of the guided wave,so that it can excite and detect the torsional wave and realize the defect detection for non-contact detection of pipes. |
PDF Full Text Request