Research On Integrated Detection Method Of Electromagnetic Acoustic SV Wave And Surface Wave For Thick-Walled Pipes

Thick-walled steel pipes are widely used in pipeline transportation,construction,machinery and other fields in industry,and are known as industrial vessels.Because it is prone to produce various metallurgical defects in production and processing,it needs to be tested in the factory.After leaving the factory,in the process of long-term service,it will be affected by the corrosion of environmental media and external loads such as high temperature and pressure.So various defects will be formed on the surface and inside of it.EMATs have the advantages of non-contact,no coupling agent,flexible excitation of body waves and long-distance surface waves.So EMATs have shown great advantages.Although lots of researches on EMATs technology have been carried out at home and abroad,there are still three problems.First,the bias magnetic field is limited,and the magnetic force is too large.Second,the detection coil of the steel pipe basically ignores the influence of the curvature of the steel pipe.Third,the current EMATs basically only generate one type wave,and there will be blind spots in the detection of thick-walled steel pipes.In order to in-depth study of the EMATs detection technology for thick-walled steel pipe defects,this paper has made the following three aspects of research work.(1)An EMAT with controllable magnetic force of bias magnetic field was designed.The combination of electromagnet and permanent magnet was adopted to achieve the purpose of bias magnetic field controllable.The feasibility of magnetic force controllable was verified by the simulation and experiment.The feasibility of electromagnetic acoustic signal enhancement was verified by experiment in the enhancement mode.Relative to off mode,the thickness measurement signals of aluminum and steel plates were increased by 46.91% and 62.01%,respectively.And the detection signals of internal defects and surface defects of thick-walled aluminum pipes were increased by 29.78% and 119.06%,respectively.(2)A design method of line-focusing curved meander-line coil was researched.Compared with the traditional plane coil,the designed curved coil could improve the directivity accuracy of the excited line-focusing SV(Shear Vertical)wave and the amplitude of defect detection signal.And as the curvature of the curved workpiece increases,the optimization effect was better.Among the simulation research objects,the directivity accuracy was increased maximumly by 14.53%,and the defect detection amplitude was increased maximumly by 37.42%.In the experiments of thick-walled aluminum pipes and thick-walled steel pipes,the internal through-hole defects were detected,and the defect detection signal amplitude was increased maximumly by338.84%.(3)A multi-dimensional integrated detection method of electromagnetic acoustic SV wave and surface wave was proposed.A single electromagnetic acoustic meander-line coil was loaded with pulse signals containing two different frequency bands for excitation.And SV waves and surface waves were generated at the same time and the surface and internal deep defects of thick-walled parts were detected respectively.In the experiment of thick-walled aluminum pipes,detected the inner through hole and the surface groove for the four specifications.Finally,using the combination of magnetic controllable magnetizer with bias magnetic field and meander-line detection coil with line-focusing curved surface,combined with the proposed multi-dimensional integrated detection method of electromagnetic ultrasonic SV wave and surface wave,the inner through hole and surface groove of thick wall steel pipe are detected respectively.Compared with the permanent magnet,the detection signal amplitude was enhanced by 134.8% and 578.26% respectively.