The Testing Mechanism And Propagation Characteristic Of Magnetostrictive Guided Waves Of Pipelines

Magnetostrictive sensor of guided wave is a new type of ultrasonic testingdevice which is useful for the structural health monitoring (SHM) of systems. Thetraditional NDT tools, such as magnetic flux leakage testing, piezoelectric ultrasonictesting and X-ray testing, have shortcomings, it is slowly and costing. Whilemagnetostrictive sensor of guided wave have overcome these defects and realize theefficient nondestructive testing which is non-contact. While the shortcoming ofmagnetostrictive sensor of guided wave, such as low SNR and the output affected bythe nonlinear magneto-mechanical coupling performance under magnetic field, canlimit the use of it. In order to solve this problem, the magnetostrictive guided wavestesting mechanism and propagation characteristic for steel tubes were analyzed.Making clear of main influence factors of sensor can improve the energy transitionefficiency. By the method of simulation and experiment, the mechanism ofmagnetostrictive guided waves testing and propagation characteristics of guidedwaves which under stress and magnetic are studied.Firstly, using the law of conservation of momentum, the governing equations ofmagnetostriction guided wave sensor are got which basing on the Lorentz force,magnetizing force and magnetostrictive force. And all these are deduced from theclassical theory such as electro-magnetics, mechanics of elasticity andelectrodynamics and so on. By electromagnetic field, force field and ultrasonic field,the working principle of magnetostrictive guided waves sensor is got andcorresponding mathematical description is given. The relationship betweenmagnetostrictive force and Lorentz force is discussed by the equations given before.And the leading role of the magnetostrictive force in magnetostrictive guided wavessensor is determined. The result is that with the change of the magnetostrictive force,the efficiency of magnetostrictive guided waves sensor is much affected when thebias magnetic paralleled the surface of pipe.Secondly, the research on the magnetostrictive guided waves sensor was usually regarded the material constitutive as linear which could only suit the bias magneticnear the linear range. So these models can’t satisfy the actual working conditions. Inorder to solve this problem, the paper set up the dynamics mechanical model formagnetostrictive guided wave sensor generation on the nonlinearmagneto-mechanical coupling performance of ferromagnetic. The effect of the biasmagnetic field, excitation frequency and excitation current on the particle amplitudewas analyzed. The difference of particle amplitude between nonlinear and linearmodel was discussed, which proved the applicability of the model further. Theanalysis indicates that by increasing of excitation current or decreasing of excitationfrequency and setting the bias magnetic around the biggest tangent slope of themagnetostrictive curve can improve the magnetic conversion efficiency.Thirdly, in past research the effect of stress or magnetic on the guided waves isneglected which caused the inaccurate in defect locational. So, according to thenonlinear constitutive equations of magnetostrictive material, in the free constraincondition, the effect of magnetic and stress on the material is studied. The relationshipbetween magnetic, stress and elastic modulus of ferromagnetic materials is set up. ByFEM software, the propagation characteristics of magnetostrictive guided wave undermagnetic or stress is simulated. The result is the group velocity of magnetostrictiveguided wave is changed which can influence the defect location.At last, the weak form of balance equations for permanent magnet, pulsed eddycurrent by coil and the test sample under axisymmetric coordinates are got by thevirtual displacement method. At the same time, the formulas of the magnetostrictiveforce and magnetostrictive current density are deduced which having effect on themagnetostrictive guided wave system. In order to optimize the system, a FEM modelis set up. Using the model, the factors, such as the strength and uniform of biasmagnetic and the characteristics of excitation current, are analyzed. All these resultsare verified by experiments.