Reseacrh On Propagation Characetristics For Magnetstrictive Guided Waves Under Sterss And Magnetic Fields

The magnetostrictive guided waves inspection technique for crack in pipeline is oneof the most emerging and front development of pipeline diction technique. However,due to the residual stresses of pipeline in Manufacture and molding, the pressurecoming from conveying fluid delivery and stress concentration on the crack in pipeline,sensor recognition ability and localization accuracy is reduced. This paper focuses onthe research of the influence relationship between applied stresses, external magneticfield and magnetostrictive guided wave propagation characteristics, and scatteringcharacteristics of the different types of cracks with applied stress, theoretical support isprovided in order to improve the accuracy of magnetostrictive guided wave detectionsystem.For the first time, According to the macroscopic thermodynamic, themagnetostrictive material non-linear constitutive model is derived. Based on this model,the relationship between stress and magnetostrictive strain are derived, and then thematerials of Yang’s modulus theoretical model under applied stress is derived. Andbecause the dispersion characteristics of magnetostrictive guided waves can be affectedby the elastic modulus, the effects of stresses on the dispersion characteristics wasmainly simulated and investigated using numerical method. The simulation andexperimental results demonstrate that the stress have a great influence on the groupvelocity of longitude and torsional guided-wave apparently. And all the cut-offfrequencies of L(0,2), L(0,3), T(0,2) and T(0,3) increase with the applied stressgradually. Moreover, the wave-packet duration of L(0,1) and L(0,2) increase with theapplied stress if given a constant exciting frequency.Secondly, According to the nonlinear constitutive equations of magnetostrictivematerials, in the free constrain condition，the relationship between external magneticfield and magnetostrictive strain and the relationship between bias magnetic field andpiezomagnetic coefficient are derived，base on this, the theory model between biasmagnetic field and materials of Yang’s modulus were built. The dispersioncharacteristics of magnetostrictive guided waves are simulated under the condition ofdifferent bias magnetic fields. The simulation and experimental results demonstrate thatthe bias magnetic fields have a great influence on the group velocity of longitude andtorsional guided-wave apparently. And all the cut-off frequencies of L(0,2), L(0,3),T(0,2) and T(0,3) increase with applied bias magnetic fields gradually. Moreover, thewave-packet duration of L(0,1) and L(0,2) increase with the applied bias magnetic fieldsif given a constant exciting frequency.Finally, using the finite element transient dynamics method and the model ofdisplacement-magnetostrictive force, the function relation between L(0,1)、 L(0,2)guided wave and crack are simulated and analyzed. By means of building typicalpermeability longitudinal and circumferential crack, and load local stress on the nodesin the crack to simulate the stress concentration phenomenon in pipeline. Theexperimental results show that L (0,2) guided wave is sensitive to circumferential crack.If the loading stress is zero, the reflectivity of L (0,2) guided wave increase linearlywith the length of circumferential crack; and if the loading stress is not zero, thereflectivity no longer increase linearly with the length of circumferential crack, and the reflectivity decrease because of the increasing stress, especially showes on the length ofthe crack which accounte for1/4of circumference in pipeline. T(0,1) guided wave issensitive to longitudinal crack. If the loading stress is zero, the reflectivity of T(0,1)guided wave changes like M shape while the length of longitudinal crack less than theguided wave length; but loading stress has different effect on different length oflongitudinal crack, when the longitudinal crack length occupies30%and80%of thewavelength, the local stress has more obvious effect on the reflectivity of crack, and thereflectivity decrease with applied stress. Generally speaking, the local stress hasrelatively smaller effect on the reflectivity of T(0,1) guided wave.