| The guided wave nondestructive testing (NDT) technique for pipeline based on magnetostrictive effect, which can overcome such limitations as low-efficiency and high-cost of some conventional NDT techniques and realize the non-contact, long distance and quick inspection, has attracted more and more attention from the pipeline inspection industry. However, being influeneed by much more factors like the local stress and the working pressure of internal and external medium, it is difficult to size and locate the defects accurately yet. Hence, it is important to study the effects of the applied stress on magnetostrictive material properties for magnetostrictive transducer design and optimization.Firstly, the magnetic-elastie coupling behaviour of magnetostrictive material was studied systematically in this dissertation, and a nonlinear constitutive model to deseribe magnetic-elastie coupling property was derived. The model considered here was built on the Gibbs free energy function G(o,H) and thermodynamic relations were used to obtain the constitutive expressions. Based on the magnetie-elastic model, magnetic induction model and magnetic permeability model of magnetostrictive material were derived by variable substitution. And the characteristic curves of magnetostrictive material, including the magnetic intensity-magnetic induction curves under different compressive stress and stress-magnetic permeability curves under different external magnetic fields, were studied. The magnetic-elastie coupling behaviour and the influence of magnetostriction strain on different stress situation was described systematically.Secondly, the basic principle of pipes NDT based on magnetostrictive effect is described by using the magnetostrictive property in magnetostrictive materials, i.e. guided wave generation based on the magnetostrictive effect; guided wave detection based on the inverse-magnetostrictive effect. And the magnetizing technique of steel pipeline and the basic principle of static bias magnetic field were synthetically summarized, and the calculation formulas of the static bias magnetic field generated by the solenoid coils were given.Under the different external magnatic and stress situation, the H-B constitutive relation has been studied by the theoretical constitutive relation expression formula of magnetic-elastie coupling effect and the coupling parameter. And the H-B constitutive relation is numerical calculated by the finite element program ANSYS. By setting up the runway coil, an external magnetic can be induction by the runway coils. According to the optimum polarized magnetic field, the characteristic table of the runway coils-stress was derived by the magnetic intensity-stress corresponding relation.The generation of guided waves in pipes is derived from the magnetostrictive force, while the magnetostrictive force is derived from the varied magnetic-elastic energy. Therefore, the magnetostrictive force model was derived, and the effects of the set static bias magnetic field and alterative current magnetic field on the magnetostrictive force was analyzed systematically. On the basis of the above result, a longitudinal magnetostric-tive guided wave transducer was designed, and the detailed scheme was provided yet.Finally, some numerical simulation investigation on the magnetostrictive guided wave transducer was carried out. The alterative current magnetic field and static bias magnetic field were applied respectively by small sine current and direct current. And the energy conversion between magnetic and mechanical was simulated. Using the piezoelectric element of ANSYS, simulation of the magnetostrictive guided wave transducer is realized by equivalent the ampere-turn of excitor coil to the input voltage of piezoelectric element. And the vibrogram of the longitudinal elastic guided waves in the pipeline was showed by modal analysis. The results of numerieal simulation showed that the magnetostrictive transducer was effective and reliable to detect the pipe and locate the defect.
|
PDF Full Text Request