Research On Steel Plate Defects Inspection By Electromagnetic Acoustic Technology

Rising electromagnetic acoustic (EMA) technique, as a type of ultrasonic with electromagnetic excitating principle, attracts highly attention in Nondestructive Testing field, because it doesn't need any couplant during inspection. EMA technique using in steel plate defects inspection is emphatically researched in this dissertation.On the basis of inrtroducing the EMAT technology development and application of history, the theoretical of EMAT is discussed. Finite element analysis is used to research the electromagnetic acoustic testing system, and establishes the finite element model of E MA exciting system. The coil above tested conductor is excitated by both transient current and high voltage pulse in order to investigate the induction of alternating magnetic field, induced eddy current and Lorentz caused by two different incentives. Taking into account the magnetic field and eddy current in the course of the importance of testing , the impact caused by permeability rate, resistivity and so on is inspected. A planar model with defect is established so as to research and analyze different phenomenon of magnetic field and induced eddy current on the surface which were caused by various defects. EMAT signals are often highly polluted by noise.The structure and circuit model of EMAT system is designed. The characteristics of permanent magnet, DC electromagnet and the AC electromagnetic are compared, and through analyzing different shapes of incentive coil derives, its structure parameters is optimized. The inspection system is designed. Based on Direct Digital Synthesis technology, the sinusoidal pulse bunch signal generator and the power amplifier system are designed. The electromagnetic acoustic transducer excitation is studied, and the pre-amplifier is designed as well as the main amplifier in order to enlarge the weak voltage signal.Practical signals are obtained by using EMA detector, and processed by a nonlinear adaptive time domain signal processing method to de-noise EMA signals, aimed at the feature that received signals belong to non-stationary signals. Some analysis and researches are taken on some factors affecting experimental results which are found in the simulation study.