| According to the advantages of compact size, versatile, couplant free and fairly in expensive, the development of electromagnetic ultrasonic technology has a great future in the field of Non-destructive Detection. Among all the acoustic waves, Lamb waves is espectially suitable for the detection of metal plates. However, during the detection work, errors can often occur due to the characters of multi-modes and disperation, and electromagnetic acoustic transducers of Lamb waves can ’t meet the requirements of Military grade for thin plate detection, which means the transducers should be redesigned to improve the inspection accuracy. Besides, t he efficiengy is low if only one transducer is used, this can be solved by taking use of an EMAT array. But there is no reasonable method for the arrangement of transducer array presently. In this paper, to solve the problems above, a plate inspection meth od based on transducers array of electromagnetic ultrasonic Lamb wave is studied.To solve the problem that multi-mode and dispersion of Lamb wave would interfer the detection, a method that can avoid the high mode and inhibit frequency dispersion is proposed. This method is based on Two-Dimensional(2-D) finite element(FE) models, and has fully considered the excitation curves and mechanisms of EMAT. By taking use of the FE models, the propagation characteristics of Lamb waves are studied, and the high mode elimination method based on receiver coil designment is presented, as well as the selectable region of operating points and the method of using wave structure.To make up for the shortage of study on Lamb wave acoustic field distribution generated by an EMAT, a three-dimentional(3-D) FE model on frequency domain is proposed. This model can perfectly combine the electromagnetic field and structure field together. The static magnetic field, eddy current and Lorenze force can be calculated respectively, then the 3-D Lamb wave acousitic field and the radiation pattern is achieved. Further more, by studying the acoustic field of meander coil, an new coil structure which can focus the Lamb wave field is designed, and coil parameters that have the best convergence of the acoustic field are achieved. Due to simulated Lamb wave field, the effective detection region of an EMAT is calculated, based on it, a method for arranging the EMAT array with no miss dectection is worked out.To meet the requirement of high detection accuracy in aluminum plate, interactions of Lamb waves with different flaws are studied by taking use of FE models. Methods that based on 2-D models with gap flaw and 3-D models with through-hole are proposed. Incident waves and echo reflection waves can be achieved by simulations in time domain. Based on these simulation models, the detection capability of Lamb waves and influence of inclined gap are studied, as well as the coil design method to improve the accuracy of through-hole inspection.A platform for experiments of electromagnetic acoustic Lamb waves is built to verify the simulation results. Then an inspection system based on Lamb wave EMAT array is developed, and the system can effectively detect a gap with 0.4mm depth and 4mm length, as well as a through-hole with diameter of Φ1mm, shown by the experiment results.In summary, a plate inspection method, which based on the electromagnetic acoustic Lamb wave transducers, is studied in this paper. This method can provide a good foundation for aluminum plate detection with high speed and accuracy. |
PDF Full Text Request