Research On Thick Aluminum Allloy Plate Inspection Method Using EMAT

Thick aluminum alloy plates are widely used in a variety of areas, such as aerospace and defense. The demand for thick aluminum alloy plates sharply rises as our country's aviation blooms and military modernization process accelerates. Flaws inevitably appear during the production of thick aluminum alloy plates, since the process technology and plants are inadequate in our country. At present, delivery inspection of thick aluminum alloy plates are commonly performed by piezo ultrasonic technique, which needs couplant during testing, thus efficiency and accuracy are influenced. Electromagnetic acoustic technique is couplant-free, which can favorably generate several types of ultrasonic waves, contributing to a better performance in efficiency and accuracy. Hence, research on thick aluminum alloy plate inspection method using electromagnetic acoustic transducers is of great significance.Characteristics of ultrasonic waves as well as advantages and disadvantages of each inspection method were investigated. Normal incidence shear horizontal wave in pulse-echo mode was utilized to perform thick aluminum alloy plate flaw detection. Moreover, dead zone was circumvented by relying on the 2nd bounce from the wall.As aluminum is nonferromagnetic material, Lorentz mechanism for electromagnetic acoustic transducers (EMATs) was studied. Furthermore, the model for butterfly and spiral coil EMATs, which were widely used to excite shear horizontal wave, and corresponding sound fields were established respectively by 3-dimentional finite element method. Simulation results showed that butterfly EMATs yielded better performance in flaw detection when compared to spiral coil EMATs.Flaw detection device using EMATs for thick aluminum alloy plates was designed and realized, which consisted of a high frequency and high power isolated driver, high gain low noise cascaded amplifiers with filters, a variable gain amplifier and a multi-channel data acquisition circuit. Experiment indicated that the device could verify the validity of modeling and simulation results proposed in this paper, and it presented a preliminary flaw detection ability, which established a solid foundation for the further application of electromagnetic acoustic technique in thick aluminum alloy plate inspection.Flaw detection signal processing algorithm was developed. Flaw detection signal was pre-processed by averaging technique which was able to greatly improve the signal-to-noise ratio. Adaptive noise cancellation technique was utilized to counteract unexpected echoes which could be troublesome to the identification of flaws. Adaptive LMS, NLMS and RLS algorithms were studied and used to process the flaw detection signal respectively. The results showed that RLS algorithm was able to minimize the influence of unexpected echoes.