Characterization of Aluminum and Steel Thin Plates Using Electromagnetic Acoustic Transducers

Abstract

Several techniques are currently used for the noncontact thickness mapping of thin-walled structures, which include laser contour mapping and electrical capacitance measurement, but very few methods are available when one side of the structure is not accessible. A popular technique when only one side of the structure is accessible is the use of ultrasonics in either a resonance approach or a through transmission approach. This study will focus on the use of the through transition approach using ultrasonic guided waves transmitted and received by Electro Magnetic Acoustic Transducers (EMATs) in thin aluminum and steel plates. To validate results a Fourier analysis was preformed verifying thickness resonant frequencies predicted by dispersion curves. Because the medium is a thin plate, symmetric and antisymmetric Lamb waves as well as shear horizontal waves are the candidates for analysis in the thickness measurements. The EMATs are used for both transmission and reception of guided waves of different types. The major challenge with this type of transducers is power requirement which is achieved with the high-power ultrasonic pulse generator and a transformer circuit. The temporal difference in transmitted and received signals of various wave types were used to calculate the average ultrasonic speed of propagation in several regions of the plates. The speed of propagation is a function of plate thickness as well as several physical parameters, allowing an average thickness to be calculated over the path of the guided waves. These values can then used to produce a map of the thickness over the entire structure as a precursor to the identification and localization of damage in thin-walled structures such as large scratches, corrosion pitting, and holes. If further quantization of plate thickness is desirable, the guided waves can be explored in several orientations allowing for a finer map of sound speeds over the plate to be created. Guided wave thickness calculations where preformed on both steel and aluminum plates proving the validity of the approach to both ferrous and non-ferrous metals while providing accuracy and precision values for the methodology and hardware used. This is in preparation for future work detecting both uniform and pitting type corrosion using similar techniques.