Research On Eddy Current Testing Technology For Aircraft Multilayered Structure Inner Corrosion

The corrosion hidden in aircraft multilayered structure is concealed, such as the inner corrosion among the lap joint structure of aircraft wing and fuselage skin, it seriously affects the integrity, safety and reliability of aircraft structure. Eddy current testing is widely used in aviation and aerospace sector as its advantages of strong applicability, non-contact, high speed and accurate. This paper study the eddy current testing technology of inner corrosion among aircraft multilayered structure. The main results of research are as follow:(1) Build the analytic model of a mutual inductive eddy current sensor above an infinite multilayered conductive structure, according to Maxwell equations and boundary conditions, with the eigenfunction series expansion method of truncated region, deduce the series expression of coil’s impedance variation. Take the single conductive plate model as example, use Mathematica and CIVA calculate the coil’s impedance variation when the eddy current sensor is placed above the single conductive plate with different thickness. The results got from the two calculating methods are almost the same. Verify the correctness of series expression of coil’s impedance variation.(2) Use CIVA construct the numerical model of three layers conductive structure,calculate the eddy current intensity distribution in conductive structure, improve the sensor’s structure. Then, construct the eddy current sensor models with different size and calculate their response to the defects in three layers conductive structure, optimize the coil’s size by comparing the calculating results. At last, calculate the response of sensors with different detection frequency and different lift off. Conclude the optimum detecting parameters, improve the sensitivity.(3) According to the feature of common corrosion existed in aircraft multilayered structure, use CIVA add the rectangular air defects with different size and different depth to the three layers conductive structure, calculate the influence of defects with different size and different depth on the amplitude and phase of coil’s impedance variation. Find the change in amplitude and phase of coil’s impedance variation with defects of different size and different depth. Conclude the method of judging the defect’s size and depth. These conclusions provide the theoretical help for quantitative study ofcorrosion in aircraft multilayered structure.(4) According to the feature of aircraft multilayered structure, Use LY12 make a three layer lap joint structure reference block and make an eddy current sensor for detecting corrosion in aircraft multilayered structure with the conclusions got from Chapter 3. Make up the sodium chloride acid solution to simulate the actual salt and acid gas in the air, let the acid solution react with the LY12, then use the corroded LY12 make a three layer test-piece. By comparing the coil’s impedance signals got from the defects in reference block and test-piece, accurately quantify the boundary, embedded depth and depth of the corrosion according to the method concluded from Chapter 4.Verify the correctness of the quantitative method concluded from Chapter 4, and provide help to periodic inspection and maintenance of aircraft.