Research On Methods Of Crack Parameter Estimation And Imaging In Eddy Current Array Nondestructive Testing

Eddy current array testing (ECAT) is a new branch of eddy current nondestructive testing (NDT) technique developed in recent years. In view that ECAT has the advantages of non-contact detecting which merit conventional eddy current testing (ECT) has and easy to implement large-area and fast testing which merit array testing does in the pattern of electronic scan, it has become currently a common hot subject of research in sensor technonlogy and NDT technonlogy. Supported by ministerial-level pre-research projects'××××miniaturization, integrated, rapid NDT technonlogy'and'××××××micro-injury testing technonlogy based on methods of array magnetic field measurement', this paper focuses on quantitative and visual crack detecting by ECAT. Based on time-sharing multi-excitation transimit-receive (TSMETR) eddy current array sensors, several key technology about optimal design methods of eddy current array sensors, methods of crack parameter estimation, calibration methods of crack parameter estimation precision unconsistency from different coil unit groups, methods of eddy current array C scan imaging are studied in this paper. The brief of these researches and the novel approaches are as follows:(1) To solve the problem that less sensitive zones exist when the workpieces are detected using ECAT, the ratio of effective detection area of coil unit group which can quantitatively reflect the less sensitive zones is proposed, and this parameter can guide eddy current array sensors'optimal design for reducing the range of less sensitive zones. Based on electromagnetic field theory, ECAT analytical calculation method and numerical method are discussed. Detailed calculation processes of induced voltage of TSMETR ECAT are described. The influence of crack parameters, such as length, depth and position, to induced voltage are researched, and the physical explanation of the results is given. With the help of simplified double coil unit model, finite element simulations of the mutual inductance between eddy current array sensor coil units are implemented, and the elimination methods of mutual inductance are discussed. Using coil unit group (an eddy current array sensor can be seen as integrated by many coil unit groups, and a coil unit group consists of several coil units) as the simplified object of study, the variations of sensitivity, spatial resolution and ratio of effective detection area of coil unit group with diffirent arranged patterns, center distances, outer radius and heights of coil unit are researched by means of finite element simulation method, and several significant conclusions which can guide the design of the eddy current array sensors are drawn. The simulation results verifies that the ratio of effective detection area of coil unit group brings important reference value to the optimal design of arranged pattern, center distance and outer radius of coil unit of eddy current array sensor. (2) The method of crack parameter estimation based on the eddy current array defect reconstruction technique is deficient in complete accurate prior database and time -consuming during the process of inverse analysis. By analysing the mapping between the scan times of crest, trough and inflection point of the signals output by ECAT system and the moments when the center of receiving coil unit reach crack edges in the length direction, this paper presents a new method of crack length estimation without any prior knowledge of the workpieces. A method of crack depth estimation which has the advantage in detecting the workpieces with little prior knowledge based on resilient BP neural network is researched by analyzing the nonlinear incremental mapping between the crest amplititude and crack depth. To solve the problem that it is difficult to improve the eatimation precision of crack position that is perpendicular to the scanning direction, a new method of crack position estimation that is perpendicular to the scanning direction based on the amplitude deviation ratio of two valleys is proposed. This method has better real-time performance compared with the method of crack parameter estimation based on the eddy current array defect reconstruction technique. Experiments are done to verify the methods of crack length, depth and position parameter estimation, and the precision of the experimental results are analysed. The results show the high precision of the above mentioned methods of crack length, depth and position parameter estimation.(3) To solve the problem that different coil unit groups have different crack parameter estimation precision, the hardware calibration period of the testing system is long and the crack parameter estimation precision are still low after completing the hardware calibration processes, a new software calibration method of ECAT instrument based on the crack parameters is proposed. The basic forms of the software calibration models are established. By researching the partial least-squares (PLS) regression based on the spline transform as well as kernel function and the algorithm based on the least squares support vector machine (LS-SVM), the software calibration models are founded. According to the precision analysis of the crack depth estimated by the software calibration models, the following conclusions can be made, after the hardware calibration process, using the proposed software calibration method can largely reduce the crack parameter estimation precision unconsistency from different coil unit groups so that significantly improving the crack parameter estimation precision. The precision of the crack depth estimated by the software calibration models which are founded by the PLS regression method based on the spline transform is higher than the precision of the crack depth estimated by the software calibration models which are founded by the other modeling approaches.(4) In NDT of components whose crack parameters need to be estimated precisely, eddy current array C scan images have the disadvantages of only qualitatively reflecting the defects, existing contaminated areas, having differences between the positions of crack zones in C scan images and the real positions of crack zones, and et al. Eddy current array modified C scan imaging, which can quantitatively reflect the crack position, direction, length and depth, is proposed. Combining the image processing algorithms, such as image segmentation, edge detection, contour tracking, thinning and et al, with the crack parameter estimation methods, the generation algorithms of the eddy current array modified C scan images are researched. Detect the Aluminum specimen with cracks in different directions, generate the eddy current array modified C scan images in U testing mode and T testing mode. The experiments show the below conclusions. As one of the derivative images of the eddy current array C scan images, eddy current array modified C scan images are adaptive for fast testing some components, whose crack parameters need to be estimated precisely, because of precisely quatitative reflection of crack length, depth, direction and position. For eliminating the contaminated areas of the crack zones in C scan images and modifying the deviations between the positions of crack zones in C scan images and the real positions of crack zones, modified C scan images can effectively avoid the error orientation due to the inspectors'visual errors. However, modified C scan imaging is poor in real-time applications because of many image processing algorithms and methods of crack parameter estimation. Therefore, in practice modified C scan imaging may be used to complete the second-step testing for estimating the crack parameters accurately after C scan imaging has determined whether cracks exist.(5) Overall design scheme of ECAT system is elaborated, and implementation methods of main hardware modules, including DDS signal generation circuit, orthogonal lock-in Amplifier circuit, analog multiplexer array circuit and et al, are researched. Based on the designed ECAT system, airplane oil tubes with circumferential and axial cracks and aircraft engine blades with cracks are detected. The crack parameters are estimated from the detected signals. Eddy current array C scan image and modified C scan image are generated. Quantitative and visual nondestructive detection to the above key components is fulfilled. Therefore, the reliability of the ECAT system's hardware and the effectiveness of the proposed algorithms are verified.