Research Of Perturbed Magnetic Fields Testing Based On Metal Materials

The Perturbed Magnetic Fields Testing technology is a newly developed electromagnetic non-destructive testing (NDT) technique, and it has wide range application prospects. The technique is used to detect surface defects and corrosion in electrically conducting based on measuring the surface magnetic distribution.In this paper, firstly we elaborated the research status and development trend both at home and abroad, and we established a mathematical model in the basis of the principle of electromagnetic induction. We analyzed the corresponding relationship between defect size and magnetic fields distribution by establishing finite element model about excitation coil and test material, exerting load and imposing boundary conditions to solve.Then we researched the factors in regard to excitation coil diameter, coil turns and coil layers by doing experiments. Sensors are optimal designed and manufactured to detect the surface magnetic fields; the results showed that the sensor had high precision and satisfying the requirements of defect detection.Secondly, we designed the magnetic field measuring system which was mainly composed of excitation signal source, sensors, amplifying and filtering circuits, data acquisition card and computer. A large number of experiments were carried out and the measurement data were basically consistent with theoretical analysis.We used the capacity of neural networks highly nonlinear, parallel information- processing, taking full account of the complex nonlinear relationship between defect size and magnetic fields distribution, we achieved the goal for accurate predicting the size of the crack by constructing a BP neural network about the disturbance magnetic fields of the crack . BP neural network model is very simple and its convergence rate is very fast, it shows strong adaptability at the case of small number of samples, and it has good promotion at quantitative prediction of defect depth and length.At last, we derived the three-dimensional magnetic fields of Nd-Fe-B permanent magnet from the theoretical of molecular circulation model and tested the surface magnetic fields distribution.