Impedance Angle Modeling Of Anisotropic Materials And Its Application In Nondestructive Testing

With the rapid development of modern industrial manufacturing and aerospace technology,traditional metal material properties are hard to meet the requirements of various technical industries.Three-dimensional carbon fiber reinforced polymer(3D-CFRP)is stronger than steel,denser than aluminum,more resistant to corrosion than stainless steel and more resistant to high temperatures than heat-resistant steel.Therefore,it is favored by the fields ranging from aerospace,defense to automotive and sporting goods.In terms of structural characteristics,3D-CFRP is an anisotropic and inhomogeneous material.The woven structure overcomes the weakness of being easy to be separated and poor mechanical properties between layers and gradually becomes the frontier and future directions of high-performance CFRP.However,structural defects occur easily in the production process,especially in the aerospace field,material quality requirements are extremely demanding.The defective products after solidification cannot be recycled,which resulting in serious economic losses.In terms of its electromagnetic properties,3D-CFRP can conduct electricity not only along the fiber direction and transverse direction,but also on adjacent piles.Eddy current testing(ECT),with its advantages of non-contact,no radiation,low power consumption,is very suitable for 3D-CFRP online quality detection.During the weaving process,the change of conductivity in different axials can be real-time monitored by ECT sensors.Before solidification forming,if the product defects can be detected and repaired,it will reduce the rate of defective products and speed up the process of large-scale industrialization of such materials.In traditional eddy current testing,when the lift-off(the distance between the sensor and the specimen)increases,the magnetic flux reaching the specimen will attenuate exponentially,and the electromagnetic coupling effect between the coil and the specimen is weaken,which resulting in leakage or fault detection.The thesis is based on eddy current field,electromagnetic field,material and mathematical physics theory to establish a nonlinear analytical phase model to study the relationship between the electromagnetic characteristics of the measured material and the variation of the impedance phase of the sensor.In the experiment,it is found that the real and imaginary parts of the impedance decrease,and the peak frequency of the impedance imaginary part is shifted obviously.Therefore,the peak frequency of impedance imaginary part is corrected by a data processing method and the phase change can be reflected by a globally fitted a₀.Ignore the influence of boundary effect,the fitted a₀ of the plate with the same thickness and electromagnetic characteristics can be maintained a constant value within a certain range of lift-off.During the detection process,a₀ can be quantized and recorded.An alarm will be generated when a₀beyond a certain threshold.It provides a new solution for studying the rule of conductivity of anisotropic materials along axial directions.Sensor structure,winding mode and geometrical parameters will be optimized through simulation and experiment.In addition,a real-time multi-frequency phase sensitive demodulation device based on field programmable gate array(FPGA)is designed to provide solutions for portable system implementation.