Research Of The Composite Coating Thickness Measurement Method Based On Electromagnetic Tomography

Many Aerospace vehicles such as the nose cone of rocket and the front wing of aircraft are made of carbon fiber composite materials. In order to prevent it from high temperature oxidation defects and cause major accidents, this paper proposes a new method technology based on electromagnetic tomography for coating thickness measurement of composite materials, and provides certain theoretical basis for the practical application of electromagnetic tomography method.Based on the theory of electromagnetic tomography technology, described the mathematical basis of EMT and the control equation of 3d current field and the theoretical calculation method of parameters. On this basis, established three-dimensional finite element model of field- road coupled of EMT matrix conductivity single direction(x direction) and both directions(x and y directions), and researched the relationship between detection coil transimpedance and matrix conductivity direction. With transimpedance radar chart shows that, when the base material is a single direction, the transimpedance had a maximum value in the vertical direction with the material sheet conductivity(90 °, 270 °). But transimpedance appears minimum in the direction of the base conductivity(0 °, 180 °). Matrix of two-direction there is a great value in the transimpedance synthesis direction(45 °, 135 °, 225 °, 315 °), and the x, y both directions(0 °, 90 °, 270 °, 180 °) appears minimum.For the problem of matrix conductivity, established 3D finite element model of coating – substrate with the uniform matrix conductivity. Through simulation analyzed the electrical signal sensed by the excitation coils and adjacent, white or relatively detection coil, and it is concluded that the magnetic field along the axis excitation coil in the EMT sensitive field distribution is symmetrical, so the detecting coil voltage value and the relative position of coil voltage distribution is symmetrical; With different distance from the excitation coil, the transimpedance mutual of adjacent detecting coil is about two orders of magnitude than that of white and relative detecting coils.Finally, based on the researching object about the anisotropic conductivity of the substrate and the change of the thickness of the coating, this paper established the finite element model of 3D matrix – coating to explore the regular between the two laws and electromagnetic signals. The results showed that when the substrate conductivity is non-uniform, with the change of coating thickness, the change of each value on adjacent detection coil is more tangible than the interphase and relative detection coil, when the coating thickness is at the range of 100 um to 120 um. The direction of matrix composite materials can be judged by the number of extremum in the mutual resistance. With the change of coating thickness, excitation coil is sensitive to the change of the coating thickness, which is good to measure and analysis the coating thickness.