Determination of the Thickness of the Paint Coating of Metal Surfaces by the Impulse Technique

Paint coating of metal surfaces has the purpose not only to increase the aesthetic value but also to provide anticorrosive protection, and improve the mechanical qualities. The life of a paint coating depends on the adhesion between the two materials, the substrate and the paint. At the boundary of separation between the two materials, in the bending process, a jump of the bending effort occurs, which results in a shear stress. This stress depends on the elasticity of the two materials, but also on the thickness of the deposited layer. It is therefore important to be able to determine the thickness of the paint layer. Based on the analysis of the bending vibrations of a sample, before and after coating with paint using the Bernoulli equation, a ratio of natural frequencies is determined as a function of the elasticity modules of the two materials, their densities and their geometric dimensions. From this ratio can be determined the thickness of the paint layer knowing the natural vibration frequencies. The proposed method is nondestructive and is based on the analysis of vibration signals obtained by impulse excitation of samples, before and after coating with paint. By applying the Fast Fourier Transform (FFT), natural frequencies are obtained in the two situations. The experimental results come to validate numerical results obtained by applying the Finite Element Analysis (FEA).