Theory Research Of The Calculation Of A Thin Layer On An Elastic Substrate With Finite Thickness

Ultrasonic testing is an important form of nondestructive testing, ultrasonic nondestructive testing is the method which uses ultrasonic propagation in the medium to detect the presence of target, evaluating the target features. Using ultrasonic testing thickness of thin layer become relatively hot issues. Application of ultrasonic testing related to the national economy almost all industries and sectors, involving almost all the industrial sectors, such as aerospace, high-speed rail, integrated circuits, the nuclear industry, oil and mining industry and so on. Thickness is an important parameter to reflect the properties of layer, so to carry out the measurement of thickness of thin layer becomes significant.The conventional measurement methods which using ultrasonic to detect thickness of thin layer are as follows: Pulse echo method in time domain and resonance methods in frequency domain. Pulse echo method requires recording detachable before and after the interface echo of the sample, so it is failure to measure the thickness of the thin layer. Resonance method requires one or more the resonance or anti-resonance frequency covering the samples. For the thin layer, this requires ultrasonic transducer has a wide frequency range. In the inversion algorithm relation to layer thickness measurement, the form of the inverse function is complex, and the general analytically solution can not be solved. If use the numerical method, such as the Newton-Raphson iterative method, it required initial value of parameters, and it need to study and limit the convergence region of parameters, otherwise, it may face the problem that from more than one equation to search the global minimum point of zero corresponding to the optimal solution in solution corresponding local minima.This view of this situation, depending on the desired system model in layer within two adjacent frequency points phase is not greater thanπ. I derived analytical expression between layer thickness and the experimental data. By the expression can be seen the thin thickness is only concerned with measurements of the acoustic impedance and acoustic impedance, and using the two adjacent frequency points measured data can be realized on the thin layer thickness.In this paper, I use transfer function of normal incidence of the ultrasonic reflection to realize the thin elastic lining thickness on the measurement method. In two cases, the thickness of the thin layer is calculated, a situation is when the elastic lining is thicker , reflection wave of the thin layer and the reflected wave of other media can be separated. Another case is thin elastic substrate is liner, reflection wave of the thin layer and the reflected wave of other media add together, can not be separated. In this paper, in the study for both cases, select to three material with different acoustic impedance as the lining: aluminum, steel and plexiglas to conduct research and study their anti-noise ability.In the actual project, thin layer is often not independent of existence or emergence, generally lining with one or more layers,and the elastic lining very thick, eflection wave of the thin layer and the reflected wave of other media can be separated. a thin layer of this type typically measured, usually reflected in the time domain signal from the ultrasound to reflected wave from the thin layer extracted separately, Extracted from reflection wave of the thin layer for the Fourier transform, obtain transmission function of the signal of this part, and then inversion or characterization parameters of thin layer. In the inversion layer thickness, the use of one or more of the frequency interval to calculate the thickness of materials of thin layer .In order to study its anti-noise ability, plus a 1% random noise in the Ultrasound reflected signal, the measurements obtained and the relative error between the true values of about 2%, indicating that this method has some anti-noise ability.and confirmed the feasibility of the method.If thickness of the thin layer gradually decreases, Whether in the time domain or frequency domain, Information of reflective of Thin layer material and Information of the interface echo of the lining can not be separated. Therefore, the reflected signal of the thin layer can not be extracted from the ultrasound reflected signal ,so do not inversion thickness of the thin layer. Then using the resonant frequency point in the reflection coefficient of the lining calculation thickness of the thin layer. As before we select aluminum, steel and plexiglass as the thin lining material, and analysis a few points near the resonant transmission frequency. through the graphics by obtaining the same frequency interval points connected into line, can be found a straight line with minimum frequency in the graphics. This line is close to the true value point;,it Is the estimate of the thickness of thin materials. After analysis its stability and anti-noise ability. plus a 1% random noise in the Ultrasound reflected signal, further validates the feasibility of measuring the thickness of thin layers.