在太赫兹厚度无损检测中,针对薄涂层上、下表面回波不可分的问题,测定频率为0.2~7 THz的普通面漆涂层的光学参数,根据其在频率为0.2~2 THz时的光谱特性,采用折射率常数模型和消光系数常数模型建立了反射式太赫兹波传播仿真模型。将仿真波形与实际太赫兹检测波形的差值方程组作为目标函数,提出最小二乘厚度优化方法,同时引入太赫兹发射器与样品间的距离参数对目标函数进行优化。设计具有不同阶梯厚度的面漆涂层样件进行无损检测厚度成像实验。实验结果表明:所提方法对定量无损检测精度有显著提高效果,对面漆涂层来说,厚度检测精度达到了7.9 μm,相较于现有方法提升了2个数量级;距离参数的引入实现了对检测过程中太赫兹发射器与样品间距离分布的可视化分析。

In this study, to address the problem of inseparability of echoes from the upper and lower surfaces of a thin coating during terahertz nondestructive thickness testing, the optical parameters of the common top coating were measured in the frequency range of 0.2-7 THz. According to spectral characteristics in the range of 0.2-2 THz, constant models of refractive index and extinction coefficient were used to establish the reflecting terahertz wave propagation simulation model. The difference equations between the simulation waveforms and the actual terahertz testing waveforms were taken as the objective function, which were optimized by constructing a least-squares thickness optimization method and introducing a distance parameter between the terahertz transmitter and the sample. Samples with different step thicknesses of the top coating were designed for nondestructive thickness testing. The experimental results reveal that the proposed method considerably enhance the accuracy of quantitative nondestructive thickness testing. For the top coating, the accuracy of thickness testing reach 7.9 μm, which is two orders of magnitude compared existing methods. The introduction of the distance parameters enables the visual analysis of the distance distribution between the terahertz transmitter and the sample during the testing process.