Nondestructive evaluation of the initial surface damage of structural elements by using nonlinear surface wave

本课题的目的是建立一种基于非线性表面波的近闭浅表面裂纹超声评价方法。该项目包括(a)利用线聚焦表面波传感器检测近闭合裂纹处漏出的瑞利波的二次谐波，(b)采用有限元分析方法对近闭合表面裂纹进行非线性表面波传播的数值模拟。现将本项目的主要成果总结如下：1)利用泄漏瑞利波5MHz的二次谐波幅值检测到接近闭合的疲劳裂纹。采用线聚焦PVDF传感器发射5MHz的入射波，接收10MHz的二次谐波。接收波形的振幅谱识别二次谐波振幅，通过二次谐波振幅比10MHz振幅/5MHz振幅计算。2)施加弱压应力闭合疲劳裂纹后，二次谐波幅值比有明显变化。从而识别出疲劳裂纹。3)有限元分析结合接触单元模拟近闭合裂纹，模拟瑞利波在裂纹处激发二次谐波的产生和传播。4)倾斜表面裂纹的二次谐波振幅比远小于垂直表面裂纹的二次谐波振幅比。5)二次谐波振幅比与表面波长的四分之一以下的裂缝深度成正比，但对于深度超过该深度的裂缝，二次谐波振幅比是饱和的。

The purpose of this project is to establish an ultrasonic evaluation method of nearly closed shallow surface cracks with nonlinear surface wave. The project is composed of (a) the detection of the second harmonic of the leaky Rayleigh wave exited at nearly closed cracks with a line-focused surface wave sensor, and (b) numerical simulation of the nonlinear surface wave propagation by using FEM analysis in which contact elements are introduced to model nearly closed surface cracks.Main results obtained by this project are summarized in the followings.1) A nearly closed fatigue crack is detected by the second harmonic amplitude of the leaky Rayleigh wave of 5MHz. A line-focused PVDF sensor was used to transmit 5MHz incident wave to receive the second harmonic wave of 10MHz. The amplitude spectrum of the received waveforms identifies the second harmonic amplitude, by which the second harmonic amplitude ratio, 10MHz amplitude/5MHz amplitude, is evaluated.2) By applying weak compressive stress to close the fatigue cracks, the second harmonic amplitude ratio shows marked change. Thus, fatigue cracks are identified.3) FEM analyzes combined with contacts element to model nearly closed cracks simulate generation and propagation of the second harmonic wave excited at the cracks by the Rayleigh wave.4) The second harmonic amplitude ratio of an inclined surface cracks is far smaller than that ofa perpendicular crack.5) The second harmonic amplitude ratio is proportional to the crack depth up to a quarter of the surface wavelength, but it is saturated for deeper cracks over that depth.