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

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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.