Development of a nonlinear acoustic microscope and evaluation of bond integrity

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The purpose of this project is to detect nondestructively minute micro-cracks and/or disbonding in structural materials and bonded interface by using nonlinear ultrasonic waves. The main idea is to detect higher harmonics generated at micro cracks under burst ultrasonic waves of large amplitude of a few tens of nanometers. When the surfaces of minute micro-cracks and/or disbonding are repeatedly dapped or rubbed, then higher harmonics, of which frequency are integral times of the incident wave frequency, are excited.This project is composed of the followings : (a) measurement technique of 2^<nd> harmonic without bonding piezo elements on the surfaces of materials tested, (b) elimination of nonlinearity of coupling water for immersion measurement, (c) numerical simulation of 2^<nd> harmonic wave with dynamic finite element method in which contact elements are introduced to model nearly dosed cracks or interfaces.Main results obtained by this projects are summarized in the followings.1) … More A 2^<nd> harmonic measurement system has built in terms of a burst wave of large amplitude, commercial piezo transducers, and digital signal processing. This system detects A_2/A_1, the 2^<nd> harmonic amplitude ratio normalized by the incident wave amplitude, greater than 0.4% with a resolution of 0.1%.2) In immersion measurement, a transmission technique has been developed, which exclude the acoustic nonlinearity of the coupling water. This is very effective for scanning to identify the locations of weak bonds.3) With the 2^<nd> harmonic of the leaky Rayleigh wave, depth of a dosed fatigue crack is evaluated for depth range of 1-2mm. This is particularly useful for sizing of initial fatigue cracks depth in pressure vessels.4) FEM codes for nonlinear wave propagation is developed to simulate crack surface clapping and rubbing with introducing special contact elements. With this simulation, we understand quantitative nature of harmonic generation.5) A patent on evaluation method and apparatus for bond interface evaluation is going to be submitted.6) This project is interested in some material makers, and two joint projects of nonlinear ultrasonic measurement are currently running. Less