报告摘要：

It is well recognized that nonlinear ultrasound has the capability to determine certain microstructural changes in materials that occur during various degradation processes including fatigue, creep, and exposure to high temperatures. This talk will first introduce the fundamentals on the nonlinear ultrasonic nondestructive evaluation (NDE) and then some examples of its applications are given. It is shown how the nonlinear ultrasound can be applied to assess damage in materials with distributed microcracks. As an example of cracked material, thermally damaged sandstone specimens are examined using a nonlinear ultrasonic technique. This research attempts to experimentally determine the correlation between the crack density and the acoustic nonlinearity in thermally damaged sandstone. The dynamic Young’s modulus and longitudinal wave speed are measured in sandstone samples which are exposed to high temperatures, from which the crack density is calculated using a micromechanics model. A recently proposed standing wave based ultrasonic technique is employed to measure the acoustic nonlinearity parameters in these samples, as the ratio of the second harmonic vibration amplitude to the square of the fundamental amplitude. This research compares the sensitivities of the linear and nonlinear elastic parameters to the thermally-induced microcracking in sandstone. It is shown that the crack density and acoustic nonlinearity are related by an approximately linear relation. This result demonstrates that the acoustic nonlinearity is not only a means that is highly sensitive to microcracking in rock materials but also can be used to evaluate quantitatively the evolution of microcracks.

报告人简介：

Dr. Kim received his PhD and Master’s degree from Korea Advanced Institute of Science and Technology (KAIST) in Mechanical Engineering with a minor of Applied Physics, in 1997 and 1994, respectively. Since 2004, Dr. Kim has been a research faculty in School of Mechanical Engineering at Georgia Institute of Technology. Before joining Georgia Tech, he was one of the founders of NanoTech Inc., where he was developing nano indentation devices for nondestructive evaluation (NDE) of small scale material specimens. He has been working in the area of nondestructive evaluation (NDE) for past thirty years, participating in a number of important projects on NDE, health monitoring, and maintenance of aging aircraft and infrastructures which were supported by US government agencies. He has developed a few working NDE techniques that are used in the field and laboratory. He published more than 200 articles in journals and conference proceedings in the various areas including acoustics, wave scattering, interface mechanics, functional materials, granular materials, and scientific instrumentation. A few of his articles have been highly cited by other authors. The major thrust of his current research is on nonlinear ultrasound, nonlinear vibration, and nonlinear wave propagation for characterizing damage in complex material systems.

GWW School of Mechanical Engineering & School of Civil and Environmental Engineering,
Georgia Institute of Technology, USA. E-mail: jinyeon.kim@me.gatech.edu

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