Diffuse Ultrasonic Concrete Crack Depth Measurement Device

时    间：3月22日 14:00  

地    点：交通科学与工程学院404会议室

主持人：陈军副教授  

报告人：Dr. Jin-Yeon Kim  

报告一: Diffuse Ultrasonic Concrete Crack Depth Measurement Device

摘    要:

Surfce breaking cracks in reinforced  concrete (RC) structures can be critical to the long term reliability of these structures. Especially when the crack tip reaches the rebars water penetration will cause corrosion of rebars, which will then lead to multiple cracks, spalling, and finally significant reduction in the strength of the RC structures. This talk will first review the existing NDE techniques to measure the depth of surface breaking cracks in concrete. Then will introduce a new more reliable technique based on the recently developed diffuse ultrasound concept. The diffuse ultrasound technique is validated on small laboratory specimens and full scale RC beams.

报告二: Relation between crack density and acoustic nonlinearity in thermally damaged sandstone

摘   要：

Numerous investigations have demonstrated that microcracks or lightly compressed interfaces in solids can generate strong higher harmonic signals in a propagating harmonic wave. A simple deduction would postulate that distributed cracks in rock will generate such nonlinearity and thus measurement of the nonlinearity can be a useful experimental means to quantify microcracking damage. In fact, there are papers that show increasing nonlinearity with the progress of microcracking damage in different rocks and concrete. 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 then finds 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.