Browsing by Subject "air-coupled sensor"

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Air-Coupled Surface Wave Transmission Measurement Across A Partially Closed Surface-Breaking Crack In Concrete
(2011-06) Kee, S. H.; Zhu, J. Y.; Kee, S. H.; Zhu, J. Y.
Previous researchers have demonstrated that the transmission of surface waves is effective to estimate the depth of a surface-breaking crack in solids. However, most of the results were obtained using a well-defined crack (or notch) in laboratory. In fact, there is a critical gap to apply the theory to surface-breaking cracks in concrete structures subjected to external loadings where the cracks are generally ill-defined, and partially closed. In this study, the authors investigated transmission coefficients of surface waves across a partially closed surface-breaking crack in concrete subjected to monotonically increasing compressive loadings. First, a concrete beam (0.5 X 0.154 X 2.1 m(3)) having two surface-breaking cracks with various crack widths was prepared in laboratory. Second, transmission coefficients of impact-induced surface waves were measured across a surface-breaking crack in the concrete beam with increasing compressive loadings from 0 to 140kN (10% of the ultimate compressive strength of the concrete beam). External post-tensioning was used to apply the compression. For comparison purpose, sensitivity of surface wave velocity to compressive loading was also investigated. As a result, observations in this study reveal that transmission coefficient is a more sensitive acoustic parameter than phase velocity to evaluate a surface-breaking cracking in concrete subjected to compressive loadings.
Using Air-Coupled Sensors To Measure Depth Of A Surface-Breaking Crack In Concrete
(2009-03) Kee, S. H.; Zhu, J. Y.; Kee, S. H.; Zhu, J. Y.
Previous studies show that surface wave transmission ratio across a surface breaking crack in concrete can be used as an indicator of the crack depth. However, due to inconsistent sensor coupling condition on rough concrete surface, reliable measurement of the transmission ratio is still a challenging task. In this study, the air-coupled sensing method is proposed as a solution to this problem. Without direct contact between sensors and the testing surface, the air-coupled sensing not only allows rapid testing speed, but also enables more consistent signal measurement owing to removal of sensor coupling variation. The latter feature is especially valuable to wave transmission measurement. This paper first presents results from a numerical analysis (FEM model). Based on the results, a simplified algorithm is proposed for surface wave transmission ratio calculation. A calibration curve between the transmission ratio and normalized crack depth (actual crack depth/wavelength) is obtained. Experimental study using the air-coupled sensing method verifies the validity of the curve.