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Signatures of acoustic emission and bond breakages in cemented sands

Authors Ma, Chun Hung
Issue Date 2007
Summary Bond breakages provide all-important information for construction of a mechanistic picture of cemented sands. The purpose of this study is to characterize the bond breakages in cemented sands utilizing acoustic emission technique. In the first part of this study, bond breakages in cemented sands subject to isotropic loading, unloading, and triaxial shearing are characterized by the measurement of acoustic emission (AE) using piezoelectric AE transducers. In all these different loading stages, the AE that is initiated by bond breakages can be detected. The AE rate demonstrates a great resemblance to the associated stress-strain response. In addition, the secant Young’s moduli decrease with an increase in the corresponding cumulative AE and the decreasing trend reflects the different stress-strain responses. These observations suggest an intimate relationship between the bond breakages and the mechanical response in cemented sands. Bond breakages and the associated AE rates are mild in the beginning of shearing and then increase to initiate yielding and ultimately a local weakness, i.e., the formation of shear banding. After shear banding becomes mature, the bond breakage and the resulted AE rate render a distinct drop. If the shearing rate is slower, the test sample becomes stiffer and the measured AE is less when compared with a case subject to a higher shearing rate. Bond breakages are captured during the sampling process of cemented sands, i.e., under an unloading-reloading cycle, and the major portion of the bond-breakage events occur during reloading. These bond breakages eventually weaken the sample and lead to lower stiffness. In the second part of this study, a MEMS accelerometer, MEMSA (1 kHz – 3 kHz), and a commercially available piezoelectric AE transducer, PZT (125 kHz – 750 kHz), are used to capture the AE responses in uncemented and cemented sands during triaxial compression tests, with particular emphasis on the former one. The AE rates measured in the cemented sands by both sensors demonstrate a similar trend and show a great resemblance to the stress-strain response. These observations suggest that the MEMS accelerometer can function as an effective AE sensor to detect the bond-breakage process in cemented sands. In addition, the PZT is more sensitive in the AE detection for cemented sands but the MEMSA can start earlier to capture AE and also continue to capture AE from the shear band at large strains, while the PZT only measures a few AE activities or null.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007
Language English
Format Thesis
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