THESIS
2005
xviii, 123 leaves : ill. ; 30 cm
Abstract
This thesis revisits the signals transmitted in the bender element test and the associated transfer functions, which is aimed to identify the central problems that bias the velocity (travel time) determination. Based on the findings in the numerical and physical experiments, it can be concluded that errors appear in the velocity estimation can be traced to (i) the near-field effect, and (ii) the influence caused by the transfer function of receiver system H
R. The second factor also creates distinguished changes between the source and receiver signals. For a two-receiver measurement system, the accurate travel time (velocity) can be identified by the cross-correlation of two receiver signals only if the near-field is not pronounced and the two receivers possess very similar transfer func...[
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This thesis revisits the signals transmitted in the bender element test and the associated transfer functions, which is aimed to identify the central problems that bias the velocity (travel time) determination. Based on the findings in the numerical and physical experiments, it can be concluded that errors appear in the velocity estimation can be traced to (i) the near-field effect, and (ii) the influence caused by the transfer function of receiver system H
R. The second factor also creates distinguished changes between the source and receiver signals. For a two-receiver measurement system, the accurate travel time (velocity) can be identified by the cross-correlation of two receiver signals only if the near-field is not pronounced and the two receivers possess very similar transfer functions H
R. The near-field effect can be avoided by placing the sensors with the regulation, r
1/λ≥2 and r
1/r
2≥2, where r
1 and r
2 are the distance between the source and the first and second receivers, and λ is the wavelength. For a one-receiver measurement system, the inherent time-delay caused by H
R has to be quantified and corrected. The near-field effect which leads to underestimate the velocity, still, only can be avoided by placing the receiver at least 2λ away from the source.
In the second part of this thesis, bender elements are used as sensors for the damping ratio measurement by the spectral ratio method. The numerical and physical experiments indicate the adequate precision can be achieved by minimizing the two types of inherent biases coming from (i) the near-field effect, and (ii) the different transfer functions of the two receiver bender elements. The first factor can be avoided by setting sensors following the regulations: r
1/λ=2.0 and r
1/r
2=2.0 where r
1 and r
2 are the distance between the source and the first and second receivers, and λ is the wavelength. The second factor can be solved by modifying the original spectral ratio method integrated with the self-healing technique. The accuracy of this measurement technique is also proved to be adequate under different states of stress: results from the experiment carried out in the tailor-made true-triaxial apparatus are very similar to those obtained from the resonant column test.
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