This article describes a new system for high-speed and noncontact rail integrity evaluation being developed at the University of California at San Diego. A prototype using an ultrasonic air-coupled guided wave signal generation and aircoupled signal detection has been tested at the University of California at San Diego Rail Defect Farm. In addition to a real-time statistical analysis algorithm, the prototype uses a specialized filtering approach due to the inherently poor signal-to-noise ratio of the air-coupled ultrasonic measurements in rail steel. The laboratory results indicate that the prototype is able to detect internal rail defects with a high reliability. Extensions of the system are planned to add rail surface characterization to the internal rail defect detection. In addition to the description of the prototype and test results, numerical analyses of ultrasonic guided wave propagation in rails have been performed using a Local Interaction Simulation Approach algorithm and some of these results are shown. The numerical analysis has helped designing various aspects of the prototype for maximizing its sensitivity to defects.
Temperature has a significant effect on Lamb wave propagation. It is important to compensate for this effect when the method is considered for structural damage detection. The paper explores a newly proposed, very efficient numerical simulation tool for Lamb wave propagation modelling in aluminum plates exposed to temperature changes. A local interaction approach implemented with a parallel computing architecture and graphics cards is used for these numerical simulations. The numerical results are compared with the experimental data. The results demonstrate that the proposed approach could be used efficiently to produce a large database required for the development of various temperature compensation procedures in structural health monitoring applications.
The paper presents a new damage-detection method based on nonlinear crack-wave interaction. Low-frequency vibration excitation is introduced to perturb damage, and high-frequency interrogating wave is used to detect damage-related nonlinearities. However, in contrast to other crack-wave interaction approaches, localised wave packets are used for high-frequency excitation. The synchronisation of the low-frequency vibration with the interrogating high-frequency wave packets is a key element of the proposed method. Numerical simulations and simple experimental tests in cracked aluminium beams are performed to demonstrate the method. The results show that the proposed method can detect and localise damage-related and intrinsic nonlinearities, allowing for reliable damage detection. The method does not require baseline measurements representing an undamaged condition, and it is not sensitive to temperature variations.
Ultrasound shear wave elastography is an imaging modality for noninvasive evaluation of tissue mechanical properties. However, many current techniques overestimate a lesion's dimension or shape especially when small inclusions are taken into account. In this study we propose a new method called local phase velocity based imaging (LPVI) as an alternative technique to measure tissue elasticity. Two separate acquisitions with ultrasound push beams focused once on the left side and once on the right side of the inclusion were generated. A local shear wave velocity is then recovered in the frequency domain (for a single frequency or frequency band) for both acquired data sets. Finally, a two-dimensional shear wave velocity map is reconstructed by combining maps from two separate acquisitions. Robust and accurate shear wave velocity maps are reconstructed using the proposed LPVI method in calibrated liver fibrosis tissue mimicking homogeneous phantoms, a calibrated elastography phantom with stepped cylinder inclusions and a homemade gelatin phantom with ex vivo porcine liver inclusion. Results are compared with an existing phase velocity based imaging approach and a group velocity based method considered as the state-ofthe-art. Results from the phantom study showed that increased frequency improved the shape of the reconstructed inclusions and contrast-to-noise ratio between the target and background.
Capillary waves are associated with fluid mechanical properties. Optical coherence tomography (OCT) has previously been used to determine the viscoelasticity of soft tissues or cornea. Here we report that OCT was able to evaluate phase velocities of capillary waves in fluids. The capillary waves of water, porcine whole blood and plasma on the interfacial surface, air-fluid in this case, are discussed in theory, and phase velocities of capillary waves were estimated by both our OCT experiments and theoretical calculations. Our experiments revealed highly comparable results with theoretical calculations. We concluded that OCT would be a promising tool to evaluate phase velocities of capillary waves in fluids. The methods described in this study could be applied to determine surface tensions and viscosities of fluids for differentiating hematological diseases in the future potential biological applications.
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