Biomedical subjects

Tobias Leutenegger

Publications and source records attributed to Tobias Leutenegger.

3 recordsLinked to original sources

PubMed2004

Non-destructive testing of tubes using a time reverse numerical simulation (TRNS) method.

A method for the detection of defects in cylindrical structures and the determination of their positions and orientations is presented in this paper. The scattered field, which is generated by the interaction of excited guided waves with a defect, is evaluated with an approach named time reverse numerical simulation method (TRNS). Since the excited waves and the scattered field propagate along the sample, the time-consuming scanning of the whole tube can be eliminated. The scattered displacement field is measured in three dimensions over time with a laser vibrometer at different locations distributed equally around the circumference at a fixed axial coordinate far away from the defect. Instead of analyzing the complicated time signals directly, they are played back in time. If the recorded displacement histories of the scattered field are reversed in time and played back in an identical structure, the waves travel back the same path and interfere to a maximum at their origin. The result is an amplitude increase at the position of the defect where the scattered field was generated. Instead of playing back the recorded time signals in an experiment, this step is replaced by a numerical simulation. Only this enables the visualization and detection of the amplitude increase. As long as the simulation is of high accuracy, the position of the maximum interference corresponds exactly to the location of the defect in the experiment, although no defect is implemented in the simulation.

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PubMed2004

Modeling three-dimensional elastic wave propagation in circular cylindrical structures using a finite-difference approach.

Wave propagation along circular cylindrical structures is important for nondestructive-testing applications and shocks in tubes. To simulate elastic wave propagation phenomena in such structures the governing equations in cylindrical coordinates are solved numerically. To reduce the required amount of computer memory and the computational time, the stress components are eliminated in the equilibrium equations. In the resulting coupled partial differential equations, in which only the three displacement components are involved, the derivatives with respect to spatial coordinates and time are approximated using second order central differences. This leads to the present new approach, which is both accurate and efficient. In order to obtain a stable scheme the displacements must be allocated on a staggered grid. The von Neumann stability analysis is performed and the result is compared with an existing empirical criterion. Mechanical energies are observed in order to validate the finite-difference code. Since no material damping or energy dissipation is taken into account in the equations of motion, the total energy must remain constant over time. Only negligible variations are observed during long-term simulations. Dispersion relations are used to check the physical behavior of the waves calculated with the proposed finite-difference method: Theoretically calculated curves are compared with values obtained by a spectrum estimation method, applied to the results of a simulation.

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PubMed2002

Detection of defects in cylindrical structures using a time reverse method and a finite-difference approach.

The detection and characterization of defects in structures is an important issue in non-destructive testing. To avoid the scanning of large samples, guided elastic waves, which propagate along the structure, are excited. These waves interact with a defect, which results in a scattered wave field. In an experiment, the displacements of these scattered waves are recorded over time for a fixed axial coordinate at a number of circumferential positions of a circular cylindrical tube. Since in complex structures it is difficult to determine the axial and particularly circumferential position of the defect directly from the time signals, a time reversed numerical simulation is performed. There the measured displacement histories are reversed in time and used as displacement excitations in a simulation of the tested structure. A three-dimensional code in cylindrical coordinates, based on a velocity-stress finite-difference method, is used to simulate the wave propagation. As long as the geometric and material parameters are chosen equivalent to the performed experiment, the scattered waves travel back through the simulated structure and interfere, even if no defect is present in the numerical model. The result is an increase of the amplitudes of the stress and displacement components at the location where the defect was in the tested sample.

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