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- 2010 (3) (entfernen)
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- Englisch (3)
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- Wave propagation (3) (entfernen)
When dealing in ultrasonic testing with inhomogeneous and anisotropic material structure such as
diverse types of components made from austenitic or nickel based cast, which are currently used
for modern power plant concepts, data interpretation is quite difficult. For better understanding of
the complex interaction between the sound field and the component under test, the mathematical
modeling of sound propagation in solids is a substantial task to increase the probability of
detection of relevant defects.
First we present a mathematical approach for modeling the three dimensional transient
particle displacement as a function of time in each point in a half space excited by an impulsive
point load at the surface. The transient ultrasonic field of a rectangular array element is calculated
with this approach by a point source synthesis. Based on this solution we model the wave
propagation of a phased array transducer by time delayed superposition of the wave field of the
transducer elements.
Next we use an electrodynamic technique to visualize the grazing sound field at the surface
of a test block radiated by a phased array probe. By detecting the grazing beam at the samples
surface with a small electrodynamic probe, we measured the particle displacement as a function of
time. It allows for measuring the displacement in all three spatial directions. This comprises the
detection of the horizontal and vertical particle displacement with respect to the surface and thus
also the detection of longitudinal and shear waves is possible.
The calculated and measured wave fields will be compared for different delay laws in
isotropic and transversely isotropic media. The results support the theoretical activities to model
the wave propagation and to find optimal testing parameters for different components and
configurations.
Lamb waves travel in plates and hollow cylinders over large distances and propagate with multiple mode shapes. Therefore the waves can be used for integrity tests of large scale structures. Each propagating wave mode has a unique dispersive character and a frequency dependent vibration pattern. Therefore, by interacting with flaws each mode is assumed to generate an individual reflection pattern depending on the flaw type, flaw orientation and size. To extract this information a frequency as well as wave number dependent mode excitation is required ensuring a single mode excitation. The challenge, however, is to control the selective mode excitation electronically to allow a sequence of consecutive tests with different wave modes to collect the data. A method is proposed using phased array wedge transducers with fluid coupling for normal force excitation of Lamb wave modes. The excitation principle is investigated analytically on plates by modelling the wave generation in the wedge and the coupling between wedge and structure. The analytical results of the excitation principle are validated by corresponding experiments where the field patterns on the wedge surface and on the plate were scanned using a laser vibrometer. The directivity of the radiated field is investigated together with the direction dependent analysis of the mode content.
A method is presented which allows to calculate the wave-field in a homogeneous or layered soil in case of a dynamic interior load. The wave propagation along the surface, the distribution of the response over the depth, the horizontal propagation at different depths and the vertical downward propagation are shown and compared with the simpler surface solution of the half-space and the interior solution of the full-space. The complete wave-field (Green's function) is applied to the dynamic behaviour of piles and pile groups by use of a boundary element formulation. The stiffness, damping and – typically for piles – mass of different groups of piles are presented. Different group effects occur for lines, circles, grids, parallels and crosses of piles, which can be regarded as oscillations around average values. Moreover, the piles and pile groups behave almost like a damper for most of the frequencies. A building on a pile group that is excited by ground vibration due to surface or interior loads shows a reduction of the wave-field due to kinematic and inertial soil–building interaction effects. The results presented lead to simplified descriptions of the wave-field due to interior loads and of the soil–pile–building interaction which can be used for the prediction of technically induced vibration.