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Structural Boundaries (structural + boundary)

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Engineering80%

Selected Abstracts

Lamb Wave Interactions with Non-symmetric Features at Structural Boundaries

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2008
M. R. Mofakhami
The paper initially describes on a numerical basis how a Lamb wave would have to perform that has been initiated in a pure mode (either symmetric or anti,symmetric) and what the wave would have to anticipate in terms of mode conversion when being reflected at a surface not perpendicular to its traveling direction. The effects of changing in geometric specifications of non,symmetric artificial features like angle of sloping edge or partially sloping edges are studied. The results obtained from these studies are presented as the reflected and converted parts of the incident wave versus angle of the edge or percentage of the sloped edge. It has been further shown that Lamb waves being generated experimentally by a finite size transducer into a plate like structure thus most likely result in a combination of modes. Reflection of these combined modes at structural boundaries will therefore generate an even more complex coupling of modes. This situation is further aggravated if the structural boundary is not purely perpendicular to the traveling wave but has a slightly varying angle such as it might have to be anticipated at a countersunk rivet, a notch or even more extreme a crack in a metallic component. However from understanding the background of Lamb wave generation, mode separation and superposition, a systematic approach can be established that allows complex Lamb waves, such as they are observed when monitoring true structures, to be interpreted and understood. This approach has been explained on the basis of numerical result obtained from finite element analyses first before proving the findings by some fundamental experiments performed with variable angle beam transducers which demonstrates the difficulties in de,coupling Lamb wave modes and how to handle those coupled modes in terms of structural condition monitoring. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

A method for representing boundaries in discrete element modelling,part I: Geometry and contact detection

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 12 2001
M. Kremmer
Abstract The discrete element method for analysis of the dynamic behaviour of discontinuous media is well established. However, its application to engineering problems is still limited to simplified representations of structural boundaries and their kinematics. In this paper a method is developed for representing three-dimensional boundaries of arbitrary geometry and for modelling the interaction between boundary objects and particles within the discrete element modelling framework. The approach, which we term the finite wall method, uses planar triangular elements to approximate the boundary surface topology. Any number of wall elements can be used to model the shape of the structure. A contact detection scheme is presented for boundary surfaces and spheres based on a series of vector projections to reduce the problem dimensionally. The algorithm employs spatial sporting to obtain the set of potential contacts between spheres and wall elements prior to contact resolution. In a further stage, all possible contact conditions including contact with surfaces, edges and corners are explicitly determined. Part I of this two-part series of papers describes the finite wall method for representation of surface geometry and fully elaborates the method for detecting and resolving contact between boundary wall elements and spheres. In Part II the finite wall method is extended to apply kinematics to linearly independent boundary objects using combinations of translational and rotational motion. An approach is developed for coupling the DEM with the FEM for the purpose of optimising the design of structures which are dynamically interacting with particulate media. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Lamb Wave Interactions with Non-symmetric Features at Structural Boundaries

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2008
M. R. Mofakhami
The paper initially describes on a numerical basis how a Lamb wave would have to perform that has been initiated in a pure mode (either symmetric or anti,symmetric) and what the wave would have to anticipate in terms of mode conversion when being reflected at a surface not perpendicular to its traveling direction. The effects of changing in geometric specifications of non,symmetric artificial features like angle of sloping edge or partially sloping edges are studied. The results obtained from these studies are presented as the reflected and converted parts of the incident wave versus angle of the edge or percentage of the sloped edge. It has been further shown that Lamb waves being generated experimentally by a finite size transducer into a plate like structure thus most likely result in a combination of modes. Reflection of these combined modes at structural boundaries will therefore generate an even more complex coupling of modes. This situation is further aggravated if the structural boundary is not purely perpendicular to the traveling wave but has a slightly varying angle such as it might have to be anticipated at a countersunk rivet, a notch or even more extreme a crack in a metallic component. However from understanding the background of Lamb wave generation, mode separation and superposition, a systematic approach can be established that allows complex Lamb waves, such as they are observed when monitoring true structures, to be interpreted and understood. This approach has been explained on the basis of numerical result obtained from finite element analyses first before proving the findings by some fundamental experiments performed with variable angle beam transducers which demonstrates the difficulties in de,coupling Lamb wave modes and how to handle those coupled modes in terms of structural condition monitoring. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Regional teleseismic tomography of the western Lachlan Orogen and the Newer Volcanic Province, southeast Australia

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2002
Frank M. Graeber
Summary From 1998 May to September a portable array of 40 short-period digital seismograph stations was operated in western Victoria, southeast Australia, across the western end of the mid-Paleozoic Lachlan Foldbelt and the Newer Volcanic Province. Consisting of four parallel, almost W,E-oriented receiver lines, the array covered an area of about 270 × 150 km2. The major aim of the LF98 (Lachlan Foldbelt survey 1998) project is to map lateral variations in P -wave speeds (Vp) in the crust and upper mantle using teleseismic arrival time tomography, primarily in order to investigate whether the major surface structural zones are associated with seismic velocity signatures at depth. Little a priori information from seismic profiling is available. We invert 4067 relative arrival time residuals for a minimum structure Vp model in the upper few hundred km using non-linear iteration and 3-D ray tracing. The most prominent negative anomaly (,3.8 per cent) in Vp is found at a depth of about 45 km underneath the eastern part of the Newer Volcanic Province. It correlates spatially with the highest density of Pliocene and Pleistocene eruption centres northwest of Melbourne, and is therefore interpreted as a hotspot-related high-temperature anomaly causing reduced mantle velocities. The related coherent volume of significantly lower than average velocities extends down to depths greater than 100 km in the east, and extends west underneath the Newer Volcanic Province. A strong velocity contrast, with average velocities ,2 per cent greater in the west, is found down to about 100 km across the Moyston Fault Zone, which forms the major structural boundary between the early-Paleozoic Delamerian Orogen in the west and the Lachlan Orogen in the east. This result suggests that the Moyston Fault Zone should be seen as a major lithospheric boundary. In the south this boundary is also expressed by a distinct discontinuity in Sr-isotopic ratios of xenoliths (the so-called Mortlake discontinuity) and a change in the geochemistry of plutons of similar age. However, if the east to west velocity contrast originally existed in this southern zone, it is now overprinted by the thermally reduced mantle velocities beneath the Newer Volcanic Province. [source]

Lamb Wave Interactions with Non-symmetric Features at Structural Boundaries

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2008
M. R. Mofakhami
The paper initially describes on a numerical basis how a Lamb wave would have to perform that has been initiated in a pure mode (either symmetric or anti,symmetric) and what the wave would have to anticipate in terms of mode conversion when being reflected at a surface not perpendicular to its traveling direction. The effects of changing in geometric specifications of non,symmetric artificial features like angle of sloping edge or partially sloping edges are studied. The results obtained from these studies are presented as the reflected and converted parts of the incident wave versus angle of the edge or percentage of the sloped edge. It has been further shown that Lamb waves being generated experimentally by a finite size transducer into a plate like structure thus most likely result in a combination of modes. Reflection of these combined modes at structural boundaries will therefore generate an even more complex coupling of modes. This situation is further aggravated if the structural boundary is not purely perpendicular to the traveling wave but has a slightly varying angle such as it might have to be anticipated at a countersunk rivet, a notch or even more extreme a crack in a metallic component. However from understanding the background of Lamb wave generation, mode separation and superposition, a systematic approach can be established that allows complex Lamb waves, such as they are observed when monitoring true structures, to be interpreted and understood. This approach has been explained on the basis of numerical result obtained from finite element analyses first before proving the findings by some fundamental experiments performed with variable angle beam transducers which demonstrates the difficulties in de,coupling Lamb wave modes and how to handle those coupled modes in terms of structural condition monitoring. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]