1,911 research outputs found
An adaptive space-time phase field formulation for dynamic fracture of brittle shells based on LR NURBS
We present an adaptive space-time phase field formulation for dynamic fracture of brittle shells. Their deformation is characterized by the KirchhoffâLove thin shell theory using a curvilinear surface description. All kinematical objects are defined on the shellâs mid-plane. The evolution equation for the phase field is determined by the minimization of an energy functional based on Griffithâs theory of brittle fracture. Membrane and bending contributions to the fracture process are modeled separately and a thickness integration is established for the latter. The coupled system consists of two nonlinear fourth-order PDEs and all quantities are defined on an evolving two-dimensional manifold. Since the weak form requires C1-continuity, isogeometric shape functions are used. The mesh is adaptively refined based on the phase field using Locally Refinable (LR) NURBS. Time is discretized based on a generalized-α method using adaptive time-stepping, and the discretized coupled system is solved with a monolithic NewtonâRaphson scheme. The interaction between surface deformation and crack evolution is demonstrated by several numerical examples showing dynamic crack propagation and branching
Subdivision Shell Elements with Anisotropic Growth
A thin shell finite element approach based on Loop's subdivision surfaces is
proposed, capable of dealing with large deformations and anisotropic growth. To
this end, the Kirchhoff-Love theory of thin shells is derived and extended to
allow for arbitrary in-plane growth. The simplicity and computational
efficiency of the subdivision thin shell elements is outstanding, which is
demonstrated on a few standard loading benchmarks. With this powerful tool at
hand, we demonstrate the broad range of possible applications by numerical
solution of several growth scenarios, ranging from the uniform growth of a
sphere, to boundary instabilities induced by large anisotropic growth. Finally,
it is shown that the problem of a slowly and uniformly growing sheet confined
in a fixed hollow sphere is equivalent to the inverse process where a sheet of
fixed size is slowly crumpled in a shrinking hollow sphere in the frictionless,
quasi-static, elastic limit.Comment: 20 pages, 12 figures, 1 tabl
Analysis of Thin-Walled Beam and Flexible Plate Structures through the Unified Formulation
L'abstract Ăš presente nell'allegato / the abstract is in the attachmen
Nonlinear and Linearized Analysis of Vibrations of Loaded Anisotropic Beam/Plate/Shell Structures
L'abstract Ăš presente nell'allegato / the abstract is in the attachmen
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Developing a QFD-Based design-integrated structural analysis methodology
This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Design of the mechanical components greatly depends on their expected structural
performances. In modern design applications these performances are quantified by
computer-based analysis and occasionally confirmed by experimental measurements or
theoretical calculations. The dependency of the mechanical product to the structural
analysis process is more significant under the productâs multi-functionality aspect that
requires analyses for a variety of Variable Input Parameters, to obtain various structural responses and against more than one failure or design criterion. Structural analysis is known as the expert field, which requires an upfront investment and facilitation to be implemented in commercial design environment. On the other hand, the product design process is a systematic and sequential activity that put the designer in the central role of decision making. Lack of mutual understanding between these two disciplines reduces the efficiency of the structural analysis for design. This research aims to develop an integrated methodology to embed the structural analysis in the design process. The proposed methodology in this research combines the
benefits of state-of-the-art approaches, early simulation and Validation and Verification practice, towards the specified aim. Moreover the novelty of the proposed methodology is in creative implication of Quality Function Deployment method to include the productâs multi-functionality aspect. The QFD-Based Design Integrated Structural Analysis methodology produces a reliable platform to increase the efficiency of the structural analysis process for product design purpose. The application of this methodology is examined through an industrial case-study for the telescopic cantilever boom, as it appears in Access platforms, and Cranes products. Findings of the case-study create a reliable account for the structural performance in early stages of the design, and ensure the functionality of the proposed methodology.This research programme was funded by KTP organisation
Meshless Mechanics and Point-Based Visualization Methods for Surgical Simulations
Computer-based modeling and simulation practices have become an integral part of the medical education field. For surgical simulation applications, realistic constitutive modeling of soft tissue is considered to be one of the most challenging aspects of the problem, because biomechanical soft-tissue models need to reflect the correct elastic response, have to be efficient in order to run at interactive simulation rates, and be able to support operations such as cuts and sutures.
Mesh-based solutions, where the connections between the individual degrees of freedom (DoF) are defined explicitly, have been the traditional choice to approach these problems. However, when the problem under investigation contains a discontinuity that disrupts the connectivity between the DoFs, the underlying mesh structure has to be reconfigured in order to handle the newly introduced discontinuity correctly. This reconfiguration for mesh-based techniques is typically called dynamic remeshing, and most of the time it causes the performance bottleneck in the simulation.
In this dissertation, the efficiency of point-based meshless methods is investigated for both constitutive modeling of elastic soft tissues and visualization of simulation objects, where arbitrary discontinuities/cuts are applied to the objects in the context of surgical simulation. The point-based deformable object modeling problem is examined in three functional aspects: modeling continuous elastic deformations with, handling discontinuities in, and visualizing a point-based object. Algorithmic and implementation details of the presented techniques are discussed in the dissertation. The presented point-based techniques are implemented as separate components and integrated into the open-source software framework SOFA.
The presented meshless continuum mechanics model of elastic tissue were verified by comparing it to the Hertzian non-adhesive frictionless contact theory. Virtual experiments were setup with a point-based deformable block and a rigid indenter, and force-displacement curves obtained from the virtual experiments were compared to the theoretical solutions.
The meshless mechanics model of soft tissue and the integrated novel discontinuity treatment technique discussed in this dissertation allows handling cuts of arbitrary shape. The implemented enrichment technique not only modifies the internal mechanics of the soft tissue model, but also updates the point-based visual representation in an efficient way preventing the use of costly dynamic remeshing operations
Virtual product development and testing for aerospace tube hydroforming industry : improved non-linear solid-shell element
Dans les recherches rĂ©alisĂ©es pour ce projet de thĂšse, il est dĂ©montrĂ© quâune traverse existante de train dâatterrissage dâhĂ©licoptĂšre Ă patins fabriquĂ©e par pliage et Ă©rosion chimique, pourrait ĂȘtre remplacĂ©e par une autre traverse, dont la forme innovante est fabricable par le procĂ©dĂ© dâhydroformage de tubes. Ce procĂ©dĂ© prĂ©sente par exemple lâavantage dâĂȘtre plus respectueux de lâenvironnement que le procĂ©dĂ© de fabrication actuel, car il ne nĂ©cessite pas lâutilisation de produits chimiques polluant. De plus, la mĂ©thodologie dĂ©veloppĂ©e dans le cadre des recherches rĂ©alisĂ©es permet de prendre en compte lâhistoire du matĂ©riau de la traverse dans toutes les Ă©tapes de son processus de fabrication. Les performances dâun train dâatterrissage Ă©quipĂ© de la nouvelle traverse ont Ă©tĂ© Ă©valuĂ©es numĂ©riquement. Des travaux, dĂ©veloppĂ©s avec le logiciel de calculs par Ă©lĂ©ments finis ABAQUS, ont permis de mettre en Ă©vidence lâintĂ©rĂȘt dâutiliser des Ă©lĂ©ments finis de coque solides fiables et prĂ©cis. Ces Ă©lĂ©ments sont en effet capables de prendre en compte le comportement dans lâĂ©paisseur de structures minces avec une seule couche dâĂ©lĂ©ments. Une nouvelle technique de lissage appelĂ© «Smoothed finite element method» ou «SFEM» a retenu lâattention pour sa simplicitĂ© de mise en Ćuvre et son insensibilitĂ© Ă la distorsion de maillage parfois rencontrĂ©e dans les simulations de formage de formes complexes. Un Ă©lĂ©ment de coque solide rĂ©sultant linĂ©aire dĂ©veloppĂ© en utilisant cette mĂ©thode SFEM pour traiter de la cinĂ©matique en membrane et en flexion a Ă©tĂ© testĂ© avec succĂšs au travers dâexemples classiques identifiĂ©s dans la littĂ©rature. Ce nouvel Ă©lĂ©ment a montrĂ© un niveau de prĂ©cision souvent supĂ©rieur Ă celui dâautres Ă©lĂ©ments dĂ©jĂ existants. En outre, un Ă©lĂ©ment de coque solide Ă intĂ©gration rĂ©duite, capable de fonctionner avec la plupart des lois de comportement en trois dimensions et cela mĂȘme en prĂ©sence de structures minces a Ă©tĂ© dĂ©veloppĂ©. Cet Ă©lĂ©ment, libre de tout blocage a montrĂ© un bon niveau de prĂ©cision par rapport aux Ă©lĂ©ments existants dans le cas de problĂšmes implicites gĂ©omĂ©triquement linĂ©aires et non-linĂ©aires. LâĂ©lĂ©ment a Ă©tĂ© Ă©tendu en formulation explicite puis couplĂ© avec une loi de comportement hyper Ă©lastoplastique en trois dimensions. Il a enfin Ă©tĂ© testĂ© dans une simulation dâhydroformage de tubes en prĂ©sence de pressions Ă©levĂ©es, de frottement et de grandes dĂ©formations.In the current work, it is shown that an existing helicopter skid landing gear cross tube, made by tube bending and chemical milling, could be replaced by another cross tube, whose innovative shape is producible by tube hydroforming. This method has for example the advantage of being more environmentally friendly than the current manufacturing process, because it does not require the use of hazardous chemicals. In addition, the methodology developed in this project takes into account the cross tube materialâs history throughout the manufacturing process. Moreover, the performance of a skid landing gear equipped with this new cross tube has been evaluated numerically. This thesis simulation work has been developed with the finite element analysis software ABAQUS. It highlights the potential gains of using a reliable and accurate solid-shell finite element which is capable to take into account the through-thickness behavior of thin structures with a single layer of elements. A new smoothing technique called «Smoothed finite element method» or «SFEM» has been considered for its simplicity and insensitivity to mesh distortion, sometimes encountered while simulating complex shapes forming. A new resultant linear solid-shell element using this SFEM to deal with membrane and bending kinematics has been developed and successfully tested through classical benchmark problems found in the literature. This new element has often shown much greater level of accuracy than other existing elements. In addition, a novel reduced integration solid-shell element, able to work with most three dimensions constitutive laws even in the presence of thin structures is also discussed. This element, free of locking, shows a good accuracy level with respect to existing elements in implicit geometrically linear and non-linear benchmark problems. Its extension to explicit formulation is coupled with a three dimensions hyper elastoplastic constitutive law and tested in a tube hydroforming simulation involving high pressures, friction and large deformations
Stresses In The End Zones Of Precast Inverted T-Beams With Tapered Webs
Short to medium span composite bridges constructed with adjacent precast inverted T-beams and cast-in-place topping are intended to provide more resiliency against reflective cracking and time dependent effects compared to voided slabs and adjacent box girder systems. This thesis investigates the stresses in the end zones of such a uniquely shaped precast element. The transfer of prestressing force creates vertical and horizontal tensile stresses in the end zones of the girder. A series of 3-D finite element analyses were performed to investigate the magnitude of these tensile stresses. Hoyer effect is captured by modelling the strands as solid elements and defining the interaction between strands and concrete in the tangential and normal behavior using friction coefficient and hard contact, respectively. The modelling protocol captures spalling, splitting, and bursting stresses. It was found that, stresses in the end zones of precast inverted T-beams with tapered webs are not likely to cause any significant cracking if the beam is reinforced based on AASHTOâs provisions for pretensioned anchorage zones. Various modeling techniques were evaluated, and it was found that linear elastic models with truss elements are adequate for design purposes in terms of mapping where the end zone reinforcing needs to be located. However, if such modeling capabilities are not available AASHTOâs provisions suffice in terms of reinforcing the critical areas in the end zones
Linear and non-linear dynamic analyses of sandwich panels with face sheet-tocore debonding
Đ survey of recent developments in the dynamic analysis of sandwich panels with face sheet-to-core
debonding is presented. The finite element method within the ABAQUSTM code is utilized. The emphasis
is directed to the procedures used to elaborate linear and non-linear models and to predict dynamic response
of the sandwich panels. Recently developed models are presented, which can be applied for structural
health monitoring algorithms of real-scale sandwich panels. First, various popular theories of intact
sandwich panels are briefly mentioned and a model is proposed to effectively analyse the modal dynamics
of debonded and damaged (due to impact) sandwich panels. The influence of debonding size, form and
location, and number of such damage on the modal characteristics of sandwich panels are shown. For
nonlinear analysis, models based on implicit and explicit time integration schemes are presented and dynamic
response gained with those models are discussed. Finally, questions related to debonding progression
at the face sheet-core interface when dynamic loading continues with time are briefly highlighted
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