Filtern
Volltext vorhanden
- ja (56) (entfernen)
Dokumenttyp
- Dissertation (56) (entfernen)
Institut
- Institut für Strukturmechanik (ISM) (56) (entfernen)
Schlagworte
- Finite-Elemente-Methode (12)
- Isogeometric Analysis (4)
- Optimierung (4)
- Peridynamik (4)
- finite element method (4)
- Beton (3)
- Isogeometrische Analyse (3)
- Mehrskalenmodell (3)
- Modellierung (3)
- Peridynamics (3)
- Phasenfeldmodell (3)
- Polymere (3)
- Strukturmechanik (3)
- Batterie (2)
- Bruch (2)
- FEM (2)
- Fracture (2)
- Fracture mechanics (2)
- Mehrgitterverfahren (2)
- Mehrskalenanalyse (2)
- NURBS (2)
- Nanoverbundstruktur (2)
- Neuronales Netz (2)
- Nichtlineare Finite-Elemente-Methode (2)
- Optimization (2)
- Partielle Differentialgleichung (2)
- Phase-field modeling (2)
- Simulation (2)
- Staumauer (2)
- Strukturdynamik (2)
- Uncertainty (2)
- Unsicherheit (2)
- Variational principle (2)
- XFEM (2)
- continuum mechanics (2)
- crack (2)
- multiphase (2)
- multiscale (2)
- nanocomposite (2)
- stochastic (2)
- 2D/3D Adaptive Mesh Refinement (1)
- Abaqus (1)
- Adaptive central high resolution schemes (1)
- Adaptives System (1)
- Adaptives Verfahren (1)
- Aerodynamic Stability (1)
- Akkumulator (1)
- Auswirkung (1)
- Autogenous (1)
- Autonomous (1)
- B-Spline (1)
- B-Spline Finite Elemente (1)
- B-spline (1)
- Battery (1)
- Battery development (1)
- Bayes (1)
- Bayes neuronale Netze (1)
- Bayesian Inference, Uncertainty Quantification (1)
- Bayesian method (1)
- Bayesian neural networks (1)
- Bayes’schen Inferenz (1)
- Berechnung (1)
- Beschädigung (1)
- Biomechanics (1)
- Biomechanik (1)
- Bridges (1)
- Bruchverhalten (1)
- Brustkorb (1)
- Brücke (1)
- Brückenbau (1)
- Capsular clustering; Design of microcapsules (1)
- Carbon nanotubes (1)
- Cohesive surface technique (1)
- Computational fracture modeling (1)
- Computermodellierung des Bruchverhaltens (1)
- Computersimulation (1)
- Concrete (1)
- Concrete catenary pole (1)
- Continuous-Time Markov Chain (1)
- Continuum Mechnics (1)
- Control system (1)
- Cost-Benefit Analysis (1)
- Damage (1)
- Damage Identification (1)
- Damage identification (1)
- Dams (1)
- Data-driven (1)
- Deal ii C++ code (1)
- Defekt (1)
- Diskontinuumsmechanik (1)
- Diskrete-Elemente-Methode (1)
- Dissertation (1)
- Dynamik (1)
- Electrochemical properties (1)
- Elektrochemische Eigenschaft (1)
- Elektrode (1)
- Elektrodenmaterial (1)
- Energiespeichersystem (1)
- Entwurf von Mikrokapseln (1)
- Erdbeben (1)
- Erdbebensicherheit (1)
- Erweiterte Finite-Elemente-Methode (1)
- Festkörpermechanik (1)
- Fiber Reinforced Composite (1)
- Finite Element Method (1)
- Finite Element Model (1)
- Flattern (1)
- Flexoelectricity (1)
- Fluid-Structure Interaction (1)
- Flutter (1)
- Fracture Computational Model (1)
- Full waveform inversion (1)
- Fuzzy logic (1)
- Fuzzy-Logik (1)
- Gasleitung (1)
- Geometric Modeling (1)
- Geometric Partial Differential Equations (1)
- Geometry Independent Field Approximation (1)
- Gewebeverbundwerkstoff (1)
- Goal-oriented A Posteriori Error Estimation (1)
- Grauguss (1)
- HPC (1)
- Healing (1)
- High-speed railway bridge (1)
- Homogenisieren (1)
- Homogenisierung (1)
- Hydrodynamik (1)
- Hyperbolic PDEs (1)
- Impact (1)
- Incompressibility (1)
- Instandhaltung (1)
- Inverse Probleme (1)
- Inverse Problems (1)
- Inverse analysis (1)
- Inverse problems (1)
- Isogeometrc Analysis (1)
- Kapselclustern (1)
- Keramik (1)
- Kirchoff--love theory (1)
- Klüftung (1)
- Kohlenstoff Nanoröhre (1)
- Kohäsionsflächenverfahren (1)
- Konjugierte-Gradienten-Methode (1)
- Kontinuierliche Simul (1)
- Kontinuumsmechanik (1)
- Kosten-Nutzen-Analyse (1)
- Lebensdauerabschätzung (1)
- Local maximum entropy approximants (1)
- Lösungsverfahren (1)
- Markov-Kette mit stetiger Zeit (1)
- Maschinenbau (1)
- Mass Tuned Damper (1)
- Material (1)
- Materialversagen (1)
- Mechanical properties (1)
- Mechanik (1)
- Mechanische Eigenschaft (1)
- Mesh Refinement (1)
- Meso-Scale (1)
- Mikro-Scale (1)
- Mikrokapsel (1)
- Model assessment (1)
- Modellbildung (1)
- Modellkalibrierung (1)
- Modezuordung (1)
- Molecular Dynamics Simulation (1)
- Molekulardynamik (1)
- Monte-Carlo-Integration (1)
- Monte-Carlo-Simulation (1)
- Multi-scale modeling (1)
- Multiphysics (1)
- Multiscale modeling (1)
- Muscle model (1)
- Muskel (1)
- Nanomaterial (1)
- Nanomechanical Resonators (1)
- Nanomechanik (1)
- Nanostructures (1)
- Nanostrukturiertes Material (1)
- Nichtlokale Operatormethode (1)
- Numerical Simulation (1)
- Numerical Simulations (1)
- Numerische Berechnung (1)
- Numerische Mathematik (1)
- Oberleitungsmasten (1)
- Operante Konditionierung (1)
- Optimization problems (1)
- PDEs (1)
- PU Enrichment method (1)
- Parameteridentification (1)
- Parameteridentifikation (1)
- Partial Differential Equations (1)
- Passive damper (1)
- Phase field method (1)
- Phase field model (1)
- Phase-field model (1)
- Physics informed neural network (1)
- Piezoelectricity (1)
- Polykristall (1)
- Polymer compound (1)
- Polymer nanocomposites (1)
- Polymers (1)
- Polymerverbindung (1)
- Polynomial Splines over Hierarchical T-meshes (1)
- RC Buildings (1)
- Railway bridges (1)
- Rapid Visual Assessment (1)
- Recovery Based Error Estimator (1)
- Referenzfläche (1)
- Rehabilitation (1)
- Reliability Theory (1)
- Residual-based variational multiscale method (1)
- Resonator (1)
- Riss (1)
- Rissausbreitung (1)
- SHM (1)
- Schaden (1)
- Schadenerkennung (1)
- Schadensdetektionsverfahren (1)
- Schadensmechanik (1)
- Schwingung (1)
- Schwingungsanalyse (1)
- Schädigung (1)
- Schätztheorie (1)
- Seismic Vulnerability (1)
- Selbstheilendem Beton (1)
- Selbstheilung (1)
- Self-healing concrete (1)
- Semi-active damper (1)
- Sensitivity (1)
- Sensitivitätsanalyse (1)
- Simulationsprozess (1)
- Sprödbruch (1)
- Stabilität (1)
- Standsicherheit (1)
- Staudamm (1)
- Strömungsmechanik (1)
- Super Healing (1)
- Surface effects (1)
- System Identification (1)
- Talsperre (1)
- Taylor Series Expansion (1)
- Thermal Fluid-Structure Interaction (1)
- Thermoelastic damping (1)
- Thermoelasticity (1)
- Thermoelastizität (1)
- Thin shell (1)
- Thorax (1)
- Tichonov-Regularisierung (1)
- Tikhonov regularization (1)
- Uncertainty analysis (1)
- Unschärfequantifizierung (1)
- Variationsprinzip (1)
- Verbundwerkstoff (1)
- Vesicle dynamics (1)
- Vesikel (1)
- Vortex Induced Vibration (1)
- Wasserbau (1)
- Wave propagation (1)
- Wavelet (1)
- Wavelet based adaptation (1)
- Wechselwirkung (1)
- Werkstoffdämpfung (1)
- Werkstoffprüfung (1)
- Wärmeleitfähigkeit (1)
- Zementbeton (1)
- Zuverlässigkeitstheorie (1)
- adaptive simulation (1)
- atomistic simulation methods (1)
- beton (1)
- brittle fracture (1)
- building information modelling (1)
- ceramics (1)
- cohesive elements (1)
- concrete (1)
- conjugate gradient method (1)
- continuum damage mechanics (1)
- crack identification (1)
- damage (1)
- dams (1)
- decay experiments (1)
- deep neural network (1)
- defects (1)
- diskontinuum mechanics (1)
- dissimilarity measures (1)
- domain decomposition (1)
- earthquake (1)
- effective properties (1)
- energy dissipation (1)
- finite element (1)
- gas pipes (1)
- gradient elasticity (1)
- grid-based (1)
- heterogeneous material (1)
- high-performance computing (1)
- intergranular damage (1)
- isogeometric analysis (1)
- isogeometric methods (1)
- jointed rock (1)
- level set method (1)
- machine learning (1)
- material failure (1)
- matrix-free (1)
- maximum stress (1)
- mehrphasig (1)
- modal damping (1)
- mode pairing (1)
- model updating (1)
- mortar method (1)
- multigrid (1)
- multigrid method (1)
- multiscale method (1)
- nanosheets (1)
- numerical methods (1)
- optimal sensor positions (1)
- optimale Sensorpositionierung (1)
- optimization (1)
- parameter identification (1)
- phase field (1)
- phase field fracture method (1)
- quasicontinuum method (1)
- recovery-based and residual-based error estimators (1)
- reinforcement learning (1)
- scalable smeared crack analysis (1)
- scale transition (1)
- seismic control (1)
- self healing concrete (1)
- smoothed particle hydrodynamics (1)
- solver (1)
- structural control (1)
- thermal conductivity (1)
- tuned mass damper (1)
- weighted residual method (1)
- woven composites (1)
The phenomenon of aerodynamic instability caused by the wind is usually a major design criterion for long-span cable-supported bridges. If the wind speed exceeds the critical flutter speed of the bridge, this constitutes an Ultimate Limit State. The prediction of the flutter boundary, therefore, requires accurate and robust models. The complexity and uncertainty of models for such engineering problems demand strategies for model assessment. This study is an attempt to use the concepts of sensitivity and uncertainty analyses to assess the aeroelastic instability prediction models for long-span bridges. The state-of-the-art theory concerning the determination of the flutter stability limit is presented. Since flutter is a coupling of aerodynamic forcing with a structural dynamics problem, different types and classes of structural and aerodynamic models can be combined to study the interaction. Here, both numerical approaches and analytical models are utilised and coupled in different ways to assess the prediction quality of the coupled model.
This study permits a reliability analysis to solve the mechanical behaviour issues existing in the current structural design of fabric structures. Purely predictive material models are highly desirable to facilitate an optimized design scheme and to significantly reduce time and cost at the design stage, such as experimental characterization.
The present study examined the role of three major tasks; a) single-objective optimization, b) sensitivity analyses and c) multi-objective optimization on proposed weave structures for woven fabric composites. For single-objective optimization task, the first goal is to optimize the elastic properties of proposed complex weave structure under unit cells basis based on periodic boundary conditions.
We predict the geometric characteristics towards skewness of woven fabric composites via Evolutionary Algorithm (EA) and a parametric study. We also demonstrate the effect of complex weave structures on the fray tendency in woven fabric composites via tightness evaluation. We utilize a procedure which does not require a numerical averaging process for evaluating the elastic properties of woven fabric composites. The fray tendency and skewness of woven fabrics depends upon the behaviour of the floats which is related to the factor of weave. Results of this study may suggest a broader view for further research into the effects of complex weave structures or may provide an alternative to the fray and skewness problems of current weave structure in woven fabric composites.
A comprehensive study is developed on the complex weave structure model which adopts the dry woven fabric of the most potential pattern in singleobjective optimization incorporating the uncertainties parameters of woven fabric composites. The comprehensive study covers the regression-based and variance-based sensitivity analyses. The second task goal is to introduce the fabric uncertainties parameters and elaborate how they can be incorporated into finite element models on macroscopic material parameters such as elastic modulus and shear modulus of dry woven fabric subjected to uni-axial and biaxial deformations. Significant correlations in the study, would indicate the need for a thorough investigation of woven fabric composites under uncertainties parameters. The study describes here could serve as an alternative to identify effective material properties without prolonged time consumption and expensive experimental tests.
The last part focuses on a hierarchical stochastic multi-scale optimization approach (fine-scale and coarse-scale optimizations) under geometrical uncertainties parameters for hybrid composites considering complex weave structure. The fine-scale optimization is to determine the best lamina pattern that maximizes its macroscopic elastic properties, conducted by EA under the following uncertain mesoscopic parameters: yarn spacing, yarn height, yarn width and misalignment of yarn angle. The coarse-scale optimization has been carried out to optimize the stacking sequences of symmetric hybrid laminated composite plate with uncertain mesoscopic parameters by employing the Ant Colony Algorithm (ACO). The objective functions of the coarse-scale optimization are to minimize the cost (C) and weight (W) of the hybrid laminated composite plate considering the fundamental frequency and the buckling load factor as the design constraints.
Based on the uncertainty criteria of the design parameters, the appropriate variation required for the structural design standards can be evaluated using the reliability tool, and then an optimized design decision in consideration of cost can be subsequently determined.
Encapsulation-based self-healing concrete (SHC) is the most promising technique for providing a self-healing mechanism to concrete. This is due to its capacity to heal fractures effectively without human interventions, extending the operational life and lowering maintenance costs. The healing mechanism is created by embedding capsules containing the healing agent inside the concrete. The healing agent will be released once the capsules are fractured and the healing occurs in the vicinity of the damaged part. The healing efficiency of the SHC is still not clear and depends on several factors; in the case of microcapsules SHC the fracture of microcapsules is the most important aspect to release the healing agents and hence heal the cracks. This study contributes to verifying the healing efficiency of SHC and the fracture mechanism of the microcapsules. Extended finite element method (XFEM) is a flexible, and powerful discrete crack method that allows crack propagation without the requirement for re-meshing and has been shown high accuracy for modeling fracture in concrete. In this thesis, a computational fracture modeling approach of Encapsulation-based SHC is proposed based on the XFEM and cohesive surface technique (CS) to study the healing efficiency and the potential of fracture and debonding of the microcapsules or the solidified healing agents from the concrete matrix as well. The concrete matrix and a microcapsule shell both are modeled by the XFEM and combined together by CS. The effects of the healed-crack length, the interfacial fracture properties, and microcapsule size on the load carrying capability and fracture pattern of the SHC have been studied. The obtained results are compared to those obtained from the zero thickness cohesive element approach to demonstrate the significant accuracy and the validity of the proposed simulation. The present fracture simulation is developed to study the influence of the capsular clustering on the fracture mechanism by varying the contact surface area of the CS between the microcapsule shell and the concrete matrix. The proposed fracture simulation is expanded to 3D simulations to validate the 2D computational simulations and to estimate the accuracy difference ratio between 2D and 3D simulations. In addition, a proposed design method is developed to design the size of the microcapsules consideration of a sufficient volume of healing agent to heal the expected crack width. This method is based on the configuration of the unit cell (UC), Representative Volume Element (RVE), Periodic Boundary Conditions (PBC), and associated them to the volume fraction (Vf) and the crack width as variables. The proposed microcapsule design is verified through computational fracture simulations.
Encapsulation-based self-healing concrete has received a lot of attention nowadays in civil engineering field. These capsules are embedded in the cementitious matrix during concrete mixing. When the cracks appear, the embedded capsules which are placed along the path of incoming crack are fractured and then release of healing agents in the vicinity of damage. The materials of capsules need to be designed in a way that they should be able to break with small deformation, so the internal fluid can be released to seal the crack. This study focuses on computational modeling of fracture in encapsulation-based selfhealing concrete. The numerical model of 2D and 3D with randomly packed aggreates and capsules have been developed to analyze fracture mechanism that plays a significant role in the fracture probability of capsules and consequently the self-healing process. The capsules are assumed to be made of Poly Methyl Methacrylate (PMMA) and the potential cracks are represented by pre-inserted cohesive elements with tension and shear softening laws along the element boundaries of the mortar matrix, aggregates, capsules, and at the interfaces between these phases. The effects of volume fraction, core-wall thickness ratio, and mismatch fracture properties of capsules on the load carrying capacity of self-healing concrete and fracture probability of the capsules are investigated. The output of this study will become valuable tool to assist not only the experimentalists but also the manufacturers in designing an appropriate capsule material for self-healing concrete.
The key objective of this research is to study fracture with a meshfree method, local maximum entropy approximations, and model fracture in thin shell structures with complex geometry and topology. This topic is of high relevance for real-world applications, for example in the automotive industry and in aerospace engineering. The shell structure can be described efficiently by meshless methods which are capable of describing complex shapes as a collection of points instead of a structured mesh. In order to find the appropriate numerical method to achieve this goal, the first part of the work was development of a method based on local maximum entropy (LME)
shape functions together with enrichment functions used in partition of unity methods to discretize problems in linear elastic fracture mechanics. We obtain improved accuracy relative to the standard extended finite element method (XFEM) at a comparable computational cost. In addition, we keep the advantages of the LME shape functions,such as smoothness and non-negativity. We show numerically that optimal convergence (same as in FEM) for energy norm and stress intensity factors can be obtained through the use of geometric (fixed area) enrichment with no special treatment of the nodes
near the crack such as blending or shifting.
As extension of this method to three dimensional problems and complex thin shell structures with arbitrary crack growth is cumbersome, we developed a phase field model for fracture using LME. Phase field models provide a powerful tool to tackle moving interface problems, and have been extensively used in physics and materials science. Phase methods are gaining popularity in a wide set of applications in applied science and engineering, recently a second order phase field approximation for brittle fracture has gathered significant interest in computational fracture such that sharp cracks discontinuities are modeled by a diffusive crack. By minimizing the system energy with respect to the mechanical displacements and the phase-field, subject to an irreversibility condition to avoid crack healing, this model can describe crack nucleation, propagation, branching and merging. One of the main advantages of the phase field modeling of fractures is the unified treatment of the interfacial tracking and mechanics, which potentially leads to simple, robust, scalable computer codes applicable to complex systems. In other words, this approximation reduces considerably the implementation complexity because the numerical tracking of the fracture is not needed, at the expense of a high computational cost. We present a fourth-order phase field model for fracture based on local maximum entropy (LME) approximations. The higher order continuity of the meshfree LME approximation allows to directly solve the fourth-order phase field equations without splitting the fourth-order differential equation into two second order differential equations. Notably, in contrast to previous discretizations that use at least a quadratic basis, only linear completeness is needed in the LME approximation. We show that the crack surface can be captured more accurately in the fourth-order model than the second-order model. Furthermore, less nodes are needed for the fourth-order model to resolve the crack path. Finally, we demonstrate the performance of the proposed meshfree fourth order phase-field formulation for 5 representative numerical examples. Computational results will be compared to analytical solutions within linear elastic fracture mechanics and experimental data for three-dimensional crack propagation.
In the last part of this research, we present a phase-field model for fracture in Kirchoff-Love thin shells using the local maximum-entropy (LME) meshfree method. Since the crack is a natural outcome of the analysis it does not require an explicit representation and tracking, which is advantageous over techniques as the extended finite element method that requires tracking of the crack paths. The geometric description of the shell is based on statistical learning techniques that allow dealing with general point set surfaces avoiding a global parametrization, which can be applied to tackle surfaces of complex geometry and topology. We show the flexibility and robustness of the present methodology for two examples: plate in tension and a set of open connected
pipes.
In recent years increasingly consideration has been given to the lifetime extension of existing structures. This is based on the fact that a growing percentage of civil infrastructure as well as buildings is threatened by obsolescence and that due to simple monetary reasons this can no longer be countered by simply re-building everything anew. Hence maintenance interventions are required which allow partial or complete structural rehabilitation. However, maintenance interventions have to be economically reasonable, that is, maintenance expenditures have to be outweighed by expected future benefits. Is this not the case, then indeed the structure is obsolete - at least in its current functional, economic, technical, or social configuration - and innovative alternatives have to be evaluated. An optimization formulation for planning maintenance interventions based on cost-benefit criteria is proposed herein. The underlying formulation is as follows: (a) between maintenance interventions structural deterioration is described as a random process; (b) maintenance interventions can take place anytime throughout lifetime and comprise the rehabilitation of all deterioration states above a certain minimum level; and (c) maintenance interventions are optimized by taking into account all expected life-cycle costs (construction, failure, inspection and state-dependent repair costs) as well as state- or time-dependent benefit rates. The optimization is performed by an evolutionary algorithm. The proposed approach also allows to determine optimal lifetimes and acceptable failure rates. Numerical examples demonstrate the importance of defining benefit rates explicitly. It is shown, that the optimal solution to maintenance interventions requires to take action before reaching the acceptable failure rate or the zero expected net benefit rate level. Deferring decisions with respect to maintenance not only results, in general, in higher losses, but also results in overly hazardous structures.
Identification of flaws in structures is a critical element in the management of maintenance and quality assurance processes in engineering. Nondestructive testing (NDT) techniques based on a wide range of physical principles have been developed and are used in common practice for structural health monitoring. However, basic NDT techniques are usually limited in their ability to provide the accurate information on locations, dimensions and shapes of flaws. One alternative to extract additional information from the results of NDT is to append it with a computational model that provides detailed analysis of the physical process involved and enables the accurate identification of the flaw parameters. The aim here is to develop the strategies to uniquely identify cracks in two-dimensional 2D) structures under dynamic loadings.
A local NDT technique combined eXtended Finite Element Method (XFEM) with dynamic loading in order to identify the cracks in the structures quickly and accurately is developed in this dissertation. The Newmark-b time integration method with Rayleigh damping is used for the time integration. We apply Nelder-Mead (NM)and Quasi-Newton (QN) methods for identifying the crack tip in plate. The inverse problem is solved iteratively, in which XFEM is used for solving the forward problem in each iteration. For a timeharmonic excitation with a single frequency and a short-duration signal measured along part of the external boundary, the crack is detected through the solution of an inverse time-dependent problem. Compared to the static load, we show that the dynamic loads are more effective for crack detection problems. Moreover, we tested different dynamic loads and find that NM method works more efficient under the harmonic load than the pounding load while the QN method achieves almost the same results for both load types.
A global strategy, Multilevel Coordinate Search (MCS) with XFEM (XFEM-MCS) methodology under the dynamic electric load, to detect multiple cracks in 2D piezoelectric plates is proposed in this dissertation. The Newmark-b method is employed for the time integration and in each iteration the forward problem is solved by XFEM for various cracks. The objective functional is minimized by using a global search algorithm MCS. The test problems show that the XFEM-MCS algorithm under the dynamic electric load can be effectively employed for multiple cracks detection in piezoelectric materials, and it proves to be robust in identifying defects in piezoelectric structures. Fiber-reinforced composites (FRCs) are extensively applied in practical engineering since they have high stiffness and strength. Experiments reveal a so-called interphase zone, i.e. the space between the outside interface of the fiber and the inside interface of the matrix. The interphase strength between the fiber and the matrix strongly affects the mechanical properties as a result of the large ratio of interface/volume. For the purpose of understanding the mechanical properties of FRCs with functionally graded interphase (FGI), a closed-form expression of the interface strength between a fiber and a matrix is obtained in this dissertation using a continuum modeling approach according to the ver derWaals (vdW) forces. Based on the interatomic potential, we develop a new modified nonlinear cohesive law, which is applied to study the interface delamination of FRCs with FGI under different loadings. The analytical solutions show that the delamination behavior strongly depends on the interphase thickness, the fiber radius, the Young’s moduli and Poisson’s ratios of the fiber and the matrix. Thermal conductivity is the property of a material to conduct heat. With the development and deep research of 2D materials, especially graphene and molybdenum disulfide (MoS2), the thermal conductivity of 2D materials attracts wide attentions. The thermal conductivity of graphene nanoribbons (GNRs) is found to appear a tendency of decreasing under tensile strain by classical molecular dynamics (MD) simulations. Hence, the strain effects of graphene can play a key role in the continuous tunability and applicability of its thermal conductivity property at nanoscale, and the dissipation of thermal conductivity is an obstacle for the applications of thermal management. Up to now, the thermal conductivity of graphene under shear deformation has not been investigated yet. From a practical point of view, good thermal managements of GNRs have significantly potential applications of future GNR-based thermal nanodevices, which can greatly improve performances of the nanosized devices due to heat dissipations. Meanwhile, graphene is a thin membrane structure, it is also important to understand the wrinkling behavior under shear deformation. MoS2 exists in the stable semiconducting 1H phase (1H-MoS2) while the metallic 1T phase (1T-MoS2) is unstable at ambient conditions. As it’s well known that much attention has been focused on studying the nonlinear optical properties of the 1H-MoS2. In a very recent research, the 1T-type monolayer crystals of TMDCs, MX2 (MoS2, WS2 ...) was reported having an intrinsic in-plane negative Poisson’s ratio. Luckily, nearly at the same time, unprecedented long-term (>3months) air stability of the 1T-MoS2 can be achieved by using the donor lithium hydride (LiH). Therefore, it’s very important to study the thermal conductivity of 1T-MoS2.
The thermal conductivity of graphene under shear strain is systematically studied in this dissertation by MD simulations. The results show that, in contrast to the dramatic decrease of thermal conductivity of graphene under uniaxial tensile, the thermal conductivity of graphene is not sensitive to the shear strain, and the thermal conductivity decreases only 12-16%. The wrinkle evolves when the shear strain is around 5%-10%, but the thermal conductivity barely changes.
The thermal conductivities of single-layer 1H-MoS2(1H-SLMoS2) and single-layer 1T-MoS2 (1T-SLMoS2) with different sample sizes, temperatures and strain rates have been studied systematically in this dissertation. We find that the thermal conductivities of 1H-SLMoS2 and 1T-SLMoS2 in both the armchair and the zigzag directions increase with the increasing of the sample length, while the increase of the width of the sample has minor effect on the thermal conductions of these two structures. The thermal conductivity of 1HSLMoS2 is smaller than that of 1T-SLMoS2 under size effect. Furthermore, the temperature effect results show that the thermal conductivities of both 1H-SLMoS2 and 1T-SLMoS2 decrease with the increasing of the temperature. The thermal conductivities of 1HSLMoS2 and 1T-SLMoS2 are nearly the same (difference <6%) in both of the chiral orientations under corresponding temperatures, especially in the armchair direction (difference <2.8%). Moreover, we find that the strain effects on the thermal conductivity of 1HSLMoS2 and 1T-SLMoS2 are different. More specifically, the thermal conductivity decreases with the increasing tensile strain rate for
1T-SLMoS2, while fluctuates with the growth of the strain for 1HSLMoS2. Finally, we find that the thermal conductivity of same sized 1H-SLMoS2 is similar with that of the strained 1H-SLMoS2 structure.
This dissertation is devoted to the theoretical development and experimental laboratory verification of a new damage localization method: The state projection estimation error (SP2E). This method is based on the subspace identification of mechanical structures, Krein space based H-infinity estimation and oblique projections. To explain method SP2E, several theories are discussed and laboratory experiments have been conducted and analysed.
A fundamental approach of structural dynamics is outlined first by explaining mechanical systems based on first principles. Following that, a fundamentally different approach, subspace identification, is comprehensively explained. While both theories, first principle and subspace identification based mechanical systems, may be seen as widespread methods, barely known and new techniques follow up. Therefore, the indefinite quadratic estimation theory is explained. Based on a Popov function approach, this leads to the Krein space based H-infinity theory. Subsequently, a new method for damage identification, namely SP2E, is proposed. Here, the introduction of a difference process, the analysis by its average process power and the application of oblique projections is discussed in depth.
Finally, the new method is verified in laboratory experiments. Therefore, the identification of a laboratory structure at Leipzig University of Applied Sciences is elaborated. Then structural alterations are experimentally applied, which were localized by SP2E afterwards. In the end four experimental sensitivity studies are shown and discussed. For each measurement series the structural alteration was increased, which was successfully tracked by SP2E. The experimental results are plausible and in accordance with the developed theories. By repeating these experiments, the applicability of SP2E for damage localization is experimentally proven.
Isogeometric analysis (IGA) is a numerical method for solving partial differential equations (PDEs), which was introduced with the aim of integrating finite element analysis with computer-aided design systems. The main idea of the method is to use the same spline basis functions which describe the geometry in CAD systems for the approximation of solution fields in the finite element method (FEM). Originally, NURBS which is a standard technology employed in CAD systems was adopted as basis functions in IGA but there were several variants of IGA using other technologies such as T-splines, PHT splines, and subdivision surfaces as basis functions. In general, IGA offers two key advantages over classical FEM: (i) by describing the CAD geometry exactly using smooth, high-order spline functions, the mesh generation process is simplified and the interoperability between CAD and FEM is improved, (ii) IGA can be viewed as a high-order finite element method which offers basis functions with high inter-element continuity and therefore can provide a primal variational formulation of high-order PDEs in a straightforward fashion. The main goal of this thesis is to further advance isogeometric analysis by exploiting these major advantages, namely precise geometric modeling and the use of smooth high-order splines as basis functions, and develop robust computational methods for problems with complex geometry and/or complex multi-physics.
As the first contribution of this thesis, we leverage the precise geometric modeling of isogeometric analysis and propose a new method for its coupling with meshfree discretizations. We exploit the strengths of both methods by using IGA to provide a smooth, geometrically-exact surface discretization of the problem domain boundary, while the Reproducing Kernel Particle Method (RKPM) discretization is used to provide the volumetric discretization of the domain interior. The coupling strategy is based upon the higher-order consistency or reproducing conditions that are directly imposed in the physical domain. The resulting coupled method enjoys several favorable features: (i) it preserves the geometric exactness of IGA, (ii) it circumvents the need for global volumetric parameterization of the problem domain, (iii) it achieves arbitrary-order approximation accuracy while preserving higher-order smoothness of the discretization. Several numerical examples are solved to show the optimal convergence properties of the coupled IGA–RKPM formulation, and to demonstrate its effectiveness in constructing volumetric discretizations for complex-geometry objects.
As for the next contribution, we exploit the use of smooth, high-order spline basis functions in IGA to solve high-order surface PDEs governing the morphological evolution of vesicles. These governing equations are often consisted of geometric PDEs, high-order PDEs on stationary or evolving surfaces, or a combination of them. We propose an isogeometric formulation for solving these PDEs. In the context of geometric PDEs, we consider phase-field approximations of mean curvature flow and Willmore flow problems and numerically study the convergence behavior of isogeometric analysis for these problems. As a model problem for high-order PDEs on stationary surfaces, we consider the Cahn–Hilliard equation on a sphere, where the surface is modeled using a phase-field approach. As for the high-order PDEs on evolving surfaces, a phase-field model of a deforming multi-component vesicle, which consists of two fourth-order nonlinear PDEs, is solved using the isogeometric analysis in a primal variational framework. Through several numerical examples in 2D, 3D and axisymmetric 3D settings, we show the robustness of IGA for solving the considered phase-field models.
Finally, we present a monolithic, implicit formulation based on isogeometric analysis and generalized-alpha time integration for simulating hydrodynamics of vesicles according to a phase-field model. Compared to earlier works, the number of equations of the phase-field model which need to be solved is reduced by leveraging high continuity of NURBS functions, and the algorithm is extended to 3D settings. We use residual-based variational multi-scale method (RBVMS) for solving Navier–Stokes equations, while the rest of PDEs in the phase-field model are treated using a standard Galerkin-based IGA. We introduce the resistive immersed surface (RIS) method into the formulation which can be employed for an implicit description of complex geometries using a diffuse-interface approach. The implementation highlights the robustness of the RBVMS method for Navier–Stokes equations of incompressible flows with non-trivial localized forcing terms including bending and tension forces of the vesicle. The potential of the phase-field model and isogeometric analysis for accurate simulation of a variety of fluid-vesicle interaction problems in 2D and 3D is demonstrated.