175 research outputs found
Martin Carol [probably Martine Carol]
No full textBlack-and-white "glamour" photograph of French actress Martine Carol, from a collection by Waldo Ruess
On the structural design of imperfection sensitive laminated composite shell structures subjected to axial compression
No full textShell structures are used as primary structures of space launch vehicles. These structures are thin-walled and are thus prone to buckling when loaded in compression. Because of the imperfection sensitivity of these structures, small deviations of the real shell from the theoretically perfect shell may result in a tremendous decrease in load carrying capacity. For this reason, geometrical imperfections need to be taken into account. When designing unstiffened composite shells, the laminate stacking sequence influences both, the buckling load of the geometrically perfect shell and the imperfection sensitivity of the shell. Consequently, to derive laminate stacking sequences that maximize the buckling load of real shell structures, geometrical imperfections need to be taken into account already in an early design phase. In this paper, two laminate stacking sequences that were derived to maximize the buckling load of the geometrically perfect and imperfect shell structure are studied using stochastic methods. To this end, combination of non-rotational symmetric imperfections derived from measured data and variations of the ply orientation are studied in a stochastic analysis on basis of Monte Carlo simulations. The results of this study will be used to evaluate the influence of the stacking sequence as one of the essential properties dominating the structural response of geometrically imperfect laminate composite shell structures
Effects of management intensity, soil properties and region on the nematode communities in temperate forests in Germany
No full textIsogeometric analysis for thin-walled composite structures
No full textThe conceptual ideas behind isogeometric analysis (IGA) are aimed at unifying computer aided design (CAD) and finite element analysis (FEA). Isogeometric analysis employs the non-uniform rational B-spline functions (NURBS) used for the geometric description of a structure to approximate its physical response in an isoparametric sense. Due to the tensor product property of multi-variate NURBS, it is difficult to represent complex topological shapes with a single NURBS patch. Multiple, often non-conforming patches are needed to tackle increasing complexity of the geometry. To further facilitate the modeling of complex shapes and geometric features trimming technology is widely used in CAD software, however, the trimmed domain is only visually unseen and the trimming features can not be utilized directly for the analysis. To overcome these difficulties, extra efforts are needed to make isogeometric methods adapted to engineering related cases. Thin-walled structures, such as plates and shells, excel in optimal load-carrying behavior and are of major importance in the design of aerospace components and the automotive engineering. Isogeometric analysis is an ideal candidate for the modeling and simulation of shell structures, especially for rotation-free Kirchhoff-Love type shells, which profit from the exact description of the geometry and from the higher continuity properties of NURBS. Furthermore, it favorably supports continuity requirements for flexible through-the-thickness design of laminate composites. Laminated composite materials are increasingly used in the aerospace industry this asks for reliable and computationally efficient lamina theories. The classical lamination theory belongs to the class of equivalent-single-layer methods (ESL), it is computationally efficient but often fails to capture the 3D stress state accurately. The demand for an accurate 3D stress state within laminates is mainly driven by the need to identify and to evaluate potential damage of lamina structures. While a full 3D layerwise (LW) model is computationally expensive, a combined approach considering both concepts, ESL and LW, seems to be a natural choice to tackle the computational costs of increasing model size and model complexity. In this thesis, a layerwise method for laminated composite structures is proposed in the framework of isogeometric analysis. A highly accurate prediction of the state of stress for thick and moderately thick laminate composite shells including transverse normal and shear stresses is demonstrated. The layerwise theory is successfully extended to linear buckling analysis of delaminated composites where a contact formulation is added to eliminate physically inadmissible buckling states which may result from overlapping plies. Furthermore, a Nitsche type formulation is introduced to enforce both weakly, essential boundary conditions and multi-patch coupling constraints for trimmed and non-conforming isogeometric rotation-free Kirchhoff-Love shell patches. The proposed formulation is variationally consistent and excels in a high level of stability and accuracy. A built-in stabilization, used to ensure coercivity of the formulation, prevents ill-conditioning of the physical problem. The inherent trimming problem is tackled with a fictitious domain extension for the trimming domain following the principles of the finite cell method to facilitate the workflow for geometrically complex structures in engineering practice. Computational efficiency is significantly increased with a blended coupling, taking continuum-like shell elements and thin shells elements, according to the theory of Kirchhoff-Love, into account. The blended approach provides access to the full 3D state of stress within selected subdomains while preserving the computational efficiency of the overall analysis.Aerospace Structures and Computational MechanicsAerospace Engineerin
Dr. Lillien J. Martin on a tour of inspection at the Salvaging Old Age Farm
No full textBlack and white photograph of Dr. Lillien J. Martin on a tour of inspection at the Salvaging Old Age Farm in Alameda County, California, in 1938
Stacking sequence influence on imperfection sensitivity of cylindrical composite shells under axial compression
No full textSpace launcher vehicle structures are designed as thin walled cylindrical and conical structures which are prone to buckling and are sensitive towards geometrical imperfections. Small deviations in dimensions, which still are within manufacturing tolerances, may lead to a tremendous decrease in load carrying capacity. Thus, imperfections have to be considered during the design phase and this is commonly done using empirical knock down factors. Besides this approach, imperfections can be considered by applying numerical or analytical structural models. Composite materials are used to exploit the light weight potential of unstiffened thin walled structures. For this type of shell structure, the buckling load of the geometrically perfect shell and the imperfection sensitivity are significantly influenced by the laminate stacking sequence. In this paper, the influence of the laminate stacking sequence of composite shells with rotational symmetric imperfections on the buckling behavior is studied and laminate stacking sequences leading to the highest buckling loads of an imperfect shell structure are identified. These stacking sequences are evaluated further by applying non-rotational symmetric imperfections and localized imperfections and the stacking sequences leading to optimum designs of the geometrical perfect shell structure are considered as reference structures
Variational Germano Optimization of Arbitrary Unresolved-Scale Models
No full textThis thesis demonstrated how the Newton and BFGS algorithm could be used to solve the standard VGM relations and least-squares formulation respectively, for arbitrary forms of the _ parameter, including nonlinear _ s, appearing in the VMM. The proposed procedure were also shown to be able to handle arbitrary projectors that are compatible with the VGM. When applied to the advection-di_usion equation, Burgers' equation and Stokes equations the algorithms always reached the speci_ed stopping criteria and did not exceed the maximum alloted iterations. Additionally the increase in computational e_ort required was shown to be limited. However, it was shown that the Newton procedure for the VGM could not be used in cases where the local VGM residuals had varying signs. In that case the BFGS and least-squares VGM were successful. Application of the proposed procedures to di_erent SGS models showed that nonlinear models tended to outperform linear models in terms of the L2 and projected error. It was also shown that the parametrization of an SGS model can greatly inuence how well the VGM will be able to optimize its coe_cients.Aerospace Engineering | Aerodynamics and Wind Energ
Topology optimization using the Finite Cell Method
No full textThe ongoing demand for better performing designs, has resulted in an increase in the complexity of topology optimization problems. Traditionally, the majority of the corresponding computational cost comes from solving the analysis equations using linear finite elements (FE). In this thesis a topology optimization method is presented, that is based on the finite cell method (FCM). This higher-order fictitious domain method is, due to its decoupled geometry-, integration-, and analysis-mesh well suited for large-scale topology optimization, and reducing its corresponding computational cost. The use of a decoupled density and analysis mesh greatly reduced the computational cost of topology optimization compared to linear FEM. Especially in 3D topology optimization examples, the computational cost has been decreased by more than a factor 10, while maintaining a high-resolution in the density field. The use of a larger length-scale can reduce the computational cost even more, which is especially beneficial for robust topology optimization. It is identified that the choice of the analysis system completely depends on the complexity of the optimization problem. Simple optimization problems showed great increase in computational efficiency using relatively low polynomial degree (p= 1, 2, 3), combined with more density elements per finite cell. For more difficult topology optimization examples, such as problems were the boundary conditions have to be enforced in the weak sense, or stress-constrained topology optimization, a more accurate analysis system is required, hence a larger polynomial degree should be used.Mechanics, Aerospace Structures & MaterialsAerospace Engineerin
Multiscale Modelling of Discontinuities: Towards accurate computations of shock-turbulence interactions
No full textAerospace Engineering | Aerodynamics and Wind Energ
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