1,721,208 research outputs found
The nonlinear, third-order thickness and shear deformation theory for statics and dynamics of laminated composite shells
No full textThe present study introduces a rigorous higher-order polynomial in the thickness coordinate to develop a theory with thickness and shear deformations for doubly curved, laminated composite shells. In this theory, the nonlinear terms in all the kinematic parameters are kept. By applying the conditions of zero transverse normal and shear stresses at the top and bottom surfaces of shells, a third-order thickness and shear deformation theory with six kinematic parameters is derived. This is a particularly interesting result since the developed theory presents a single additional parameter to describe the thickness deformation with respect to the popular Reddy's third-order shear deformation theory, which has five parameters. The accuracy of the proposed six-parameter theory is tested for static and dynamic benchmark cases. Results are also very satisfactorily compared to those obtained with a more sophisticated and computationally onerous nine-parameter theory. The considered cases are isotropic and cross-ply laminated circular cylindrical shells under radial forces and pressure, and nonlinear forced vibrations of a cross-ply laminated shell under harmonic radial excitation
Nonlinear mechanics of sandwich plates: Layerwise third-order thickness and shear deformation theory
No full textThe nonlinear mechanics of sandwich plates is studied using a layerwise third-order thickness and shear deformation theory. In particular, the two face sheets are modelled using a second-order shear deformation theory (well-justified in case of a zero-shear stress only at one outer surface) and the core is modelled with a third-order thickness and transverse shear deformation theory. After introducing continuity of displacements at the interfaces between the core and the face sheets, 16 independent kinematic parameters are retained. An independent parameter is kept to describe the thickness deformation, which allows for introduction of the corresponding boundary condition; this represents a significant novel contribution. Geometric nonlinearity in all the kinematic parameters is introduced. Numerical results are presented for deflection of a sandwich plate under pressure. A thin core was considered, since it is numerically more critical, and a wide range of core stiffnesses was studied in order to verify the validity of the proposed model till the limit case of two plates joined by a core of negligible stiffness (i.e., two independent plates). Numerical results are compared to those obtained by a commercial finite element (FE) program with three-dimensional solid elements till the limit when the FE code fails for large deformations of the extremely weak core
Numerical study of the mixed-mode behavior of generally-shaped composite interfaces
No full textThe use of high-performance laminated composites and adhesively bonded structures has become very common for many engineering applications. Laminated structures are known to suffer from the non-linear and irreversible delamination process, including the formation and propagation of cracks, up to the complete detachment of the adhering parts. In this context, a new numerical formulation based on the generalized differential quadrature (GDQ) approach, is developed to determine the peeling and shear stresses along interfaces of arbitrary shape, made of laminated composite structures and subjected to mixed-mode conditions, as well as to examine the internal distribution of reactions and the kinematic response of the adherends. In line with a Linear Elastic-Brittle Interface Model (LEBIM), the specimen is considered as an assemblage of two sublaminates, partly bonded together by an elastic interface. This is, in turn, modeled as a continuous distribution of elastic-brittle springs acting along the normal and/or tangential direction, depending on the selected mixed-mode condition. A large parametric study is performed to predict the effect of the geometrical shape and curvature of the specimen on its structural response. The feasibility of the proposed formulation is also verified through a convergence analysis, for the simplest case of straight composite adherends, for which we provide a closed form analytical solution. The excellent agreement between the GDQ approach and the analytical predictions, confirms the reliable accuracy of the novel numerical formulation for the treatment of the mixed-mode fracture of composite materials or laminated joints of general shapes, as useful for practical strengthening requirements
FGM and Laminated Doubly-Curved and Degenerate Shells Resting on Nonlinear Elastic Foundations: A GDQ Solution for Static Analysis with a Posteriori Stress and Strain Recovery
No full textThis work focuses on the static analysis of functionally graded (FGM) and laminated doubly-curved shells and panels resting on nonlinear and linear elastic foundations using the Generalized Differential Quadrature (GDQ) method. The First-order Shear Deformation Theory (FSDT) for the aforementioned moderately thick structural elements is considered. The solutions are given in terms of generalized displacement components of points lying on the middle surface of the shell. Several types of shell structures such as doubly-curved shells (elliptic and hyperbolic hyperboloids), singly-curved (spherical, cylindrical and conical shells), and degenerate panels (rectangular plates) are considered in this paper. The main contribution of this paper is the application of the differential geometry within GDQ method to solve doubly-curved FGM shells resting on nonlinear elastic foundations. The linear Winkler-Pasternak elastic foundation has been considered as a special case of the nonlinear elastic foundation proposed herein. The discretization of the differential system by means of the GDQ technique leads to a standard nonlinear problem, and the Newton-Raphson scheme is used to obtain the solution. Two different four-parameter power-law distributions are considered for the ceramic volume fraction of each lamina. In order to show the accuracy of this methodology, numerical comparisons between the present formulation and finite element solutions are presented. Very good agreement is observed. Finally, new results are presented to show effects of various parameters of the nonlinear elastic foundation on the behavior of functionally graded and laminated doubly-curved shells and panels
Nonlinear higher-order shell theory for incompressible biological hyperelastic materials
No full textIn the present study, a geometrically nonlinear theory for circular cylindrical shells made of incompressible hyperelastic materials is developed. The 9-parameter theory is higher-order in both shear and thickness deformations. In particular, the four parameters describing the thickness deformation are obtained directly from the incompressibility condition. The hyperelastic law selected is a state-of-the-art material model in biomechanics of soft tissues and takes into account the dispersion of collagen fiber directions. Special cases, obtained from this hyperelastic law setting to zero one or some material coefficients, are the Neo-Hookean material and a soft biological material with two families of collagen fibers perfectly aligned. The proposed model is validated through comparison with the exact solution for axisymmetric cylindrical deformation of a thick cylinder. In particular, the shell theory developed herein is capable to describe, with extreme accuracy, even the post-stability problem of a pre-stretched and inflated Neo-Hookean cylinder until the thickness vanishes. Comparison to the solution of higher-order shear deformation theory, which neglects the thickness deformation and recovers the normal strain from the incompressibility condition, is also presented
Laminated SMA beams finite elements
No full textShape-memory-alloys (SMA) present very special features. In particular, because of the austenite-martensite and martensite-austenite transformations, governed by the temperature and the stress state, they can undergo large deformations, showing the so-called superelastic behavior and the shape memory effect. The superelastic behavior occurs when, for a fixed value of the temperature, the material recovers its natural state after a loading-unloading stress cycle. The shape memory effect occurs when an inelastic strain is present after a loading-unloading stress cycle; this in elastic strain can be recovered by a further temperature cycle. Because of the very special material behavior, SMA are successfully adopted in many advanced systems; they are adopted as orthodontic wires, as self-expanding micro-structures in the treatment of blood vessel occlusions, as devices to control the spatial antennas opening, to name a few. Beams, plates and shells represent the most common elements for SMA applications. Several models have been proposed in the last decade to reproduce the SMA constitutive behavior. In fact, different micromechanical and macromechanical approaches are distinguished in the literature in the SMA modeling. In the present paper, a simple SMA model to simulate the superelastic behavior as well as the shape memory effect is proposed. It considers only the transformations from austenite to single variant martensite and from single variant martensite to austenite, taking into account the influence of the temperature in the constitutive relationship. The proposed SMA constitutive law is used in beam elements that neglect or include the transverse shear deformation. The new SMA beam finite elements are formulated using suitable approximation functions. The proposed finite elements are developed within a numerical procedure for the time integration of the SMA constitutive equations. In particular, the developed elements allow the use of SMA material as a reinforcement of elastic beams. Several applications are presented to assess the model and the proposed numerical procedure
A new twelve-parameter spectral/hp shell finite element for large deformation analysis of composite shells
No full textIn this paper, a new 12-parameter shell finite element for large deformation analysis of composite shell structures is developed using third-order thickness stretch kinematics. The continuum shell element is utilized in the numerical simulations of laminated composite and functionally graded materials, using a high-order spectral/hp approximations. The results obtained from the 12-parameter shell element are compared with those obtained from the 7-parameter shell element to bring out the differences. Deflections and maximum stresses are computed using the two models. The results show that the responses predicted by the two formulations (and models) are consistent with each other, with the 12-parameter model showing slight difference from those predicted by the 7-parameter model
A continuum eight‐parameter shell finite element for large deformation analysis
No full textIn this paper, a finite element formulation, using eight independent parameters and high-order spectral/hp functions, for nonlinear analysis is presented. This formulation allows the use of a third-order thickness stretch kinematics, which also avoids Poisson's locking. Full nonlinear terms up to quadratic in the Green-Lagrange strain tensorare retained. Several nontrivial problems are solved using the presented formulation. A comparison between this formulation and others found in the literature,and with shell and solid elements in commercial codes ABAQUS and ANSYS are presented and the differences are brought out
Winkler-Pasternak Foundation Effect on the Static and Dynamic Analyses of Laminated Doubly-Curved and Degenerate Shells and Panels
No full textThis work presents the static and dynamic analyses of laminated doubly-curved shells and panels of revolution resting on the Winkler–Pasternak elastic foundation using the generalized differential quadrature (GDQ) method. The analyses are worked out considering the first-order shear deformation theory (FSDT) for the aforementioned moderately thick structural elements. The solutions are given in terms of generalized displacement components of points lying on the middle surface of the shell. Several types of shell structures such as doubly-curved and revolution shells, singly-curved and degenerate shells are considered in this paper. The main novelty of this paper is the application of the differential geometry within GDQ method to solve doubly-curved shells resting on the Winkler–Pasternak elastic foundation. The discretization of the differential system by means of the GDQ technique leads to a standard linear problem for the static analysis and to a standard linear eigenvalue problem for the dynamic analysis. In order to show the accuracy of this methodology, numerical comparisons between the present formulation and finite element solutions are presented. Very good agreement is observed. Finally, new results are presented to show effects of the Winkler modulus, the Pasternak modulus, and the inertia of the elastic foundation on the behavior of laminated doubly-curved shells
Automatically Create Digital Elevation Model from Photos Captured by a Low-Cost UAV-Based System
No full textUnmanned aerial vehicles (UAVs) are commonly utilized as cost-effective devices for data collection by capturing photos of target objects. UAV images have been used for many applications, such as civil engineering, transportation, architecture, surveying, and mapping. Although commercial UAV image data processing software is suitable for generating orthoimages and dense point clouds of surfaces, it still requires extensive labor to prepare the appropriate point cloud to create a digital elevation model (DEM). This study proposes a method to automatically create DEM from a point cloud generated from UAV images. The proposed method composes of three main steps: (1) Candidate ground points, (2) Ground points extraction, and (3) Creation of a DEM model. The proposed method was tested on three datasets, covering a total area of approximately 45 hectares from 200 images captured by DJI Phantom 4 drone. As a result, the DEMs are successfully created with a spatial resolution of 1.0 m.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Optical and Laser Remote Sensin
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