567 research outputs found
Modelling stress-corrosion microcracking in polycrystalline materials by the Boundary Element Method
No full textThe boundary element method is employed in this study in conjunction with the finite element method to build a multi-physics hybrid numerical model for the computational study of stress corrosion cracking related to hydrogen diffusion in polycrystalline microstructures. More specifically a boundary integral representation is used to represent the micro-mechanics of the aggregate while an explicit finite element method is used to model inter-granular hydrogen diffusion. The inter-granular interaction between contiguous grains is represented through cohesive laws, whose physical parameters depend on the concentration of inter-granular hydrogen, diffusing along the interfaces according to the Fick's second law. The model couples the effectiveness of the polycrystalline boundary element micro-mechanics model with the generality of the finite element representation of the inter-granular diffusion process. Few numerical tests are reported, to demonstrate the potential of the proposed technique
Nonlinear free vibrations analysis of cracked composite stiffened plates via X-Ritz approach
No full textThin and moderately thick composite multi-layered plates are widely employed in naval and aerospace
structures. They can experience the presence of cracks, generated for example by corrosion, fatigue
or accidental external causes, which aect their static and dynamic behaviour. As regard the dynamic
characteristics of plates, many studies have focused on the linear vibration analysis of both isotropic
and composite thin and thick plates, providing for a comprehensive knowledge of the plate dynamic
behaviour. However, for an accurate appraisal of the plate dynamics, in some applications it is needed
to investigate the nonlinear free vibration problem; a literature survey evidences that the large amplitude
vibrations have received considerable attention for single plate congurations. Dierent approaches have
been proposed to model cracked plates and, among others, the Ritz method shows adequate accuracy
and computational eciency. To apply the Ritz method to crack problems by using standard admissible
functions the sub-structuring or multi-domain strategy has been employed. More recently, the Ritz
approach has been proposed with special trial functions, which account for the presence of the crack by
describing the discontinuity of the solution across the crack and the tip singularity [1,2]. In the present
work, nonlinear free vibrations of cracked stiened composite plates are investigated by modelling the
problem by the X-Ritz formulation recently presented by the authors [3]. In the framework of the rst
order shear deformation theory, this formulation combines the features of both the Ritz method and the
X-FEM strategy, as it employs variables approximations obtained by enriching the Ritz series expansion
with suitably dened crack functions. This modelization applies to a single quadrangular plate and is
coupled with a multi-domain approach which open towards an ecient modelization of stiened panels
and thin-walled structures. Thus, the entire structure is modelled as individual, separated plates which
can contain cracks; these plates are then joined by enforcing the continuity conditions along the common
edges through penalty techniques. This approach has: (i) the advantage of a very simple pre-processing
stage, as it only requires the geometrical information on the plates that compose the structure and the
cracks locations; (ii) the feature to incorporate the singular behaviour at the crack tips. Convergence
and accuracy studies on linear and nonlinear free vibrations of uncracked congurations are carried out
on uncracked congurations to validate the approach by comparison with literature and nite elements
results. Cracked stiened panels nonlinear free vibrations results are then presented to discuss the
potential of the method.
References
1. Milazzo, A., Benedetti, I., Gulizzi, V. (2018). An extended
Ritz formulation for buckling and post-buckling analysis of cracked multilayered plates. Composite
Structures, 201, 980-994.
2. Milazzo, A., Benedetti, I., Gulizzi, V. (2019). A single-domain Ritz
approach for buckling and post-buckling analysis of cracked plates. International Journal of Solids and
Structures, 159, 221-231.
3. Gulizzi, V., Oliveri, V., Milazzo, A. (2019), Buckling and post-buckling
analysis of cracked stiened panels via an X-Ritz method. Aerospace Science and Technology, in pres
Discontinuous Galerkin models for composite multilayered shells with higher order kinematics
No full textComposite multilayered shells are widely employed in aerospace, automotive and civil engineering as weight-saving structural components. In multilayered shells, despite its versatility, the interplay between the curved geometry and the properties of the composite layers induces a complex distribution of the mechanical fields, which must be accurately resolved to safely employ generally curved composite shells as load-bearing structures.
The problem can be addressed through the two-dimensional shell theories, which are based on suitable assumptions on the behavior of the mechanical fields throughout the thickness of the considered structures and are a viable strategy for reducing the computational complexity with respect to 3D models. After the wide investigation on the CLT and FSDT shell theories, motivated by the need of accurate models, researchers have introduced the so-called higher-order theories for plate and shells. These can be classified into Equivalent-Single-Layer (ESL) theories, whereby the layers are replaced by a single layer with equivalent mechanical properties, Layer-Wise (LW) theories, whereby each layer is treated independently, and sub-laminate theories, whereby groups of layers are replaced by groups of equivalent layers. A unified description of these approaches has been introduced by the Carrera Unified Formulation (CUF) [1], which provides a framework able to determine the best 2D theory in terms of computational efficiency versus solution accuracy for a given structural problem. In most cases, numerical models based on these theories are solved using the Finite Element Method [2]. Among the available numerical strategies alternative to FEM for solving problems governed by systems of partial differential equations, the discontinuous Galerkin (dG) method has been recognized as powerful in enabling the seamless use of high-order elements and hierarchical meshes with tunable hp-refinement. The dG approach has been successfully used for general plate theories described via the CUF [3,4].
In this work, for the first time, Equivalent-Single-Layer dG formulations for generally curved multilayered shells are proposed. To account for complex structures, the shell geometry is described by using NURBS whereas different order theories are obtained via a CUF-based description of the shell kinematic model. An Interior Penalty dG scheme, which allows for a high-order numerical solution of the governing equations throughout the shell modeling domain. The dG scheme is also coupled with the implicitly defined mesh technique, which allows to resolve curved boundaries with high-order accuracy by combining an easy-to-generate background grid and the implicit representation of the domain of analysis. Results are presented to show the accuracy and potentiality of the proposed approach.
References
[1] E. Carrera. Theories and finite elements for multilayered plates and shells: a unified compact formulation with numerical assessment and benchmarking. Archives of Computational Methods in Engineering, 10(3):215{296, 2003.
[2] M. F. Caliri, A. J.M. Ferreira, V. Tita. A review on plate and shell theories for laminated and sandwich structures highlighting the Finite Element Method. Composite Structures, 156: 63-77, 2016.
[3] V. Gulizzi, I. Benedetti, and A. Milazzo. An implicit mesh discontinuous Galerkin formulation for higher-order plate theories. Mechanics of Advanced Materials and Structures, 27(17):1494-1508, 2020.
[4] V. Gulizzi, I. Benedetti, and A. Milazzo. A high-resolution layer-wise discontinuous Galerkin formulation for multilayered composite plates. Composite Structures, 242:112137, 2020
Mixed finite elements for nonlocal elastic multilayered composite plate refined theories
No full textA novel mixed finite element formulation for the layerwise analysis of nonlocal multilayered composite plates is
presented. The finite elements are formulated starting from the weak form of a set of governing equations for the
laminate layers that were deduced via the Reissner Mixed Variational Theorem. The primary variables, namely
displacements and out-of-plane stresses, are expressed at layer level as through-the-thickness expansions of suitable
selected functions with coefficients approximated by the finite element scheme. The through-the-thickness expansion
order is considered as a free parameter. This way, finite elements for different refined higher order plate
theories can be systematically developed by assembling the layers contributions associated with the variable expansion
terms. These contributions are called fundamental nuclei and their definition is formally unique whatever
the considered expansion order. The obtained finite elements inherently ensure stresses and displacements continuity
at the layer interfaces and they allow to associate different values of the nonlocal parameter to the laminate
layers. Standard 9-node and 16-node isoparametric, quadrilateral finite elements have been implemented to verify
the viability of the proposed formulation. The obtained results compare favourably with literature solutions and
highlight the characteristics of the approach. Original results are proposed also to serve as benchmarking data
Considerazioni e prospettive storico-giuridiche sul recente accordo Santa Sede - Italia in materia di riconoscimento dei titoli ecclesiastici
No full textThe study is concerned with analyzing from both a historical and a legal point of view the recognition of ecclesiastical academic titles in light of the agreements between the Holy See and Italy. More precisely, it deals with the process that, from art. 10, paragraph 2 of the “Villa Madama Agreements”, has led, thanks to the Lisbon Convention and the so-called “Bologna Process”, to the agreement of February 13, 2019
Considerazioni e prospettive storico-giuridiche sul recente accordo Santa Sede - Italia in materia di riconoscimento dei titoli ecclesiastici.
No full textThe study is concerned with analyzing from both a historical and a legal point of view the recognition of ecclesiastical academic titles in light of the agreements between the Holy See and Italy. More precisely, it deals with the process that, from art. 10, paragraph 2 of the “Villa Madama Agreements”, has led, thanks to the Lisbon Convention and the so-called “Bologna Process”, to the agreement of February 13, 2019
Boundary Element Method for Composite Laminates
No full textThe boundary element method (BEM) is a numerical technique to solve engineering/physical problems formulated in terms of boundary integral equations. Composite laminates are assemblages of stacked different materials layers, generally consisting of variously oriented
fibrous composite material
Nonlinear free vibrations of composite structures via the X-Ritz method
No full textThe analysis of large amplitude vibrations of thin-walled cracked structures build as plate assembly
is considered in this study. The problem is addressed via a Ritz approach, called X-Ritz, based
on the first order shear deformation theory and von K ́arm ́an’s geometric nonlinearity assumptions.
The trial functions are expressed as series of regular orthogonal polynomial products supplemented
with special functions able to represent the crack behaviour; boundary functions are used to guarantee
the fulfillment of the kinematic boundary conditions. Results are presented, which illustrate
the influence of cracks on the stiffening effect due to large amplitude vibrations
Refined equivalent single layer formulations and finite elements for smart laminates free vibrations
No full textA family of 2D refined equivalent single layer models for multilayered and functionally graded smart magneto-electro-elastic plates is presented. They are based on variable kinematics and quasi-static behavior for the electromagnetic fields. First, the electromagnetic state of the plate is determined by solving the strong form of the electromagnetic governing equations coupled with the corresponding interface continuity conditions and external boundary conditions. The electromagnetic state is then condensed into the plate kinematics, whose governing equations can be written using the generalized principle of virtual displacements. The procedure identifies an effective elastic plate kinematically equivalent to the original smart plate. The effective plate is characterized by inertia, stiffness and loading properties which take the multifield coupling effects into account through their definitions, which involve the electromagnetic coefficients appearing in the smart materials constitutive law. The proposed model extends the techniques and tools available for the assessment of the mechanical behavior of multilayered composite plates to smart laminates. Additionally, finite elements for the proposed single layer models are formulated and validated against available benchmark 3D solutions. © 2014 Elsevier Ltd. All rights reserved
Computational Analysis of the Active Control of Incompressible Airfoil Flutter Vibration Using a Piezoelectric V-Stack Actuator
No full textThe flutter phenomenon is a potentially destructive aeroelastic vibration studied for the
design of aircraft structures as it limits the flight envelope of the aircraft. The aim of this work is to
propose a heuristic design of a piezoelectric actuator-based controller for flutter vibration suppression
in order to extend the allowable speed range of the structure. Based on the numerical model of a three
degrees of freedom (3DOF) airfoil and taking into account the FEM model of a V-stack piezoelectric
actuator, a filtered PID controller is tuned using the population decline swarm optimizer PDSO
algorithm, and gain scheduling (GS) of the controller parameters is used to make the control adaptive
in velocity. Numerical simulations are discussed to study the performance of the controller in the
presence of external disturbances
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