Publications
62 results found
, 2023, Multiphysics Simulation of Particle-Surface Interaction and its Effect on Powder Patterns and Process Optimization, The International Journal of Multiphysics, Vol: 17, ISSN: 1750-9548
Yavari A, Safa Y, Soleiman Fallah A, 2023, Finite extension of accreting nonlinear elastic solid circular cylinders, Continuum Mechanics and Thermodynamics, ISSN: 0935-1175
Gorshkov VN, Bereznykov OV, Boiger GK, et al., 2023, Acoustic metamaterials with controllable bandgap gates based on magnetorheological elastomers, International Journal of Mechanical Sciences, Vol: 238, Pages: 1-12, ISSN: 0020-7403
Acoustic metamaterials allow for creating selective pass- and stop-bands on the frequency spectrum. We demonstrate the possibility of designing acoustic metamaterials as core-shell 2D-phononic media with an extremely simple morphology, the frequency spectrum of which contains many real-time tunable bandgaps. The connected shells of such metamaterials form a grid with square cells filled with nuclei partitionable into two subsystems. Both subsystems are characterized by their frequency spectra, and it is the coupling between them that generates the bandgaps. If the structural elements of the metamaterial are built based on magnetoelastomers, then bandgaps can be easily controlled by an external magnetic field that changes the elastic moduli of shells/cores. We have shown the possibility of manipulating single bandgaps in different parts of the spectrum, and simultaneous control of all bandgaps up to their complete disappearance. This manipulation can be carried out, specifically, with no change in the maximum achievable frequency in the metamaterial. The results obtained can be used for selective filtering of damaging wave components, active control of seismic or blast waves, sonar systems, ultrasound imaging, impact-resistant structures, and noise cancellation protocols. The physical concepts developed are extendable to 3D-structures in a similar fashion so can benefit a wider community.
Gorshkov VN, Tereshchuk VV, Bereznykov OV, et al., 2022, Self-Ordering in the Dynamics of Forming Periodic 1D-Structures Controlled by External Irradiation, Prime Archives in Nanotechnology, ISBN: 978-81-953047-8-3
We have studied the striking effect of external irradiation of nanowires on the dynamics of their surface morphology at elevated temperatures that do not destroy their crystal lattice. Numerical experiments performed on the basis of the Monte Carlo model revealed new possibilities for controlled periodic modulation of the cross section of quasi-one-dimensional nanostructures for opto- and nanoelectronic elements. These are related to the fact that external irradiation stimulates the surface diffusion of atoms. On the one hand, such stimulation should accelerate the development of the well-known spontaneous thermal instability of nanowires (Rayleigh instability), which leads to their disintegration into nanoclusters. On the other hand, Prime Archives in Nanotechnology3 www.videleaf.comthis leads to the forced development of the well-known roughening transition (RT) effect. Under normal circumstances, this manifests itself on selected crystal faces at a temperature above the critical one. The artificial stimulation of this effect on the lateral surface of quasi-one-dimensional structures determines many unpredictable scenarios of their surface dynamics, which essentially depend on the orientation of the nanowire axis relative to its internal crystal structure. In particular, long-wave Rayleigh breakup observed in absence of external irradiation transforms into strongly pronounced short wave metastable modulations of the cross section (a chain of unduloids). The effect of the self-consistent relationship between the Rayleigh instability and RT is dimensional and can be observed only at relatively small nanowire radii. The fact is analyzed that for the manifestation of this effect, it is very important to prevent significant heating of the nanowire when surface diffusion is stimulated. A number of developed theoretical concepts have already found confirmation in real experiments with Au- and Ag-nanowires irradiated by electrons and Ar+ -ions, respectively.
Gorshkov VN, Tereshchuk VV, Bereznykov OV, et al., 2022, Dynamics of quasi-one-dimensional structures under roughening transition stimulated by external irradiation, Nanomaterials, Vol: 12, Pages: 1411-1411, ISSN: 2079-4991
We studied the striking effect of external irradiation of nanowires on the dynamics of their surface morphology at elevated temperatures that do not destroy their crystal lattice. Numerical experiments performed on the basis of the Monte Carlo model revealed new possibilities for controlled periodic modulation of the cross-section of quasi-one-dimensional nanostructures for opto- and nanoelectronic elements. These are related to the fact that external irradiation stimulates the surface diffusion of atoms. On the one hand, such stimulation should accelerate the development of the well-known spontaneous thermal instability of nanowires (Rayleigh instability), which leads to their disintegration into nanoclusters. On the other hand, this leads to the forced development of the well-known roughening transition (RT) effect. Under normal circumstances, this manifests itself on selected crystal faces at a temperature above the critical one. The artificial stimulation of this effect on the lateral surface of quasi-one-dimensional structures determines many unpredictable scenarios of their surface dynamics, which essentially depend on the orientation of the nanowire axis relative to its internal crystal structure. In particular, the long-wave Rayleigh breakup observed in absence of external irradiation transforms into strongly pronounced short-wave metastable modulations of the cross-section (a chain of unduloids). The effect of the self-consistent relationship between the Rayleigh instability and RT is dimensional and can be observed only at relatively small nanowire radii. The fact is analyzed that, for the manifestation of this effect, it is very important to prevent significant heating of the nanowire when surface diffusion is stimulated. A number of developed theoretical concepts have already found confirmation in real experiments with Au and Ag nanowires irradiated by electrons and Ag+ ions, respectively.
Mehreganian N, Toolabi M, Zhuk YA, et al., 2021, Dynamics of pulse-loaded circular Föppl-von Kármán thin plates- Analytical and numerical studies, Journal of Sound and Vibration, Vol: 513, ISSN: 0022-460X
Materials such as modern armour steel, benefit from appreciably high elastic energy storage capacity prior to failure. Such a capacity contributes to absorption of the impulse generated during an extreme pulse pressure loading event such as a localised blast. As the plate deforms within the bounds of the elastic region without plastic dissipation, the probability of catastrophic failure is mitigated while large deformations compared to conventional metallic panels are encountered. No studies have proposed, to date, a closed-form solution for nonlinear elastic response of thin circular plates subject to localised pulse loads. The present work aims at deducing, from the minimization of the Föppl-von Kármán (FVK) energy functional, explicit solutions for the response of dynamically (pulse) loaded thin clamped circular plates undergoing large deformations. The solutions were derived from a presumed kinematically admissible displacement field together with an associated stress tensor potential as an infinite polynomial series, which was truncated into a multiplicative decomposition of temporal parts and spatial parts, representative of a Multiple Degrees-of-Freedom (MDOF's) system. In the case of static loading, using the Frobenius method, an exact recursive solution to each mode of defamation was obtained. In the event of dynamic loading, useful expressions for stress tensor components were delineated, corresponding to a multimode multiplicative product, and a series of coupled Ordinary Differential Equations (ODE's) were derived, using the Ritz-Galerkin variational method. The explicit solutions were sought using the Poincaré-Lindstedt (PL) perturbation method. The closed-form solutions obtained were corroborated with FE results including the Fluid-Structure Interaction (FSI) effects and showed convergence when the first few modes were considered. The influence of higher modes, however, on the peak deformation was negligible and the solution w
Goroshko IO, Zhuk YA, Fallah AS, et al., 2021, Dynamic Behavior of Composite Wind Turbine Blades with Different Material Combinations: a Numerical Study*, International Applied Mechanics, Vol: 57, Pages: 635-643, ISSN: 1063-7095
Navadeh N, Sareh P, Basovsky VG, et al., 2021, Nonlinear vibrations in homogeneous non-prismatic timoshenko cantilevers, Journal of Computational and Nonlinear Dynamics, Vol: 16, Pages: 1-9, ISSN: 1555-1415
Deep cantilever beams, modelled using Timoshenko beam kinematics, have numerous applications in engineering. This study deals with the nonlinear dynamic response in a non-prismatic Timoshenko beam characterized by considering the deformed configuration of the axis. The mathematical model is derived using the extended Hamilton's principle under the condition of finite deflections and angles of rotation. The discrete model of the beam motion is constructed based on the finite difference method (FDM), whose validity is examined by comparing the results for a special case with the corresponding data obtained by commercial finite element (FE) software ABAQUS 2019. The natural frequencies and vibration modes of the beam are computed. These results demonstrate decreasing eigenfrequency in the beam with increasing amplitudes of nonlinear oscillations. The numerical analyses of forced vibrations of the beam show that its points oscillate in different manners depending on their relative position along the beam. Points close to the free end of the beam are subject to almost harmonic oscillations, and the free end vibrates with a frequency equal to that of the external force. When a point approaches the clamped end of the beam, it oscillates in two-frequency mode and lags in phase from the oscillations of the free end. The analytical model allows for the study of the influence of each parameter on the eigenfrequency and the dynamic response. In all cases, a strong correlation exists between the results obtained by the analytical model and ABAQUS, nonetheless, the analytical model is computationally less expensive.
Soleiman Fallah A, Mehreganian N, 2021, Blast loading effects on aircraft fuselage, Multiphysics Simulations in Automotive and Aerospace Applications, Publisher: Elsevier, ISBN: 9780128178997
Given the book's comprehensive coverage, automotive and aerospace engineers, designers, graduate students and researchers involved in the simulation of practical coupling problems will find the book useful in its approach.
Mehreganian N, Fallah AS, Sareh P, 2021, Structural mechanics of negative stiffness honeycomb metamaterials, Journal of Applied Mechanics, Vol: 88, Pages: 051006-1-051006-10, ISSN: 0021-8936
The development of multi-stable structural forms has attracted considerable attention in the design of architected multi-materials, metamaterials, and morphing structures, as a result of some unusual properties such as negative stiffness and, possibly, negative Poisson's ratio. Multi-stability is achieved through a morphological change of shape upon loading, and in doing so multi-stable structures undergo transitions from one equilibrium state to another. This paper investigates the structural performance of the negative stiffness honeycomb (NSH) metamaterials made of double curved beams which are emerging in various applications such as sensors, actuators, and lightweight impact protective structures with structural tunability and recoverability. An analytical treatment is pursued using the Euler–Lagrange theorem and the stability of the honeycomb has been studied. Based on a static analysis of the nonlinear elastic system, the developed tangent stiffness matrix and ensuing deformation curve were assessed through multiple phases of deformation. The closed-form solution was in good agreement with the numerical finite element (FE) model at different bistability ratios. It was shown that the bistability ratio had a pronounced effect on the overall response of the honeycomb and the desired negativity in the stiffness matrix could be achieved with high bistability ratios.
Gorshkov V, Sareh P, Navadeh N, et al., 2021, Multi-resonator metamaterials as multi-band metastructures, Materials & Design, Vol: 202, Pages: 1-13, ISSN: 0264-1275
Fallah AS, 2021, Dynamic analysis of cylindrical shells subject to multiple blasts using FSI, The International Journal of Multiphysics, Vol: 15, ISSN: 1750-9548
Localised pressure pulse loads can pose a significant threat to structural elements and critical equipment and may cause failure and damage in the target due to the concentrated energy delivered upon a localised area of the target. The impulse impinged upon the local area at the contact interface can exceed 80% of the total impulse that the charge can deliver upon the infinite target, leading to potential perforation of the structural element. When multiple charges are detonated, the advection of gaseous products depends, among other parameters such as fluid density, on the type of blast wave interference and superposition.This work examines the influence of multiple charge detonations blasted in the air on the external surface of cylindrical shells. Two types of detonations were considered, viz. simultaneous and sequential. In both cases the charges were positioned at 50mm and 75mm stand-off to the right and left of the shell. The Fluid-Structure Interaction (FSI) phenomenon was investigated in each scenario. The pressure registered with the gauge points of the rigid target was implemented in an uncoupled study on a flexible target which demonstrated different mode shapes occurring in the shell due to each blast scenario.A dimensionless impulse parameter was defined based on the Gaussian distribution function associated with the load shape, which renders the probability of the impulse as the total impulse that can potentially be imparted to the target.
Fallah AS, Giannakeas IN, Mella R, et al., 2020, On the computational derivation of bond-based peridynamic stress tensor, Journal of Peridynamics and Nonlocal Modeling, Vol: 2, Pages: 352-378, ISSN: 2522-896X
The concept of ‘contact stress’, as introduced by Cauchy, is a special case of a nonlocal stress tensor. In this work, the nonlocal stress tensor is derived through implementation of the bond-based formulation of peridynamics that uses an idealised model of interaction between points as bonds. The method is sufficiently general and can be implemented to study stress states in problems containing stress concentration, singularity, or discontinuities. Two case studies are presented, to study stress concentration around a circular hole in a square plate and conventionally singular stress fields in the vicinity of a sharp crack tip. The peridynamic stress tensor is compared with finite element approximations and available analytical solutions. It is shown that peridynamics is capable of capturing both shear and direct stresses and the results obtained correlate well with those obtained using analytical solutions and finite element approximations. A built-in MATLAB code is developed and used to construct a 2D peridynamic grid and subsequently approximate the solution of the peridynamic equation of motion. The stress tensor is then obtained using the tensorial product of bond force projections for bonds that geometrically pass through the point. To evaluate the accuracy of the predicted stresses near a crack tip, the J-integral value is computed using both a direct contour approximation and the equivalent domain integral method. In the formulation of the contour approximation, bond forces are used directly while the proposed peridynamic stress tensor is used for the domain method. The J-integral values computed are compared with those obtained by the commercial finite element package Abaqus 2018. The comparison provides an indication on the accurate prediction of the state of stress near the crack tip.
Giannakeas IN, Papathanasiou TK, Fallah AS, et al., 2020, Coupling XFEM and Peridynamics for brittle fracture simulation: part II-adaptive relocation strategy, COMPUTATIONAL MECHANICS, Vol: 66, Pages: 683-705, ISSN: 0178-7675
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Giannakeas IN, Papathanasiou TK, Fallah AS, et al., 2020, Coupling XFEM and peridynamics for brittle fracture simulation—part I: feasibility and effectiveness, Computational Mechanics, Vol: 66, Pages: 103-122, ISSN: 0178-7675
A peridynamics (PD)–extended finite element method (XFEM) coupling strategy for brittle fracture simulation is presented. The proposed methodology combines a small PD patch, restricted near the crack tip area, with the XFEM that captures the crack body geometry outside the domain of the localised PD grid. The feasibility and effectiveness of the proposed method on a Mode I crack opening problem is examined. The study focuses on comparisons of the J integral values between the new coupling strategy, full PD grids and the commercial software Abaqus. It is demonstrated that the proposed approach outperforms full PD grids in terms of computational resources required to obtain a certain degree of accuracy. This finding promises significant computational savings when crack propagation problems are considered, as the efficiency of FEM and XFEM is combined with the inherent ability of PD to simulate fracture.
Soleiman Fallah A, Ghajari M, Safa Y, 2019, Computational modelling of dynamic delamination in morphing composite blades and wings, The International Journal of Multiphysics, Vol: 13, Pages: 393-430, ISSN: 1750-9548
Morphing blades have been promising in lifting restrictions on rated capacity of wind turbines and improving lift-to-drag ratio for aircraft wings at higher operational angles of attack. The present study focuses on one aspect of the response of morphing blades viz. dynamic delamination. A numerical study of delamination in morphing composite blades is conducted. Both components i.e. the composite part and the stiffener are studied. The eXtended Finite Element Method (XFEM) and nonlocal continuum mechanics (peridynamics) have both been used to study fracture in the isotropic stiffener used in conjunction with the blade. As for the composite morphing blade, cohesive elements are used to represent the interlaminar weak zone and delamination has been studied under dynamic pulse loads. Intraply damage is studied using the nonlocal model as the peridynamic model is capable of addressing the problem adequately for the necessary level of sophistication.The differences and similarities between delamination patterns for impulsive, dynamic, and quasi-static loadings are appreciated and in each case detailed analyses of delamination patterns are presented. The dependence of delamination pattern on loading regime is established, however; further parametric studies are not included as they lie beyond the scope of the study. Through the use of fracture energy alone the nonlocal model is capable of capturing intra- and interlaminar fractures. The proposed modelling scheme can thus have a major impact in design applications where dynamic pulse and impact loads of all natures (accidental, extreme, service, etc.) are to be considered and may therefore be utilised in design of lightweight morphing blades and wings where delamination failure mode is an issue.
Mehreganian N, Fallah AS, Louca LA, 2019, Nonlinear dynamics of locally pulse loaded square Föppl-von Kármán thin plates, International Journal of Mechanical Sciences, Vol: 163, ISSN: 0020-7403
Modern armour graded thin steel plates benefit from significant elastic strength with high elastic energy storage capacity, which contributes to dissipation of total impulse from extensive blast loads within the bounds of the elastic region. Higher elastic energy storage capability mitigates the probability of catastrophic damage and ensuing large deformations compared to the conventional graded metallic panels. While blast assessment of such structures is important to design and application of protective systems, limited studies are available on their response to localised blasts.The present paper aims at deducing, from the minimization of Föppl-von Kármán (FVK) energy functional, the dynamic response of localised blast loaded thin elastic square plates undergoing large deformations. The presumed blast load function is a multiplicative decomposition of a prescribed continuous piecewise smooth spatial function and an arbitrary temporal function which may assume various shapes (e.g. rectangular, linear, sinusoidal, exponential).A kinematically admissible displacement field and the associated stress tensor were considered as a truncated cosine series with multiple Degrees-of-Freedom (DoF's). From the prescribed displacement field, having simply supported boundary conditions, useful expressions for stress tensor components were delineated corresponding to a unique mode and a series of differential equations were derived. The explicit solutions were sought using the Poincaré-Lindstedt perturbation method. The closed form solutions of each mode were corroborated with the numerical FE models and showed convergence when the first few modes were considered. The influence of higher modes, however, on the peak deformation was negligible and the solution with 3 DOF's conveniently estimated the blast response to a satisfactory precision.
Soleiman Fallah A, Zhuk Y, Gorshko I, et al., 2019, Approximate Mode-based Simulation of Composite Wind Turbine Blade Vibrations using a Simplified Beam Model, European Journal of Computational Mechanics, ISSN: 1779-7179
Gorshkov VN, Sareh P, Tereshchuk VV, et al., 2019, Dynamics of Anisotropic Break‐Up in Nanowires of FCC Lattice Structure, Advanced Theory and Simulations, Vol: 2, Pages: 1900118-1900118, ISSN: 2513-0390
Soleiman Fallah A, Louca L, Mehreganian N, et al., 2019, Dynamic plastic response of beams subjected to localised pulse loads, International Journal of Protective Structures, Vol: 10, Pages: 198-228, ISSN: 2041-4196
Localised blast loads give rise to high gradients of overpressure detrimental to structural elements as beams and plates. This article presents an analytical study on the dynamic plastic response of beams made of a ductile metallic material due to close-in pulse pressure loading. The close-in pressure load is characterised by a spatially varying function constant over a central region and exponentially decaying beyond it. The temporal pulse shape is assumed to take different forms. The exact static plastic collapse load was obtained for the characteristic load using the framework of plastic limit analysis, whereby the analysis was then extended to the dynamic case by considering the appropriate yield surface and inclusion of inertia forces. The yield surfaces considered were representative of pure bending, the interactions between the bending moment and transverse shear, and bending moment and membrane force, each corresponding to a special case depending on the geometry of the beam. A time-dependent, kinematically admissible velocity profile was assumed to treat the dynamic formulations in interaction of each phenomenon. A study on the strain-rate sensitivity was also presented, and existence of a critical pressure triggering the apparition of travelling plastic hinges was hence highlighted. For blast loads of high magnitude, the expressions for normalised deflection were furnished in terms of the impulsive velocity. The analytical models were validated by performing a parametric study on the two-dimensional representative of the beam model in commercial finite element software ABAQUS 6.14. The numerical results show a good correlation with the analytical results in each case.
Mehreganian N, Fallah AS, Louca LA, 2019, Plastic dynamic response of simply supported thick square plates subject to localised blast loading, International Journal of Impact Engineering, Vol: 126, Pages: 85-100, ISSN: 0734-743X
Localised blast loads due to proximal charges are encountered in a variety of circumstances. This paper proposes an analytical solution for the dynamic plastic response of a rigid-perfectly plastic thick square plate subject to a localised explosion. The proposed model is an extension of the analytical model proposed by Micallef et al [1] to study circular plates which is adopted and modified in order to study impulsively loaded square plates where the effect of shear deformation is included. A piecewise continuous blast load function was assumed with axisymmetric spatial distribution of constant pressure in the central zone and exponentially decaying beyond it. Using the constitutive framework of limit analysis and incorporating the interactions between bending moment and transverse shear forces in the analyses, transverse displacement and response duration were examined on three classes of plates, classified according to the length to thickness ratio parameter ν. The results were furnished in terms of the impulsive velocity, which is a function of the localised blast load parameters. A theoretical solution for plates with ν > 2 was sought for the non-impulsive blast loads. Parametric studies were performed to elucidate the effect of loading parameters and plate thickness on the permanent deformation. The theoretical solutions have been found generic and can predict, by the correct choice of the load parameters, the dynamic response of most blast load scenarios brought about by proximal or distal charges. It was found that, for proximal impulsive blasts, the effect of transverse shear becomes irrelevant for even moderate values of ν which effect is inconsequential on both central and endpoint displacements at discontinuous interface in the range of ν > 5. Since the short duration pulse is of concern, localised pressure loads affect only a small area of the plated structures. Thus, whilst the theoretical treatments also examine the fully clamped pl
Fallah AS, Navadeh N, Tereshchuk VV, et al., 2019, Phononic dispersion in anisotropic pseudo-fractal hyper-lattices, Materials & Design, Vol: 164, Pages: 107560-107560, ISSN: 0264-1275
Fractal and pseudo-fractal microstructures have proved promising in increasing the range of detectable frequencies for devices used in the realm of electromagnetism. Due to mechanical-electrical duality it is conjectured they may provide flexible solutions capable of closing/widening bandgaps and increasing tailorability in phononic lattices. Pseudo-fractal hyper-lattices have been considered in this work and different aspects of dispersion surface morphology and frequency band structure are studied. It has been observed that higher frequencies that can be excited in the simple square lattice are almost the same as those in the pseudo-fractal structures, however; through introduction of higher levels the pseudo-fractal hyper-lattice presents new features not observable in the ordinary lattice. By increasing the order of pseudo-fractal structure the number of degrees-of-freedom increases and dispersion surfaces morphologies change thus frequency gaps are eliminated. This phenomenon can be of advantage for acoustic/phononic visibility/detectability e.g. in designing sensors. In the classical analogy to quantum level repulsion surfaces flatten which sufficiently decreases the sound group velocity in the pseudo-fractal structure, and can be used for numerous practical applications.
Chen Y, Sareh P, Yan J, et al., 2019, An integrated geometric-graph-theoretic approach to representing origami structures and their corresponding truss frameworks, Journal of Mechanical Design, ISSN: 1050-0472
Origami has provided various interesting applications in science and engineering. Appropriate representations and evaluation on crease patterns play an important role in developing an innovative origami structure with desired characteristics. However, this is generally a challenge encountered by scientists and engineers who introduce origami into various fields. As most practical origami structures contain repeated unit cells, graph products provide a suitable choice for the formation of crease patterns. Here, we will employ undirected and directed graph products as a tool for the representation of crease patterns and their corresponding truss frameworks of origami structures. Given that an origami crease pattern can be considered to be a set of directionless crease lines which satisfy the foldability condition, we demonstrate that the pattern can be exactly expressed by a specific graph product of independent graphs. It turns out that this integrated geometric-graph-theoretic method can be effectively implemented in the formation of different crease patterns, and provide suitable numbering of nodes and elements. Furthermore, the presented method is useful in constructing the involved matrices and models of origami structures, and thus enhances configuration processing for geometric, kinematic or mechanical analysis on origami structures.
Mehreganian N, Fallah AS, Louca LA, 2019, Dynamic Performance of Simply Supported Rigid-Plastic Square Plates Subject to Localized Blast Loading, Journal of Engineering Mechanics, Vol: 145, ISSN: 0733-9399
This paper presents the theoretical solution to the response of a square plate undergoing plastic deformation due to a generic localized blast pulse. A localized blast load function was assumed multiplicative of its spatial distribution and temporal pulse shape. The spatial distribution was representative of constant pressure over the central zone, while exponentially decaying outside that zone. Considering an appropriate moment function and ignoring the membrane, transverse shear, and rotary inertia effects, the static plastic collapse was found, whereby the analysis was extended to the dynamic case by assuming a kinematically admissible, time-dependent velocity profile. The analytical model, which was validated against the numerical results obtained through ABAQUS hydrocode, showed close correlation in terms of the permanent transverse deflection profile. In order to consider the effect of temporal pulse shape, the results were formulated for rectangular as well as exponentially and linearly decaying pulses. For blast loads of high magnitude, the pressure load was replaced by an impulsive velocity. The calculations were simplified by utilizing the dimensionless form, and the results were corroborated with theoretical and experimental results from the literature. The model showed improvements in predicting the final deformation of square plates over previous models of simplified loading function.
Rubiella C, Hessabi CA, Soleiman Fallah A, 2018, State of the art in fatigue modelling of composite wind turbine blades, International Journal of Fatigue, Vol: 117, Pages: 230-245, ISSN: 0142-1123
This paper provides a literature review of the most notable models relevant to the evaluation of the fatigue response of composite wind turbine blades. As wind turbines spread worldwide, ongoing research to maximize their lifetime – and particularly that of wind turbine blades – has increasingly popularized the use of composite materials, which boast attractive mechanical properties. The review first presents the wind turbine blade environment, before distributing fatigue models broadly between three categories: life-based failure criterion models, which are based on S-N curve formulations and constant-life diagrams to introduce failure criteria; residual property calculation models, which evaluate the gradual degradation of material properties; and progressive damage models, which model fatigue via the cycle-by-cycle growth of one or more damage parameters. These are then linked to current testing standards, databases, and experimental campaigns. Among the fatigue modelling approaches covered, progressive damage models appear to be the most promising tool, as they both quantify and qualify physical damage growth to a reasonable extent during fatigue. The lack of consensus and shortcomings of literature are also discussed, with abundant referencing.
Mehreganian N, Soleiman Fallah A, Louca L, 2018, Inelastic dynamic response of square membranes subjected to localised blast loading, International Journal of Mechanical Sciences, Vol: 148, Pages: 578-595, ISSN: 0020-7403
Extensive shock and highly localised blast waves generated by detonation of near field explosives (such as improvised explosive devices (IEDs)) are catastrophic to structures and humans, resulting in injury or death, progressive damage, or perforation through the structure and collapse. Mitigating the effects of such waves is paramount in various aspects of design engineering. A theoretical model is presented here to predict the large inelastic deformation of ductile thin square membranes induced by a generic, short pulse pressure load, comprising a piecewise function of spatial and temporal parts. Using the constitutive framework of limit analysis and incorporating the influence of finite displacements, two patterns of kinematically admissible, time dependent velocity profiles were investigated. These patterns included stationery and moving plastic hinges. The results were investigated in two cases: once with the interaction between bending moment and membrane forces retained in the analyses, and then when the response was solely governed by membrane forces.For blast loads of high magnitude, the pressure was replaced by an impulsive velocity and the results were expressed in terms of dimensionless form of initial kinetic energy. The effects of boundary conditions and visco-plasticity have also been investigated. The theoretical results corroborated well with various experimental results in the literature, on ductile metallic plates such as high strength ARMOX steel and mild steel.
Toolabi M, Fallah AS, Louca L, 2018, Enhanced mixed interpolation XFEM formulations for discontinuousTimoshenko beam and Mindlin-Reissner plate, International Journal for Numerical Methods in Engineering, Vol: 115, Pages: 714-737, ISSN: 0029-5981
Shear locking is a major issue emerging in the computational formulation of beam and plate finite elements of minimal number of degrees of freedom as it leads to artificial overstiffening. In this paper, discontinuous Timoshenko beam and Mindlin‐Reissner plate elements are developed by adopting the Hellinger‐Reissner functional with the displacements and through‐thickness shear strains as degrees of freedom. Heterogeneous beams and plates with weak discontinuity are considered, and the mixed formulation has been combined with the extended finite element method (FEM); thus, mixed enrichment functions are used. Both the displacement and the shear strain fields are enriched as opposed to the traditional extended FEM where only the displacement functions are enriched. The enrichment type is restricted to extrinsic mesh‐based topological local enrichment. The results from the proposed formulation correlate well with analytical solution in the case of the beam and in the case of the Mindlin‐Reissner plate with those of a finite element package (ABAQUS) and classical FEM and show higher rates of convergence. In all cases, the proposed method captures strain discontinuity accurately. Thus, the proposed method provides an accurate and a computationally more efficient way for the formulation of beam and plate finite elements of minimal number of degrees of freedom.
Navadeh N, Hewson RW, Fallah AS, 2018, Dynamics of transversally vibrating non-prismatic Timoshenko cantilever beams, Engineering Structures, ISSN: 0141-0296
Soleiman Fallah A, Mehreganian N, Boiger GK, et al., 2017, Response of Armour Steel Plates to localised Air Blast Load – A Dimensional Analysis, International Journal of Multiphysics, Vol: 11, Pages: 387-412, ISSN: 1750-9548
We report on the results of dimensional analyses on the dynamic plasticresponse of square armour steel plates due to detonation of proximalcylindrical charges and ensued air blast loading. By assuming a genericfunction for the blast load, which is multiplicative comprising its spatial andtemporal parts, a set of 14 dimensionless parameters, representative of theload and plate deformation, were identified and recast in the form ofdimensionless functions of stand-off to charge diameter ratio. Parametricstudies were performed using commercial code ABAQUS’s module of FiniteElement hydrocode using MMALE and MMAE techniques, and combinedwith regression analyses to quantify the dimensional parameters and theexpressions for dimensionless functions. A few numerical studies withvarious FE mesh types were also performed to validate the transientdeflections against the small-scale experiments. For pulse loading due toproximal charges of small orders of stand-off/charge diameter ratio, themagnitude of the transverse deflection increased abruptly with incrementaldecrease in stand-off, in contradistinction to the plate deformations athigher stand-offs where variations in displacement are smooth. Thisconfirmed the existence of a stand-off at which a transition in behaviourtakes place. For stand-off values less than charge diameter, a dimensionlessenergy absorbing effectiveness factor was considered to investigate theprediction of rupture in the plate corresponding to different charge masses.This factor is measured as a baseline parameter to predict, using solelynumerical means, the blast loads which ensue rupture on full-scaleprototypes.
Navadeh N, Gorshko IO, Zhuk YA, et al., 2017, An FEM-based AI approach to model parameter identification for low vibration modes of wind turbine composite rotor blades, European Journal of Computational Mechanics, Vol: 26, Pages: 541-556, ISSN: 1779-7179
An approach to construction of a beam-type simplified model of a horizontal axis wind turbine composite blade based on the finite element method is proposed. The model allows effective and accurate description of low vibration bending modes taking into account the effects of coupling between flapwise and lead–lag modes of vibration transpiring due to the non-uniform distribution of twist angle in the blade geometry along its length. The identification of model parameters is carried out on the basis of modal data obtained by more detailed finite element simulations and subsequent adoption of the ‘DIRECT’ optimisation algorithm. Stable identification results were obtained using absolute deviations in frequencies and in modal displacements in the objective function and additional a priori information (boundedness and monotony) on the solution properties.
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