Optimal design for active damping in sandwich structures using the Direct MultiSearch method

This paper addresses the problem of optimal positioning of surface bonded piezoelectric patches in sandwich plates with viscoelastic core and laminated face layers. The objective is to maximize a set of modal loss factors for a given frequency range using multiobjective topology optimization. Active damping is introduced through co-located negative velocity feedback control. The multiobjective topology optimization problem is solved using the Direct MultiSearch Method. An application to a simply supported sandwich plate is presented with results for the maximization of the first six modal loss factors. The influence of the finite element mesh is analyzed and the results are, to some extent, compared with those obtained using alternative single objective optimization. (C) 2013 Elsevier Ltd. All rights reserved.

Optimal design for active damping in sandwich structures using the Direct MultiSearch method

This paper addresses the problem of optimal positioning of surface bonded piezoelectric patches in sandwich plates with viscoelastic core and laminated face layers. The objective is to maximize a set of modal loss factors for a given frequency range using multiobjective topology optimization. Active damping is introduced through co-located negative velocity feedback control. The multiobjective topology optimization problem is solved using the Direct MultiSearch Method. An application to a simply supported sandwich plate is presented with results for the maximization of the first six modal loss factors. The influence of the finite element mesh is analyzed and the results are, to some extent, compared with those obtained using alternative single objective optimization.

Multiobjective optimization of viscoelastic laminated sandwich structures using the Direct MultiSearch method

A multiobjective approach for optimization of passive damping for vibration reduction in sandwich structures is presented in this paper. Constrained optimization is conducted for maximization of modal loss factors and minimization of weight of sandwich beams and plates with elastic laminated constraining layers and a viscoelastic core, with layer thickness and material and laminate layer ply orientation angles as design variables. The problem is solved using the Direct MultiSearch (DMS) solver for derivative-free multiobjective optimization and solutions are compared with alternative ones obtained using genetic algorithms.

Performance Assessment of Sandwich Structures with Debonds and Dents

A sandwich construction is a special form of the laminated composite consisting of light weight core, sandwiched between two stiff thin face sheets. Due to high stiffness to weight ratio, sandwich construction is widely adopted in aerospace industries. As a process dependent bonded structure, the most severe defects associated with sandwich construction are debond (skin core bond failure) and dent (locally deformed skin associated with core crushing). Reasons for debond may be attributed to initial manufacturing flaws or in service loads and dent can be caused by tool drops or impacts by foreign objects. This paper presents an evaluation on the performance of honeycomb sandwich cantilever beam with the presence of debond or dent, using layered finite element models. Dent is idealized by accounting core crushing in the core thickness along with the eccentricity of the skin. Debond is idealized using multilaminate modeling at debond location with contact element between the laminates. Vibration and buckling behavior of metallic honeycomb sandwich beam with and without damage are carried out. Buckling load factor, natural frequency, mode shape and modal strain energy are evaluated using finite element package ANSYS 13.0. Study shows that debond affect the performance of the structure more severely than dent. Reduction in the fundamental frequencies due to the presence of dent or debond is not significant for the case considered. But the debond reduces the buckling load factor significantly. Dent of size 8-20% of core thickness shows 13% reduction in buckling load capacity of the sandwich column. But debond of the same size reduced the buckling load capacity by about 90%. This underscores the importance of detecting these damages in the initiation level itself to avoid catastrophic failures. Influence of the damages on fundamental frequencies, mode shape and modal strain energy are examined. Effectiveness of these parameters as a damage detection tool for sandwich structure is also assessed

Analysis of functionally graded sandwich plate structures with piezoelectric skins, using B-spline finite strip method

Functionally graded materials are composite materials wherein the composition of the constituent phases can vary in a smooth continuous way with a gradation which is function of its spatial coordinates. This characteristic proves to be an important issue as it can minimize abrupt variations of the material properties which are usually responsible for localized high values of stresses, and simultaneously providing an effective thermal barrier in specific applications. In the present work, it is studied the static and free vibration behaviour of functionally graded sandwich plate type structures, using B-spline finite strip element models based on different shear deformation theories. The effective properties of functionally graded materials are estimated according to Mori-Tanaka homogenization scheme. These sandwich structures can also consider the existence of outer skins of piezoelectric materials, thus achieving them adaptive characteristics. The performance of the models, are illustrated through a set of test cases. (C) 2012 Elsevier Ltd. All rights reserved.

Analysis of sandwich beam structures using kriging based higher order models

Functionally graded composite materials can provide continuously varying properties, which distribution can vary according to a specific location within the composite. More frequently, functionally graded materials consider a through thickness variation law, which can be more or less smoother, possessing however an important characteristic which is the continuous properties variation profiles, which eliminate the abrupt stresses discontinuities found on laminated composites. This study aims to analyze the transient dynamic behavior of sandwich structures, having a metallic core and functionally graded outer layers. To this purpose, the properties of the particulate composite metal-ceramic outer layers, are estimated using Mod-Tanaka scheme and the dynamic analyses considers first order and higher order shear deformation theories implemented though kriging finite element method. The transient dynamic response of these structures is carried out through Bossak-Newmark method. The illustrative cases presented in this work, consider the influence of the shape functions interpolation domain, the properties through-thickness distribution, the influence of considering different materials, aspect ratios and boundary conditions. (C) 2014 Elsevier Ltd. All rights reserved.

Dynamic behaviour of soft core sandwich beam structures using kriging-based layerwise models

Sandwich structures with soft cores are widely used in applications where a high bending stiffness is required without compromising the global weight of the structure, as well as in situations where good thermal and damping properties are important parameters to observe. As equivalent single layer approaches are not the more adequate to describe realistically the kinematics and the stresses distributions as well as the dynamic behaviour of this type of sandwiches, where shear deformations and the extensibility of the core can be very significant, layerwise models may provide better solutions. Additionally and in connection with this multilayer approach, the selection of different shear deformation theories according to the nature of the material that constitutes the core and the outer skins can predict more accurately the sandwich behaviour. In the present work the authors consider the use of different shear deformation theories to formulate different layerwise models, implemented through kriging-based finite elements. The viscoelastic material behaviour, associated to the sandwich core, is modelled using the complex approach and the dynamic problem is solved in the frequency domain. The outer elastic layers considered in this work may also be made from different nanocomposites. The performance of the models developed is illustrated through a set of test cases. (C) 2015 Elsevier Ltd. All rights reserved.

Uumajäykistetyt sandwich-rakenteet

Tässä työssä tutkittiin uumajäykistettyä sandwich-rakennetta. Tutkimuksen tavoitteena oli kehittää kuormitetulle uumajäykistetylle sandwich-elementille taipuman analyyttinen laskentamenetelmä. Sellaisen kehitykselle oli tarve, koska kyseisen tyyppiselle sandwich-rakennetyypille ei ole standardisoitua taipuman laskentamenetelmää. Uumajäykistetyn sandwich-elementin taipuma on kiinnostava, koska käyttörajatilan mukainen taipumaraja on tärkein rakennetta mitoittava tekijä. Työn soveltavassa osiossa kehitettiin uumajäykistetyn sandwich-elementin taipuman analyyttinen laskentamenetelmä. Taipuman analyyttinen laskentamenetelmä kehitettiin sandwich-palkkiteorian perusteella. Keskeisimpiä ongelmia oli uumajäykisteiden taivutus- ja leikkausjäykkyyden huomioonottaminen laskennassa. Analyyttisen laskennan tuloksia verrattiin uumajäykistetylle sandwich-elementille suoritettujen taivutuskokeiden tuloksiin sekä FE-analyysin tuloksiin. Tutkimustulosten perusteella voitiin todeta kehitetyn uumajäykistetyn sandwich-elementin taipuman analyyttisen laskentamenetelmän soveltuvan taipuman laskentaan tutkitulle uumajäykistetylle sandwich-elementille. Laskentamenetelmän luotettavuuden ja toimivuusrajojen määrittämiseksi uumajäykistetty sandwich-rakenne vaatii jatkotutkimuksia.

Finite element homogenization technique for the characterization of d(15) shear piezoelectric macro-fibre composites

A finite element homogenization method for a shear actuated d(15) macro-fibre composite (MFC) made of seven layers (Kapton, acrylic, electrode, piezoceramic fibre and epoxy composite, electrode, acrylic, Kapton) is proposed and used for the characterization of its effective material properties. The methodology is first validated for the MFC active layer only, made of piezoceramic fibre and epoxy, through comparison with previously published analytical results. Then, the methodology is applied to the seven-layer MFC. It is shown that the packaging reduces significantly the shear stiffness of the piezoceramic material and, thus, leads to significantly smaller effective electromechanical coupling coefficient k(15) and piezoelectric stress constant e(15) when compared to the piezoceramic fibre properties. However, it is found that the piezoelectric charge constant d(15) is less affected by the softer layers required by the MFC packaging.

Estudo, fabrico e caracterização de painéis sanduíche com núcleos em materiais compósitos de cortiça

Trabalho Final de Mestrado para obtenção do grau de Mestre em Engenharia Mecânica

Synthesis of bone-like structured foams

Here we present a processing route to produce multi-structured ceramic foams based on the combination of particle-stabilized foams with polymeric sponges to produce positive and negative templating structures. Polyester sponges are infiltrated with freshly produced calcium aluminate alumina foams and upon sintering either positive templating structures are produced when wetting the sponges, or negative templating foams with a percolating pore network are obtained when completely filling the sponges. Additionally, by combining different layers of these particle-stabilized foam infiltrated sponges, various different structures can be produced, including sandwich structures, pore size gradients, and ceramic bone-like structures applying to different types of bone. The particle-stabilized foams used were in situ self-hardening calcium aluminate cement enriched alumina foams to obtain crack-free samples with pore interconnections and tailorable pore sizes.

Ultrakevyen rahtialuksen kansiluukkujen sekä luukkujen siirto- ja varastointijärjestelmän suunnittelu hyödyntäen kevytrakennekonsepteja

Tämän työn tarkoituksena oli tutkia miten rahtialuksen kansiluukut voitaisiin valmistaa mahdollisimman kevyiksi. Katettavan ruuman pinta-ala on n. 10 m x 40 m. Luukkujen suuresta jännevälistä johtuen, rakenteelta vaaditaan suurta jäykkyyttä. Erilaisina vaihtoehtoina tutkittiin vaahtomaista alumiinia, alumiinisia kennorakenteita ja polyuretaanisia sandwich-rakenteita. Työssä vertailtiin myös erilaisia geometrisia ratkaisuja, joilla kansiluukkujen jäykkyyttä pyrittiin lisäämään ja sitä kautta pääsemään pienempään materiaalin tarpeeseen. Geometriaa suunniteltaessa huomioitiin myös vaikutukset ruuman tilavuuteen ja lainsäädännön asettamat reunaehdot. Lainsäädännöstä saatiin esimerkiksi turvakaiteiden minimikorkeus, joka vaikuttaa suoraan ruuman tilavuuteen, kun aluksen korkeimmaksi kohdaksi on valittu laivan keskilinja ja tämä korkeus on annettu suunnittelun lähtötietona. Tietokoneavusteisen lujuuslaskennan avulla eri vaihtoehdoista muodostettiin elementtimallit. Malleja varioimalla ja tuloksia vertailemalla saatiin selville kevyin mahdollinen rakenne ja geometria. Malleista saatiin selville myös luukkujen tukireaktiovoimat, eli voimat, jotka luukut kohdistavat aluksen muihin rakenteisiin. Lisäksi työssä mietittiin erilaisia tapoja ruuman avaamiseen ja avaamistavan vaikutusta kansiluukkujen painoon, geometriaan ja ruuman tilavuuteen.

Polymer light emitting devices with Langmuir Blodgett (LB) films of a polyfluorene derivative

Polyfluorenes are promising materials for the emitting layer of polymer light emitting devices (PLEDs) with blue emission. In this work, we report on PLEDs fabricated with Langmuir-Blodgett (LB) films of a polyfluorene derivative, namely poly(9,9-di-hexylfluorenediyl vinylene-alt-1,4-phenylenevinylene) (PDHF-PV). Y-type LB films were transferred onto ITO substrates at a surface pressure of 35 mN m-1 and with dipping speed of 3 mm min -1. A thin aluminum layer was evaporated on top of the LB film, thus yielding a sandwich structure (ITO/PDHF-PV(LB)/Al). Current-voltage (I vs V) measurements indicate that the device displays a classical behavior of a rectifying diode. The threshold value is approximately 5 V, and the onset for visible light emission occurs at ca. 10 V. From the a.c. electrical responses we infer that the active layer has a typical behavior of PLEDs where the real component of ac conductivity obeys a power-law with the frequency. Cole-Cole plots (Im(Z) vs. Re(Z)) for the device exhibit a series of semicircles, the diameter of which decreases with increasing forward bias. This PLED structure is modeled by a parallel resistance and capacitance combination, representing the dominant mechanisms of charge transport and polarization in the organic layer, in series with a resistance representing the ITO contact. Overall, the results presented here demonstrate the feasibility of LEDs made with LB films of PDHF-PV.

Application of UTHSCSA-Image ToolTM to estimate concrete texture

The texture of concrete blocks is very important and is often the decisive factor when choosing a product, particularly if the building specifications does not dispense with the high resistance of the blocks, but has the purpose of reducing costs with finishing, therefore preferring exposed blocks with a closer texture. Furthermore, a closer texture, especially for exteriors,may be the vital factor of the building's pathology.However, there is so far no standard to quantify the texture of a structural block. This article proposes to apply the freely available UTHSCSA-Image ToolTM program developed by the University of Texas Health Science Center at San Antonio to evaluate the texture of masonry blocks. One aspect that should never be overlooked when studying masonry blocks is compressive strength. Therefore, this work also gets the compressive strength of the blocks with and without the addition of lime. The addition of small quantities of lime proved beneficial for both texture and compressive strength. However, increasing the amount of lime proved to be feasible only to improve texture. © 2012 Taylor & Francis Group.

Parametric Analysis of Effective Material Properties of Thickness-Shear Piezoelectric Macro-Fibre Composites

A previous study on the characterization of effective material properties of a d(15) thickness-shear piezoelectric Macro-Fibre Composite (MFC) made of seven layers (Kapton, Acrylic, Electrode, Piezoceramic Fibre and Epoxy Composite, Electrode, Acrylic, Kapton) using a finite element homogenization method has shown that the packaging reduces significantly the shear stiffness of the piezoceramic material and, thus, leads to significantly smaller effective electromechanical coupling coefficient k(15) and piezoelectric stress constant e(15) when compared to the piezoceramic fibre properties. Therefore, the main objective of this work is to perform a parametric analysis in which the effect of the variations of fibre volume fraction, Epoxy elastic modulus, electrode thickness and active layer thickness on the MFC effective material properties is evaluated. Results indicate that an effective d(15) MFC should use relatively thick fibres having relatively high shear modulus and relatively stiff epoxy filler. On the other hand, the electrode thickness does not affect significantly the MFC performance.