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.

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.

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 d15 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 k15 and piezoelectric stress constant e15 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 d15 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.

Mechanical properties of chemically modified portuguese pinewood

To turn wood into a construction material with enhanced properties, many methods of chemical modification have been developed in the last few decades. In this work, mechanical properties of pine wood were chemically modified, compared and evaluated. Maritime pine wood (Pinus pinaster) was modified with four chemical processes: 1,3-dimethylol-4,5- dihydroxyethyleneurea, N-methylol melamine formaldehyde, tetra-alkoxysilane and wax. The following mechanical properties were assessed experimentally: Modulus of elasticity measured statically, stiffness stabilization efficiency in different climates (30 and 87% of relative humidity), modulus of rupture, work maximum load, impact bending strength, compression, tensile and shear strength at indoor conditions (65% of relative humidity). In both types of active principle of modification, cell wall or lumen fill, no significant changes on the bending stiffness (modulus of elasticity) were found. In the remaining properties analysed significant changes in the modified wood-material took place compared to unmodified wood control: - Cell wall modification was the most effective method to achieve high stiffness stabilization efficiency (up to 60%) and also increased compression strength (up to 230%). However, modulus of rupture, tensile, shear and the impact bending strength were reduced by both resins, but in a varying extent, where the N-methylol melamine formaldehyde endured less reduction than 1,3-dimethylol-4,5-dihydroxyethyleneurea resin. In the latter, reduction up to 60% can take place. - In the lumen fill modification: tetra-alkoxysilane has no effect in the mechanical properties. Although, a slight increase in shear strength parallel to the grain was found. Wax specimens have shown a slight increase in bending strength, compression, tensile and shear strength as well as in the absorption energy capacity.

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.

Performance of Gabion Faced Reinforced Earth Retaining Walls

Gabion faced re.taining walls are essentially semi rigid structures that can generally accommodate large lateral and vertical movements without excessive structural distress. Because of this inherent feature, they offer technical and economical advantage over the conventional concrete gravity retaining walls. Although they can be constructed either as gravity type or reinforced soil type, this work mainly deals with gabion faced reinforced earth walls as they are more suitable to larger heights. The main focus of the present investigation was the development of a viable plane strain two dimensional non linear finite element analysis code which can predict the stress - strain behaviour of gabion faced retaining walls - both gravity type and reinforced soil type. The gabion facing, backfill soil, In - situ soil and foundation soil were modelled using 20 four noded isoparametric quadrilateral elements. The confinement provided by the gabion boxes was converted into an induced apparent cohesion as per the membrane correction theory proposed by Henkel and Gilbert (1952). The mesh reinforcement was modelled using 20 two noded linear truss elements. The interactions between the soil and the mesh reinforcement as well as the facing and backfill were modelled using 20 four noded zero thickness line interface elements (Desai et al., 1974) by incorporating the nonlinear hyperbolic formulation for the tangential shear stiffness. The well known hyperbolic formulation by Ouncan and Chang (1970) was used for modelling the non - linearity of the soil matrix. The failure of soil matrix, gabion facing and the interfaces were modelled using Mohr - Coulomb failure criterion. The construction stages were also modelled.Experimental investigations were conducted on small scale model walls (both in field as well as in laboratory) to suggest an alternative fill material for the gabion faced retaining walls. The same were also used to validate the finite element programme developed as a part of the study. The studies were conducted using different types of gabion fill materials. The variation was achieved by placing coarse aggregate and quarry dust in different proportions as layers one above the other or they were mixed together in the required proportions. The deformation of the wall face was measured and the behaviour of the walls with the variation of fill materials was analysed. It was seen that 25% of the fill material in gabions can be replaced by a soft material (any locally available material) without affecting the deformation behaviour to large extents. In circumstances where deformation can be allowed to some extents, even up to 50% replacement with soft material can be possible.The developed finite element code was validated using experimental test results and other published results. Encouraged by the close comparison between the theory and experiments, an extensive and systematic parametric study was conducted, in order to gain a closer understanding of the behaviour of the system. Geometric parameters as well as material parameters were varied to understand their effect on the behaviour of the walls. The final phase of the study consisted of developing a simplified method for the design of gabion faced retaining walls. The design was based on the limit state method considering both the stability and deformation criteria. The design parameters were selected for the system and converted to dimensionless parameters. Thus the procedure for fixing the dimensions of the wall was simplified by eliminating the conventional trial and error procedure. Handy design charts were developed which would prove as a hands - on - tool to the design engineers at site. Economic studies were also conducted to prove the cost effectiveness of the structures with respect to the conventional RCC gravity walls and cost prediction models and cost breakdown ratios were proposed. The studies as a whole are expected to contribute substantially to understand the actual behaviour of gabion faced retaining wall systems with particular reference to the lateral deformations.

Verificação de pilares metálicos compostos constituídos por perfis formados a frio por meio de programa computacional

In this work, we discuss the procedures adopted for the design of built-up columns (laced and battened columns). Built-up columns are widely used in steel construction generally when the compression forces are relatively low and the column buckling lengths are large. They are commonly used in industrial buildings, for example, as posts for cladding, or as columns supporting a crane girder. Unlike columns with full section, in the case of built-up columns, it is necessary to evaluate the shear stiffness. In fact, the shear strength leads to a significant reduction of the critical load. In the context of this work, the components of the columns (chords, diagonals, posts, etc.) are formed by cold-formed members. In order to systematize and rationalize the verification of the built-up columns, this work aim to develop a computer program based on the standards NBR 14762, NBR 6355 and Eurocode 3, basically the considerations of the part EN 1993-1-1

Verificação de pilares metálicos compostos constituídos por perfis formados a frio por meio de programa computacional

In this work, we discuss the procedures adopted for the design of built-up columns (laced and battened columns). Built-up columns are widely used in steel construction generally when the compression forces are relatively low and the column buckling lengths are large. They are commonly used in industrial buildings, for example, as posts for cladding, or as columns supporting a crane girder. Unlike columns with full section, in the case of built-up columns, it is necessary to evaluate the shear stiffness. In fact, the shear strength leads to a significant reduction of the critical load. In the context of this work, the components of the columns (chords, diagonals, posts, etc.) are formed by cold-formed members. In order to systematize and rationalize the verification of the built-up columns, this work aim to develop a computer program based on the standards NBR 14762, NBR 6355 and Eurocode 3, basically the considerations of the part EN 1993-1-1

Behaviour of grouted dowels used in precast concrete connections

This research deals with the behaviour of grouted dowels used in beam-to-column connections in precast concrete structures. The research focuses primarily on the theoretical and experimental analysis of the resistance mechanism of the dowels. The experimental programme included 15 models for analysing the following variations in dowel parameters: a) dowel diameters of 16, 20 and 25 mm, b) dowel inclinations of 0 degrees (i.e. perpendicular to the interface), 45 degrees and 60 degrees, c) compressive strength of classes C35 and C50 for the concrete adjacent to the dowels, and d) the absence or presence of compressive loads normal to the interface. The experimental results indicate that the ultimate capacity and shear stiffness of the inclined dowels are significantly higher than those of the perpendicular dowels. Based on these results, an analytical model is proposed that considers the influence of the parameters studied regarding the capacity of the dowel.

Study of concrete cracking during accelerated corrosion tests in reinforced concrete

Cracking of reinforced concrete can occur in certain environments due to rebar corrosion. The oxide layer growing around the bars introduces a pressure which may be enough to lead to the fracture of concrete. To study such an effect, the results of accelerated corrosion tests and finite ele- ment simulations are combined in this work. In previous works, a numerical model for the expansive layer, called expansive joint element , was programmed by the authors to reproduce the effect of the oxide over the concrete. In that model, the expansion of the oxide layer in stress free conditions is simulated as an uniform expansion perpendicular to the steel surface. The cracking of concrete is simulated by means of finite elements with an embedded adaptable cohesive crack that follow the standard cohesive model. In the present work, further accelerated tests with imposed constant cur- rent have been carried out on the same type of specimens tested in previous works (with an embedded steel tube), while measuring, among other things, the main-crack mouth opening. Then, the tests have been numerically simulated using the expansive joint element and the tube as the corroding electrode (rather than a bar). As a result of the comparison of numerical and experimental results, both for the crack mouth opening and the crack pattern, new insight is gained into the behavior of the oxide layer. In particular, quantitative assessment of the oxide expansion relation is deduced from the ex- periments, and a narrower interval for the shear stiffness of the oxide layer is obtained, which could not be achieved using bars as the corroding element, because in that case the numerical results were insensitive to the shear stiffness of the oxide layer within many orders of magnitude

Delamination Arrestment in Bonded-Bolted Composite Structures by Fasteners

Thesis (Ph.D.)--University of Washington, 2016-06

Vibrational characteristics of brain magnetic resonance elastography

This thesis aims to investigate vibrational characteristics of magnetic resonance elastography (MRE) of the brain. MRE is a promising, non-invasive methodology for the mapping of shear stiffness of the brain. A mechanical actuator shakes the brain and generates shear waves, which are then imaged with a special MRI sequence sensitive to sub-millimeter displacements. This research focuses on exploring the profile of vibrations utilized in brain elastography from the standpoint of ultimately investigating nonlinear behavior of the tissue. The first objective seeks to demonstrate the effects of encoding off-frequency vibrations using standard MRE methodologies. Vibrations of this nature can arise from nonlinearities in the system and contaminate the results of the measurement. The second objective is to probe nonlinearity in the dynamic brain system using MRE. A non-parametric decomposition technique, novel to the MRE field, is introduced and investigated.

Refined sandwich model for the vibration of beams with embedded shear piezoelectric actuators and sensors

This work extends a previously presented refined sandwich beam finite element (FE) model to vibration analysis, including dynamic piezoelectric actuation and sensing. The mechanical model is a refinement of the classical sandwich theory (CST), for which the core is modelled with a third-order shear deformation theory (TSDT). The FE model is developed considering, through the beam length, electrically: constant voltage for piezoelectric layers and quadratic third-order variable of the electric potential in the core, while meclianically: linear axial displacement, quadratic bending rotation of the core and cubic transverse displacement of the sandwich beam. Despite the refinement of mechanical and electric behaviours of the piezoelectric core, the model leads to the same number of degrees of freedom as the previous CST one due to a two-step static condensation of the internal dof (bending rotation and core electric potential third-order variable). The results obtained with the proposed FE model are compared to available numerical, analytical and experimental ones. Results confirm that the TSDT and the induced cubic electric potential yield an extra stiffness to the sandwich beam. (C) 2007 Elsevier Ltd. All rights reserved.

Texture alignment in simple shear

We illustrate the flow behaviour of fluids with isotropic and anisotropic microstructure (internal length, layering with bending stiffness) by means of numerical simulations of silo discharge and flow alignment in simple shear. The Cosserat theory is used to provide an internal length in the constitutive model through bending stiffness to describe isotropic microstructure and this theory is coupled to a director theory to add specific orientation of grains to describe anisotropic microstructure. The numerical solution is based on an implicit form of the Material Point Method developed by Moresi et al. [1].

Using neural networks and support vector regression to relate marchetti dilatometer test parameters and maximum shear modulus

In the last two decades, small strain shear modulus became one of the most important geotechnical parameters to characterize soil stiffness. Finite element analysis have shown that in-situ stiffness of soils and rocks is much higher than what was previously thought and that stress-strain behaviour of these materials is non-linear in most cases with small strain levels, especially in the ground around retaining walls, foundations and tunnels, typically in the order of 10−2 to 10−4 of strain. Although the best approach to estimate shear modulus seems to be based in measuring seismic wave velocities, deriving the parameter through correlations with in-situ tests is usually considered very useful for design practice.The use of Neural Networks for modeling systems has been widespread, in particular within areas where the great amount of available data and the complexity of the systems keeps the problem very unfriendly to treat following traditional data analysis methodologies. In this work, the use of Neural Networks and Support Vector Regression is proposed to estimate small strain shear modulus for sedimentary soils from the basic or intermediate parameters derived from Marchetti Dilatometer Test. The results are discussed and compared with some of the most common available methodologies for this evaluation.