2,089 research outputs found
Numerical analysis of a transmission problem with Signorini contact using mixed-FEM and BEM
No full text© EDP Sciences, SMAI 2011This paper is concerned with the dual formulation of the interface problem consisting of a linear partial differential equation with variable coefficients in some bounded Lipschitz domain Ω in
Rn (n ≥ 2) and the Laplace equation with some radiation condition in the unbounded exterior domain Ωc := Rn\￣Ω. The two problems are coupled by transmission and Signorini contact conditions on the interface Γ = ∂Ω. The exterior part of the interface problem is rewritten using a Neumann to Dirichlet mapping (NtD) given in terms of boundary integral operators. The resulting variational formulation becomes a variational inequality with a linear operator. Then we treat the corresponding numerical scheme and discuss an approximation of the NtD mapping with an appropriate discretization of the inverse Poincar´e-Steklov operator. In particular, assuming some abstract approximation properties and a discrete inf-sup condition, we show unique solvability of the discrete scheme and obtain the corresponding a-priori error estimate. Next, we prove that these assumptions are satisfied with Raviart- Thomas elements and piecewise constants in Ω, and continuous piecewise linear functions on Γ. We suggest a solver based on a modified Uzawa algorithm and show convergence. Finally we present some numerical results illustrating our theory
Analytical Description of the Bond Behavior of Thermally Preconditioned Carbon FRCM Applied onto Masonry Substrates
No full textIn the last few years, externally bonded fabric-reinforced cementitious matrix (FRCM) composites have been increasingly employed as externally bonded (EB) reinforcement of existing concrete and masonry structures. Failure of EB FRCM reinforcement is generally caused by composite debonding at the matrix-fiber interface. The stress transfer mechanism at the joint interface is analytically described within the fracture mechanics framework assuming a pure Mode-II loading condition and a zero-thickness interface, which allows for formulating the bond differential equation. Its solution requires the knowledge of the cohesive material law (CML) of the specific interface studied, often obtained by calibration of direct shear (DS) test results. In this paper, a rigid-trilinear CML is used to analytically describe the bond behavior of four control and four thermally preconditioned carbon FRCM-masonry joints subjected to single-lap direct shear test. Thermal preconditioning consists of 250-min-long exposure up to 300 °C. Stress responses obtained from the DS tests are used to calibrate the CML of the matrix-fiber interface, which is then used to solve the bond differential equation. Comparison between the analytical and experimental stress responses of control and conditioned specimens sheds light on the effect of temperature exposure on the bond behavior of carbon FRCM-masonry joints
Tensile Behavior of Epoxy-Impregnated Carbon FRCM Exposed to High Temperatures
No full textExternally bonded (EB) fabric-reinforced cementitious matrix (FRCM) composites, also referred to as textile reinforced concrete (TRC), are getting increasingly popular in the field of structural retrofit- ting as they combine good mechanical properties with important advantages in terms of cost effectiveness, ease of intervention, and reversibility. In addition, FRCMs have good resistance to relatively elevated temperature, also by vitue of the protecting role exerted by the inorganic matrix on the embedded textile. While polymeric impregnation of multifilament textiles maximizes the mechanical response of FRCM, it also introduces an organic element into an otherwise fully inorganic composite and, accordingly, raises some concerns in terms of high temperature vulnerability of the material. In this paper, the influence of thermal preconditioning on the tensile properties of carbon FRCM in a cementitious mortar is investigated, with special regard to the role of epoxy-impregnation of the open-mesh textile. Eight FRCM specimens are subjected to a 250-minute-long thermal preconditioning up to 300∘C, and their mechanical behavior is assessed and compared with eight specimens in the control group. It is found that, unexpectedly, the greatest performance loss is associated with the control, as opposed to the epoxy-coated, group
Carbon Nanotubes Strengthened Interphase in Textile Reinforced Mortar (TRM) Composites
No full textPerformance of inorganic matrix composite materials for structural purposes is strongly dependent on the matrix-to-fabric interphase bond strength. Consequently, owing to lack of congruence between the fabric and the matrix, design performance parameters are strongly penalised. Besides, yarn inner filaments (the core) easily slide over outer filaments (the sleeve) in the so-called telescopic failure. Broad experimental evidence supports the adoption of epoxy coatings to improve matrix-to-fabric strength and prevent telescopic failure, although the presence of the organic phase partially impairs the remarkable advantages associated to the inorganic matrix, such as thermal stability and water vapour permeability. Silica coatings appear as a promising alternative to traditional epoxy, by inducing localised pozzolanic reactivity, firmly linking synthetic fibres and hydraulic lime through the formation of highly cementing products at the interphase. In this work, the effect of a dispersion of multi-walled carbon nanotubes (MWCNT) in a silica nano-coating is assessed in uni-axial traction tests. The silica coating is prepared through sol-gel deposition in which carbon nanotubes are dispersed. The overall amount of carbon nanotubes in the silica sol is fixed at 0.5% wt. Silica-coated AR-glass and carbon fabric composite specimens, embedded in a commercially-available lime mortar matrix, are tested and compared. Carbon nanotubes provide a remarkable enhancement in both ultimate strength and elongation for AR-glass TRM, yielding an impressive two-fold increase in terms of strength. Differently, coated carbon fabrics composites show an increase up to 31% in terms of ductility, in view of an unexpected strength loss
Assessment of the Behaviour of Low-Modulus Polyurethane Foams Subjected to Severe Shear Deformation Conditions
No full textPolymeric materials are broadly employed as buildings materials because of a number of interesting properties for specific applications. Among them, polyurethane (PU) takes advantage of outstanding mechanical properties, such as high deformability and dissipation, as well as remarkable thermal and chemical stability. As a foam, PU is arguably the most popular insulating materials, used as a supplementary layer in precast concrete panels, infill walls and roofs. Experimental assessment of the mechanical behaviour of PU foams is therefore a long-standing issue, which is demanded to validate analytical models and provide reliable parameters in FEM modelling. In particular, reliable experimental assessment to large deformations is still difficult to attain. In the present study, we carry out a preliminary mechanical characterisation of a single low-modulus PU foam by means of a testing machine prototype, which performs simple shear and shear-per-traction deformations of a square-shaped sample, according to the restraining system adopted. Simple curve-fitting of the response leads to different mechanical parameters for the same material. Shear test results are related to compressive tests and microstructural investigation of the PU foam, through Scanning Electron (SEM) microscopy. The proposed polynomial laws for the tangential and normal net forces are applicable for calibrating FEM models aimed to predict the behaviour of soft materials subjected to high deformations
Scheda codicologica
No full textDescrizione dei 7 manoscritti che contengono la copia per la stampa dei diari di Pietro della Valle relativi ai diversi viaggi in Persia e in India che questi fece tra 1614 e 1626; se ne esaminano le diverse mani e se ne ricompoe la storia
Biohydrogen production from buffalo manure codigested with agroindustrial by-products in an anaerobic reactor
No full textMost on-farm biogas plants in Europe use animal waste and co-substrates for biogas production (CH4 and CO2 mixture). However combined hydrogen and methane production in a twostage process is a concept which has been developed in recent years and it is more promising from an energy perspective. The aim of this research was to explore hydrogen production from buffalo manure alone and in co-digestion with suitable feedstocks (low protein cheese whey and crude glycerol from biodiesel manufacturing). Experiments were carried out in batch reactor at 37 °C using a microbial consortia as inoculum. In a first set of batch trials the sterilization effect on hydrogen production from each individual substrate was explored. Results showed that sterilization increased hydrogen production yield (mL H2/g VS) in all substrates, even reaching a three times higher yield in the case of buffalo manure. In a second set of experiments hydrogen production using different mixing ratios of sterilized substrates were investigated. Results showed that the hydrogen production yield from manure codigested with 10% glycerol or 10% of low protein cheese whey (LPCW), was increased from 37.7 mL H2/g VS to 47.2 and 65.4, respectively. Moreover the co-digestion decreased H2 production time from 114 hrs to 24 hrs. The yields further increased up to 116 mL H2/g VS when a combination of 70% manure, 20% LPCW and 10% glycerol was used. During the experiments CH4 was not detected. Buffalo manure, when codigested with LPCW and glycerol gave interesting yield together with an optimum buffering capacity avoiding the use of any external buffer/pH control system
Boundary augmented Lagrangian method for the Signorini problem
No full textsummary:An augmented Lagrangian method, based on boundary variational formulations and fixed point method, is designed and analyzed for the Signorini problem of the Laplacian. Using the equivalence between Signorini boundary conditions and a fixed-point problem, we develop a new iterative algorithm that formulates the Signorini problem as a sequence of corresponding variational equations with the Steklov-Poincaré operator. Both theoretical results and numerical experiments show that the method presented is efficient
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