Search

Made available in DSpace on 2020-12-12T02:02:27Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-06-01 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) The creep behavior of hybrid carbon/glass laminate composites was investigated using a dynamic mechanical analyzer and different temperature profiles. Dynamic analysis and porosity were carried out prior to the creep test applying Findley's power law. The dynamic curves were not influenced by porosity but were affected by the architecture of the composites. The creep tests showed porosity dependence at temperatures below Tg while at higher temperatures, the architecture of the reinforcement plays a major role. Time-temperature superposition was conducted for long-term creep prediction and the results showed lower creep strain for the carbon and interleaved hybrid composites. Fatigue and Aeronautical Materials Research Group Department of Materials and Technology Sao Paulo State University (Unesp) School of Engineering, Guaratingueta Postgraduate Program in Mining Metallurgical and Materials Engineering (PPGE3M) Federal University of Rio Grande do Sul, Av. Bento Gonçalves 9500 Postgraduate Program in Engineering of Processes and Technologies (PGEPROTEC) University of Caxias do Sul (UCS) Fatigue and Aeronautical Materials Research Group Department of Materials and Technology Sao Paulo State University (Unesp) School of Engineering, Guaratingueta CAPES: 001 CNPq: 153335/2018-1 FAPESP: 2006/02121-6 FAPESP: 2015/19967-4

To further investigate the effects of in-plane and out-of-plane stresses on the delamination initiation for composite laminates under out-of-plane loading, this paper reports a joint experimental and numerical study, in which the fully clamped rectangular CFRP composite laminates were subjected to out-of-plane quasi-static indentations. The results show that the combination of the out-of-plane shear and in-plane tensile stresses together determined the initiation of delamination, whereas the influences of the out-of-plane compressive stress on the delamination initiation can be neglected. For the purpose of understanding the effects of deformations on the out-of-plane shear and compressive stress distributions, a concise analytical model was developed, which was validated against the numerical and experimental results. As a key take-away, this study reveals that the common impact tests at the geometric centre of the panel may not resemble sufficient similitude with stiffened panels where panel flexure is suppressed by various geometrical stiffening concepts.

Being immune to corrosion, and having a tensile strength up to three times higher than structural steel, glass fiber reinforced polymer (GFRP) bars are suitable for reinforcing concrete structures exposed to aggressive environmental conditions. However, a relatively low elasticity modulus of GFRP bars (in respect to the steel) favors the occurrence of relatively large deformability of cracked reinforced concrete. Lack of ductility and degradation of properties under high temperature can be also identified as debilities of GFRP bars over steel ones. Combining GFRP and steel bars can be a suitable solution to overcoming these concerns. Nevertheless, the application of such hybrid reinforcement systems requires reliable material models. The influence of the relative area of GFRP and steel bars on the tensile capacity of cracked concrete (generally known as tension-stiffening effect), was never investigated from the experimental point of view, mainly crossing results from different tools on the assessment of the cracking process. This paper experimentally investigates deformations and cracking behavior of concrete prisms reinforced with steel bars and GFRP bars in different combinations. The test results of 11 elements are reported. A tensile stress-strain diagram is conceptually proposed for modelling the tension-stiffening effect in elements with such hybrid combination of the reinforcement. The cracking process in terms of crack width and crack spacing is analyzed considering the hybrid reinforcement particularities and a preliminary approach is proposed for the prediction of the crack width for this type of reinforced concrete elements, Research Council of Lithuania (Research Project S-MIP-17-62). The second author also 590 wish to acknowledge the support provided by FCT through the PTDC/ECM591 EST/1882/2014 project

Mode II delamination testing for measuring the critical strain-energy release rate has reached a high level of maturity with the recent publication of ISO and ASTM standards. Cohesive zone modelling has also become a well-established design tool implemented in many commercial finite element analysis codes. The particularly large fracture process zone in mode II delamination increases the relevance of the traction-separation law. This paper proposes a fairly simple inverse method to evaluate an approximate trilinear traction-separation law from quasi-static end-notched flexure test results. The method employs an analytical beam cohesive zone model and consists of evaluating the parameters that fit the effective crack length versus strain-energy release rate curve. Preliminary studies showed that specific features of the latter curve were quite sensitive to some model parameters. Therefore, accurate approximate trilinear traction-separation laws could be obtained without the extensive numerical calculations typical of inverse methods.

The measurement of the fracture toughness of fiber reinforced composites at high rates of loading is still, despite years of research, not well established. This can be related to challenges in applying appropriate high rate loading on the specimen, accurately measuring the load, and in-situ determination of the crack length. In this work these challenges are addressed by using a direct wedge-on-specimen type loading of a double cantilever beam (DCB) specimen, high resolution optical deformation tracking, and a beam theory based analysis of the specimen deflection and crack length. This approach results in symmetric mode I opening of the crack and a robust analytical determination of the fracture toughness without the need to measure the external forces acting on the specimen nor to visually estimate the crack length. Tests carried out on carbon fiber reinforced epoxy composite at quasi-static and high rates (relative velocity up to 15 m/s) show the validity of the approach.

The stress-driven nonlocal integral model of elasticity for 1D nano-structures (Romano & Barretta, 2017a) is extended in this paper to Kirchhoff axisymmetric nano-plates. The nonlocal formulation, relating elastic principal flexural curvatures and moments, provides an effective methodology to assess size effects in 2D nano-structures. The associated elastostatic problem of nano-plates is conveniently expressed by differential relations equipped with constitutive boundary conditions involving nonlocal curvature fields. The proposed approach is illustrated by examining case-studies of engineering interest. In particular, nonlocal displacement solutions of axisymmetric nano-plates are detected for a variety of loading systems and kinematic boundary conditions. Merits and implications of the stress-driven strategy are elucidated by comparing the achieved results with those of the strain gradient model of elasticity generated by Reissner's variational principle. The outcomes can be useful for design and optimization of plate-like components of ground-breaking Nano-Electro-Mechanical-Systems (NEMS).

An analytical pore-scale model is proposed for predicting the pressure drop of a biofilter by taking the effect of biofilm development into account. Besides the average particle radius and fluid parameters, the pressure drop is expressed in terms of the initial bed porosity, the biofilm affected porosity, particle sphericity, surface roughness coefficient, coordination number and biofilm thickness. The coordination number represents the number of particles in contact with a single one in the bed. The biofilm affected porosity is based on the assumption of biofilm overlap and particle contact. The sphericity for a cluster of particles in contact and biofilm overlap is quantified in terms of the biofilm thickness and coordination number and its effect on the pressure drop investigated. The model predictions are verified against experimental pressure drop data obtained from a biofilter in operation for 107 days. Expanded schist was used as packing material. The proposed model is also compared to a modified Ergun equation from the literature of which the empirical coefficients and porosity exponents were adjusted to be applicable to a biofilter. The model proposed in this study contains no empirical coefficients. An equation is also presented for predicting the biofilm affected specific surface area which requires the measured pressure drop and superficial velocity values as input parameters. The effect of the biofilm affected porosity on the pressure drop is also investigated in the case of no biofilm overlap and no particle contact. Lastly a sensitivity analysis is performed on the initial bed porosity and coordination number.

The main aim of this work is to propose and validate an analytical strategy to correlate the electric and mechanical response of bi-directional laminates in the presence of both off-axis cracks and delamination. To this end, the attention is initially focused on the electrical problem, developing and validating against numerical results a new, fully analytical, model to properly assess the electrical resistance increase in the laminate as a function of the damage state. Subsequently, an analytical model to predict the laminate stiffness as a function of the damage scenario is derived and later combined with the electric one, thus establishing a direct correlation between the resistance change and the stiffness degradation.

The time-depending stress transfer in existing structures strengthened with externally bonded fiber reinforced polymer (FRP) systems, due to viscous effects of reinforcement, is analysed in this paper. A new modelling strategy for assessing the long-term response of these composite beams was developed, by assembling two one-dimensional components (existing structural element and external strengthening) and accounting for thin-walled sectional geometry of FRPs. Several numerical experiments dealing with FRP laminates-reinforced concrete (RC) composite beams were performed and the corresponding results were compared with the predictions of the Effective Modulus Method (EM). A relevant stress redistribution between the components of the examined strengthened structures was observed.

The shear response of fabrics and fabric reinforced materials is primarily characterised by means of Picture Frame and Bias Extension experiments. Normalisation methods have been proposed earlier to enable comparison between different measurement results. Here, a continuum mechanics based analysis is presented for biaxial fabric materials without intra-ply slip, subject to constraints of fibre inextensibility. The fibre stresses are separated from the constitutively determined extra stresses, leading to a scalar equivalent stress resultant which makes the analysis of these tests more transparent. The equations are elaborated for both shear characterisation experiments and a direct measurement evaluation is proposed, without the need for iterative parameter identification methods.

A consistent stress-driven nonlocal integral model for nonisothermal structural analysis of elastic nano- and microbeams is proposed. Most nonlocal models of literature are strain-driven and it was shown that such approaches can lead toward a number of difficulties. Following recent contributions within the isothermal setting, the developed model abandons the classical strain-driven methodology in favour of the modern stress-driven elasticity theory by G. Romano and R. Barretta. This effectively circumvents issues associated with strain-driven formulations. The new thermoelastic nonlocal integral model is proven to be equivalent to an adequate set of differential equations, accompanied by higher-order constitutive boundary conditions, when the special Helmholtz averaging kernel is adopted in the convolution. The example section provides several applications, thus enabling insight into performance of the formulation. Exact nonlocal solutions are established, detecting also new benchmarks for thermoelastic numerical analyses.

The use of Natural Fibers in cementitious composites is an innovative technical solution but, they are characterized by a complex microstructure and significant heterogeneity, which influence their interaction with cementitious matrices, whose identification requires further advances in the current state of knowledge. The present study summarizes the results of a wide series of pull-out tests carried out on sisal, curaua and jute fibers. Then, the experimental results are employed in an inverse identification procedure aimed at unveiling the key features of the aforementioned bond-slip laws. Morphological, chemical, physical and mechanical characterization of the natural fibers were correlated with the resulting bond properties within the embedding matrix. The obtained results in terms of relevant parameters, such as bond strength and fracture energy (under pull-out stresses) of the fiber-matrix interface, pave the way for future studies intended for a better understanding of the structural response of Natural Fiber Reinforced Cementitious Composites.

This paper presents a one-dimensional, steady-state, analytical and a two-dimensional, steady-state, numerical approach to describe heat transfer in the complex solid structure of a skin evaporator used in household refrigerators. Both models are based on a specific form of the energy equation (the heat diffusion equation), which is solved to obtain the temperature distributions and heat flows within the different parts of the structure. Heat transfer is limited by a number of thermal resistances, such as the inner plastic wall of the refrigerator, the aluminum plate, the foil, the tube of the evaporator, and the air gaps. The insulation that prevents heat flowing into the evaporator from the surroundings is also taken into consideration. A comparison of the models showed that the root-mean-square deviation of the calculated temperatures ranged from 0.34 K to 0.60 K and that the area-weighted discrepancy of the calculated heat flows was 4.6%. Experimental validation of the model showed that the calculated and measured evaporator capacities were within ±12%. This demonstrates that several simplifications to the analytical model can be made, so drastically reducing the complexity of modeling a skin evaporator's structure, while retaining sufficient accuracy. These simplifications include reducing the heat transfer to one-dimensional heat conduction in the individual parts of the structure, neglecting the heat flux through the aluminum–air-gap contact, and linearizing the aluminum foil.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2017-10-31, 資料番号: PA1820032000

Ultra-High Performance Fiber-Reinforced Concretes (UHP-FRC) subjected to uniaxial tensile loads are investigated in the present paper. The study comprises a new procedure to assess the effectiveness of the hybridization, herein obtained by reinforcing UHP-FRC with micro and macro steel fibers. A comprehensive experimental campaign is also performed on monofiber and hybrid UHP-FRC. In all the concretes, the distance between the cracks and the minimum fiber volume fraction, which produces strain hardening response and multiple cracking, are theoretically and experimentally evaluated. If the bond parameter of the macro-fibers is properly calculated, the results of the analytical model, in terms of crack-spacing vs. fiber volume fraction, are in good agreement with the test data. Moreover, to increase the number of the cracks, and to reduce crack spacing, the hybridization is suitable only when the amount of macro-fibers is within a well-defined range.

Determination of the effective elastic properties of unidirectional short fiber reinforced composites (SFRCs) is a key fundamental requirement which is usually estimated by mean-field homogenization (MFH) methods. Available classical MFH methods only account for volume fraction and aspect ratio of the fibers and do not consider the physical cylindrical geometry of the fibers. Moreover, classical MFHs are not designed for predicting the effective properties of SFRCs with distinct packing configurations and order of fibers. In this work, new modified Eshelby tensors associated with an enhanced MFH are developed which resolve the mentioned limitations of the available MFH methods and preserve their computational efficiency. While classical Eshelby tensors used in MFHs depend only on the aspect ratio of the fictitious ellipsoidal inclusion and material properties of the matrix, the introduced modified Eshelby tensors additionally account for physical cylindrical geometry as well as interactions of the short fibers with different packing configurations. Moreover, non-uniform homogenizing eigenstrains required for accurate homogenization of the short fibers are also incorporated by the proposed analytical treatment. Predictions of the developed enhanced MFH for various packing configurations are compared with those obtained from Finite Element Method implementing periodic boundary conditions and very good agreements are observed. It is shown that packing configuration and geometrical specifications of short fibers can significantly affect the effective properties of the SFRC.

This paper aims to present an experimental investigation on the behavior of web-flange junctions (WFJs) rotational stiffness of pultruded fiber-reinforced polymer composites (FRP). Channels and I-sections were tested using a simple set-up, which was developed in order to experimentally characterize the junctions in a direct manner. The Digital Image Correlation (DIC) technique was used, allowing overall deflections and relative rotations between web and flange to be monitored. The WFJs' imperfections were analyzed through an optical microscope and correlated with the cracks' formation. Further, damage thresholds are identified using available stress equations for curved composite members and lower bound functions are proposed to simulate the junction stiffness retention. Finally, two Equations are developed in order to analytically predict pultruded junctions' rotational stiffness per unit of width. In general, the theoretical and experimental results agreed fairly well, with a maximum difference of 24% for I-sections and 38% for channels.

The mitigation of electromagnetic pollution is becoming an increasing issue due to the proliferation and miniaturization of electronic components. Electromagnetic interference (EMI) shields must be considered to bring about electromagnetic compatibility (EMC), where analyzing the shielding effectiveness (SE) is of great importance. Graphene-enhanced polymer composite materials are recently introduced as a replacement for metal-based EMI shielding materials due to their low cost, resistance to corrosion, lightweight, versatile, and straightforward processability, as well as broad bandwidth properties compared to conventional metal-based materials. The SE can be determined from the bulk material properties, including permittivity, permeability, and conductivity, using analytical or numerical solutions to Maxwell's equations. Though, Due to the heterogeneity of graphene-based polymer composites, characterization of these values remains difficult. This article attempts to summarise and critically review the state-of-the-art progress in the fabrication and characterization of these materials. Also, this review elaborates on accurate modelling techniques based on the underlying micromechanics, e.g., percolation, electron tunnelling, agglomeration, imperfect interface, frequency-dependent nanocapacitance and electron hopping. The model is designed to predict the electromagnetic properties of the nanocomposite from the pre-determined properties of the polymer matrix and carbon-based filler. This understanding may ultimately lead to the development of graphene/polymer composites with optimized EMI shielding properties to develop emerging EMI shielding materials. This paper focuses heavily on Graphite Nanoplatelet (GnP)/Epoxy as the choice of graphene/polymer.

This work is concerned with the geometrical characterisation of bistable composite plates in order to investigate their curvatures interdependence. The main non linear feature of bistable asymmetric laminates concerns large deflection with small energy input. This is an aspect of particular interest for several applications. Accordingly, there is an increasing importance placed on accuracy/sensitivity of predicting techniques to model bistable deformation shapes. This paper presents a novel forecast model to map the surface profiles of bistable laminates. By Timoshenko and Ashwell approaches, an analytical model is developed to provide an interpretation of the bistable shapes, in terms of principal and anticlastic curvatures. The experimental data (via laser acquisitions), with analytical and Finite Element models, help to understand the relation between material properties, plate dimensions, stacking sequence, curing features of the composite laminates and the bistability phenomenon.

This paper presents an analysis of the performance of different existing formulations to quantify the FRP contribution to the shear strength of RC elements strengthened in shear by externally bonded FRP sheets. A large database of 555 tests has been assembled distinguishing between the shape of the section, the existence of internal transverse reinforcement and the FRP configurations. In general, predictions are more conservative for beams without transverse reinforcement. In addition, in some cases predictions are unsafe for beams with transverse reinforcement, showing a possible interaction with the internal transverse reinforcement which is not considered in the experimental FRP contribution to the shear strength. For wrapped FRP configurations, models generally assumed failure at the bottom corner of the section and results are very conservative in some cases where failure was experimentally observed along the web. For U-shaped and side-bonded configuration, results depend mainly on the assumed bond model and are more accurate than in the previous case, showing for some models unsafe predictions for the continuous FRP system applied in beams with transverse reinforcement. This paper has been developed under the framework of the Research Projects “BIA2015-64672-575 C4-1-R” and “RTI2018-097314-B-C21” funded by the Spanish Ministry of Economy and Competitiveness (MINECO) and co-funded by the European Regional Development Funds (ERDF). The authors would also like to thank the Cost Action TU1207 which funded a short term scientific mission of the first author in Lodz University of Technology.

The input flow of groundwater from the seabed to the coastal ocean, known as Submarine Groundwater Discharge (SGD), has been only recently recognized as an important component of continental margin systems. It potentially impacts physical, chemical and biological marine dynamics. Independently of its specific nature (seepage, submarine springs, etc.) or fluid chemical composition, a SGD is generally characterized by low flow rates, hence making its detection and quantification very difficult, and explaining why it has been somewhat neglected by the scientific community for a long time. Along with the growing interest for SGDs emerged the need for in-situ observations in order to characterize in details how these SGDs behave. In this work, we describe the morphology of a pockmark field, detected in the Southern Tyrrhenian Sea (Mediterranean Sea), and provide observational evidences of the presence of active submarine springs over the coastal shelf area. We describe the effect of the fluid seeps on the water column stratification close to the main plumes and in the neighbouring areas, providing quantitative estimates of the intensity of the turbulent mixing and discussing their potential impact on the seabed morphology and pockmark formation in the context of turbulent buoyant plumes analytical modelling.

The paper discusses the restrictions the positive definite character of the strain energy imposes to the values of the Poisson's ratios in a linear elastic orthotropic constitutive law. By exploiting the reciprocity principle and the conservation of energy, non-trivial constraints arise, leading to a particularly expressive form of the domain of existence, named Tetrahedron-Ellipsoid locus, in the space of three independent Poisson's ratios. The presented analysis establishes several notable restrictions for the elastic behavior of orthotropic composite materials, in all the cases in which there are great differences in the Young's moduli. For instance, near the edge of this locus, some Poisson ratios may belong to an interval not including the zero value; a very interesting peculiarity, perhaps not yet figured out in literature. In order to assess the coerciveness of the formulation, the results are applied to common wood essences, to several Italian growing species of bamboo and Arundo donax, natural composite functionally graded orthotropic materials. In the latter cases, an extensive experimental investigation is carried out to detect several Poisson's ratios, not studied in literature. Poisson ratios are obtained directly by strains measured by means of both physical strain gauges and Digital Image Correlation (DIC).

Auxetic materials are gaining special interest in technical sectors due to their attractive mechanical behaviour. This paper reports a systematic investigation on missing rib design based auxetic structures produced from braided composites for civil engineering applications. The influence of various structural and material parameters on auxetic and mechanical properties was thoroughly investigated. The basic structures were also modified with straight longitudinal rods to enhance their strengthening potential in structural elements. Additionally, a new analytical model was proposed to predict Poisson’s ratio through a semi empirical approach. Auxetic and tensile behaviours were also predicted using finite element analysis. The auxetic and tensile behaviours were observed to be more strongly dependent on their structural parameters than the material parameters. The developed analytical models could well predict the auxetic behaviour of these structures except at very low or high strains. Good agreement was also observed between the experimental results and numerical analysis., University of Minho – UMINHO/BI/146/2012, under the scheme of ‘‘Strategic plan of school of engineering – Agenda 2020: Multidisciplinary projects.”

This paper presents a static analysis of laminated cross-ply beams by using an unavailable unified higher order shear deformation theory (HSDT) for composite beams which includes the thickness stretching effect. The generalized governing equations are derived by employing the principle of virtual work. Navier-type closed-form solution is obtained for several beams subjected to several kinds of loads (sinusoidal, uniform, linear and point load). Infinite HSDTs for beams can be further developed just by modifying the shear strain shape functions. Several hybrid type shear strain shape functions were introduced to evaluate the generality of the proposed theory. The stretching effects for the accurate prediction of the transverse stresses are analyzed. Numerical results of some HSDTs for beams are compared with the elasticity solutions and other HSDTs. Convergence studies were carried out to determine the necessary half-wave number for the different models.

Carrera Unified Formulation (CUF) is used for free vibration analysis of Levy-type thick rectangular porous–cellular plates. The variation of porosity through the thickness causes mechanical properties to change along the thickness. Properties are modeled by a simple cosine rule. The equations of motion are solved analytically for Levy-type rectangular porous–cellular plates using state space method. The correctness of this approach is confirmed through comparison studies with published results based on highly accurate approaches. Different orders of model are used to enhance the accuracy of the method in order to obtain the exact frequencies of thick and very thick porous–cellular plates. The effects of the coefficient of plate porosity and the thickness–length ratio as well as the aspect ratio, on the frequencies are investigated for Levy-type boundary conditions. It is found that natural frequencies of the porous–cellular thick plates decrease as the coefficient of plate porosity increases in all studied boundary conditions.

This paper presents a tension stiffening model for Fiber Reinforced Concrete (FRC) tensile member reinforced by hybrid glass fiber reinforced polymer (GFRP) and steel bars. The model is developed through an explicit analytical bond formulation by considering a four-linear bond shear stress-slip relationship to simulate the bond behavior between reinforcing bars and surrounding FRC. The model is also capable of simulating both the fiber reinforcement contribution and the yielding stage of steel bar at cracked section. Additionally, a FE Model is carried out using a multi-directional smeared crack approach for modeling cracking process in FRC, and adopting interface finite elements to simulate the bond behavior between reinforcements and FRC, whose constitutive model was defined from the aforementioned bond law. Both the analytical and numerical approaches showed a good agreement with some recent experimental results on tension-stiffening in the literature. Finally, an extensive parametric study is performed by using the analytical model, and the influence of the involved parameters on the tension-stiffening and cracking behavior of hybrid GFRP/steel FRC tensile member is investigated., QREN project n. 30367 – UrbanCrete, Portuguese Foundation for Science and Technology (FCT) - SFRH/BD/77409/2011

The Embedded Through-Section (ETS) is a recent strengthening technique that has been developed to retrofit existing reinforced concrete (RC) elements with shear reinforcement deficiencies. This technique is based on the execution of holes drilled through the element cross section, in which steel or fiber reinforced polymer (FRP) bars are inserted and bonded to the surrounding concrete with an epoxy adhesive. An experimental program was carried out with RC T-cross section beams strengthened in shear using steel ETS bars. The influence of the inclination and shear strengthening ratio of ETS on the shear strengthening efficiency was evaluated, as well as the interaction of ETS bars with existing steel stirrups. Two different analytical models are presented in this paper in order to calculate the contribution of ETS to shear resistance. The first model follows an empirical approach (experimental-based approach), while the second model takes into account the physical and mechanical principles of the technique (mechanical-based approach). The predictive performance of both models is assessed by using the experimental results., The authors wish to acknowledge the support provided by the CutInov project QREN n. 38780 supported by ADI, and by the Engineering Department of the University of Ferrara.

International audience; The evaluation of the predictive capabilities of models proposed in the literature for laminated composites calls for experimental testing providing detailed results of both the global and local response in terms of degradation mechanisms, such as delamination, transverse cracking and fibre breaking. Scaled tests, in which one or more characteristic dimensions are modified, allow variation of the different mechanisms. In this paper, a unique series of scaled indentation tests are performed on quasi-isotropic composite plates, and a detailed assessment of the damage evolution is carried out through non-destructive techniques, including ultrasonic C-scan and X-ray Computed Tomography (CT). Four different configurations are tested, presenting changes in both in-plane dimensions and fully three dimensional scaled cases. The latter are performed with sublaminate and ply scaling to show the effect of ply thickness on response. A detailed set of results for both global behaviour and the damage evolution is provided to demonstrate the mechanisms controlling behaviour and to create a reference set of data for model validation. The scaling effects observed are also discussed making use of simplified analytical models.

In this paper, a novel analytical model for predicting the post-crushing compressive response of progressively crushable sandwich foam cores is presented. The calibration of the model is performed using experimental measurements obtained exclusively from standard monotonic compressive tests. Therefore, the need for performing time-consuming compressive tests including multiple unloading–reloading cycles is avoided. Model predictions have been validated against experimental measurements available for three different foam materials. The model is shown to accurately predict the thickness of the crushed material layer during progressive crushing and the residual after-crushing strain (with a maximum error of 12.1 % ). The proposed model is capable of predicting the residual after-crushing strain with a significantly smaller error (error-reduction over 56 % ) than existing models, whose calibrations require the same experimental measurements as the present model. The results presented in this work demonstrate the relevance of the proposed model for a damage-tolerant design of foam-cored composite sandwich structures.

In this paper a piecewise exponential model is proposed for analysis of three-dimensional elastic deformation of rectangular sandwich panels with graded core subjected to transverse loading. The model can be applied to any functionally graded plate or sandwich panel with graded core as long as the through thickness stiffness variation can be described by a smooth function. The new piecewise exponential model is fully validated through both comparison with results from the literature and a three-dimensional finite element method which employs user-implemented graded finite elements. As an example the new model is applied to three dimensional elasticity analysis of sandwich panels with power law variation in stiffness properties of the core and a comparative study is carried out to examine the effect of panel thickness and varying the power law index on panel’s response.

The paper analyses the application of high-frequency periodic timetables on single track railways. A mesoscopic approach is first of all presented, in order to provide a simple but rather complete model. On this basis, a mathematical and deterministic evaluation formula is presented, as well as a simulation tool able to evaluate the effects of traffic perturbations. All the main parameters which characterise railway operations (with a special focus on single track infrastructure) are taken into consideration and their influence is pointed out, as regards the possibility to realise high frequency periodic timetables. The analysis is subsequently extended for assessing how these timetables can behave in case of traffic perturbations, in order to provide also in these circumstances a sufficient service quality. The influence of the abovementioned parameters on buffer times and hence on timetable stability and robustness is therefore outlined, highlighting the benefits that an effective rescheduling algorithm could play. The presented calculation method is then applied to a real case, i.e. to a part of the railway network of Piedmont, Italy. This case study provides some considerations about the elements that can actually play a determinant role and what measures can be planned for achieving certain service targets. Finally, conclusions are drawn together with some considerations for future work about the possible implementation of effective rescheduling logics within advanced driver advisory systems as well as modern signalling and control systems.

© 2015 Elsevier B.V..A variational model describing the behaviour of a thin-walled I-section strut suffering from local-global buckling mode interaction is presented where global (Euler) buckling about the strong axis is the critical mode. A system of differential and integral equations is derived that describe the equilibrium states from variational principles and are solved numerically using the continuation and bifurcation software A. uto-07. p for the perfect case. Initially stress relieved out-of-straightness imperfections are subsequently introduced and the nonlinear response is modelled. The modelled interaction is between the critical global buckling mode about the strong axis and local buckling in the flange and web simultaneously, where the flange-web joint is assumed to be free to rotate as a rigid body. The initial eigenmode is shown to be destabilized at a secondary bifurcation where interactive buckling is triggered. A progressive change in the buckling mode is then observed, initially with local buckling localizing at the mid-span of the compression flange, which also triggers sympathetic local buckling in the web. The results from the analytical model have been validated using the commercial finite element (FE) software A. baqus with good comparisons presented for the initial post-buckling behaviour. The strut also exhibits sensitivity to initial out-of-straightness imperfections, with a notable decrease in the ultimate load as the imperfection size increases. The ultimate loads for a range of imperfection amplitudes are found using both analytical models and FE analysis, with very good correlation observed.

Fibre reinforced polymer laminates have low electrical conductivity compared to metals due to the high resistivity of the matrix phase, fibre-matrix interface, and fibres. This paper presents a new method for improving the electrical conductivity of fibre-polymer composites by through-thickness weaving of continuous filaments with high electrical conductivity (e.g. copper and other materials). Experimental testing and analytical modelling reveal that orthogonally weaving of conductive z-filaments in the through-thickness direction of carbon-epoxy composite increases the electrical conductivity by creating interconnected pathways for current flow. In particular, it is possible to increase the electrical conductivity by over two to three orders of magnitude by using a very low volume fraction (under 1.3 vol%) of metal z-filaments with a diameter of 0.51 mm. The improvements to both the in-plane and through-thickness electrical conductivities can be designed by controlling the volume fraction and by the judicious choice of the z-filament material.

With the continuous development of new supplemental seismic control systems, recent research advancements can be applied to structures located in earthquake-prone zones. Several energy dissipation devices have been conceived and experimentally investigated. However, until a comprehensive design procedure that incorporates all parameters of the control system is formulated, the applications of these devices remain limited. The present study is part of a research project about a particular shear-link (SL) dissipative device. It has been recently introduced for an effective seismic protection of structures, also installed for many real applications, especially in South America. Starting from experimental data, an ABAQUS finite-element nonlinear model of such device is presented. It is suitably tuned to simulate the combined isotropic/kinematic hardening response of the SL device. It also provides a deeper understanding of the overall nonlinear behavior of the device, so as to laid the foundations for a simplified modelling approach the authors propose in the second part of the work. The latter is an approximate analytical model able to predict initial stiffness of the SL device, yielding force, as well as the equivalent damping ratio at large displacement. Alternative models for stiffness are also introduced and compared for complexity and accuracy. The final aim is that of providing a reliable yet simple tool the professionals can use to include SL devices in a fully integrated and manageable design process. The assessment of the accuracy of both analytical and numerical models is based on the available experimental data. The results as well as the involved approximation are presented and discussed.

Despite the well-known advantages of fabric-reinforced cementitious matrix (FRCM), few applications of this composite material have been proposed apart from structural strengthening. To exploit material corrosion resistance, seven FRCM pipes were produced with mortar and dry concrete matrixes. They were experimentally tested under diametral compression configuration, and their cracking load and loadbearing capacity were compared with both analytical and numerical simulations results, which indicated the better accuracy (between 10% and 16% of relative error) of the proposed analytical approach, proving the efficiency of the latter as a calculation tool for this innovative structural application of FRCM.

This study investigated the efficacy of near surface mounted (NSM) hybrid carbon/glass fibre reinforced polymer (FRP) strips for strengthening of shear-deficient reinforced-concrete (RC) rectangular deep beams. A specific focus was the interaction of FRP with steel stirrups, which were arranged in both aligned and unaligned configurations for different FRP and stirrup volume fractions. Five unstrengthened reference beams and seven strengthened beams were tested. The NSM-FRP increased the beam shear strength up to 55.8%, alleviated debonding between concrete and strengthening material, and enhanced the deformational characteristics of the strengthened beams. The FRP/stirrups interaction also affected the contribution of NSM-FRP to the shear strength of the beams. Moreover, this study proposes a model based on the modified compression field theory (MCFT) to determine the shear capacity of the beams. The proposed formulation accounts for the FRP/stirrups interaction, which is not considered in the currently available models for the NSM-FRP strengthened RC beams. The average theoretical to experimental shear capacities ratio was 0.96 with a standard deviation of 3.19%. 2020 Elsevier Ltd This paper was made possible by NPRP grant # NPRP 7-1720-2-641 from the Qatar National Research Fund (a member of Qatar Foundation) and by Qatar University grants QUST-CENG-SPR-14/15-15 and QUST-CENG-SPR-14/15-16 . The authors would also like to acknowledge Qatar University for the Graduate Assistantship, students code GTRA-CENG-2019-16 and GTRA-CENG-2019-09. The findings achieved herein are solely the responsibility of the authors. Scopus

The use of steel reinforced grout (SRG) has emerged as a promising technique for the strengthening and retrofitting of reinforced concrete (RC) structures, however, the experimental evidence on its application for shear-critical RC beams is rather limited. Accordingly, this study focuses on the structural performance of SRG-strengthened RC beams critical in shear. At this aim, an experimental investigation has been conducted on eleven RC T cross-section beams. Eight beams were strengthened with different types of SRG (different for the density, namely 1.57 and 3.14 cords per centimeter) comprised of ultra-high tensile strength steel fabrics, and three, which were unstrengthened, were used as reference specimens. The test parameters investigated were as follows: (a) steel fabric density, (b) bond scheme, and (c) amount of internal shear reinforcement within the critical shear span, SRG/stirrups interaction. The experimental results revealed the high potential of SRG system for the strengthening of shear-critical RC beams. It has significantly improved the shear capacity and deformation characteristics of the strengthened beams. The percentage increase in the load-carrying capacity over the reference beam of up to 71% was observed. In addition, an analytical model based on the simplified modified compression field theory (SMCFT) has been proposed to predict the shear capacity of SRG-strengthened beams. The SMCFT-based model has resulted in an accurate and safe prediction of the shear capacity of the SRG-strengthened beams with an average ratio of Vth/Vex of 0.92 and a standard deviation of 4.07%. - 2019 Elsevier Ltd This paper was made possible by NPRP grant # NPRP 9-110-2-052 and UREP grant # UREP24-045-2-013 from the Qatar National Research Fund (a member of Qatar Foundation) and Qatar University grant # QUST-CENG-SPR- 14/15-15 . The authors would also gratefully acknowledge Kerakoll SpA for providing the strengthening material. The findings achieved herein are solely the responsibility of the authors. Scopus

Laminated bamboo in structural applications has the potential to change the way buildings are constructed. The fibrous microstructure of bamboo can be modelled as a fibre-reinforced composite. This study compares the results of a fibre volume fraction analysis with previous experimental beam bending results. The link between fibre volume fraction and bending stiffness shows that differences previously attributed to preservation treatment in fact arise due to strip thickness. Composite theory provides a basis for the development of future guidance for laminated bamboo, as validated here. Fibre volume fraction analysis is an effective method for non-destructive evaluation of bamboo beam stiffness.

This paper presents a novel composite production cost estimation model. The strength of the model is its modular construction, allowing for easy implementation of different production methods and case studies. The cost model is exemplified by evaluating the costs of a generic aeronautical wing, consisting of skin, stiffeners and rib feet. Several common aeronautical manufacturing methods are studied. For studied structure, hand layup is the most cost-effective method for annual volumes of less than 150 structures per year. For higher production volumes automatic tape layup (All) followed by hot drape forming (HDF) is the most cost-effective choice. QC 20150918

This article introduces new methods for static and free vibration analyses of functionally graded annular and circular micro-plates, which can take into account spatial variation of the length scale parameter. The underlying higher order continuum theory behind the proposed approaches is the modified couple stress theory. A unified way of expressing the displacement field is adopted so as to produce numerical results for three different plate theories, which are Kirchhoff plate theory (KPT), Mindlin plate theory (MPT), and third-order shear deformation theory (TSDT). Governing partial differential equations and corresponding boundary conditions are obtained following the variational approach and the Hamilton's principle. Derived systems of differential equations are solved numerically by utilizing the differential quadrature method (DQM). Comparisons to the results available in the literature demonstrate the high level of accuracy of the numerical results generated through the developed methods. Extensive analyses are presented in order to illustrate the influences of various geometric and material parameters upon static deformation profiles, stresses, and natural vibration frequencies. In particular, the length scale parameter ratio -which defines the length scale parameter variation profile-is shown to possess a profound impact on both static and dynamic behaviors of functionally graded annular and circular micro-plates. (C) 2015 Elsevier Ltd. All rights reserved.

© 2015 Elsevier B.V..An analytical model for axially loaded thin-walled stringer stiffened plates based on variational principles is exploited to study the sensitivity to initial geometric imperfections and the effects of altering geometric properties. Studies on different forms of global and local imperfections indicate that the post-buckling response governs the worst case imperfections. The investigation also focuses on the effect of changing the global and the local slendernesses on the post-buckling behaviour. The parametric space in which the stiffened plates are imperfection sensitive and susceptible to highly unstable cellular buckling is identified.

The purpose of this paper is to study the vibrational behavior of advanced composite plates by using a novel first shear deformation theory (FSDT). This theory contains only four unknowns, with is even less than the classical FSDT. The governing equations are derived by employing the Hamilton's principles and solved via Navier's solution. The present results were validiated by comparing it with the 3D, classical FSDT and other solutions available in the literature. Shear correction factor apper to be unfovarable in some cases (case dependent). Finally, authors recommend further study of this new manner to model the displacement field.

Milling forces play an importantrole in the milling process and are generally calculatedbythe mechanistic or numerical methods which are considered time-consuming and impractical forvarious cutting conditions and workpiece-toolpair. Therefore, this paper proposes an analytical method for modelling themilling forcesin helical end milling process based on a predictive machining theory, which regards the workpiece material properties, tool geometry, cutting conditions and types of milling as the input data. In this method,each cutting edge is discretized into a series of infinitesimal elements along the cutter axis and the cutting action of each elementis equivalent to the classical oblique cutting process. The three dimensional cutting force components applied on eachelement are predicted analytically using this predictive oblique cutting modelwith the effect of cutting edge radius. Finally, the proposed analytical model of milling forcesis verified by the publishedresultsandthe simulation values using the software AdvantEdgeFEM.

This work describes the manufacture and characterisation of a PEEK-based zero Poisson’s ratio honeycomb (SILICOMB) produced using Kirigami-inspired cutting and folding techniques. The flatwise compression and transverse shear properties of the structure are determined through ASTM mechanical testing, and the results compared against commercially available honeycombs, including several other zero Poisson’s ratio cellular structures. An analytical model to predict the shear strength is compared to the results. SILICOMB specimens are found to have lower stiffness compared to other honeycomb configurations, but comparable strength. Factors influencing the results and variations to the manufacturing process are discussed.

Mechanical properties and failure modes of carbon fiber composite egg and pyramidal honeycombs cores under in plane compression were studied in the present paper. An interlocking method was developed for both kinds of three-dimensional honeycombs. Euler or core shear macro-buckling, face wrinkling, face inter-cell buckling, core member crushing and face sheet crushing were considered and theoretical relationships for predicting the failure load associated with each mode were presented. Failure mechanism maps were constructed to predict the failure of these composite sandwich panels subjected to in-plane compression. The response of the sandwich panels under axial compression was measured up to failure. The measured peak loads obtained in the experiments showed a good agreement with the analytical predictions. The finite element method was used to investigate the Euler buckling of sandwich beams made with two different honeycomb cores and the comparisons between two kinds of honeycomb cores were conducted. Fil: Xiong, Jian. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Zhang, Meng. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Stocchi, Ariel Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigación En Ciencia y Tecnología de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingeniería; Argentina Fil: Hong, Hu. The Hong Kong Polytechnic University; China Fil: Ma, Li. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Wu, Linzhi. Harbin Institute of Technology. Center for Composite Materials and Structures; China Fil: Zhang, Zhong. Beijing Institute of Structure and Environment Engineering; China

Mobile clouds are a new paradigm enabling mobile users to access the heterogeneous services present in a pervasive mobile environment together with the rich service offers of the cloud infrastructures. In mobile computing environments mobile devices can also act as service providers, using approaches conceptually similar to service-oriented models. Many approaches implement service provisioning between mobile devices with the intervention of cloud-based handlers, with mobility playing a disruptive role to the functionality offered by of the system. In our approach, we exploit the opportunistic computing model, whereby mobile devices exploit direct contacts to provide services to each other, without necessarily go through conventional cloud services residing in the Internet. Conventional cloud services are therefore complemented by a mobile cloud formed directly by the mobile devices. This paper exploits an algorithm for service selection and composition in this type of mobile cloud environments able to estimate the execution time of a service composition. The model enables the system to produce an estimate of the execution time of the alternative compositions that can be exploited to solve a user's request and then choose the best one among them. We compare the performance of our algorithm with alternative strategies, showing its superior performance from a number of standpoints. In particular, we show how our algorithm can manage a higher load of requests without causing instability in the system conversely to the other strategies. When the load of requests is manageable for all strategies, our algorithm can achieve up to 75% less time spent in average to solve requests.

The static analysis of Kirchhoff nano plates subjected to uniformly (UDL) and sinusoidally (SSL) distributed load is computed. The strain gradient nonlocal theory has been employed in order to involve the size effects of nanostructures in classical continuum theory. The governing equation of motion of Kirchhoff in weak form are applied to nano plates, involving second-order strain gradient nonlocal theory. Thus, the obtained partial differential equations have an increased order of derivation respect to the classical theory, from the fourth to the sixth. The displacements are carried out following the Navier procedure for simply supported boundary conditions. Isotropic and antisymmetric orthotropic laminates, both cross- and angle-ply are studied, for different layouts involving different material properties. Dimensionless outcomes in terms of transverse displacements, and normal and shear stresses, are given to changing aspect ratio and non local ratio, also making a comparison with the classical theory.

The problem of the pullout of a viscoelastic synthetic fibre embedded in a cementitious matrix and subjected to an external time-dependent axial load is considered in the present work. A 1D phenomenological model able to simulate the contribution of viscoelastic relaxation as well as the hardening behavior due to abrasion phenomena during slippage is developed. The cement matrix compliance is neglected with respect to the fibre elongation. The interfacial shear stress between the fibre and the surrounding matrix is assumed to depend on the slippage distance through a second degree polynomial law, thus involving three constitutive parameters. Two distinct phases are recognized: An earlier debonding stage followed by the effective fibre pullout process. Two different creep functions have been assumed for modelling the viscous response of polymeric fibres: A function based on the fraction-exponential Rabotnov operator and a classical exponential model. Identification of the governing constitutive parameters allows obtaining the relation between the external strain and the axial displacement, which has been compared with experimental results provided by pullout tests both on plain and treated fibres, finding a good agreement. It is shown that the proposed approach can predict the whole pullout process of discrete synthetic macrofibres.