1,721,052 research outputs found
An electromechanical micropolar peridynamic model
Full text linkA micropolar peridynamic model for in-plane electro-mechanical behavior of isotropic solids is presented in this paper. The conceived analytical implicit formulation of the electrical part of the model is based on the definition of a proper microelectrical energy function and a specific bond electrical field measure. A compatibility condition and a constitutive relationship have been derived and thus the electrical stiffness operator has been obtained. The electrical formulation is then coupled with a mechanical micropolar peridynamic formulation with adjustable Poisson's ratio. The obtained unified model is capable to predict the elastic response and the electrical conduction of elastic brittle materials taking into account the influence of cracks and other defects. The accuracy of the proposed model has been assessed by several problems including the simulation of fracture propagation and damage sensing in a lamina under tensile loading and the piezoresistive response of nanocomposite materials
High-cycle fatigue numerical modelling of bond between FRP rebar and concrete
No full textExperimental studies have evidenced that the use of Fibre Reinforced Polymer (FRP) composite materials to reinforce or strengthen the RC structures exposed to repeated cyclic loading can improve their fatigue life. To exploit the good fatigue performance of these composite materials, the bond between the FRP reinforcement and the concrete must remain effective. The current study aims to simulate the nonlinearity in the bond of FRP rebar and concrete under high-cycle fatigue, firstly, by developing a damage-based model for reproducing the bond stiffness degradation and residual slip growth due to fatigue load effects, and then, developing a 3D finite element (FE) model in a commercial software. The FE model considers the nonlinear behaviour of the materials coupled with the developed damage-based model to simulate the bond deterioration due to high number of cycles. Moreover, to reduce the computational cost for modelling each cyclic loading, a cycle jump approach is implemented in the FE model. The developed numerical model is validated by comparing with the relevant results of an experimental program involving eccentric pull-out fatigue tests
Humidity responsive bi-layered composites
No full textAmong cellulose-based materials, which are known for their hygroscopic behaviour, cellulose acetate (53.3 % of acetylation) has been chosen for this investigation. Membranes of cellulose acetate have been prepared via solvent evaporation of cellulose acetate and ethyl lactate solutions. Membranes with thicknesses within the range of 66÷200 μm have been manufactured. Via gravimetric measurement of preliminary dried membranes, the moisture absorption at room temperature and different relative humidity (RH = 21÷53%) have been monitored. A moisture diffusion coefficient of 3.35×10-6 mm2⁄s and a relaxation factor of 0.026 s-1 have been determined. A dependency of moisture concentration at saturation on relative humidity has been observed. The induced expansion due to the moisture absorption has been determined by thermomechanical analysis. The hygroscopic expansion coefficient of cellulose acetate has been evaluated as 192.5 (mm3/g). The experimental measurements have been considered as input of a detailed finite element model that couples moisture diffusion and hygroscopic expansion. The model allows to predict the response to the changes of relative humidity of a bi-layered composite made up of cellulose acetate and a non-hygroscopic textile
Effect of fiber hybridization on mechanical properties of concrete
Full text linkTen concrete mixtures, using long and medium length hooked-end and short wave-shaped steel fibers, were designed to experimentally investigate the effect of hybrid reinforcement on workability, drying shrinkage, and mechanical properties of hybrid steel fiber reinforced concrete. The steel fibers reduced the workability and drying shrinkage. Hybrid fibers, including long hooked-end steel fiber, can produce a synergistic effect on compressive strength. For the adopted materials, a linear relationship was observed between shrinkage strain and compressive strength. The tensile splitting strength increased with the volume fraction of the hybrid fibers. The hybrid steel fibers generated a synergistic effect on the tensile splitting strength, with an almost constant ratio of tension splitting strength to compressive strength of hooked-end steel fiber reinforced concrete. The use of long hooked-end steel fiber reinforcement led to a higher modulus of rupture, residual strength, and toughness than other mono fibers. Flexural strength increased with the increasing volume fraction of hybrid fibers. Residual strength of hybrid steel fiber reinforced concrete varied with fiber hybridization. Overall, a concrete reinforced with a hybrid mix of all the considered steel fibers had the best performance among the considered ones
Modelling the bond in GFRP bar reinforced concrete thin structural members
Full text linkThis paper analyses the accuracy of FE (Finite Element) modelling of concrete thin slabs, reinforced with GFRP (Glass Fiber Reinforced Polymer) bars, under bending loading conditions. It considers two strategies of concrete/bar interaction approach – direct and indirect. Indirect bond approach was combined with concrete tension stiffening model, whereas the direct bond one employed plain concrete constitutive model. Apart from selection of material and bond models, FE mesh sensitivity was examined. The efficiency of both bond modelling strategies was proven for the slab of standard thickness (100 mm) failed due to concrete crushing, whereas direct bond method was shown indispensable when simulating very thin slabs (40 mm) failed due to bar debonding
Numerical modelling of GFRP reinforced thin concrete slabs
No full textWith the development of new glass fiber reinforced polymer (GFRP) bars for RC structures, their application extends simultaneously. The non-corrosive nature of GFRP bars enables maximal lowering of the concrete cover, thus making them very suitable as a reinforcement in thin RC plate elements. Such thin members are usually prefabricated and used as façade panels, pavement or components of sandwich panels. Along with experimental studies, the finite element (FE) numerical modeling represents very useful tool for assessing and predicting the structural member behavior. Proper choice of material constitutive models and strategy of concrete/bar bond implementation always presents challenge when dealing with numerical FE modelling of RC structures. This study considers FE modelling of thin GFRP RC slabs’ flexural behavior under three-point bending test setup. It uses direct bond approach, that is, explicit simulation of the bond-slip effect between concrete and reinforcing bars. For this purpose, the experimental bond-slip law was used, obtained from the pull-out test having the same GFRP bar, same concrete cover and similar concrete properties as simulated RC slab. Since the slab failed for concrete crushing, the study assesses the importance of concrete compressive model selection on the numerical analysis results. Two different models were employed in the numerical analysis, in combination with three FE mesh densities. The main differences between the models comprise post-peak capacity and mesh dependency. The FE modelling strategy developed in the study was shown successful in reproducing the experimental outcome. Both concrete models showed convergence tendency when refining the mesh, whereas only one of them succeeded to reproduce the experimental results
Percolation of fatigue damage in plain-weave textile reinforced composites
No full textSeveral studies have investigated the mechanical response of textile reinforced composites under cyclically repeated loadings (fatigue). However, there is still a need for accurate quantitative evaluation of the fatigue damage accumulation and accurate predictive models to ensure the safe design of textile composite components. In this study, a thermography based method has been developed to detect the evolution of the fatigue damage in plain-weave carbon fiber/epoxy composites. The method adopts the unit cell as the unit of damage evolution (damaged unit cell). The fatigue damage has been related to the development of clusters of damaged unit cells and their distribution on the surface of the tensile fatigued specimen, considering the thermoelastic damage analysis (TDA). Percolation theory was also applied to demonstrate how the number and properties of the clusters, formed by irregularly distributed damaged unit cells, affect the entire system. The method showed that the size ratio of the maximum damaged unit cell cluster, Cmax, increased rapidly after the damaged unit cell ratio exceeded the ‘percolation threshold’ pc. This threshold announced the fatigue final failure, which occurred with rapid accumulation of transverse cracks, wider local delamination and fibers breakage. The arrangement of the damaged unit cells was also quantitatively evaluated using the fractal concept. The fractal dimension D of the damaged unit cells cluster increased up to a steady state value Dc. Beyond this value, the fatigue damage had a faster evolution when p exceeded pc. Finally, the fatigue damage evolution was predicted using the concept of percolation and mutual interference between damaged units
Fatigue damage characterization and percolation in plain-weave carbon fiber-epoxy composites
No full textThe concept of ‘damaged unit cell’ was employed to describe the tensile fatigue damage evolution in plain-weave carbon-epoxy composites. The fatigue damage was connected to the development of cluster of damaged unit cells and their distribution, considering the measurements by thermoelastic damage analysis (TDA). Internal damage state was also observed by SEM. The experimental results showed that the size ratio of the maximum damaged unit cell cluster, Cmax, rapidly increased after the damaged unit cell ratio p exceeded 0.527, called ‘percolation threshold’ pc. This threshold announced the fatigue final failure, which occurred with fast accumulation of transverse cracks, wider local delamination and fibers breakage, once p exceeded pc. Since the relationship between the averaged thermoelastic damage response and Cmax was nearly linear, the fatigue damage evolution involved clustering of damaged unit cells. The fractal dimension D of the damaged unit cells cluster increased up to a steady state value Dc, when p exceeded pc, showing the faster fatigue damage evolution. Finally, the fatigue damage evolution was predicted using the concept of percolation and mutual interference between damaged units
Moisture absorption measurement and modelling of a cellulose acetate
Full text linkWith a view toward the application of highly hygroscopic polymers as a humidity responsive self-actuator, the evaluation of the real time moisture concentration in the material becomes a priority. In this paper, the moisture diffusion process in a cellulose acetate (53.3% of acetylation) has been studied. Membranes of cellulose acetate (thickness within the range 66–200 μm) have been prepared, and the moisture absorption at room temperature and at a different relative humidity (RH within the range 21–53%) has been monitored. An analytical model has been used to describe the observed non-Fickian sigmoidal behavior of moisture diffusion. A relaxation factor (β) of about 0.026 s−1 and a moisture diffusion coefficient (D) of 3.35 × 10–6 mm2/s have been determined. At constant room temperature, the moisture concentration at saturation (Csat) has shown a linear relation with relative humidity. The identified values β, D and Csat of the analytical model have been used as input for the finite element simulation of the non-Fickian diffusion. The reliability of the finite element simulations has been confirmed with a second set of experiments
FATIGUE BEHAVIOUR OF OPEN HOLE CARBON TEXTILE COMPOSITE WITH MICROFIBRILS CELLULOSE MODIFIED EPOXY
No full textThe strength and the failure mechanisms of notched composite materials are of considerable importance in several industrial applications where joints and bolts are unavoidable. They become the main concern when notched composite components are subjected to repeated cyclic loadings. To enhance the fatigue life of composite materials, research efforts were also dedicated to the toughening mechanisms of resin systems. Thermoset systems were modified to overcome their brittle nature by a second phase consisting of nano- or micro- sized fillers (such as nanotubes, nano-fibres, nano-particles, etc.). To this purpose, the family of hybrid nano/micro enhanced resin systems has been extended exploiting microfibrils cellulose (MFC). Cellulose is the most abundant natural homo-polymer and one of the most promising renewable and environmentally friendly resources. The aim of this experimental study was to contribute on the understanding the effect of hybrid microfibrils cellulose epoxy resin (MFC content 0.3% of the resin weight) on the tensile fatigue performance of open hole carbon textile composites. This comprehensive study had three steps. The first dealt with the pre-fatigue quasi-static tensile behavior to measure the relevant mechanical properties, and understanding the damage modes initiation and development around the hole by digital image correlation (DIC) and scanning electron microscope (SEM) observations. The second aimed to detect the effect of the hybrid microfibrils cellulose epoxy resin on the fatigue life diagram of the open hole textile composite by tensile-tensile cyclic loading. Moreover, in this step, the damage development during cyclic loading was observed by DIC, SEM and X-ray micro-CT. Finally, the third step was dedicated to post-fatigue quasi-static tensile behavior, after one million cycles, to assess the retention of the tensile strength and the distribution of the damage comparing to the pre-fatigue quasi-static results. The MFC hybridization of the matrix improved the damage tolerance of the open hole carbon textile composite leading to the extension of the fatigue life. The enhanced performance was mainly connected to the bridging effect of cellulose microfibrils preventing or delaying the cracks propagation in the matrix and along the fiber-matrix interface
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