1,720,997 research outputs found
A rate dependent cohesive model for the analysis of concrete-FRP bonded interfaces under dynamic loadings
No full textReinforced concrete structures, strengthened with fibre-reinforced polymers materials (FRP), are frequently subjected to dynamic loadings, due to, e.g., earthquake, blast, or impact events. The definition of proper cohesive laws to model the bond between the fibre-reinforced polymer sheet and concrete, under high deformation rates, is a crucial issue because the typical failure mode of these joints is debonding of the composite from the concrete substrate. Although numerous studies have already investigated the quasi-static interface response, experimental and numerical investigations, concerning the effect of deformation rate on the bond behaviour between a fibre-reinforced polymer sheet and concrete, are still few. This paper presents a cohesive law for the modelling of interfaces under mixed-mode dynamic loadings, considering the effect of deformation rate. The formulation is based on the decomposition of the discontinuity displacement vector across the interface into elastic and viscoplastic components, with the evolution of the latter being governed by a viscoplastic law formulated according to the overstress approach. Experimental results available in literature, related to double- and single-lap shear tests, performed on FRP reinforced concrete specimens, are exploited to validate the proposed model and to show its capacity to simulate closely the experimental behaviour
A new cohesive law for the simulation of crack propagation under cyclic loading. Application to steel- and concrete-FRP bonded interface
No full textThis paper presents a new cohesive law for modelling interfaces under mixed-mode cyclic loading. The formulation is based on the definition of a free energy function which governs the interface behaviour under monotonic loading, which is then extended to cyclic-driven decohesion through the introduction of a scalar damage variable, whose evolution in time is governed by a phenomenological rate equation. The cohesive model is formulated for a mixed-mode problem and then it is applied for the simulation of debonding phenomena occurring at the interface under a pure a shear stress state. Experimental results available in literature, related to single-lap shear tests, performed on both concrete and steel specimens reinforced by fibre reinforced composite (FRP), are used to validate the proposed model and to show its effectiveness to simulate very closely the observed experimental behaviour
Stochastic and recursive estimation of the hygro-thermo-chemical-mechanical parameters of concrete through Monte Carlo analysis and extended Kalman filter
No full textHygro-thermo-chemical-mechanical models, used to determine the variations over time of temperature, relative humidity and shrinkage induced deformations in concrete components, are characterised by the presence of a large number of input parameters. Some of these parameters can be evaluated on the basis of the concrete mix specifications or from literature data, while the others present a large variability and, in some cases, do not have a precise physical meaning and, for this reason, require the implementation of proper identification strategies. The experimental work involved for this characterisation can be time-consuming and costly because based on the long-term monitoring of the time evolution of the field quantities in specific positions within concrete components. The aim of this paper is to propose and validate recursive identification strategies that exploit, in a step by step fashion, the information coming from the experimentation for the identification of the model input parameters. The influence of different exposure conditions and of different concrete thicknesses are investigated and, for each scenario considered, the expected identification error of each parameter is estimated, within a stochastic context implemented through Monte Carlo analyses and Kalman Filter, as a function of the monitored time
Identification of the parameters contained in a cyclic cohesive zone model for fatigue crack propagation
No full textCyclic cohesive zone models provide a useful tool to describe fatigue driven crack propagation,
covering a wide range of engineering applications. For a proper use of these models, particular
attention must be devoted to the correct calibration of the parameters contained, considering that
some of these can be characterized by a large variability and/or by the absence of a precise
physical meaning so that they are not amenable to a direct measurement. This paper proposes a
robust inverse analysis procedure, to investigate the identifiability of the model parameters
governing the fatigue induced damage evolution, in a recently proposed cyclic cohesive zone
model. A novel control for compact test specimens providing more meaningful experimental
information is proposed. The identification problem is formulated by considering fatigue crack
propagation curve and deformations measurements in a discrete number of points of the specimen
surface as input data of the inverse algorithm. The finite element operator, adopted to simulate
the experimental tests, has been substituted by a proper calibrated meta model to reduce the
computational cost of the forward operator and, thus, to solve the inverse problem in a stochastic
context through Monte Carlo like procedures. Representative results, obtained starting from
virtual data affected by different levels of noise, are reported to highlight the identifiability of the
model parameters on the basis of the experimental data adopted. Indications regarding the
minimum number of measurements needed to make the inverse problem well-posed are also
provided, supporting possible planning of measurements setups for laboratory investigations
A hygro-thermo-chemical-mechanical model for the service response of composite steel-concrete floor systems
No full textComposite steel-concrete floor systems are widely used throughout the world for building applications. This paper focuses on the service behavior of composite floors that usually consist of composite slabs in steel framed construction and of post-tensioned composite slabs in concrete structures. In the initial part of the paper, recent experimental work carried out on composite floor slabs is outlined and it includes the description of the occurrence of a non-uniform shrinkage profile through the slab thickness due to the inability of the concrete to dry from its underside because of the presence of the profiled steel sheeting. A hygro-thermo-chemical-mechanical model is then presented to predict the service response of composite floors. The proposed approach is validated against experimental data collected on post-tensioned composite samples. For this purpose, an inverse analysis procedure is applied for the characterisation of the numerous material parameters and the multi-physics model is used to predict the non-uniform shrinkage gradient. Based on this, a design model available in the literature is then used to evaluate the long-term deflections and compared the calculated values against long-term experimental measurements. Simplified models have also been used to determine the shrinkage profile for benchmarking purposes. It has been shown that the numerical and experimental deflection comparisons show good agreement when determined using a non-uniform shrinkage distribution, while the calculated values underestimate the observed deflections when obtained with a uniform shrinkage profile
Algebrized approach for the finite element analysis of heterogeneous viscoelastic structures
No full textThe analysis of heterogeneous structures in the linear viscoelastic field is a common problem for reinforced concrete and prestressed concrete structures. The analysis is complicated by the form of the constitutive law that includes a Volterra integral, whose analytical evaluation is possible only for simple models and simple histories of the prescribed variables. For general applications, proper numerical methods are thus needed which require the use of specific software usually available in research environments only. This paper suggests an approach that makes use of the method originally proposed by Trost and actually referred to as Age Adjusted Effective Modulus (AAEM) to overcome the difficulties related to the integration over time. It is shown that the proposed method can be easily coupled with standard linear elastic finite element strategies in order to convert them to the analysis of viscoelastic structures as well. Some illustrative examples prove the simplicity of this approach and the good level of accuracy achieved
Numerical insight on the interaction effects of a confined masonry tower
No full textThe present paper provides a numerical insight into the structural behavior of a confined masonry tower. The study is carried out in relation to the Gabbia Tower, the tallest historical masonry tower in the city of Mantua (northern Italy), which is surrounded by multistory masonry buildings on all its sides. Three 3D FE numerical models are critically compared to investigate the sensitivity of the structural behavior of the tower with respect to the adjacent buildings, both in the linear and non-linear fields. Firstly, the tower is studied as a fictitious building isolated from the context, then two different constraints are considered for modeling the interaction between tower and buildings. In the first case, linear elastic springs are inserted along the height of the tower; while the other one focuses on the effect provided by the adjacent buildings, whose walls are modeled as monolithic elements joined to the main body of the tower. The validation of the FE models is carried out in the linear field by comparing mode shapes and frequencies resulting from the numerical modal analysis with the available experimental data provided by the ambient vibration test conducted in 2012. The effect of the different constraints on the seismic response of the tower is then investigated by performing a set of non-linear dynamic analyses and considering the accelerograms recorded during the earthquake of May 29, 2012 by the fixed station located in Mantua as seismic input. Numerical results indicate the modeling of the surroundings as an essential step for a realistic analysis of confined towers. In particular, the simulations highlight advantages and limitations of the proposed modeling approaches of the constraints, both in the linear and non-linear fields
Stochastic Calibration of a Cyclic Cohesive Zone Model Through Monte Carlo Analysis
No full textFatigue induced crack propagation is still an open issue, relevant to many engineering applications. Cyclic loading produces damage accumulation at a localized region which results first in the formation of micro-cracks and finally leads to the creation of macro-cracks. The modeling of fatigue induced crack propagation can be done according to different approaches, such as the Paris law, where the rate of crack growth is dependent on fracture mechanics parameters, e.g., the stress intensity factor or the strain energy release rate. Other approaches are the empirical methods, typically S-N approach and the micromechanical models describing the accumulation of damage based on material microstructure changes. Another approach involves the definition of phenomenological models where the fatigue crack growth is described by adopting cohesive zone laws. In this paper a damage-based irreversible cyclic cohesive zone model is adopted, where damage healing is considered during the fatigue damage evolution. The model presents different parameters, some of which are characterized by a large variability, do not possess a precise physical meaning and then they are not amenable to direct measurement. In this context, this paper aims to provide a robust procedure for the calibration of these model parameters, based on a Monte Carlo stochastic approach. Indications regarding the minimum number of experimental measurements are also provided, to support the planning of tests setups for laboratory investigations. Finally, the definition of a well posed inverse problem allows an efficient identification of all the sought model parameters reducing then the experimental costs
A method to calculate the support length of beams resting on masonry walls
Full text linkRehabilitation, strengthening, and retrofitting of existing masonry buildings represent an important challenge for the construction engineering field. Often, slab strengthening/retrofitting is performed by replacing existing timber and steel beams or by adding new beams to improve the slab load-carrying capacity. The computation of the stresses at the beam–masonry interface (i.e., the contact pressure) is crucial to properly design the beam support length, preventing local failure of masonry and beam. This paper presents a simple analytical procedure to compute the contact pressure at the beam–masonry interface. The analytical procedure is validated by comparison between analytical and corresponding numerical results obtained by finite element modeling. Different types of beam (solid and laminated timber beams and steel beams) were considered, as well as different support conditions (simply resting on the wall considering different support lengths or fully embedded). The results obtained show that the method proposed is simple and reliable, which makes it suitable for professional practice
Assessment of the shock adsorption properties of bike helmets: a numerical/experimental approach
Full text linkIn this paper, a numerical and experimental study of the shock absorption properties of bike helmets is presented. Laboratory compression and tensile tests were carried out on samples of expanded polystyrene (EPS) and polycarbonate (PC), respectively constituting the internal shock absorption layer and the external hard shell of composite helmets. The measured responses of the two materials were then exploited to calibrate the relevant elasto-plastic constitutive models, adopted in full-scale finite element analyses of a helmet subject to standardized impacts. The simulations allowed assessing the time evolution of the acceleration measured inside the headform (according e.g., to EN 1078) and the failure mechanisms of the helmet, if any, as induced by the localization of plastic deformations
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