Abstract: In this study, the results of an in-situ experimental program on the performance of concrete taxiways are presented. The experimental program has been undertaken at the Guglielmo Marconi airport of Bologna (Italy). It concerns two portions of the taxiway, one built with plain concrete and one with rubberized concrete. Each portion has been instrumented with strain gauges embedded in concrete for the acquisition of vertical strains. The results of the experimentation are discussed in view of possible applications to the computational analysis of the stress field induced into pavements by aircrafts.
Abstract: In this study, the problem of finding the complete trajectory of propagation and the limiting load in plates with internal straight cracks is extended to the non-linear field. In particular, results concerning concrete plates in bi-axial tensile loading are shown. The concrete constitutive law adopted for this purpose is monotonic non-decreasing, as following according to previous studies of the author on monotonic mono-axial loading. The analysis is performed in a discrete form, by means of the Cell Method (CM). The aim of this study is both to test the new concrete constitutive law in biaxial tensile load and to verify the applicability of the CM in crack propagation problems for bodies of non-linear material. The discrete analysis allows us to iden-tify the crack initiation without using the stress intensity factors.
Abstract: The main assumption on the basis of the identifying model of the effective law, developed by the Author, is the impossibility of considering the specimen as a continuum, when an identifying procedure from load-displacement to stress-strain in uniaxial compression is attempted. Actually, a failure mechanism with propagation of a macro-crack was found to activate from the very beginning of the uniaxial compression test forth. This leads to considering the acquired dis-placements as composed by two quotes: one constitu-tive, due to the material strain, and one of crack open-ing. Since the ratio between these two quotes is not constant during the compression test, the properties of the displacement field (which attains to structural prop-erties) cannot be transposed to the strain field (which attains to material properties) through a mere scale fac-tor. In this context, also creep takes on a different meaning, in the sense that time-dependence is an effect observed in the displacement field that does not neces-sarily correspond to a property of the strain field, i.e., the creep. In other words, it is not possible to exclude a-priori that the time-dependence of displacements is induced by crack propagation alone. A time-dependent motion of crack opening could activate and affect the displacements acquisition. The aim of the present work is to investigate the role played in displacement time-dependence both by creep and crack propagation. Re-sults of an experimental program are presented here, stating the strict relationship existing between the in-creasing of displacement and the propagation of cracks at constant load.
Abstract: In this study, the Cell Method (CM) is applied in order to investigate the failure mechanisms of masonry walls under shear force. The direction of propagation is computed step-wise by the code, and the domain is updated by means of a propagation technique of intra-element nodal relaxation with re-meshing. The crack extension condition is studied in the Mohr/Coulomb plane, using the criterion of Leon. The main advantage of using the CM for numerical analyses of masonry is that the mortar, the bricks and the interfaces between mortar and bricks can be modeled without any need to use homogenization techniques, simply providing each of them with their own constitutive properties. The capability of the CM to handle domains with more than one material is exploited to capture how the propagation direction changes when the crack over-come the joints or passes from the brick to the interface and to the mortar. Also, the principal stresses and prin-cipal directions of stress are mapped for the bricks, the interfaces and the mortar. In comparison with those presented in Ferretti (2003) and Ferretti (2004a), the computational capabilities of the CM code have been improved considerably. Actually, a new version of the CM code has been implemented, which is able to self-compute the position of crack initiation and manage several cracks propagating at the same time. This al-lows us not to impose the number and the position of crack initiations a-priori, letting the code estimate them as the imposed displacement is increased. Interactions between propagating cracks are simply taken into ac-count by the code, leading to modification of the failure direction or to crack arrest as soon as a new crack acti-vates. The code is also able to self-estimate whether or not one or more cracks bifurcate and to follow the propagation of each branch of bifurcation.
Abstract: In this study nonlocality is discussed with regard to the differential and discrete formulations. Here, nonlocality is found to be a concept attaining not to the description of the material, but to the governing equations. This has made it possible to discuss the op-portunity of introducing nonlocality in the constitutive equations, in order to give respectability to strain-softening damage models. When using the differential formulation, a length scale must be introduced into the material description of a strain-softening modeling, par-ticularly when the size-effect is involved. In the opin-ion of the Author, this need lies in the basics themselves of the differential formulation, performing the limit process. Actually, with the reduction of global vari-ables to point (and instant) variables, we loose metrics. Consequently, metrics must be reintroduced a-posteriori, by means of a length scale, if we want to de-scribe more than 0-dimensional (nonlocal) effects. Here it is shown how a length scale is intrinsic in Phys-ics. Avoiding the limit process, that is, using a discrete formulation, we preserve the length scale of Physics and do not need to recover it. In this sense, it may be asserted that the discrete formulation is nonlocal in it-self and does not require nonlocal constitutive relation-ships for modeling nonlocal effects. Obtaining a nonlo-cal formulation by using local constitutive laws and discrete operators seems to be possible and physically appealing. Numerical results are provided here, show-ing how a formulation using discrete operators and a local constitutive law is able to model softening and size-effect, which is impossible for differential local approaches. The mathematical and physical well-posedness and the existence itself of strain-softening are also discussed.
Abstract: Nonlocality is discussed in differential and discrete formulations. When modeling heterogeneous materials, a length scale must be introduced into the material description of the differential formulation. This happens since metrics is lost in performing the limit process. Avoiding the limit process, that is, using a discrete formulation, the length scale is intrinsically taken into account. Moreover, nonlocality seems to characterize global variables rather than material. This made it possible to move the length scale from constitutive to governing equations.
Abstract: The Cell Method (CM) code with automatic remeshing for crack propagation analysis [Ferretti (2003)] is here used for modeling the pullout test. Particular emphasis is given to the analysis in the Mohr-Coulomb plane, since previous numerical models were not decisive in describing failure mechanism in pullout tests. The interpretations of experimental and analytical studies vary widely, and none of the existing explanations offer a complete description of the progressive failure of the concrete medium [Yener (1994)]. Nor do most existing interpretations appear to be totally compatible with the experimental evidence. Analysis of the failure mechanism for the pullout test requires a failure criterion accurately describing crack initiation in tension loading. The Mohr-Coulomb criterion of the first code [Ferretti (2003)] has therefore been abandoned in favor of a more realistic criterion for the tensile state of stress, the Leon criterion. The failure analysis has been performed for several ratios between the counter pressure diameter and the stem length (Fig. 1). Moreover, the complete crack path has been obtained for the geometry of the Lok-test. The evolving state of stress-strain for the Lok-test is also provided. The identification of the directions of principal stress completes the stress analysis. Modeling is performed both on the concrete specimen and on the steel insert, showing how the CM can easily handle domains with several materials.
Abstract: Defining the crack path in brittle and non-brittle crack is not easy, due to several unknowns. If the direction of crack propagation can be computed by means of one of the existing criteria, it is not known whether this direction will remain constant during crack propagation. A crack initiation leads to an enhanced stress field at crack tip. During propagation, the enhanced tip stress field propagates into the solid, locally interacting with the pre-existing stress field. This interaction can lead to modifications of the propagation direction, depending on the domain and crack geometry. Moreover, trajectory deviation affects the length of crack propagation. Thus, the length of crack propagation too depends on the domain and crack geometry. Finally, the local interaction between stress fields of opposite signs can return a modified condition of crack arrest. Crack stability analysis cannot be performed without considering this interaction. The problem of defining trajectory deviation, propagation length and crack stabilization is of particular interest in brittle cracks, since these cracks develop statically from the moment of crack initiation forth. It will be shown here how a numerical code for use with the CM returns an accurate crack path for brittle and non-brittle cracks. In both cases, the stress analysis has been performed on the plane of Mohr for each step of the carrying process. At crack propagation, an automatic tool of nodal relaxation with remeshing is used to update the domain geometry.
Abstract: The question of whether concrete strain-softening is a real material property is discussed here. The discussion is given from both the physical and the analytical point of view. New evaluations of the actual nature of strain-softening are added to those existing in literature.
Abstract: A procedure is proposed for identifying uniaxial stress-strain relationship in compressed concrete. By considering the specimen as a structure, the procedure identifies effective properties from experimental data. This involves a modification of traditionally identified uniaxial stress-strain relationship. Results are presented for cylinders of varying slenderness.
Abstract: A new procedure is proposed for identifying uniaxial stress-strain relationship and Poissonâs ratio in compressed plain concrete. The procedure is based on the assumption of an internal core of intact material always present inside a specimen in uniaxial-compression. This involves a modification of the traditionally identified uniaxial stress-strain relationship and Poissonâs ratio, which turns out to be almost independent of the loading step. The main finding concerns the volumetric strain, since it appears to be no real increase in the volume of a concrete solid when the solid is placed under pressure.
Abstract: On the basis of a discussion on the existence of strain-softening in concrete, a monotonic constitutive law, the effective law, has been proposed by the present author. The identification of the effective parameters has been performed for quasistatic load processes. Here, we address applications of the effective law to modeling the response of structures to dynamic loadings.
Abstract: A numerical code for modeling crack propagation using the cell method is proposed. The Mohr-Coulomb criterion is used to compute the direction of crack propagation, and the new crack geometry is realized by an intra-element propagation technique. Automatic remeshing is then activated. Applications in Mode I and Mixed Mode are presented to illustrate the robustness of the implementation.
Abstract: A novel constitutive law for concrete in monoaxial loading was developed in previous studies. This law has been extended here to the triaxial field, so as to model composite (FRP) wrapped concrete cylinders. A numerical code is presented, which is able to reproduce experimental results for unwrapped and wrapped cylinders by only setting the number of wrapping sheets. No parameter is introduced to take into account triaxial stress. No calibration is therefore needed. A tool for crack propagation description is proposed, so as to analyse stiffness decreasing during loading. Numerical simulations have been carried out by means of a Cell Method code
Abstract: The Cell Method (CM) code with automatic remeshing for crack propagation analysis [6] is here used for modelling the pullout test. Particular emphasis is given to the analysis in the Mohr-Coulomb plane, since previous numerical models were not decisive in describing failure mechanism in pullout tests. The interpretations of experimental and analytical studies vary widely, and none of the existing explanations offer a complete description of the progressive failure of the concrete medium [21]. Nor do most existing interpretations appear to be totally compatible with the experimental evidence. Analysis of the failure mechanism for the pullout test requires a failure criterion accurately describing crack initiation in tension loading. The Mohr-Coulomb criterion of the first code [6] has therefore been abandoned in favour of a more realistic criterion for the tensile state of stress, the Leon criterion. The failure analysis has been performed for several ratios between the counterpressure diameter and the stem length (Fig. 1). Moreover, the complete crack path has been obtained for the geometry of the Lok-test. The evolving state of stressstrain for the Lok-test is also provided. The identification of the directions of principal stress completes the stress analysis. Modelling is performed both on the concrete specimen and on the steel insert, showing how the CM can easily handle domains with several materials.
Abstract: A numerical code for modeling crack propagation using the Cell Method is proposed. The Leon failure surface is used to compute the direction of crack propagation, and the new crack geometry is realized by an intra-element propagation technique. Automatic remeshing is then activated. Applications in Mode I, Mode II and Mixed Mode are presented to illustrate the robustness of the implementation.
Abstract: The present study is part of an identifying programme for constitutive parameters in damaged materials, termed the âeffective parametersâ. The programme starting point is that the experimental response depends not only on constitutive parameters, but also on structural mechanics and interaction with the test-machine. It is showed how the load-displacement diagram of compressed concrete cylinders is affected by crack propagation, through the resistant structure modification. Moreover, it is analytically demonstrated that the effective stress (Ïeff )-effective strain (²eff ) curve exhibits a strictly positive derivative at the point corresponding to the average stress (Ï)-average strain (ε) curve peak. Finally, it is proposed a new identification procedure which provided satisfactory results, giving monotone strictly non-decreasing, size-effect insensitive and failure mechanism insensitive Ïeff â εeff curves.
Abstract: A numerical code for modeling crack propagation using the Cell Method (CM) has been implemented. The crack geometry is updated with an intra-element propagation technique. Automatic remeshing is activated after each update. The code was implemented in Matlab on EIDOS. Results for Mixed Mode crack propagation are presented.
Abstract: In this study, the results of an in-situ experimental program on the performance of concrete taxiways are presented. The experimental program has been undertaken at the Guglielmo Marconi airport of Bologna (Italy). It concerns two portions of the taxiway, one realized in plain concrete and one realized in rubberized concrete. Each portion has been instrumented with strain gauges for the acquisition of vertical strains inside concrete.
Abstract: In this paper an exact solution methodology, based on the coupling of the dynamic stiffness matrix and the line-spring, enabling one to analyze the coupled bending-torsion free vibration of Timoshenko beams weakened by multiple non-propagating part-through surface cracks is presented. The changes introduced by the presence of three transverse open cracks, regarding the modal response, are investigated. A parametric study has been carried out for various crack parameters such as crack depth and location.
Abstract: One of the main research fields in past years concerns the modeling of heterogeneous materials. For these materials, the use of the classical local continuum concept does not seem to be adequate. The classical local continuum concept leads to constitutive models falling within the category of simple nonpolar materials (Noll 1972). For these materials, the stress at a given point uniquely depends on the current values, and possibly also the previous history, of deformation and temperature at that point only (Bažant and Jirásek 2002).
Beginning with Krumhansl (1965), Rogula (1965), Eringen (1966), Kunin (1966), and Kröner (1968), the idea was promulgated that heterogeneous materials should properly be modeled by some type of nonlocal continuum. Some preliminary ideas on nonlocal elasticity can be traced back to the late 19th century (Duhem 1893). Nonlocal continua are continua in which the stress at a certain point is not a function of the strain at the same point, but a function of the strain distribution over a certain representative volume of the material centered at that point (Bažant and Chang 1984). Thus, nonlocality is tantamount to an abandonment of the principle of the local action of classical continuum mechanics (Bažant and Jirásek 2002).
Local constitutive relations between stress and strain tensors are not adequate for describing the mechanical behavior of solids in the classical differential formulation, since no material is an ideal continuum, decomposable into a set of infinitesimal material volumes, each of which can be described independently. All materials, natural and man-made, are characterized by microstructural details whose size ranges over many order of magnitude (Bažant and Jirásek 2002). In constructing a material model, one must select a certain resolution level below which the microstructural details are not explicitly visible. Instead of refining the explicit resolution level, it is often more effective to use various forms of generalized continuum formulation, dealing with material that are nonsimple or polar, or both. A list of enriched continuum models is provided in Bažant and Jirásek (2002). Among these, a great variety of nonlocal models was developed.
The aim of the present study is to show that nonlocal constitutive relations between stress and strain tensors are not strictly needed to construct a material model. They are required only if a differential formulation is used for modeling nonlocality, since differential operators are local. The physical well-posedness of nonlocality is discussed with regard to the differential and discrete formulations. Nonlocality was found to be a concept not attaining to the description of the material, but of the phenomenon. This made it possible to discuss the opportunity of using nonlocality in order to give respectability to strain-softening damage models. The mathematical and physical well-posedness and the existence of strain-softening are also discussed. When using the differential formulation, a length scale must be introduced into the material description of a strain-softening modeling. This need has been here justified on the basis of the geometrical information which has been lost in performing the limit process. It was shown how, avoiding the limit process, a length scale is intrinsically taken into account into a discrete formulation. Thus, the discrete formulation turns out to be more appealing than the differential formulation with nonlocal approach, from the physical point of view.
Abstract: One of the main research fields in past years concerns the modeling of heterogeneous materials. For these materials, the use of the classical local continuum concept does not seem to be adequate. The classical local continuum concept leads to constitutive models falling within the category of simple nonpolar materials (Noll 1972). For these materials, the stress at a given point uniquely depends on the current values, and possibly also the previous history, of deformation and temperature at that point only (Bažant and Jirásek 2002). Beginning with Krumhansl (1965), Rogula (1965), Eringen (1966), Kunin (1966), and Kröner (1968), the idea was promulgated that heterogeneous materials should properly be modeled by some type of nonlocal continuum. Some preliminary ideas on nonlocal elasticity can be traced back to the late 19th century (Duhem 1893). Nonlocal continua are continua in which the stress at a certain point is not a function of the strain at the same point, but a function of the strain distribution over a certain representative volume of the material centered at that point (Bažant and Chang 1984). Thus, nonlocality is tantamount to an abandonment of the principle of the local action of classical continuum mechanics (Bažant and Jirásek 2002). Local constitutive relations between stress and strain tensors are not adequate for describing the mechanical behavior of solids in the classical differential formulation, since no material is an ideal continuum, decomposable into a set of infinitesimal material volumes, each of which can be described independently. All materials, natural and man-made, are characterized by microstructural details whose size ranges over many order of magnitude (Bažant and Jirásek 2002). In constructing a material model, one must select a certain resolution level below which the microstructural details are not explicitly visible. Instead of refining the explicit resolution level, it is often more effective to use various forms of generalized continuum formulation, dealing with material that are nonsimple or polar, or both. A list of enriched continuum models is provided in Bažant and Jirásek (2002). Among these, a great variety of nonlocal models was developed. The aim of the present study is to show that nonlocal constitutive relations between stress and strain tensors are not strictly needed to construct a material model. They are required only if a differential formulation is used for modeling nonlocality, since differential operators are local. The physical well-posedness of nonlocality is discussed with regard to the differential and discrete formulations. Nonlocality was found to be a concept not attaining to the description of the material, but of the phenomenon. This made it possible to discuss the opportunity of using nonlocality in order to give respectability to strain-softening damage models. The mathematical and physical well-posedness and the existence of strain-softening are also discussed. When using the differential formulation, a length scale must be introduced into the material description of a strain-softening modeling. This need has been here justified on the basis of the geometrical information which has been lost in performing the limit process. It was shown how, avoiding the limit process, a length scale is intrinsically taken into account into a discrete formulation. Thus, the discrete formulation turns out to be more appealing than the differential formulation with nonlocal approach, from the physical point of view.
Abstract: The Cell Method (CM) code with auto-matic remeshing for crack propagation analysis [6] is here used for modelling the pullout test. Particular emphasis is given to the analysis in the Mohr-Coulomb plane, since previous numerical models were not decisive in describing failure mechanism in pullout tests. The interpretations of ex-perimental and analytical studies vary widely, and none of the existing explana-tions offer a complete description of the progressive failure of the concrete medium [21]. Nor do most existing interpretations appear to be totally compatible with the experimental evidence.
Analysis of the failure mechanism for the pullout test requires a failure criterion ac-curately describing crack initiation in ten-sion loading. The Mohr-Coulomb criterion of the first code [6] has therefore been abandoned in favour of a more realistic criterion for the tensile state of stress, the Leon criterion.
The failure analysis has been performed for several ratios between the counterpres-sure diameter and the stem length (Fig. 1). Moreover, the complete crack path has been obtained for the geometry of the Lok-test. The evolving state of stress-strain for the Lok-test is also provided. The identification of the directions of principal stress completes the stress analy-sis. Modelling is performed both on the concrete specimen and on the steel insert, showing how the CM can easily handle domains with several materials.
Abstract: The Cell Method (CM) code with automatic remeshing for crack propagation analysis is here used for modeling the pullout test. The interpretations of experimental and analytical studies on the pullout test vary widely, and none of the existing explanations offer a complete description of the progressive failure of the concrete medium. Here, failure analysis has been performed for several ratios between the counter-pressure diameter and the stem length. Moreover, the complete crack path has been obtained for the geometry of the Lok-test. The evolving state of stress-strain for the Lok-test is also provided. The identification of the directions of principal stress completes the stress analysis.
Abstract: The present study is part of an identifying programme for constitutive parameters in damaged materials, termed the âeffective parametersâ. The programme starting point is that the experimental response depends not only on constitutive parameters, but also on structural mechanics and interaction with the test-machine. In previous studies, it was showed how the load-displacement diagram of compressed concrete cylinders is affected by crack propagation, through the resistant structure modification. Moreover, it was analytically demonstrated that the effective stress ( )-effective strain ( ) curve exhibits a strictly positive derivative at the point corresponding to the average stress ( )-average strain ( ) curve peak. Finally, it was proposed a new identification procedure which provided satisfactory results for cylindrical specimens of varying slendernesses, giving monotone strictly non-decreasing and size-effect insensitive curves. In the present paper, the proposed identification procedure is tested on specimens of the same geometry, but with different failure mechanisms.
Abstract: In the present paper fracture criteria for predicting crack initiation angles in an orthotropic homogeneous plate, with an inclined crack and subjected at infinity to a biaxial uniform load, are studied. The crack initiation angle can be calculated as a function of crack geometry and external loading applied at infinity. The numerical analysis is performed for a wide range of anisotropic material properties and applied loads. Singular solution to specify elastic fields is generally incorrect and the effect of non-singular terms of the series expansion for the stress and the crack tip region is underlined. The estimation of the error associated with the singular terms representation is pointed out for some calculated parameters involved in the numerical analysis. Stress and displacement components including non-singular terms are calculated making use of an unconventional approach to the derivation of the complex variable expressions of the elastic fields.
Abstract: In this study, the problem of finding the complete trajectory of propagation in plates with internal straight cracks is extended to the non-linear field. In particular, results concerning concrete plates in bi-axial tensile loading are shown. The concrete constitutive law adopted for this purpose is monotonically non-decreasing, as following according to previous studies of the Authors on monotonic mono-axial loading. The analysis is performed in a discrete form, by means of the Cell Method (CM). The aim of this study is both to test the new concrete constitutive law in biaxial tensile load and to verify the applicability of the CM in crack propagation problems for bodies of non-linear material. The discrete analysis allows us to identify the crack initiation without using the stress intensity factors. Moreover, previous studies showed how the boundary conditions are no longer a problem with the CM. These two circumstances involve computational simplifications in cracked solids of finite dimensions. An example of computation in finite solids, the skew-symmetric four-point bending beam, is provided.
Abstract: In this study, the problem of finding the limiting load in infinite plates with internal cracks is extended to the non-linear field. In particular, results concerning concrete plates in bi-axial loading are shown. The analysis is performed in discrete form, by means of the Cell Method. The discrete analysis allows us to identify the crack initiation without using the stress intensity factors. This simplifies the computation in cracked solids of finite dimensions. An example of computation in finite solids, the skew-symmetric four-point bending beam, is provided.
Abstract: An automatic remeshing technique for studying plane crack propagation problems with the Cell Method [1] is proposed. This theme is part of a more comprehensive study [2], which intends to test the applicability of the Cell Method in structural analysis. Since this is its aim, the present work does not present any comparison between The Cell Method and other already consolidated methods (e.g. Finite Element Method). Once a failure criterion has been introduced, which is adequate to the material in studying, the crack propagation criterion uses a procedure of (a) nodal releasing, (b) crack geometry updating, and (c) automatic remeshing. The identification approach of the âintra-elementâ crack propagation direction starts from the study of the stress field in the tip neighbourhood, and passes through the physical meaning linked to the dual mesh of the Cell Method. An adaptive mesher leads to a tip response, which can be refined with the desired precision. Results concerning the application of the proposed technique for crack propagation problems in concrete solids in Mode I ([2], [3]) and in Mixed Mode ([2], [4]) are presented.
Abstract: In this work, a new approach for identifying the stress intensity factors (SIFs) is proposed, using the method of energetic compliance. The new approach uses the âCells Methodâ (Tonti) which gives the continuum media equations in discrete form directly. Problems relating to the appropriate geometric model for determining the opening SIF (KI) are discussed in detail. Experimental results are used to calibrate the numerical model. Numerical results are presented that show the change in KI with the crack depth/ligament ratio, and these are compared with those of Brown (1966) and Tada (1973).
Abstract: The results of a numerical simulation of the macroscopic behavior in axial compression of concrete cylindrical specimens were presented. According with the results of previous works [1, 2, 3, 4], the introduced constitutive law was a monotonically increasing one. Stresses and displacements fields were evaluated by discrete formulation of the elastic equilibrium problem. When the Mohr/Coulomb cracking criterion was verified, the cylinder geometry was modified with the introduction of new free surfaces due to crack propagation. The macroscopic result in terms of load-displacement was affected by the decreasing of stiffness due to crack propagation. It was showed that the softening behavior is a macroscopic effect bonded to the structural behavior and that it doesnât correspond to a material constitutive property.
Abstract: The results of a numerical simulation of the macroscopic behavior in axial compression of concrete cylindrical specimens are presented. According with the results of a previous work [1], the introduced constitutive law is a monotonically increasing one. Stress and displacements fields were evaluated by discrete formulation of the elastic equilibrium problem. When the Coulomb cracking criterion is verified, the cylinder geometry is modified with the introduction of new free surfaces due to the crack propagation. The macroscopic result in terms of load-displacement is affected by the decreasing of rigidity due to the crack propagation. We shown that the softening behavior is a macroscopic effect bonded to the structural mechanics and that it doesnât correspond to a material constitutive property.
Abstract: A numerical code for modeling crack propagation using the Cell Method is proposed. The Leon failure surface is used to compute the direction of crack propagation, and the new crack geometry is realized by an intra-element propagation technique. Automatic remeshing is then activated. Applications in Mode I, Mode II and Mixed Mode are presented to illustrate the robustness of the implementation.
Abstract: The present study is part of an identifying programme for constitutive parameters in damaged materials, termed the âeffective parametersâ. The programme starting point is that the experimental response depends not only on constitutive parameters, but also on structural mechanics and interaction with the test-machine. It is showed how the load-displacement diagram of compressed concrete cylinders is affected by crack propagation, through the resistant structure modification. Moreover, it is analytically demonstrated that the effective stress (Ïeff )-effective strain (²eff ) curve exhibits a strictly positive derivative at the point corresponding to the average stress (Ï)-average strain (ε) curve peak. Finally, it is proposed a new identification procedure which provided satisfactory results, giving monotone strictly non-decreasing, size-effect insensitive and failure mechanism insensitive Ïeff â εeff curves.
Abstract: La ricerca si riferisce ad un'applicazione dei metodi di analisi basati sulla propagazione di onde elastiche di vibrazione, finalizzata a valutare lâinsorgenza e la propagazione di fessure durante prove di compressione su cilindri in calcestruzzo ad alta resistenza. La metodologia di indagine si basa sullâimpiego simultaneo di un elevato numero di trasduttori piezoelettrici e sullâelaborazione dei risultati per ottenere immagini tomografiche, a partire da determinazioni di valori di velocità di propagazione e di attenuazione.
Abstract: Viene introdotta una differenziazione concettuale tra andamento dei diagrammi carico/spostamento e andamento dei diagrammi tensione/deformazione per i calcestruzzi compressi, interpretando il comportamento
degradante dei primi, compreso il ramo softening, come effetto strutturale e proponendo una nuova legge di
evoluzione per i secondi. A tal fine, si è valutato, tramite analisi microsismica, lâeffetto sulle non linearitÃ
macroscopiche della propagazione di superfici di frattura coesive. Il comportamento macroscopico del calcestruzzo viene reinterpretato a partire da nuove leggi costitutive e parametri di danno opportunamente definiti.
