1,721,179 research outputs found
Nonlinear Interface Crack Propagation in Concrete Dams Under Seismic Loading
No full textIn this paper, the phenomenon of interface crack propagation in concrete gravity dams under seismic
loading is addressed. This problem is particularly important from the engineering point of view. In fact,
besides Mixed-Mode crack growth in concrete, dam failure is often the result of crack propagation along
the rock-concrete interface at the dam foundation. To analyze such a problem, the generalized interface
constitutive law recently proposed by the first author is used to proper modelling the phenomenon of
crack closing and reopening at the interface. A damage variable is also introduced in the cohesive zone
formulation in order to predict crack propagation under repeated loadings. Numerical examples will
show the capabilities of the proposed approach applied to concrete gravity dams
Effetti di scala sulla resistenza a trazione dei materiali
Full text linkThe dissertation analyses the scale effects on the tensile strength of materials. By the term scale effects it is meant the variation in a mechanical property as a function of structural size. In particular, it has been observed by numerous investigators that the nominal tensile strength of many materials decreases with increasing size of the specimen tested.
This phenomenon is more evident in disordered materials, that is, materials that are macroscopically heterogeneous and damaged. On the basis of Weibull's statistical theory and the principles of Linear Elastic Fracture Mechanics, a self-similarity distribution for defect size is presented (Chapter 4). With this distribution the length of the most critical defect is taken to be proportional to the linear size of the specimen. It is shown that the assumption of self-similarity represents the instance of maximum disorder that can be encountered in real materials, and it supplies, in a strength-size bilogarithmic plane, a linear scaling law with an inclination of -1/2, corresponding to the power of the stress singularity envisaged by LEFM.
This formulation contains the fractal concept of self-similarity, even though it is limited to maximum defect dimension.
In order to consider the real nature of the micro-structure of the materials, a more complex fractal model is presented (Chapter 5) in which the property of self-similarity is extended to the entire population of defects. This topological law, based on fractal theory and on the so-called renormalisation procedure, states that in order to obtain a nominal constant strength for the material it is
necessary to refer to surface areas with non integer physical dimensions. For disordered materials, such as for instance concrete and rocks, renormalised tensile strength is given by a force acting on a surface having a fractal dimension lower than 2. The dimensional decrease, always comprised in the [0, 1/2] range, represents self-similar vacancies in the undamaged section associated with the presence of pores, voids, defects, cracks, aggregate and inclusions, and it approaches the 1/2 limit only for extremely brittle and disordered materials, as is assumed, incidentally, in statistical approaches.
As a rule, the scale variation taken into consideration in experimental investigations does not exceed one order of magnitude.
In such circumstances, it is only possible to determine a single tangential inclination in the bilogarithmic diagram.
Only by taking into account scale variations higher than one order of magnitude it proves possible to detect the transition from disordered to ordered conditions, and a continuous transition from -1/2 to zero inclination may be seen to appear. In physical reality, the peak load resistant section can be viewed as multifractal, of dimension 1.5 on small scales and dimension 2 at large scales. This clearly shows a transition from the extreme disorder that is associated with small scales, where a self-similar distribution of Griffith cracks predominates, to the extreme order of large scales, where the disorder of the microstructure is no longer visible, on account of the limited dimensions of the defects and heterogeneities. The assumption of multifractality for the microstructure of the damaged material (Chapter 7) is the basis of the so-called Multifractal Scaling Law (MFSL).
Such law consists of an approximation method which imposes the concavity of the bilogarithmic curve facing upward, which
contradicts Bazant's size effect law (SEL). To verify this scaling law and to determine experimentally the variation in nominal tensile strength and fracture energy, a totally innovative testing set-up has been created, involving the use of three servo-controlled jacks (Chapter 5). The interaction of the three jacks, arranged in L formation, makes it possible to centre instant by instant the resultant of the load with the respect to the undamaged section even in the presence of cracks which make the latter asymmetrical. The main goal of this instrumentation is to determine the parameters of the concrete subjected to uniform tension, eliminating any secondary bending effect which may affect the results and lead to erroneous explanations of the scale effect
Applicazioni di meccanica della frattura all’analisi di stabilitá delle fessure nelle dighe in calcestruzzo
No full textRIASSUNTO. Nel presente lavoro si propone una disamina delle applicazioni di meccanica della frattura
all’analisi del processo fessurativo nelle dighe in calcestruzzo. In tale contesto, ripercorrendo gli studi
pionieristici e i casi di studio affrontati negli anni 1980 e 1990, si illustra nel dettaglio come applicare le
metodologie proprie della meccanica della frattura elastica lineare alla valutazione della stabilità delle fessure e
della loro lunghezza critica. Tale disamina riguarderà sia sollecitazioni quasi-statiche, quali il peso proprio e la
pressione idrostatica esercitata dall’acqua, che sollecitazioni sismiche, tema di particolare complessità ed
attualità. Infine, si illustreranno le problematiche relative alla corretta valutazione dei parametri meccanici per
strutture ciclopiche quali le dighe, tenendo propriamente in conto i forti effetti di scala osservati
sperimentalmente.
ABSTRACT. In the present study, a detailed analysis of the applications of fracture mechanics to the
phenomenon of fracture taking place in concrete dams is proposed. In this context, recalling the pioneering
approaches and the case studies proposed in the 1980s and in the 1990s, it will be shown how to apply the
methodologies of linear elastic fracture mechanics to the assessment of crack stability and to the determination
of the corresponding critical crack length. Such an analysis will concern both quasi-static loads, such as the
weight load and the hydraulic pressure, as well as seismic actions, a topic of high complexity and actuality.
Finally, the problems related to the proper evaluation of the mechanical parameters of huge structures such as
dams will be analyzed, taking into account the strong size-scale effects observed in experimental tests
Seismic analysis of concrete gravity dams: nonlinear fracture mechanics models and size-scale effects
No full textThe phenomenon of interface crack propagation in concrete gravity dams underseismic loading is herein addressed. This problem is particularly important from the engineeringpoint of view. In fact, besides Mixed-Mode crack growth in concrete, dam failure is oftenthe result of crack propagation along the rock-concrete interface at the dam foundation. Toanalyze such a problem, the generalized interface constitutive law recently proposed by the¯rst author is used to proper modelling the phenomenon of crack closing and reopening at theinterface. A damage variable is also introduced in the cohesive zone formulation in order topredict crack propagation under repeated loadings. Special attention is given to the complexityresulting from the solution of the nonlinear dynamic problem and to the choice of the interfaceconstitutive parameters, taking into account the important size-scale e®ects observed in thesecyclopic structures. Numerical examples will show the capabilities of the proposed approachwhen applied to concrete gravity dams
Effetti di scala sulla resistenza dei materiali dovuti alla dispersione statistica dei difetti
No full textApparent tensile strength and fictitious fracture energy of concrete: a fractal geometry approach to related size effects
No full text
Effect of specimen size on the dissipated energy density in compression
Full text linkThe size effects in compression on drilled cylindrical concrete specimens obtained from a unique concrete block over a large scale range (1:19) are analyzed. The experimental results show scale effects on dissipated energy density rather than on the compressive strength. A theoretical explanation for such a phenomenon is presented, assuming a noninteger physical dimension of the subdomain where dissipation occurs. A comparison between experimental and theoretical values is discussed and a renormalization procedure to obtain a scale-independent constitutive law is presente
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