Bruno Gérard

Calibration of nonlocal damage model from size effect tests

The calibration of nonlocal models which contain an internal length has been among the major issues conditioning the implementation of this kind of failure models. Direct calibration from uniaxial testing, where the state of strain remains homogeneous throughout the specimen, is impossible. The softening law is not directly accessible because the strains cannot remain homogeneous during the entire test. In the absence of local information on the displacement field and on micro cracking in the fracture process zone, the calibration has to rely on inverse analysis. This paper presents such a procedure based on three point bend size effect tests on notched specimens. The complete load deflection curves are used for the identification of the constitutive relations. Manual calibration is discussed first. It is emphasised that calibration on the load deflexion curve from a single experiment is not objective. We show that Bazants size effect law, which is related to peak loads only, may serve as a helpful guide to reach the closest fit. Then, automatic calibration is described. An optimal set of model parameters can be obtained within a reasonable number of iterations.

Coupled diffusion-damage modelling and the implications on failure due to strain localisation

Abstract For underground cement-based structures, the leaching of pore solution ions by pure water is one of the reference scenario of degradation. The penetration of this chemical attack leads to a loss of mechanical properties of the material. Under mechanical loading, progressive damage can develop too. In order to evaluate the coupling between these chemical and mechanical mechanisms of degradation (increase of porosity and cracking) a model is developed. The chemical attack kinetics follows a non-linear diffusion equation based on the mass conservation of the calcium ions in concrete. The mechanical behaviour is modelled according to a scalar damage constitutive relation. After a brief presentation of the model, the influence of damage development on the chemical attack kinetics is analysed. It is shown that due to the chemical degradation (activated by diffusion mechanisms) localisation due to strain softening can be at the origin of the failure. In spite of the chemical coupling, the problem of equilibrium is ill posed at the inception of strain localisation. A non local damage model is implemented in order to alleviate this problem. A preliminary example of finite element computations illustrates the different regimes of damage on a bending beam subjected to a chemical attack and to a variable deflection.

Identification d'un modèle non local en utilisant les effets d'échelle

ABSTRACT The calibration of non local models which contain an internal length has been among the major issues conditioning the implementation of this kind of failure models. In the absence of local information on the displacement field and on microcracking in the fracture process zone, the calibration has to rely on inverse analysis. This paper presents such a procedure based on three point bend size effect tests on notched specimens. The complete load deflection curves are used for the identification of the constitutive relations. Manual calibration is discussed first and we show that Bazants size effect law, which is related to peak loads only, may serve as a helpful guide to reach the closest fit. Then, automatic calibration is described. It is shown that an optimal set of model parameters can be obtained within a reasonable number of iterations.

Experimental Investigations on Degradation of Concrete Structures: Time-dependent Microstructure and Material Properties

ABSTRACT Most models on Durability of concrete structures have focussed on chemical aspects of the degradations. The last years have been at the origin of the development of Durability Mechanics in order to be able to predict the time-dependent evolution of structural properties of a construction. Experimental techniques have been proposed to assess coupling data between physico-chemistry and mechanics. This paper introduces recent data and techniques on leaching and steel corrosion effects on mechanical behavior of concrete materials and structures.

Endommagement et rupture des bétons dégradés : Etude d'une modélisation simplifiée

ABSTRACT Chemo-mechanical coupled problems are important for assessing the serviceability and the life time of concrete structures. In the development of constitutive relations, it is also necessary to address the problem of failure due to strain softening and damage localisation. An internal length is necessary in order to obtain well-posedness of boundary value problems. In this paper, we present a simplified generic problem for the study of failure due to coupled actions. We exhibit the influence of the internal length of the material and of the kinetics of chemical/physical degradations on the inception of failure.