A computational linear elastic fracture mechanics-based model for alkali-silica reaction

International audienceA fracture mechanics model for alkali-silica reaction (ASR) is presented that deals with the case of a concrete made up of dense spherical aggregates. Chemistry and diffusion (of ions and gel) are not modelled. The focus is put on the mechanical consequences of the progressive replacement of the aggregates by a less dense gel. A ring-shaped crack then appears in the cement paste depending on the pressure build-up, according to an incremental energy criterion. The stored elastic energy and deformation of each configuration are determined assuming that each aggregate is embedded in an infinite cement paste matrix, through Finite Element Analysis. We note a very different behaviour of aggregates of different sizes. Adding the contributions of different aggregates leads to an estimate of the free expansion of a concrete of given aggregate size distribution. Parameters of the model are identified, providing a good fit to experiments taken from Multon's work

A linear elastic fracture mechanics-based study of the influence of aggregate size and gel mechanical properties during alkali-silica reaction

International audienceWe present a fracture mechanics model for Alkali-Silica Reaction for concretes made up of dense aggregates. Chemistry and diffusion (ions and gel) are not modelled. The focus is put on the mechanical consequences of the replacement of the outer layer of the aggregate by a less dense gel. A schematic cracking pattern is assumed: a ring-shaped crack appears in the cement paste surrounding the spherical aggregate depending on the pressure build-up. The onset of cracking is determined using an incremental energy criterion. The stored elastic energy and deformation of a given configuration are determined (using FEA) assuming that each aggregate behaves as if it was embedded in an infinite cement paste matrix. We note a very different behaviour of aggregates of different sizes. The macroscopic deformation is estimated by adding the aggregate's individual contributions. A rate of attack is identified that leads to recover the usual sigmoid ASR expansion curve

A computational linear elastic fracture mechanics-based model for alkali-silica reaction - Nuwcem

International audienceThis article presents a fracture mechanics model for Alkali-Silica Reaction. The model deals with the case of a concrete made up of dense aggregates submitted to chemical attack. The chemistry and diffusion (of ions and gel) are not modelled. The focus is put on the mechanical consequences of the progressive replacement of the outer layer of the aggregate by a less dense gel. A schematic cracking pattern is assumed: a ring-shaped crack appears in the cement paste surrounding the spherical aggregate depending on the pressure build-up. The onset of cracking is determined using an incremental energy criterion. The stored elastic energy and deformation of a given configuration are determined assuming that each aggregate behaves as if it was embedded in an infinite cement paste matrix. The calculations are performed by Finite Element Analysis. We note a very different behaviour of aggregates of different sizes. Adding the contributions of different aggregate sizes leads to an estimation of the global free expansion of a concrete of given aggregate size distribution. A rate of attack is identified that leads to recover the usual sigmoid ASR expansion curve

Estimating the poroelastic properties of cracked materials

International audienceWe present a comparative study of the ability of some micromechanics estimates to predict the overall properties of heterogeneous materials. We focus mainly on cracked materials, for which this task is difficult and many estimates fail. We study particularly the interaction direct derivative estimate, proposed by Zheng and Du, which is an approximation of the generalized self-consistent scheme, but has the very convenient property to be always explicit. A modified version of this estimate, called full-range IDD by Zheng and Du, yields good results when comparing all poromechanical coefficients predicted by the estimate to finite element simulations of a 2D cracked material in plane strain, up to crack density factors of 1 for aligned cracks and 0.60 for randomly oriented cracks. The accuracy of finite element computations of the overall moduli is also commented by plotting the convergence of the average of the properties as well as the confidence intervals on these averages

Simplified Model for the Transport of Alkali-Silica Reaction Gel in Concrete Porosity

International audienceIn this work, we focus on estimating the proportion of gel created by Alkali-Silica Reaction (ASR) in concrete that can enter the Interface Transition Zone (ITZ) surrounding an aggregate at a given pressure. We consider the gel as a non-Newtonian fluid following a Darcy law with a threshold pressure gradient while invading a porous medium. We show that at equilibrium, the mass of gel that penetrates in the ITZ can be considered as proportional to the pressure surrounding the aggregates while the ITZ is not full yet, and then constant once the ITZ is full. This is fairly consistent with the common use of a "connected porosity" in the models dealing with mechanical features of ASR to estimate the volume of gel lost in the porosity. We also study briefly the kinetics of this process and the influence of the gel compressibility

A new sensor for the evaluation of contact stress by inverse analysis during steel strip rolling

International audienceKnowledge of the contact stress between roll and strip becomes a critical factor in modern, high-speed rolling mills. In this paper, an inverse analytical method is developed to determine the contact stress in the roll gap by measuring the stress tensor with fibre optics at only one point inside the roll. Unlike many inverse methods, no matrix inversion is needed because the very small contact length would lead to ill-conditioned matrices. Iterative methods are also not studied because short computation times are desired. This approach uses the theory of elasticity on the assumption that the problem is isothermal and planar and relies on the expansion of holomorphic functions into a power series. On the other hand, the computation time is studied to rapidly optimise the industrial parameters during the rolling process. Hot, cold and temper-rolling simulations are given to demonstrate the accuracy of the method and the feasibility of this new kind of sensor, taking into account the restrictions (e.g., frequency of acquisition) of the local measurement system

Curvature of an elasto-plastic strip at finite strains : application to fast simulation of coils

International audienceThis work is part of the framework of a fast modeling of winding aiming at improving knowledge of residual stress evolution in steel strips and therefore their flatness during the coiling process. An exact analytical solution of an elasto-plastic strip with isotropic hardening at finite strains under an imposedtransformation of curvature is developed. Issues related to flow rules for non-differentiable yield functions (Tresca) have been broached and a unique solution is obtained. The equivalence for this transformation, between von Mises and Tresca yield functions is demonstrated. This solution contributes to an efficient model by terms of computation times that aims at simulating coiling by taking into account inelastic deformations and enabling parametric studies in order to improve the process

Comparison of Homogenization Schemes to Periodic and Random Simulations

Assessment of the efficiency of the Interaction Direct Derivative homogenization scheme by comparison to Finite Element Simulations

Non-linear simulation of coiling accounting for roughness of contacts and multiplicative elastic-plastic behavior

International audienceIn this paper numerical simulations of coiling (winding of a steel strip on itself) and uncoiling are developed. Initial residual stress field is taken into account as well as roughness of contacts and elastic-plastic behavior at finite strains, considering the Tresca yield function and isotropic hardening. The main output is the residual stress field due to plastic deformations during the process. This enables to quantify additional flatness defects. The presented coiling simulation relies on a modeling strategy that consists in dividing each time step into two sub-steps. Each sub-step can be solved semi-analytically and numerical optimizations enable to obtain a general solution. Thus reasonable computation times are reached and parametric studies can be performed in order to develop coiling strategies considering the process parameters. Comparisons with previous models from the literature are presented. Moreover the comparison with a Finite Element simulation presents the same order of magnitude, however it shows that direct computations using classical FE codes are difficult to perform in terms of computation times and stability if an explicit integration scheme is chosen. Numerical results are also given in order to determine the effect of some parameters such as roughness, yield stress, applied force, strip crown or mandrel's radius