Jithender J. Timothy

Cascade Lattice Micromechanics Model for the Effective Permeability of Materials with Microcracks

AbstractWithin the framework of mean-field homogenization methods, a lattice version of the cascade micromechanics model for the estimation of the effective permeability of microcracked materials w...

Diffusion in Fracturing Porous Materials: Characterizing Topological Effects using Cascade Micromechanics and Phase-Field Models

The diffusion properties of fracturing porous materials, such as concrete or geological materials, are strongly influenced by the complex and random topological structure of the pore space, the state of distributed micro-cracks inevitably caused by processes such as autogenous and drying shrinkage of concrete, and finally by propagating cracks caused by various loading conditions. Information on macroscopic diffusion properties of the porous material requires up-scaling of transport processes within nano- and micro-pores over several spatial scales. The macroscopic transport coefficients are computed using a cascade continuum micromechanics model recently proposed by the authors. The cascade continuum micromechanics model recursively embeds shape information in the form of the ESHELBY matrix-inclusion problem to obtain the homogenized effective diffusion coefficient. The model is able to predict mathematically and physically consistent percolation thresholds. To consider the effects of oriented, diffusely distributed micro-cracks on the diffusion properties, the homogenization scheme for in intact concrete is enhanced by representing micro-cracks as additional ellipsoidal inclusions within the aforementioned homogenized porous matrix. Finally, the effect of propagating macro-cracks on the diffusion process is taken into consideration by weakly coupling the diffusion model and a fracture energy based staggered phase-field model to simulate brittle fracture.

A Multiscale Model for High Performance FRC

The behavior of high performance fiber reinforced cementitious composites is simulated using semi-analytical and computational sub-models specified at multiple scales. At the scale of a single fiber, a semi-analytical model is developed to characterize the microslip behavior at the interface between the matrix and the fiber. The microcrack bridging and arresting mechanisms of fiber bundles is taken into account within the framework of linear elastic fracture mechanics. Upscaling from the level of distributed microcracks to the macroscopic level is achieved using continuum micromechanics. According to the proposed model, the macroscopic hardening and softening constitutive characteristics is resulting from the microcrack-fiber interaction, the microcrack growth and the evolution of the microcrack density. Model predictions for FRC concrete are validated against experimental data. For the finite element analyses of failure behavior at the structural level, interface solid elements supplemented by a fiber bridging law specified according to the fiber pull-out mechanics are used to represent the cracking process. Selected numerical examples demonstrate that the crack pattern as well as the structural response can be well replicated by the proposed model.

Computational Modeling of Concrete Degradation Due to Alkali Silica Reaction

This paper presents a computational multi-level model for the description of alkali and moisture transport in concrete structures coupled to a macroscopic alkali silicate reaction (ASR) induced phase-field damage model. Concrete is modeled as a heterogeneous material consisting of a partially saturated pore space with diffusively distributed microcracks and the solid skeleton (cement paste and potentially reactive and inert aggregates). The influence of the topology of the pore space and the presence of oriented microcracks on ion diffusion and moisture transport is taken into account through a novel continuum micromechanics homogenization model. The transport model is connected to a phenomenological reaction kinetics model to account for the ASR induced volume expansion of the affected aggregates. At the macroscopic scale, crack propagation and effects of induced topological changes on the fluid and ion transport are taken into account using a phase-field model.

Effective Diffusivity of Porous Materials with Microcracks: Self-Similar Mean-Field Homogenization and Pixel Finite Element Simulations

We investigate the influence of distributed microcracks on the overall diffusion properties of a porous material using the self-similar cascade continuum micromechanics model within the framework of mean-field homogenization and computational homogenization of diffusion simulations using a high-resolution pixel finite element method. In addition to isotropic, also anisotropic crack distributions are considered. The comparison of the results from the cascade continuum micromechanics model and the numerical simulations provides a deeper insight into the qualitative transport characteristics such as the influence of the crack density on the complexity and connectivity of crack networks. The analysis shows that the effective diffusivity for a disordered microcrack distribution is independent of the absolute length scale of the cracks. It is observed that the overall effective diffusivity of a microcracked material with the microcracks oriented in the direction of transport is not necessarily higher than that of a material with a random orientation of microcracks, independent of the microcrack density.

Multiscale modeling FRC Composites

We propose a multiscale model for FRC composites that is a combination of semianalytical and computational sub-models specified at multiple scales. At the scale of the single fiber, a semi-analytical model is developed that characterizes the microslip behavior at the interface between the matrix and the fiber in terms of the overall composite stresses. The influence of fiber bundles on microcrack bridging and arrest is taken into account within the framework of the linear elastic fracture mechanics. Upscaling to the macroscopic level is achieved by using continuum micromechanics. We show that the macroscopic deformation of the FRC composite is governed by a ’TERZHAGI’ like effective stress. Selected numerical experiments provide insight into the role of the interface property, resulting on the macroscopic level in a brittle, softening behaviour in case of weak bond and a rather ductile, hardening behavior in case of a relatively strong interface bond that is completely described by simple microslip laws. For the finite element analyses of failure behavior at the structural level, the so-called ’interface solid element’ (ISE) is used to represent the cracking process. The softening behavior of ISE is governed by the crack bridging law obtained above. The implicit-explicit integration scheme is implemented to enhance the robustness of computation. Selected numerical examples demonstrate that the crack pattern as well as the structural responses under tension-dominant stress conditions can be well simulated