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Strain hardening ultra high performance fibre reinforced cementitious composites (UH-PFRCC) exhibit increased strength, ductility, and energy absorption capacity when compared to their quasi-brittle, unreinforced counterparts. A mesoscale finite element model can depict the underlying causes for the structural response of UHPFRC and thus help to optimize the fibre content, the fibre dimensions, and the fibre orientation.
Furthermore, it facilitates the investigation of strain rate effects in UHPFRCC under dynamic loading. The mesoscale model can either be used directly or as a representative volume element for a macroscale model.
This work proposes a two-dimensional and a three- dimensional mesoscale finite element model to simulate the structural response of strain hardening UHPFRCC. The mesoscale model employs an implicit gradient enhanced damage model, proposed by Peerlings et al., for the cement matrix and a local bond stress-slip model, proposed by Elige-hausen, Popov, and Bertero, for the bond between the cement matrix and the steel fibres. The steel fibres are modeled discretely as one-dimensional truss elements that are coupled to the cement matrix via bond elements. The implementation of hooked end fibres is realized in the constitutive equations of the bond elements.
The tensile stress-strain response of UHPFRCC is a consequence of local matrix cracking and bond failure. Both phenomena can be depicted when modeling the cement matrix, the steel fibres, and the fibre-to-matrix bond explicitly. In this work, the parameters for the constitutive equations of each constituent are determined through uniaxial Tension tests, bending tests, and fibre pullout tests. Additionally, UHPFRCC specimens are simulated with the same parameters and compared to experimental results.
Simulations of concrete on the mesoscale require complex geometry models that resolve single aggregates. Since they fill up to 60-80% of the virtual specimen, efficient algorithms for their placement are necessary.
Event-driven molecular dynamics (EDMD) algorithms using growing spheres are proven to create dense polydisperse sphere packings. In this paper, a modified EDMD algorithm is presented and compared to the widely used random sequential addition algorithm. It reaches denser aggregate packings and saves computation time at high volume fractions.
When meshing the geometry, a minimal distance between the aggregates ensures undistorted elements. It is obtained by increasing the aggregate size during the placement process, which makes the packing problem more challenging. For a given aggregate volume fraction, the presented algorithm maximizes this distance and meshable mesoscale geometries for concrete specimens with more than 70% aggregate content are produced.
A finite element tearing and interconnecting (FETI) approach for phase-field models and Gradient enhanced damage models is presented. These diffusive crack models can solve fracture mechanics problems by integrating a set of partial differential equations and thus avoid the explicit treatment of discontinuities. However, they require a fine discretization in the vicinity of the crack. FETI methods distribute the computational cost among multiple processors and thus speed up the computation.
A finite element tearing and interconnecting (FETI) approach for phase-field models and gradient enhanced damage models is presented. These diffusive crack models can solve fracture mechanics problems by integrating a set of partial differential equations and thus avoid the explicit treatment of discontinuities. However, they require a fine discretization in the vicinity of the crack. FETI methods distribute the computational cost among multiple processors and thus speed up the computation.
A finite element tearing and interconnecting (FETI) approach for phase-field models and gradient enhanced damage models is presented. These diffusive crack models can solve fracture mechanics problems by integrating a set of partial differential equations and thus avoid the explicit treatment of discontinuities. However, they require a fine discretization in the vicinity of the crack. FETI methods distribute the computational cost among multiple processors and thus speed up the computation.
Strain hardening ultra high performance fiber reinforced cementitious composites (UHPFRCC) exhibit increased strength, ductility, and energy absorption capacity when compared to their quasibrittle, unreinforced counterparts. A mesoscale finite element model can depict the underlying causes for the structural response of UHPFRCC and thus help to optimize the fiber content, the fiber dimensions, and the fiber orientation. The mesoscale model can either be used directly or as a representative volume element for a macroscopic model. We present a two-dimensional and a threedimensional mesoscale finite element model to simulate the structural response of strain hardening UHPFRCC. The mesoscale model employs an implicit gradient enhanced damage model for the cement matrix and a local bond stress-slip model for the bond between the cement matrix and the steel fibers. The steel fibers are modeled discretely as one-dimensional truss elements that are coupled to the cement matrix via bond elements. The tensile stress-strain response of UHPFRCC is a consequence of local matrix cracking and bond failure. Both phenomena can be depicted when modeling the cement matrix, the steel fibers, and the fiber-to-matrix bond explicitly. The second part of the talk deals with the efficient modeling of fracture and the prediction of crack initiation, propagation, merging, and branching through the computational domain. Phase-field models and gradient enhanced damage models can solve fracture mechanics problems by integrating a set of partial differential equations for the system and thus avoid the explicit treatment of discontinuities. The main attributes of these approaches are their simplicity and generality. However, they require a fine discretization in the region where the crack evolves. A finite element tearing and interconnecting (FETI) approach for the diffusive crack models is presented to distribute the computational cost among multiple processors and thus speed up the overall computation.
The objective is to apply FETI methods to the phase-field model for brittle fracture in order to speed up the computation time. Due to the fact that the phase-field model yields nonsymmetric, ill-conditioned matrices, special solvers and preconditioners need to be applied to improve robustness and convergence.