Multiscale Modeling of Tensile Behavior of High Performance Fiber-Reinforced Cementitious Composites PDF Download

Author: Jingu Kang
Publisher:
ISBN: 9781369615074
Category :
Languages : en
Pages :

Get Book Here

Book Description
High Performance Fiber-Reinforced Cementitious Composites (HPFRCC) exhibit strain-hardening behavior and multiple cracking up to relatively high levels of tensile strain. Due to its high toughness, and ability to control crack openings, HPFRCC has numerous applications within the civil infrastructure. Most notably, HPFRCC serves as durable construction and repair materials. The objective of this research is the development and validation of lattice models for the analysis of HPFRCC under tensile loadings. The validation exercises include simulations of the finely distributed cracking patterns, the crack counts and widths, and stress-strain responses. This is achieved through a hierarchical, multiscale accounting of the constitutive behaviors of the matrix, fiber, and fiber-matrix interface. The multiscale model is implemented using the concept of a Rigid-Body-Spring Network, in which the individual fibers within the material volume are explicitly modeled. After matrix cracking, the bonded, debonding, and fiber pullout stages are represented according to a micromechanical model. Fibers can be placed within the computational domain irrespective of the discretization of the matrix phase. The approach is computationally efficient since supplementary degrees of freedom are not introduced with the addition of fibers to matrix. An innovative approach is proposed to achieve objective results with respect to discretization of the matrix, in which force transfer along the embedded lengths of fibers is distributed to the associated matrix elements. This is in contrast to models that lump the pullout force at the crack surfaces, which can lead to spurious break-off of matrix particles as the discretization of the matrix is refined. To verify the distributed force approach, simulated pullouts of single fibers are compared with theory and test results for the cases of perfectly-plastic and slip-hardening behavior of the fiber-matrix interface. The process of fiber pullout is simulated and compared to that of a fully discretized fiber modeling approach through the study of pullout forces and conditions local to the fiber embedment lengths. With respect to fracture in multi-fiber composites, the proposed model matches theoretical predictions of post-cracking strength and pullout displacement corresponding to the traction-free condition (i.e. complete fiber pullout). The modeling approach is also applicable to simulating early-age concrete behavior, as observed during restrained ring tests. The steel and concrete rings are represented by an irregular lattice model. The evolution of concrete properties, including stiffness and strength, is based on simple models and experimental results. For simulating cases of steel fiber-reinforced concrete, each fiber is explicitly represented within the concrete ring. The simulation results compare well with the experimental data, including readings from strain gauges attached to the steel rings. As expected, the addition of short fibers prolongs the time to cracking and reduces crack widths. Viability of the model, as a means for analyzing the early-age behavior of FRCC, is discussed. The multiple cracking of HPFRCC specimens produces islands of material interconnected by fiber bridges, which places demands on solution convergence. For this reason, a special event-by-event solution strategy is newly developed in this study. Local to a developing crack, force transfer from the fibers to matrix is updated according to the event-based procedure, resulting in improved numerical stability and the simulation of realistic crack patterns. Crack count and crack size are also simulated for progressively larger levels of tensile strain. Finally, the lattice model is used to simulate the tensile behavior of HPFRCC depending on the distribution of fibers within a specimen. Fibers are distributed according to simple functions or, more realistically, spatially correlated random fields. Simulated cracking behavior is compared with experimental results for increasing levels of tensile strain. It is seen that regions of lower fiber content act as defects that promote larger crack openings and lower resistance to fracture localization. This dissertation presents a computationally efficient approach to representing individual fibers, and their composite behavior, within lattice models of cement-based materials. A hierarchical, multiscale model for HPFRCC is introduced, in which force transfer along each fiber length evolves during crack formation and opening. The proposed spatial representation of the fiber bridging forces provides realistic representations of stress transfer between the fiber and matrix, which is essential for simulating crack openings and crack spacing in HPFRCC.