Three-dimensional modeling of failure in quasi-brittle materials and structures

fracture mechanics, cohesive zone model;quasi-brittle materials, concrete materials;three-dimensional modeling;multi-scale modeling

Evangelista, Francisco, Jr.

关键词: fracture mechanics, cohesive zone model;    quasi-brittle materials, concrete materials;    three-dimensional modeling;    multi-scale modeling;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/29543/EvangelistaJunior_Francisco.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

In most serviceability conditions, concrete structures present quasi-brittle behavior and failure due to the presence of a nonlinear fracture process zone ahead of the crack front. Predictive models and design methodologies have to be able to reliably calculate the load capacity, or structural strength, of structures while taking into account the nonlinearity of the material behavior and the consideration of realistic boundary conditions such as geometry, size, and loading configuration. The main objective of this study is to develop models and to apply numerical tools to predict the cracking potential in three-dimensional concrete structures. Firstly, a cohesive zone model is formulated and implemented to simulate mode I crack growth in quasi-brittle materials based on the thermodynamics of irreversible processes allowing for loading, unloading, and re-loading. The model is a step forward to improve existing cohesive zone formulations to consider three-dimensional geometries and also overcome numerical instability, lack of convergence, and oscillations in the traction profile commonly reported in cohesive models. This study also explores the novel computational framework of the generalized finite element method (GFEM) to predict the potential for crack propagation in large scale problems such as three-dimensional airfield concrete slabs. A multi-scale approach, using the global-local concept within the GFEM framework (GFEMg-l), is applied to multi-site damage problems (MSD), where several crack geometries are placed simultaneously at different positions in a slab and loaded by different aircraft gears. This approach efficiently simulates multiple cracks not discretized in the global mesh, but only modeled in the local problem domains. The GFEMg-l enrichment functions allow the kinematics to be represented in the global domain through enrichment function from the local problems rather than explicitly modeling each crack discretely in the global domain. This research effort also proposes an integrated approach called nonlinear strength fracture model (NLSFM) to predict the structural strength or load capacity of three-dimensional concrete structures considering the structure geometry, loading configuration, and the nonlinearities ahead of the crack front. In this approach, the extraction of crack front quantities, such as stress intensity factors, are performed through finite element analysis, and then a high-order approximation based on the equivalent elastic crack approach for quasi-brittle materials accounts for the FPZ effects on the nominal strength of the structure under mode I fracture. The NLSFM uses the size- and shape-independent fracture properties definedthrough the critical energy release rate, Gf, and size of fracture process zone, c (as provided by the two-parameter fracture model and size effect model for quasi-brittle materials). As a result, the model predicts a material independent strength-curve, given the structural geometry, boundary conditions, loading, initial crack length are computationally defined through a pre-defined geometric function for the three-dimensional structure of interest. The model is validated with large scale slab tests predicting the ultimate load for cracked and uncracked slabs test setups.The NLSFM advances the state-of-the-art of computational modeling of failure in quasi-brittle materials, currently limited to 2-D structures or laboratory test specimens, to large scale 3-D problems with realistic boundary conditions and loading configurations.The proposed numerical tools are used as a computational platform to analyze the cracking potential for airfield concrete slabs with existing surface- and bottom-initiated cracks. The results show that starter cracks can induce unstable crack propagation under specific loading configurations and material fracture properties. Given existing surface and bottom starter cracks of the same geometry in the concrete slabs, it was much easier to propagate surface cracks under triple dual tandem gear loading relative to the traditional design assumption that bottom-up fatigue cracks are the critical failure mode for airfield concrete slabs.