Browsing by Author "Aidi, Bilel"

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    Experimental and Numerical Analysis of Damage in Notched Composites
    Aidi, Bilel (Virginia Tech, 2016-09-30)
    This dissertation contains the results from an experimental study, numerical, and analytical study conducted on quasi-isotropic carbon fiber laminates containing a center hole (notch) subjected to constant amplitude tension-tension fatigue loading in order to investigate the effect of fatigue damage development on the residual properties. Quasi-static tests were initially performed on notched composites using digital image correlation (DIC) to determine the strain profiles at selected transverse sections of the outer ply of the laminates and the static strength of the laminates. Subsequently, tension-tension fatigue tests were carried out at 70%, 75% and 80% of the nominal static failure load. A finite element model was developed using Abaqus and Digimat in which Digimat was used to implement the damage evolution model via a user-defined material subroutine. Damage initiation has been assessed using Hashin's failure criteria and the Matzenmiller model was adopted for damage evolution. A second finite element model was developed using Abaqus and Autodesk Simulation Composite Analysis (ASCA) in which ASCA was used to implement the user-material subroutine. The subroutine includes a failure initiation criterion based upon multi-continuum theory (MCT) and an energy-based damage evolution law. Numerical and experimental strain results were presented and compared for different section lines on the outer surface of the laminate at different loading stages. Additionally, the experimentally measured notched composite strength was compared with the predictions from the finite element solutions. These results are used as baseline for subsequent comparison with strain profiles obtained using DIC for specimens fatigued at different stress levels and fatigue lifetime fractions. The results showed a significant effect of fatigue damage development on strain redistribution even at early stages of fatigue. The results also showed the capability of DIC technique to identify damage initiation and its location. Furthermore, X-ray computed tomography (CT) was used to examine the sequence of damage development throughout the fatigue lifetime and to connect the observed damage mechanisms with the occurred change of strain profiles. Experimental vibrational modal analysis tests were also conducted to assess the effect of fatigue damage on the residual frequency responses (RFRs). Vibrational measurements were initially performed on pristine notched composites. The results are used as baseline for subsequent comparison with strain profiles obtained with DIC. Finite element models based on the classical plate theory (Kirchhoff) and the shear deformable theory (Mindlin) within the framework of equivalent single-layer and layer-wise concepts as well as the three-dimensional theory of elasticity are developed to predict the natural frequencies of non-fatigued specimen. These models are implemented using the finite element software, Abaqus, to determine the natural frequencies and the corresponding mode shapes. In addition, an analytical model based on Kirchhoff plate theory is developed. Using this approach, an equivalent bending-torsion beam model for cantilever laminated plates is extracted taking into account the reduction in local stiffness and mass induced by the center hole. Numerical and analytical natural frequency values are then compared with those obtained through experimental vibrational tests, and the accuracy of each finite element (FE) and analytical model type is assessed. It is shown that the natural frequencies obtained using the analytical and FE models are within 8% of the experimentally determined values.
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    Performance and Design of Extruded Fiber-Reinforced Mortar with Preferentially Aligned Fibers
    Alarrak, Rashed (Virginia Tech, 2024-05-03)
    This dissertation presents a comprehensive investigation into the mechanical properties of fiber-reinforced concrete (FRC), focusing on fracture and flexural toughness properties, the impact of fiber orientation and distribution, and the evaluation of flexural models for predicting the behavior of functionally graded FRC. It embarks on a critical investigation aimed at bridging a significant gap in the understanding of FRC materials' behavior, particularly in terms of fracture and flexural performance. Across five distinct manuscripts, this work employs a variety of experimental methodologies, including three-point bend tests, four-point bend tests, digital image correlation, X-ray computed tomography, and the implementation of the two parameter fracture model and then size effect fracture method to explore the effects of different casting techniques – namely, conventional casting and pump-driven extrusion – on the performance of FRC. The core hypothesis tested throughout these studies suggests that the extrusion process, by aligning fibers parallel to tensile stresses, significantly enhances the concrete's ductility, post-peak behavior, and overall fracture and flexural properties. This hypothesis was corroborated across various experiments, which demonstrated that fiber alignment via extrusion not only enhances the concrete's mechanical properties but also leads to more effective crack propagation control, increased toughness, and enhanced residual strengths. The research encompasses a series of systematic investigations into the effects of fiber alignment on the mechanical properties of FRC, revealing that the extrusion process significantly enhances fracture and flexural properties and maintains residual strength after peak stress. Utilizing both extrusion-based and conventional casting methods with varying dosages of polyvinyl alcohol fibers, the study demonstrates notable improvements in fracture properties, deflection at failure, and equivalent flexural strength ratio for extrusion-based specimens compared to their conventionally cast counterparts. Moreover, the dissertation explores the impact of casting methods and fiber orientation on fracture energy, offering a size-dependent improvement in extrusion-based methods. The strategic distribution of steel fibers, employing an innovative targeted fiber injection for creating Functionally Graded FRC (FG-FRC), is shown to significantly enhance the structural integrity and resilience of the material. The analysis of flexural models applied to FG-FRC specimens, proposing a novel functionally graded factor to improve model predictability, further advances the understanding of the predictability and reliability of these models in assessing FRC's structural behavior. This dissertation advances academic knowledge in the field of FRC casting and offers significant implications for the construction industry, demonstrating a profound understanding of the challenges and opportunities in extrusion-based FRC casting. Through its innovative approach and detailed investigations, this work contributes significantly to the advancement of the FRC casting field, paving the way for the development of more resilient and efficient construction materials.