Use of advanced composite materials for innovative building design solutions/

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2009.Includes bibliographical references (leaves 90-98).Advanced composite materials become popular in construction industry for the innovative building design solutions including strengthening and retrofitting of existing structures. The interface between different materials is a key issue of such design solutions as the structural integrity relies much on the bond. Knowledge on durability of concrete/epoxy interface is becoming essential as the use of these systems in applications such as FRP strengthening and retrofitting of concrete structures is becoming increasingly popular. Prior research studies in this area have indicated that moisture affected debonding in a FRP-bonded concrete system is a complex phenomenon that may often involve a distinctive dry-to-wet debonding mode shift from material decohesion (concrete delamination) to interface separation (concrete/epoxy interface) in which concrete/epoxy interface becomes the critical region of failure. Such premature failures may occur regardless of the durability of the individual constituent materials forming the material systems. Thus, the durability of FRP-bonded concrete is governed by the microstructure of the concrete/epoxy interface as affected by moisture ingress. In this work, fracture toughness of concrete/epoxy interfaces as affected by combinations of various degrees of moisture ingress and temperature levels is quantified. For this purpose, sandwich beam specimens containing concrete/epoxy interfaces are tested and analyzed using the concepts of fracture mechanics.(cont.) Experimental results have shown a significant decrease in the interfacial fracture toughness of concrete/epoxy bond with selected levels of moisture and temperature conditioning of the specimens. The strength of adhesive joint degrades as implied by the failure mode shift from concrete decohesion in controlled specimens to interface separation in conditioned specimens. In this thesis, primary data on the mixed mode fracture toughness of concrete/epoxy interfaces are presented as a basis for use in the design improvement of material systems containing such interfaces for better system durability, and issues related to the structural implications are also discussed.by Tak Bun Denvid Lau.S.M

Debonding in bi-layer material systems under moisture effect : a multiscale approach

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 88-96).Bi-layer material systems are found in various engineering applications ranging from nano-scale components, such as thin films in circuit boards, to macro-scale structures such as adhesive bonding in aerospace and civil infrastructure applications. They are also found in many natural and biological materials such as nacre or bone. One of the most human-related applications of bi-layer material systems is the artificial tooth involving the bonding between the natural tooth and the metal cap glued with a polymer based material. The structural integrity of a bi-layer system depends on properties of both the interface and the constitutive materials. In particular, interfacial delamination has been observed as a major integrity issue. In this research, a comprehensive investigation on the interfacial debonding mechanism has been conducted both computationally and experimentally using an epoxy-silica system. In the computational approach, a multiscale model which can predict the intrinsic strength between organic and inorganic materials, based on a molecular dynamics simulation approach, is presented. The intrinsic strength between epoxy and silica derived from the molecular level can be used to predict the structural behavior of epoxy-silica interface at the macroscopic length-scale by invoking a finite element approach using a cohesive zone model developed in this research. In order to understand the moisture effect in a more comprehensive way, the free energy profile of the epoxy-silica bonded system describing the debonding process has been reconstructed for both dry and wet conditions and it is found that the adhesion between epoxy and silica, which is dominated by the van der Waals force and Coulombic interaction, can be weakened significantly (more than 68% reduction) in the presence of water. Experimental work involving two different approaches, namely "nanoindentation" and "superlayer" approaches, in characterizing the interfacial fracture toughness are presented and the advantages and disadvantages of these two approaches are discussed. The morphology of material in the vicinity of the interface has also been captured using the scanning electronic microscope (SEM). Experimental results show that the interface fracture energy decreases significantly after 4 weeks of moisture conditioning. Both the experimental and computational results show that water plays a main role in the interfacial deterioration. The mechanism of interfacial deterioration is explained using molecular dynamics simulation and a multiscale model of the epoxy-silica bonded system which is capable of predicting the macro-scale structural behavior based on the reconstructed free energy profile of the bonded system at the nano-scale. The multiscale modeling used in this research provides a powerful new approach to link nano-level to macro-level for complex material behavior.by Tak Bun Denvid Lau.Ph.D

Molecular dynamics study on stiffness and ductility in chitin–protein composite

Chitin–protein composite is the structural material of many marine animals including lobster, squid, and sponge. The relationship between mechanical performance and hierarchical nanostructure in those composites attracts extensive research interests. In order to study the molecular mechanism behind, we construct atomistic models of chitin–protein composite and conduct computational tensile tests through molecular dynamics simulations. The effects of water content and chitin fiber length on the stiffness are examined. The result reveals the detrimental effect on the stiffness of chitin–protein composite due to the presence of water molecules. Meanwhile, it is found that the chitin–protein composite becomes stiffer as the embedded chitin fiber is longer. As the tensile deformation proceeds, the stress–strain curve features a saw-tooth appearance, which can be explained by the interlocked zigzag nanostructure between adjacent chitin fibers. These interlocked sites can sacrificially break for energy dissipation when the system undergoes large deformation, leading to an improvement of ductility.Croucher Foundation (Start-up Allowance for Croucher Scholars Grant No. 9500012)Research Grants Council (Hong Kong, China) (Early Career Scheme Grant No. 139113

Development of a coarse-grained α-chitin model on the basis of MARTINI forcefield

At nanoscale, atomistic simulation is widely used for investigating crystalline chitin fibers, the structural component for many biological materials. However, the longitudinal dimension of naturally occurring chitin fibers exceeds hundreds of nanometer, beyond the investigation range of all-atom simulation due to the limitation of computational power. Under this context, coarse-grained simulation is a useful alternative that facilitates the investigation of a large system. We develop a coarse-grained model for describing the structural and mechanical properties of α-chitin. The developed coarse-grained model can reasonably predict these properties. Moreover, this model is consistent with existing coarse-grained force fields for proteins. The present model of α-chitin possesses good potential and applicability in the investigation of natural chitin-based materials at the length scale of several hundred nanometers.Research Grants Council (Hong Kong, China) (Early Career Scheme, Grant No. 139113)Croucher Foundation (Start-up Allowance for Croucher Scholars, Grant No. 9500012

The Sensitivity of Acoustic-Laser Technique for Detecting the Defects in CFRP-Bonded Concrete Systems

This paper presents an experimental study to evaluate the sensitivity of acoustic-laser technique in defect detection. The technique is particularly useful towards the detection of near-surface defects in fiber reinforced polymer-bonded concrete by vibrating the material with an acoustic excitation and measuring the vibration signals with a laser beam. However, relatively little is known about the sensitivity of acoustic-laser technique. More research work should be conducted to evaluate the effectiveness of the technique when adopted for defect detection. It is also important to investigate the limits of the technique performance with respect to varying operational conditions so as to determine ways of improving the detectability. For this purpose, operational conditions in terms of acoustic excitation and laser beam incidence are investigated for their effectiveness in detecting near-surface defects and a reliable defect detection scheme using our portable equipment is therefore recommended. This work provides a basis for further improving such technique which can be used in other engineering applications including quality control of materials and product development process.Croucher Foundation (Start-up Allowance for Croucher Scholars Grant No. 9500012)Research Grants Council (Hong Kong, China) (Early Career Scheme Grant No. 139113

Flexural ductility improvement of FRP-reinforced concrete members

abstractpublished_or_final_versionCivil EngineeringMasterMaster of Philosoph