Mesoscopic packing of disk-like building blocks in calcium silicate hydrate

At 100-nanometer length scale, the mesoscopic structure of calcium silicate hydrate (C-S-H) plays a critical role in determining the macroscopic material properties, such as porosity. In order to explore the mesoscopic structure of C-S-H, we employ two effective techniques, nanoindentation test and molecular dynamics simulation. Grid nanoindentation tests find different porosity of C-S-H in cement paste specimens prepared at varied water-to-cement (w/c) ratios. The w/c-ratio-induced porosity difference can be ascribed to the aspect ratio (diameter-to-thickness ratio) of disk-like C-S-H building blocks. The molecular dynamics simulation, with a mesoscopic C-S-H model, reveals 3 typical packing patterns and relates the packing density to the aspect ratio. Illustrated with disk-like C-S-H building blocks, this study provides a description of C-S-H structures in complement to spherical-particle C-S-H models at the sub-micron scale.Croucher Foundation (Start-up Allowance for Croucher Scholars with the Grant No. 9500012)Research Grants Council (Hong Kong, China) (through the Early Career Scheme (ECS) with the Grant No. 139113

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

Advances in shape memory polymers and their composites: From theoretical modeling and MD simulations to additive manufacturing

Shape memory polymers (SMPs) and their composites have broad application prospects in multiple fields due to their unique shape memory effects. However, they still face challenges in accurately controlling the shape recovery process, improving the stability of shape memory loops, and achieving the manufacturing of complex shapes and functions. At present, theoretical models, molecular dynamics (MD) simulations, and additive manufacturing technologies have been widely applied. Theoretical models and MD simulations provide theoretical foundations at both macro and micro levels, respectively. Meanwhile, by combining SMPs and their composites with additive manufacturing, some complex structures can be produced. This not only verifies the accuracy of the theoretical foundation, but also further expands its application. This review aims to review the application and intersection of theoretical models, MD simulations, and additive manufacturing in the research of SMPs and their composites, and analyze how they jointly promote the leap from theory to application, providing valuable insights for future development trends

Nano- and mesoscale modeling of cement matrix

Atomistic simulations of cementitious material can enrich our understanding of its structural and mechanical properties, whereas current computational capacities restrict the investigation length scale within 10 nm. In this context, coarse-grained simulations can translate the information from nanoscale to mesoscale, thus bridging the multi-scale investigations. Here, we develop a coarse-grained model of cement matrix using the concept of disk-like building block. The objective is to introduce a new method to construct a coarse-grained model of cement, which could contribute to the scale-bridging issue from nanoscale to mesoscale.Croucher Foundation (Start-up Allowance Grant 9500012)Research Grants Council (Hong Kong, China) (Early Career Scheme Grant 139113

Evaluation of the Moisture Effect on the Material Interface Using Multiscale Modeling

Abstract Layered material systems are widely seen in various engineering applications such as thin films circuit boards in electronic engineering, lipid bilayer in biological engineering, and adhesive bonding in aerospace and civil engineering applications. However, the durability of the material interface can be seriously affected due to the prolonged exposure to water. Although the experimental studies have shown the reduction in terms of ultimate bond strength and fracture toughness for material interface, the shift in failure mode found in experiment cannot be explained using conventional fracture theory, which is related to the interaction between the water and material interface. To understand the debonding mechanism from a fundamental and comprehensive aspect and bridge knowledge from the atomistic scale to continuum scale, multiscale modeling approach has been proposed to study the debonding behavior of material interface under moisture effect. A number of studies have been conducted using multiscale modeling approach to investigate the debonding of material interface, and it is necessary to summarize these studies to understand the role of water molecules in weakening and diffusing at the material interface using different atomistic models, force fields and upscaling techniques. This paper provides a comprehensive review on the multiscale modeling of interfacial and delamination behavior of layered material system under moisture attack with the focus on the molecular dynamics simulation and finite element modeling. The FRP bonded concrete system is used as a representative to demonstrate the approach of multiscale modeling. The future research direction is recommended, which involves the consideration of roughness of substrate and structural voids at interface for the better understanding of durability issue for interface in layered material system under different environmental conditions

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