Novel Methodology for Atomistically Informed Multiscale Modeling of Advanced Composites

abstract: With the maturity of advanced composites as feasible structural materials for various applications there is a critical need to solve the challenge of designing these material systems for optimal performance. However, determining superior design methods requires a deep understanding of the material-structure properties at various length scales. Due to the length-scale dependent behavior of advanced composites, multiscale modeling techniques may be used to describe the dominant mechanisms of damage and failure in these material systems. With polymer matrix fiber composites and nanocomposites it becomes essential to include even the atomic length scale, where the resin-hardener-nanofiller molecules interact, in the multiscale modeling framework. Additionally, sources of variability are also critical to be included in these models due to the important role of uncertainty in advance composite behavior. Such a methodology should be able to describe length scale dependent mechanisms in a computationally efficient manner for the analysis of practical composite structures. In the research presented in this dissertation, a comprehensive nano to macro multiscale framework is developed for the mechanical and multifunctional analysis of advanced composite materials and structures. An atomistically informed statistical multiscale model is developed for linear problems, to estimate and scale elastic properties of carbon fiber reinforced polymer composites (CFRPs) and carbon nanotube (CNT) enhanced CFRPs using information from molecular dynamics simulation of the resin-hardener-nanofiller nanoscale system. For modeling inelastic processes, an atomistically informed coupled damage-plasticity model is developed using the framework of continuum damage mechanics, where fundamental nanoscale covalent bond disassociation information is scaled up as a continuum scale damage identifying parameter. This damage model is coupled with a nanocomposite microstructure generation algorithm to study the sub-microscale damage mechanisms in CNT/CFRP microstructures. It is further integrated in a generalized method of cells (GMC) micromechanics model to create a low-fidelity computationally efficient nonlinear multiscale method with imperfect interfaces between the fiber and matrix, where the interface behavior is adopted from nanoscale MD simulations. This algorithm is used to understand damage mechanisms in adhesively bonded composite joints as a case study for the comprehensive nano to macroscale structural analysis of practical composites structures. At each length scale sources of variability are identified, characterized, and included in the multiscale modeling framework.Dissertation/ThesisDoctoral Dissertation Aerospace Engineering 201

THEORETICAL INVESTIGATION ON OPTICAL PROPERTIES OF 2D MATERIALS AND MECHANICAL PROPERTIES OF POLYMER COMPOSITES AT MOLECULAR LEVEL

The field of two-dimensional (2D) layered materials provides a new platform for studying diverse physical phenomena that are scientifically interesting and relevant for technological applications. Theoretical predictions from atomically resolved computational simulations of 2D materials play a pivotal role in designing and advancing these developments. The focus of this thesis is 2D materials especially graphene and BN studied using density functional theory (DFT) and molecular dynamics (MD) simulations. In the first half of the thesis, the electronic structure and optical properties are discussed for graphene, antimonene, and borophene. It is found that the absorbance in (atomically flat) multilayer antimonene (group V) is comparable to or greater than that for multilayer borophene (group III) and graphene (group IV). The number of layers has a substantial impact on the electrical and optical properties of graphene, antimonene, and borophene. Unlike graphene and antimonene, however, multilayer δ6-borophene exhibits extremely anisotropic electrical and optical characteristics. Overall, our findings imply that multilayer graphene and antimonene are good optical absorbers, particularly in the infrared region of the spectrum, and could be employed as a coating to protect against mid-IR tunable lasers. However, borophene because of its high optical transparency and good metallicity, could be a promising choice for transparent conductive 2D materials with applications in photovoltaics, performance-controlled optoelectronic devices, and touch displays. Molecular-level simulations for monomers with graphene/BN were undertaken to relate the interfacial features with the corresponding mechanical response in terms of strain and stiffness. The results show that the nature of bonding at the interface determines the interaction strength between resin (or hardener) and graphene and that the mechanical response follows the hierarchical order of the interaction strength at the interface. In addition, the change in polarity from graphene to BN monolayer also leads to improved interfacial strength as well as increased transverse stiffness at the molecular level for both resins and hardeners. We have also studied the effect of BN reinforcement with representative cases of cyanate esters, epoxy, and bismaleimide (BMI) resins using molecular dynamics to characterize the bulk level properties of reinforcement/polymer interface. Calculations simulating pull-apart transverse tension experiments find that the non-fluorinated ester interface exhibits higher stiffness and toughness than the fluorinated interface. On the other hand, the epoxy/BN interface is predicted to have significantly lower toughness (or resistance to fracture) than the BMI/BN interface. BMI, thus, appears to be the polymer matrix of choice when considering the BN nanomaterials as reinforcement compared to either cyanate ester or epoxy polymers for structural applications. These results based on molecular simulations emphasize the need to use computational modeling to efficiently and accurately determine molecular-level polymer/surface combinations that yield optimal composite material mechanical performance. This is especially true when designing and developing high-performance composites with nanoscale reinforcement

Physics-Based Modeling of Material Behavior and Damage Initiation in Nanoengineered Composites

abstract: Materials with unprecedented properties are necessary to make dramatic changes in current and future aerospace platforms. Hybrid materials and composites are increasingly being used in aircraft and spacecraft frames; however, future platforms will require an optimal design of novel materials that enable operation in a variety of environments and produce known/predicted damage mechanisms. Nanocomposites and nanoengineered composites with CNTs have the potential to make significant improvements in strength, stiffness, fracture toughness, flame retardancy and resistance to corrosion. Therefore, these materials have generated tremendous scientific and technical interest over the past decade and various architectures are being explored for applications to light-weight airframe structures. However, the success of such materials with significantly improved performance metrics requires careful control of the parameters during synthesis and processing. Their implementation is also limited due to the lack of complete understanding of the effects the nanoparticles impart to the bulk properties of composites. It is common for computational methods to be applied to explain phenomena measured or observed experimentally. Frequently, a given phenomenon or material property is only considered to be fully understood when the associated physics has been identified through accompanying calculations or simulations. The computationally and experimentally integrated research presented in this dissertation provides improved understanding of the mechanical behavior and response including damage and failure in CNT nanocomposites, enhancing confidence in their applications. The computations at the atomistic level helps to understand the underlying mechanochemistry and allow a systematic investigation of the complex CNT architectures and the material performance across a wide range of parameters. Simulation of the bond breakage phenomena and development of the interface to continuum scale damage captures the effects of applied loading and damage precursor and provides insight into the safety of nanoengineered composites under service loads. The validated modeling methodology is expected to be a step in the direction of computationally-assisted design and certification of novel materials, thus liberating the pace of their implementation in future applications.Dissertation/ThesisDoctoral Dissertation Aerospace Engineering 201

Special issue: Dynamics of systems on the nanoscale (2018). Editorial

The structure, formation and dynamics of both animate and inanimate matter on the nanoscale are a highly interdisciplinary field of rapidly emerging research engaging a broad community encompassing experimentalists, theorists, and technologists. It is relevant for a large variety of molecular and nanosystems of different origin and composition and concerns numerous phenomena originating from physics, chemistry, biology, or materials science. This Topical Issue presents a collection of original research papers devoted to different aspects of structure and dynamics on the nanoscale. Some of the contributions discuss specific applications of the research results in several modern technologies and in next generation medicine. Most of the works of this topical issue were reported at the Fifth International Conference on Dynamics of Systems on the Nanoscale (DySoN) – the premier forum for the presentation of cutting-edge research in this field that was held in Potsdam, Germany in October of 2018

Hierarchical simulations of hybrid polymer-solid materials

Complex polymer-solid materials have gained a lot of attention during the last 2-3 decades due to the fundamental physical problems and the broad spectrum of technological applications in which they are involved. Therefore, significant progress concerning the simulations of such hybrid soft-hard nanostructured systems has been made in the last few years. Simulation techniques vary from quantum to microscopic (atomistic) up to mesoscopic (coarse-grained) level. Here we give a short overview of simulation approaches on model polymer-solid interfacial systems for all different levels of description. In addition, we also present a brief outlook concerning the open questions in this field, from the point of view of both physical problems and computational methodologies