Analytical Solutions and Multiscale Creep Analysis of Functionally Graded Cylindrical Pressure Vessels

This study deals with the time-dependent creep analysis of functionally graded thick-cylinders under various thermal and mechanical boundary conditions. Firstly, exact thermoelastic stress, and iterative creep solutions for a heat generating and rotating cylindrical vessel made of functionally graded thermal and mechanical properties are proposed. Equations of equilibrium, compatibility, stress-strain, and strain-displacement relations are solved to obtain closed-form initial stress and strain solutions. It is found that material gradient indices have significant influences on thermoelastic stress profiles. For creep analysis, Norton’s model is incorporated into rate forms of the above-mentioned equations to obtain time-dependent stress and strain results using an iterative method. Validity of our solutions are at first verified using finite element analysis, and numerical results found in the recent literature have been enhanced. Investigation of effects of material gradients reveals that radial variation of density and creep coefficient have significant effects on strains histories, while Young’s modulus and thermal property distributions only influence stress redistribution at an early stage of creep deformation. Next, a more realistic model of introducing microscale creep effects into a macroscopic modeling is employed to investigate the creep behavior of functionally graded hollow cylinders. Finite element (FE) simulations are employed to evaluate the position-dependent parameters associated with creep constitutive law at the microscale. A macroscopic FE model solves the non-linear boundary value problem to determine the time-varying creep stresses and strains. The framework proposed is capable of predicting the creep response of functionally graded pressure vessels based on the constitutive behavior of the creeping matrix, and volume fraction profile. Effective creep properties have been computed using three different micromechanical models and the homogenized creep response and its effect on the macroscopic behavior are compared. Considering the computational expenses associated with the large 3D finite element models, the simple 2D axisymmetric model is able to closely capture the creep behavior in such multiscale methods. Finally, a multi-objective particle swarm optimization algorithm is implemented to minimize the initial stress and final creep strain of functionally graded cylinder subjected to mechanical and thermal loads

Reactive infiltration processing and compression creep of NiAl and NiAl composites

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1998.Includes bibliographical references (p. 116-118).Reactive infiltration processing of bulk and composite NiAl was investigated with powder and wire preforms of nickel. Inhomogeneous microstructures were often obtained with powder preforms because their high surface-to-volume ratio, low permeability, and irregular infiltration paths lead to simultaneous infiltration and reaction. Homogenous NiAl could be obtained with nickel-wire preforms which had a lower surface-to-volume ratio, higher permeability, and regular infiltration paths, because infiltration was completed before the onset of reaction. Composites with continuous tungsten (W) and sapphire fibers were also successfully fabricated by reactive infiltration, while composites with molybdenum particulates and short-fibers showed significant dissolution in NiAl. The high-temperature uni-axial compression creep behavior of uni-directionally reinforced continuous fiber composite materials was investigated using NiAl-W as a model system for the case where both the NiAl matrix and the W fiber underwent plastic deformation by creep. The creep behavior of the constituents NiAl and W and NiAl composites reinforced with 5-20 volume % W was characterized at 1025 °C and 715 °C. At 1025°C, the NiAl-W composites exhibited three stage creep behavior with distinct primary, secondary, and tertiary creep, wherein the composite creep-rate decreased monotonically, remained constant, and increased rapidly, respectively. At 715C, the NiAl-W composites exhibited insignificant primary and tertiary creep but significant secondary creep. Microstructurally, primary and secondary creep were characterized by pure uni-axial compression of W fibers while brooming, bulging, buckling, and kinking were four fiber deformation modes that contributed to tertiary creep. The composite primary creep was modeled by solving for transient stress-states while loads transferred from the weaker phase (matrix) to the stronger phase (fiber) as the composite transitioned from the elastic state present at loading to steady-states attained at later times. The effects of primary creep of the constituents on the primary creep of the composite were also captured. Composite primary creep strains were predicted to be significant at high applied composite stresses and for high fiber volume fraction composites, while the composite primary time was uniquely related to the composite steady-state creep-rate by a power-law at a given temperature and for the stress range investigated. Good correlation between the primary creep model predictions and experiments was obtained when the observed composite steady-state creep behavior converged to the McLean steady-state. The composite secondary creep was observed to correlate reasonably well with the rule-of-mixtures model developed by McLean. The composite tertiary creep was modeled by solving for global or local kink-band evolution with composite deformation respectively contributing to fiber buckling or kinking. The model predicted the critical threshold strain for the onset of tertiary stage to be most sensitive to the initial kink angles while being relatively insensitive to the initial kink-band heights and varied inversely with the volume fraction of fiber in the composite. Reasonable correlation between the model and experiments was obtained when the observed composite steady-state correlated well with the McLean steady-state.by T.A. Venkatesh.Ph.D

Mesoscale simulation of the mold filling process of Sheet Molding Compound

Sheet Molding Compounds (SMC) are discontinuous fiber reinforced composites that are widely applied due to their ability to realize composite parts with long fibers at low cost. A novel Direct Bundle Simulation (DBS) method is proposed in this work to enable a direct simulation at component scale utilizing the observation that fiber bundles often remain in a bundled configuration during SMC compression molding

Mesoscale simulation of the mold filling process of Sheet Molding Compound

Sheet Molding Compounds (SMC) sind diskontinuierlich faserverstärkte Verbundwerkstoffe, die aufgrund ihrer Fähigkeit, Verbundbauteile mit langen Fasern zu geringen Kosten zu realisieren, weit verbreitet sind. Sie ermöglichen Funktionsintegration, wie etwa den Einsatz von Rippen oder metallischen Einsätzen, und können mit kontinuierlichen Kohlenstofffasern gemeinsam verarbeitet werden, um die Formbarkeit von SMC mit den überlegenen mechanischen Eigenschaften von kontinuierlichen Fasern zu kombinieren. Das Streben nach hochintegrierten und komplexeren SMC-Bauteilen erfordert jedoch ein tiefes Verständnis der Verarbeitungsmechanismen und deren Einfluss auf die Leistungsfähigkeit eines Bauteils. Prozesssimulationen adressieren diesen Punkt, indem sie mögliche Fertigungsfehler und Prozess- parameter vorhersagen. Diese Ergebnisse können nicht nur zur Prozessauslegung und zur Reduzierung von Trial-and-Error-Phasen genutzt werden, sondern auch für die anschließende Struktursimulation durch eine virtuelle Prozesskette. In dieser Arbeit wird die Prozesssimulation von SMC zunächst mit einem makroskopischen Referenzmodell auf Basis von Faserorientierungstensoren adressiert. Dies entspricht dem Stand der Forschung, aber die zugrundeliegenden Annahmen von geraden Fasern, die viel kürzer als jedes geometrische Merkmal sind, werden in anspruchsvollen SMC-Anwendungen oft verletzt. Dies führt zu der Hypothese, dass eine direkte Simulation einzelner Faserbündel erforderlich ist, um den SMC-Formfüllprozess komplexer Geometrien genau zu beschreiben. Basierend auf dieser Hypothese wird eine neuartige direkte Bündelsimulationsmethode (DBS) vorgeschlagen, die eine direkte Simulation auf Komponentenebene ermöglicht und dabei die Beobachtung nutzt, dass Faserbündel während des SMC Fließpressens oft in einer gebündelten Konfiguration verbleiben. Das entwickelte DBS Modell kann mit Patches kombiniert werden, um den Co-Molding-Prozess von SMC mit kontinuierlichen Faserverstärkungen zu simulieren. Daher wird ein Modell zur Beschreibung des Materialverhaltens von unidirektionalen Kohlenstofffaser-Patches einschließlich eines einfachen Schädigungsmodells zur Vorhersage von Defekten entwickelt. Die Parameter des makroskopischen Referenzmodells, des DBS-Modells und des Patch-Modells werden experimentell bestimmt. Dazu gehören die thermischen Eigenschaften des SMCs, die temperaturabhängige und ratenabhängige Viskosität der SMC-Paste, die Reibung an der Werkzeugwand sowie die Kompressibilität des SMCs. Ebenso werden die temperaturabhängigen und ratenabhängigen mechanischen Eigenschaften der Patches bestimmt, die jedoch große Streuungen zwischen den Proben und Chargen aufweisen. Schließlich werden die Modelle auf mehrere Validierungsfälle angewandt, um die Anwendbarkeit auf Komponentenebene zu bewerten. Die Beispiele zeigen eine gegenüber dem makroskopische Referenzmodell verbesserte Vorhersage der Faserarchitektur, insbesondere der Faserorientierung in der Nähe von Werkzeugwänden sowie der Vorhersage von Bindenähten und Fließmarken. Zusätzlich bietet das DBS Modell die Option, Krümmungen der Bündel vorherzusagen und den Faservolumenanteil zu berechnen, welche durch Mikro-Computertomographie, thermisch gravimetrische Analysen und Durchleuchtungsbilder validiert werden

Interlaminar Toughening of Fiber Reinforced Polymers

Modification in the resin-rich region between plies, also known as the interlaminar region, was investigated to increase the toughness of laminate composites structures. To achieve suitable modifications, the complexities of the physical and chemical processes during the resin curing procedure must be studied. This includes analyses of the interactions among the co-dependent microstructure, process parameters, and material responses. This dissertation seeks to investigate these interactions via a series of experimental and numerical analyses of the geometric- and temperature-based effects on locally interleaving toughening methods and further interlaminar synergistic toughening without interleaf. Two major weaknesses in composite materials are the brittle resin-rich interlaminar region which forms between the fiber plies after resin infusion, and the ply dropoff region which introduces stress concentration under loads. To address these weaknesses and increase the delamination resistance of the composite specimens, a dual bonding process was explored to alleviate the dropoff effect and toughen the interlaminar region. Hot melt bonding was investigated by applying clamping pressure to ductile thermoplastic interleaf and fiber fabric at an elevated temperature, while diffusion bonding between thermoplastic interleaf and thermoset resin is performed during the resin infusion. This method increased the fracture energy level and thus delamination resistance in the interlaminar region because of deep interleaf penetration into fiber bundles which helped confining crack propagation in the toughened area. The diffusion and precipitation between thermosets and thermoplastics also improved the delamination resistance by forming a semi-interpenetration networks. This phenomenon was investigated in concoctions of low-concentration polystyrene additive modified epoxy system, which facilitates diffusion and precipitation without increasing the viscosity of the system. Additionally, chemical reaction induced phase separation, concentration of polystyrene, and various curing temperatures are used to evaluate their effects on diffusion and precipitation. These effects were directly investigated by performing attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The diffusivity and curing kinetics experiments are performed to quantify the diffusivity coefficient of epoxy, hardener and thermoplastics, as well as the reaction rate constant of curing epoxy at various temperatures. Finally, mechanical testing and fracture surface imaging were used to quantify the improvements and characterize the toughening mechanism. Further improvement on delamination resistance was studied through the synergistic effect of combining different modification methods without the interleaf. Polysulfone molecules are end-capped with epoxide groups. Fiber surface is functionalized with amino groups to generate micro-mechanical interlocks. The interaction between two individual modifications chemically links the modified semi-interpenetration networks to the improved interfacial strength between fiber and epoxy to. The impact of the additive on the crosslinking density was examined through glass transition temperatures, and the chemical modification was characterized by Raman spectroscopy. Mode I and II fracture tests were performed to quantify the improvement of delamination resistance under pure opening and shear loads. The mechanism of synergistic effect was explained based on the fracture surface morphology and the interactions between the modification methods

Discontinuous Fiber Composites, Volume II

Discontinuous fiber-reinforced polymers have gained importance in transportation industries due to their outstanding material properties, lower manufacturing costs and superior lightweight characteristics. One of the most attractive attributes of discontinuous fiber-reinforced composites is the ease with which they can be manufactured in large numbers, using injection and compression molding processes. The main aim of this Special Issue is to collect various investigations focused on the processing of discontinuous fiber-reinforced composites and the effect that processing has on fiber orientation, fiber length and fiber density distributions throughout the final product. Papers presenting investigations on the effect that fiber configurations have on the mechanical properties of the final composite products and materials were welcome in the Special Issue. Researchers who model and simulate processes involving discontinuous fiber composites as well as those performing experimental studies involving these composites were welcomed to submit papers. The authors were encouraged to present new models, constitutive laws, and measuring and monitoring techniques to provide a complete framework on these groundbreaking materials and to facilitate their use in different engineering applications

A vector light sensor for 3D proximity applications: Designs, materials, and applications

In this thesis, a three-dimensional design of a vector light sensor for angular proximity detection applications is realized. 3D printed mesa pyramid designs, along with commercial photodiodes, were used as a prototype for the experimental verification of single-pixel and two-pixel systems. The operation principles, microfabrication details, and experimental verification of micro-sized mesa and CMOS-compatible inverse vector light pixels in silicon are presented, where p-n junctions are created on pyramid’s facets as photodiodes. The one-pixel system allows for angular estimations, providing spatial proximity of incident light in 2D and 3D. A two-pixel system was further demonstrated to have a wider-angle detection. Multilayered carbon nanotubes, graphene, and vanadium oxide thin films as well as carbon nanoparticles-based composites were studied along with cost effective deposition processes to incorporate these films onto 3D mesa structures. Combining such design and materials optimizations produces sensors with a unique design, simple fabrication process, and readout integrated circuits’ compatibility. Finally, an approach to utilize such sensors in smart energy system applications as solar trackers, for automated power generation optimizations, is explored. However, integration optimizations in complementary-Si PV solar modules were first required. In this multi-step approach, custom composite materials are utilized to significantly enhance the reliability in bifacial silicon PV solar modules. Thermal measurements and process optimizations in the development of imec’s novel interconnection technology in solar applications are discussed. The interconnection technology is used to improve solar modules’ performance and enhance the connectivity between modules’ cells and components. This essential precursor allows for the effective powering and consistent operations of standalone module-associated components, such as the solar tracker and Internet of Things sensing devices, typically used in remote monitoring of modules’ performance or smart energy systems. Such integrations and optimizations in the interconnection technology improve solar modules’ performance and reliability, while further reducing materials and production costs. Such advantages further promote solar (Si) PV as a continuously evolving renewable energy source that is compatible with new waves of smart city technology and systems

Structures Division 1994 Annual Report

The NASA Lewis Research Center Structures Division is an international leader and pioneer in developing new structural analysis, life prediction, and failure analysis related to rotating machinery and more specifically to hot section components in air-breathing aircraft engines and spacecraft propulsion systems. The research consists of both deterministic and probabilistic methodology. Studies include, but are not limited to, high-cycle and low-cycle fatigue as well as material creep. Studies of structural failure are at both the micro- and macrolevels. Nondestructive evaluation methods related to structural reliability are developed, applied, and evaluated. Materials from which structural components are made, studied, and tested are monolithics and metal-matrix, polymer-matrix, and ceramic-matrix composites. Aeroelastic models are developed and used to determine the cyclic loading and life of fan and turbine blades. Life models are developed and tested for bearings, seals, and other mechanical components, such as magnetic suspensions. Results of these studies are published in NASA technical papers and reference publication as well as in technical society journal articles. The results of the work of the Structures Division and the bibliography of its publications for calendar year 1994 are presented

Creep Behavior of a Zirconium Diboride-Silicon Carbide Composite and Preliminary ZrB2-WC Quasi-Binary Alloy Development for Long Duty Cycle Aerosurfaces and Structural Propulsion Applications

The mechanical behavior of select ultra-high temperature ceramics were studied for extreme environment aerospace applications. Hot-pressed ZrB2-20 vol% SiC composites and ZrB2-WC quasi-binary alloys were developed for assessing room temperature mechanical properties and creep behavior. A thermochemical model describing alloy phase stability and reaction equilibria, for promoting WC dissolution, is presented. Room temperature structure-property relationships were developed correlating fracture strength and KIC with microstructure constituent size. Flexural creep studies of ZrB2-20 vol% SiC were conducted over the range of 1400°C to 1820°C assessing the macroscopic creep behavior using power-law stress and temperature dependent constants. Inert environment creep experiments were conducted for probing the local grain deformation mechanism in anticipation of bridging the deformation length scales. A two decade increase in creep rate, between 1500 and 1600°C, suggests a clear transition between the low temperature (1400-1500°C) diffusion creep and high temperature (>1600°C) grain boundary sliding creep having stress exponents of unity and 1.7<n<2.2, respectively. A novel indentation deformation mapping experiment clearly defined the local ZrB2 grain boundary sliding event with its components of 80% grain translations and rotations and 20% grain deformation. EBSD and texture theory confirmed the direct observation of ZrB2 grains deforming by dislocation flow, confined to near-grain boundary (mantle) zones, accommodating the grain rotation and translation events. A transition from the grain core to mantle deformation deviated from single crystal behavior as a result of extra geometrically necessary dislocations accommodating the deformation gradient. Microstructure observations shows evidence of <5% and <20% SiC grain deformation, contributing to the macroscopic creep strain, for tension and compression bending fibers, respectively. Cavitation accounts for less than 5% contribution to the accumulated creep strain. Preliminary ZrB2-WC quasi binary alloy creep experiments reveal a decade decrease in the steady state creep rate with a 1.1 mol% increasing WC composition. Improved creep behavior is discussed in the context of solute interactions with accommodation dislocations from grain boundary sliding. Alloy creep rates of 10-7-10-6 s-1 were measured contrasting with 10-5-10-4 s-1 for the ZrB2-SiC composite approaching the design creep rate of 10-8s-1 for long duty cycle aerospace applications.Mechanical Engineering, Department o

Microstructure-based Computational Modeling of the Mechanical Behavior of Polymer Micro/Nano-composites

This dissertation is devoted to the virtual investigation of the mechanical behavior of micro/nano polymer composites (MNPCs). Advanced composite materials are favored by the automotive industry and army departments for their customizable tailored properties, especially for strength and ductility compared to pure polymer matrices. Their light weight and low finished cost are additional advantages of these composite materials. Many experimental and numerical studies have been performed to achieve the optimized behavior of MNPCs by controlling the microstructure. Experiments are costly and time consuming for micro scale. Hence, recently numerical tools are utilized to help the material scientists to customize and optimize their experiments. Most of such numerical studies are based on characterizing the MNPCs through simple microstructures, as circular particles or straight fibers embedded in a specific polymer matrix. Although these geometries are effective in virtual modeling some types of composite material behavior, they fail to address some critical key micro-structural features, which are important for our goals. Firstly, they fail to properly address the randomness of particles. Secondly, 2D analyses have limitations and they can provide qualitative insight, rather than evaluate the quantitative response of the material behavior. Thus, in order to fill this gap, a user friendly software program, REV_Maker, is developed in this project for generating 2D and 3D RVEs (representative volume elements) to precisely represent the morphology of material in microstructural level. In models, polymers are usually considered as viscoelastic-viscoplastic or hyperelastic-viscoplastic materials without taking into account viscodamage models. Therefore, in this work rate- and time-dependent damage (viscodamage) is separately considered to fully investigate the initiation and growth of damage inside polymer composites. Besides, most of the common viscoelastic and viscoplastic models assumes small deformation; therefore, in this dissertation a procedure is established, which incorporates all required modifications to generalize a small strain constitutive model to its identical large deformation range. Thus, here a straightforward generalization and implementation method based on classical continuum mechanics is proposed, which due to its simplicity, can be applied to a wide range of elastoplastic constitutive models. Then, the available viscoelastic and viscoplastic models are extended to large strain framework. By applying the generalized viscous models, one may address and measure the large deformation response of MNPCs. Numerous simulations were conducted to predict the overall responses of micro/nano composites with different morphologies (particles volume fractions, orientations, and combinations). The effect of each particle, and the combination of particles on the composite responses are compared and presented