Department of Aerospace Engineering and Mechanics
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Browsing Department of Aerospace Engineering and Mechanics by Author "Barkey, Mark E."
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Item Additive manufacturing of two phase thermoplastic composites: a process model, microstructure and performance study(University of Alabama Libraries, 2019) Papon, Md Easir Arafat; Haque, Anwarul; University of Alabama TuscaloosaFused filament fabrication (FFF) based additive manufacturing (AM) of polymers and composites is a growing interest in processing tailorable parts with functional requirements like structural integrity, lightweight, high-temperature capability, etc. In general, the parts manufactured by FFF show large void contents, weak bonding, and inferior structural performance in comparison to those produced by conventional methods. The present research focused on overcoming the shortcomings of FFF through process modeling, microstructure study, and performance analysis. An experimental and numerical study has been conducted on the FFF of carbon fiber reinforced polylactic acid (CF/PLA) composites. A computational fluid dynamics (CFD) based numerical model was developed to simulate the temperature distribution and melt flow characteristics of highly viscous polymer (single and two-phase composites) using non-Newtonian computational model. Free space bead flow geometry and bead spreading architecture on the platform were also simulated with various nozzle geometries. The effects of the circular, square, and star-shaped geometries on bead cross-sectional shapes were studied both numerically and experimentally to achieve less void contents and improve the bead/layer bonding. Different dominant FFF process variables, both in filament extrusion and part production steps were studied, and a multi-level experimentation scheme was developed to study the bead-level to part-level properties. Physics-based surrogate models were developed, and stochastic uncertainty analysis was carried out on the manufacturing process to build up an optimum process-structure relationship. Another criticality of fiber-matrix interfacial bonding in the FFF-composites was addressed by introducing proper surface treatment to the fibers and post-manufacturing thermal treatment. The numerical model showed good promise in tailoring the bead geometry with the square and star-exit nozzle that potentially enhanced the bead to bead bonding. Extensive experimental studies were conducted to characterize strength, stiffness, fracture toughness, and void contents with various printed layer orientations and fiber concentrations of the FFF coupons. An acid-based functionalization of fibers, printing using square-nozzle, and enhanced crystallinity through controlled annealing were found to improve the fiber-matrix and inter-bead bonding, reduce the inter and intra-void and improve mechanical performances. The optimization and experimental data-driven stochastic modeling of the process parameters paved the way for producing parts with greater confidence at reduced experimental affords. The investigations and strategies developed in this dissertation will help to establish a high-quality and efficient process framework to improve the performance of additively manufactured two-phase composites. The fundamental understating and knowledge exercised in this dissertation can potentially be used for any polymer-based AM processes beyond the FFF since the fundamental challenges of controlling the voids and bonding are unavoidable.Item Analysis of beveled semi-elliptical surface cracks in friction stir plug welded plates made of Al 2195 alloy(University of Alabama Libraries, 2011) Vadakke Veetil, Rahul; Barkey, Mark E.; University of Alabama TuscaloosaFriction Stir Welding (FSW) is a solid state joining process primarily used for Al alloys. Friction Stir Plug Welding (FPW) is a process in which a tapered shaped plug is friction stir welded into the hole that was left in the welded part when the initial FSW tool was removed. A rectangular plate made of Al 2195 alloy with a friction welded plug and containing a semi-elliptical surface crack was analyzed using the help of the software `FEA Crack'. Three different crack depths of deep cracks as well as shallow cracks were considered in identical plates of a quarter inch thickness. The depths were 0.08, 0.13 and 0.18 inches for deep cracks and 0.008, 0.013 and 0.018 inches for shallow cracks. A uniaxial tensile load of 1 psi was applied on one end surface with the opposite surface being fixed. For each depth, four different crack arc lengths were considered which were of 15, 30, 60 and 90 degrees. For each of these cases, the crack tube containing the crack was rotated around the plug having an inner bevel, in steps of 10 degree starting from the base position to the 90 degree (vertical) position with an additional case of 45 degree rotation in between. The stress intensity factor K was plotted against the crack front angle. The average and maximum K factor values were also plotted for each of the main crack lengths against the crack rotation angle. The same procedure was employed for shallow cracks. The results were validated using Newman-Raju equations for semi elliptical surface cracks. Non dimensional K factor plots were also made for different cases of both deep and shallow surface cracks. Researchers studying the surface cracks can now get an estimate of the value of stress intensity factor for the crack length, depth of crack and also the angular position of the crack around the plug by interpolating my results.Item Biofidelic soft composites– experimental and computational modeling(University of Alabama Libraries, 2017) Chanda, Arnab; Unnikrishnan, Vinu U.; University of Alabama TuscaloosaBiofidelic soft composites or tissues form the building blocks of the human body. Understanding the complex mechanics of these soft composites is the key to understanding the genesis and progression of disease. Biomechanically, soft composites exhibit anisotropic mechanical behavior and comprise of multiple fiber layers within a soft matrix. To date, there is a lack of understanding of the anisotropic mechanical behavior of soft composites, primarily due to unavailability of a robust characterization framework. In this dissertation, novel multiscale computational and experimental investigation models are developed to simulate and characterize anisotropic soft composite mechanical behavior. Soft composite surrogates were first developed to simulate various tissues in the human body namely the skin, brain, artery and vaginal tissues. Novel anisotropic soft composite models were also fabricated taking into consideration the tissue anisotropy and multifunctional properties. Hyperelastic anisotropic constitutive relationships were formulated to precisely characterize the mechanical behavior of soft composite considering varying fiber and matrix contributions, fiber-matrix interactions, fiber orientations and multiple fiber layers. Coupled with high fidelity experimental and computational models, microscopy, and Digital Image Correlation (DIC) studies, the damage and repair of soft composite surrogates are also discussed in this dissertation, with relevance to soft tissue wounds and suture. Computational modeling to understand the interaction between multiple soft composite systems and its effect on soft composite damage are also highlighted in this work. Some specific soft composite interaction systems modeled were the female pelvic system under abdominal loads, whole body impact due to blast, and ulceration in diabetic foot. This dissertation lays the foundation for micro and macro scale anisotropic soft composite modeling and characterization using high fidelity experimental and numerical techniques which will be indispensable for studying tissue mechanics and other soft composite applications in engineering and medicine.Item Carbon nanotube sheet scrolled fiber composite for enhanced interfacial mechanical properties(University of Alabama Libraries, 2017) Kokkada Ravindranath, Pruthul; Roy, Samit; University of Alabama TuscaloosaThe high tensile strength of Polymer Matrix Composites (PMC) is derived from the high tensile strength of the embedded carbon fibers. However, their compressive strength is significantly lower than their tensile strength, as they tend to fail through micro-buckling, under compressive loading. Fiber misalignment and the presence of voids created during the manufacturing processes, add to the further reduction in the compressive strength of the composites. Hence, there is more scope for improvement. Since, the matrix is primarily responsible for the shear load transfer and dictating the critical buckling load of the fibers by constraining the fibers from buckling, to improve the interfacial mechanical properties of the composite, it is important to modify the polymer matrix, fibers and/or the interface. In this dissertation, a novel approach to enhance the polymer matrix-fiber interface region has been discussed. This approach involves spiral wrapping carbon nanotube (CNT) sheet around individual carbon fiber or fiber tow, at room temperature at a prescribed wrapping angle (bias angle), and then embed the scrolled fiber in a resin matrix. The polymer infiltrates into the nanopores of the multilayer CNT sheet to form CNT/polymer nanocomposite surrounding fiber, and due to the mechanical interlocking, provides reinforcement to the interface region between fiber and polymer matrix. This method of nano-fabrication has the potential to improve the mechanical properties of the fiber-matrix interphase, without degrading the fiber properties. The effect of introducing Multi-Walled Carbon Nanotubes (MWNT) in the polymer matrix was studied by analyzing the atomistic model of the epoxy (EPON-862) and the embedded MWNTs. A multi-scale method was utilized by using molecular dynamics (MD) simulations on the nanoscale model of the epoxy with and without the MWNTs to calculate compressive strength of the composite and predict the enhancement in the composite material. The influence of the bias/over wrapping angle of the MWNT sheets on the carbon fiber was also studied. The predicted compressive strength from the MD results and the multiscale approach for baseline Epoxy case was shown to be in good relation with the experimental results for Epon-862. On adding MWNTs to the epoxy system, a significant improvement in the compressive strength of the PMC was observed. Further, the effect of bias angle of MWNT wrapped over carbon fiber was compared for 0, 45 and 90. This is further verified by making use of the Halpin-Tsai.Item Characterization and Modeling of Dual Phase Thermoplastic Self Healing System for Fiber Reinforced Thermoset Composite Structures(University of Alabama Libraries, 2020) Jony, Bodiuzzaman; Roy, Samit; University of Alabama TuscaloosaThe one characteristic that sets biological systems apart from human-engineered systems is its ability to heal itself repeatedly without any external intervention. In the last few decades, drawing inspiration from nature, there has been a tenacious drive towards the design and development of bio-mimetic multifunctional polymers and polymer matrix composites that possess the capability of repeatable self-healing. In this dissertation, bio-mimetic self-healing methods are explored for recover-ing the mechanical and structural performances of damaged ?ber reinforced thermoset polymer composite using thermoplastic healants. Speci?cally, repeatable Mode-I interlaminar fracture healing capabilities of thermoplastic polycaprolactone (PCL) particles and polyurethane shape memory polymer (SMP) ?brils in a thermoset unidirectional carbon/epoxy composite were investigated. During the bio-mimetic healing process, the polyurethane SMP ?brils were used to close the open crack through a thermally-activated contraction, and then the thermoplastic PCL was heated to heal the damage through melt intercalation into the crack. The chemical and thermal properties of the polymer composite and healants and interactions between the brittle epoxy and ductile healants were investigated. Further, repeatable Mode-II interlaminar shear fracture property recovery of unidirectional carbon/epoxy by a blend of the same biphasic healants was experimentally investigated. The shear crack growth phenomenon and mechanism of fracture toughness recovery were thoroughly investigated and contrasted with Mode-I failure. Finally, the real-time in-situ application of self-healing in ?ber-reinforced composite was accomplished by using a macro ?ber composite (MFC) actuator assisted healing. The parameters for generating stimulus (heat) from MFC without damaging it or the composite were calculated. Relative crack growth stability was also investigated during in-situ healing for virgin and healed cases by using R ? curve and crack growth rate phenomenon. For a comprehensive understanding of the healing mechanism and fracture behavior of the polymer composite, analytical and numerical models were generated using a bilinear cohesive law. The critical fracture parameters obtained from the analytical studies were thoroughly veri?ed with experimental results and ?nite element numerical simulations. It is envisioned that this work will provide a solid foundation for the future development and implementation of self-healing polymer composites in real life structural applications.Item Concurrently coupled multiscale modeling of polymer nanocomposites(University of Alabama Libraries, 2016) Li, Shibo; Roy, Samit; University of Alabama TuscaloosaEmbedded statistical coupling method (ESCM) was originally developed to provide computational efficiency, to decrease coupling complexities, and to avoid the need to discretize the continuum model to atomic scale resolution in concurrent multiscale modeling. ESCM scheme is relatively easy to implement within conventional FEM code and has been tested in standard solid lattice structures. However, this method encounters difficulties when being implemented for amorphous materials like polymers, due to the fact that they lack specific ordered lattice structure and atoms may not be covalently bonded with each other, which are the requirements of common coupling schemes. Therefore, a new approach needs to be developed to resolve this problem. In this paper, details of a modified ESCM approach for atomistic-continuum coupling developed to perform simulations of crack growth in polymers is presented. The presence of the continuum domain surrounding the MD region allows for the application of far-field loading, and prevents stress wave reflections from the external boundary impinging back on the crack tip. In our approach, Material Point Method (MPM), which is a meshless particle-in-cell method based on Arbitrary Euler-Lagrange (ALE) scheme and has been proven to have good performance in large deformation problems, is used to model the continuum domain. It is concurrently coupled with molecular dynamics (MD), a widely used method in atomistic simulations, using a so-called handshake region. Anchor points, the equilibrium positions of the constrained particles, which are designed to transmit displacements and forces between nanoscale and macroscale model, are defined in the handshake region. A concurrently coupled MPM-MD simulation of crack propagation inside a polymer is performed to verify this new coupling approach, thereby providing a better understanding of the fracture mechanisms at the nanoscale to predict the macro-scale fracture toughness of polymer system. Results are presented for concurrently coupled crack propagation simulation in a di-functional cross-linked thermoset polymer, EPON 862. The composite laminate open hole tension problem is also studied using concurrent multiscale approach by implementing micromechanics program MAC/GMC in FEA frame.Item Damage detection and sensor placement optimization in composite structures(University of Alabama Libraries, 2012) Zheng, Hao; Roy, Samit; University of Alabama TuscaloosaStructural health monitoring, damage identification method and sensor placement optimization for carbon fiber reinforced polymer (CFRP) composite beam were studied in this thesis. In this work, different methodologies were investigated for the damage detection process to enhance the use of current structural health monitoring systems by identifying the optimal sensor placement. Carbon fiber reinforced polymer composite materials were fabricated and the fabrication process based on vacuum assisted resin transfer molding (VARTM) is briefly introduced. Numerical analysis using finite element method was subsequently performed for a composite beam based on the material properties determined by performing experimental material characterization tests. Three benchmarking tests with different types of elements were performed to verify the best method for modeling the composite panel. Moreover, shear lag analysis was also presented to model an embedded crack in the composite panel which would be used in damage detection and optimization process. Based on the finite element analysis and static strain data extracted, a comparative study on two damage detection algorithms based on artificial neural network (ANN) and support vector machine (SVM) is presented. The viability of these two methods was demonstrated by analysis of the numerical model of composite beam with a crack embedded in it and the performance for each algorithm is also presented with different number of sensors and different noise levels. Two experiments are presented for the performance evaluation of damage detection and identification. To identify the optimal locations of sensors in the optimization process, a statistical probability based method using the combination of artificial neural networks and evolutionary strategy were developed to increase the detection rate of damage in a structure. The proposed method was able to efficiently increase the detection accuracy compared with a uniform distribution of sensors for a composite beam that was damaged in different locations. The finite element model of the composite coupon was used as a representation of the real structure. Static strain data from finite element simulation was extracted with different damage scenarios and used as feature vector for the classification process. Based on the performance of the classification for a given sensor configuration, updated sensor locations would be selected by changing the coordinates of these sensor locations using strategy parameters. The viability of this method was demonstrated by conducting different examples and significant number of simulations was performed to check the repeatability of the algorithm.Item Damage evolution in composite materials under environmental ageing: a stochastic model with experimental validation(University of Alabama Libraries, 2009) Rahman, Rezwanur; Haque, Anwarul; University of Alabama TuscaloosaThis work emphasizes on predicting probability density function of damages or "number density of damage" in graphite/epoxy polymer matrix composite materials (PMC) under hygrothermal aging condition. A coupled Forward-Backward Stochastic Differential Equation (FBSDE) is proposed as a mathematical model to predict number density of damages. The FBSDE consists of damage nucleation and annihilation rate in terms of Brownian motion. The uncertainty in damage nucleation and annihilation rate is noticed by proposing these two terms as "Brownian motion with drift". In order to verify the proposed model, a quantitative analysis was carried out on a limited number of graphite/epoxy specimens manufactured by VARTM process. The specimens were kept in a hygrothermal condition with room temperature cycling. A rigorous quantitative analysis of damages was done by optical microscopic inspection at different stages of aging period. The damages were classified based on the criteria of their size. Finally, the experimentally collected data for number density of damages were verified by using the proposed FBSDE. A detail parametric study was carried out using FBSDE and best possible predicted data was validated with the experimental observation. A reasonable estimation was observed from the model output.Item Effect of weld schedule variation on the weldability and durability of AHSS spot weld joints(University of Alabama Libraries, 2012) Weishaupt, Eric; Barkey, Mark E.; University of Alabama TuscaloosaTensile strength testing and high cycle fatigue testing of advanced high strength steel spot welded shear lap joints were performed for the various weld conditions. The materials used in this study were DP 980, DP 780 and TRIP 780. The microstructure and microhardness of the shear lap joints were examined in an effort to identify the effect of microstructural changes on the strength and fatigue durability of the spot weld specimens. The occurrence of interfacial failure was recorded for the differing weld processes. Several weld schedules were examined and used to produce shear lap spot weld joints, specifically varying the squeeze force and the average current. The weld force used to produce a spot weld does not have a significant effect on the fracture mode of the specimen given the average current is constant. The average current used to produce a spot weld has a significant effect on the fracture mode of the spot weld for several squeeze forces. Interfacial failure of spot welded TRIP 780 can be mitigated using a certain range of currents when welding. This appears to come as a tradeoff for sacrificing the strength of the joint. Higher values of weld strength were obtainable; however, welds that failed with higher strengths also experienced interfacial failure. A fracture mechanics approach to estimating the high cycle fatigue life of the shear lap specimen is also proposed and represents a conservative estimate of the shear lap specimen durability.Item Experimental and numerical characterization of the fatigue and fracture properties of friction plug welds in 2195-T8 aluminum lithium alloy(University of Alabama Libraries, 2013) Metz, David F.; Barkey, Mark E.; University of Alabama TuscaloosaThe mechanical, fatigue, and fracture properties of friction stir welded 2195-T8 Al-Li alloy plates that contained friction plug welds were investigated. The friction plug weld process is currently being developed for use in the fabrication of aerospace pressure vessels. The friction plug welds in this study used double-tapered plugs that were machined from extruded 2195-T8 Al-Li alloy. The mechanical properties were determined from uni-axial tensile tests of 2195-T8 base metal, friction stir welded 2195-T8, and friction stir welded 2195-T8 that contained friction plug welds made with 2195-T8 plugs. The microstructure and microhardness of the friction plug weld interfaces with the base metal and friction stir welded material were studied to identify the thermal, thermomechanical, and mechanical effects that the friction plug welding process introduced to the surrounding region. Microhardness measurements of the plug weld interface, base metal, friction stir weld, and heat affected zones that surround the plug weld were made with a Knoop indenter operated at a 100g load. The results of hardness tests showed the largest decrease in hardness was found at the plug weld interface and ranged from 110-130 HK100g. Metallographic images of the microhardness specimens were examined and a recrystallized zone that surrounds the plug weld was identified. The size of the recrystallized zone was measured to be 30-122 μm. The fatigue properties were examined by the use of constant amplitude fatigue tests for friction stir welded specimens and friction stir welded specimens that contained a friction plug weld. The fracture surfaces were examined to identify the crack origin and crack path. Results from the fatigue tests showed that the process parameters for the friction plug weld can directly influence the fatigue properties of friction plug welds. The unique weld geometry of the friction plug weld repair precluded the use of ASTM standard compact specimens C(T), so the fatigue crack growth test was performed in a non-conventional manner using a dog-bone style fatigue specimen and crack mouth opening displacement as a means of measuring the fatigue crack growth rate. The crack mouth opening displacement was obtained during testing from a clip gage mounted at the fatigue crack. The crack mouth opening data was then used to correlate the crack mouth opening displacement obtained experimentally with that of the crack mouth opening displacement determined from a finite element model of the specimen with a crack located at the plug weld. The crack depth, which was unobservable during testing, was determined by a correlation of the crack mouth opening data with the crack mouth opening calculated from finite element model for a specific crack geometry. The stress amplitude level used during the fatigue crack growth test was 70-90% of the ultimate strength for the friction plug welds tested.Item A finite element analysis of a boa constrictor skull and the design of a jaw bone transducer(University of Alabama Libraries, 2011) Tadi, Nava Sundeep Avinash Reddy; Barkey, Mark E.; University of Alabama TuscaloosaThe purpose of this research is to analyze the finite element model of a Boa constrictor skull and design a transducer at the jaw bone to measure the forces applied during feeding. The concepts in mechanics are applied to simulate and study the behavior of jaws while feeding.. In this study, the images of the skull of a boa constrictor were scanned to obtain the precise size and shape of each of the bones inside. The specimen was scanned using the X-ray micro tomography procedure. This process left several regions in the finite element model unconnected. These regions are corrected using the software HYPERMESH. The loading and boundary conditions along with the various properties like the Elastic modulus, Poisson's ratio are given to the FE model in ABAQUS before analyzing it. Electrical resistance strain gauges are simulated to have attached to the jawbone at two different points. The strain measured is then used to find out the forces that caused it, by using the various concepts and principles in solid and continuum mechanics. Electrical resistance strain gauges, numerical methods, moments of inertia are some of the commonly used terms in this analysis.Item Investigation of Taylor impact test of isotropic and anisotropic material through geometrical characteristics of specimens(University of Alabama Libraries, 2010) Cao, Zhiyi; Barkey, Mark E.; University of Alabama TuscaloosaIn this thesis, high strain rate properties of isotropic material (a copper alloy) and anisotropic material (2195-T8 aluminum-lithium alloy) are investigated using Taylor impact tests. Coordinate measuring machines (CMMs) are used to measure the shape of specimens after the deformation. The geometrical data enables us to determine the plastic distributions and the dynamic yield stresses of specimens. A raise in yield strength is found in the copper alloy during the impact. It means that material properties of the copper alloy are sensitive to high strain rate. Yet such phenomenon is not found in the 2195-T8 aluminum-lithium alloy. Based on the uniaxial compression strain state in the barreling regions of the specimens, the dynamic yield stresses in the rolling, transverse and short transverse directions are obtained for the 2195-T8 aluminum-lithium alloy. This enables us to determine the anisotropic coefficients in Hill's criterion and carry out the finite element analysis. The dependencies of fracture are also investigated. It is found that the fracture is sensitive to maximum shear stress, equivalent plastic strain and stress triaxiality.Item Laminar and turbulent boundary layer separation control of mako shark skin(University of Alabama Libraries, 2014) Afroz, Farhana; Lang, Amy W.; University of Alabama TuscaloosaThe Shortfin Mako shark (Isurus oxyrinchus) is one of the fastest swimmers in nature. They have an incredible turning agility and are estimated to achieve speeds as high as ten body lengths per second. Shark skin is known to contain flexible denticles or scales, capable of being actuated by the flow whereby a unique boundary layer control (BLC) method could reduce drag. It is hypothesized that shark scales bristle when the flow is reversed, and this bristling may serve to control flow separation by (1) inhibiting the localized flow reversal near the wall and (2) inducing mixing within the boundary layer by cavities formed between the scales that increases the momentum of the flow near the wall. To test this hypothesis, samples of Mako shark skin have been studied under various amounts of adverse pressure gradient (APG). These samples were collected from the flank region of a Shortfin Mako shark where the scales have the greatest potential for separation control due to the highest bristling angles. An easy technique for inducing boundary layer separation has been developed where an APG can be generated and varied using a rotating cylinder. Both the experimental and numerical studies showed that the amount of APG can be varied as a function of cylinder rotation speed or cylinder gap height for a wide range of Reynolds numbers. This method of generating an APG is used effectively for inducing both laminar and turbulent boundary layer separation over a flat plate. Laminar and turbulent boundary layer separation studies conducted over a smooth plate have been compared with the same setup repeated over shark skin. The time-averaged DPIV results showed that shark scale bristling controlled both laminar and turbulent boundary layer separation to a measurable extent. It shows that the shark scales cause an early transition to turbulence and reduce the degree of laminar separation. For turbulent separation, reverse flow near the wall and inside the boundary layer is hypothesized to bristle the shark scales thereby preventing the reverse flow from reaching higher magnitudes that leads to global flow separation.Item Life Estimation of SR-FSW Pin Tools for NASA Application(University of Alabama Libraries, 2021) Hagan, Zachary Matthew; Barkey, Mark E.; University of Alabama TuscaloosaSelf-reacting friction stir welding (SR-FSW) is one of the processes used in the fabrication of the liquid oxygen and hydrogen tanks that comprise NASA’s Space Launch System’s (SLS) rocket. The SR-FSW tool assembly both clamps and stirs the material by means of rotation and translation through the work piece being welded. One weld tool component of interest is manufactured from MP159, a cobalt-based alloy with excellent mechanical properties at elevated temperatures. Failure of the welding tool during production would result in downtime and require structural mitigation of the rocket body. The repaired weld produced may then exhibit less than desirable mechanical properties. Given the Certified Weld Procedure (CWP) used during production, the lineal inches of weld in a single production “pass”, and the requirement for frequent tool replacement, a weld pin-tool failure during production would result from low cycle fatigue (LCF) damage accumulation. Unfortunately, there is limited data available in the literature for this material’s fatigue behavior at elevated temperatures. This work has generated a statically significant strain life curve of MP159 at 800°F with fully reversed loading at a frequency of f = 1 Hz. Subsequent tests have produced data to support an exploratory strain life curve at a frequency of f = 2.4 Hz. Additionally, a combined kinematic and isotropic hardening constitutive model has been calibrated to the materials elevated temperature cyclic response. The constitutive model is used to determine the history-dependent deformation response of the tool at critical geometric locations as a function of loading conditions, boundary conditions and weld pin tool material evolution. Finally, a weld pin tool failure analysis engine has been implemented in MATLAB. This engine estimates the life of the weld pin tool as a function of accumulated damage resulting from any number of loading scenarios, or combined loading scenarios. Failure of the pin tool is predicted by use of Miner’s rule. The failure algorithm has the ability to combines damage from weld start up, steady state operation, and tool “pull out” as well as any combination of these individual operational components. The analysis engine may be used to determine safe operational conditions for the weld tool given weld procedures, or it may be used to redesign welding pin tool geometry to mitigate failures during production, thereby reducing production cost and schedule as well as ensuring the most structurally sound weld possible.Item Life prediction of composite materials subjected to long term mechanical/environmental loading condition(University of Alabama Libraries, 2010) Singh, Sushil; Roy, Samit; University of Alabama TuscaloosaA multi-scale mechanism-based life prediction model is developed for high-temperature polymer matrix composites (HTPMC) under thermo-oxidative aging conditions. Life prediction model is based on stiffness and strength degradation in unidirectional HTPMC under accelerated thermo-oxidative aging condition. A multi-scale model based on continuum damage mechanics to predict stiffness degradation and progressive failure due to degradation of inter-laminar shear strength is developed for unidirectional composite. Using continuum damage mechanics one can relate the behavior of composites at micro-level (representative volume element) to the macro-level (structural element). Thermo-oxidative aging is simulated with diffusion-reaction model in which temperature, oxygen concentration and weight loss effects are considered. For fiber/matrix debond growth simulation, a model based on Darcy's laws for oxygen permeation in the fiber-matrix interface is employed, that, when coupled with polymer shrinkage, provides a mechanism for permeation-controlled debond growth in HTPMC. Viscoelastic regularization in the constitutive equations of the cohesive layer used in this model not only mitigates numerical instability, but also enables the analysis to follow load-deflection behavior beyond the point of peak failure load. Benchmark of model prediction with experiment was carried out to establish proof-of-concept.Item Luminescent coating image analysis on a three dimensional grid(University of Alabama Libraries, 2010) Esirgemez, Ergin; Hubner, James Paul; University of Alabama TuscaloosaThe luminescent photoelastic coating (LPC) technique is a method to measure the full-field strain on three-dimensional (3D) structural components. A luminescent dye within a photoelastic binder is excited with circular polarized light, and the corresponding emission intensity for coating is detected via a CCD camera. Images are then processed to find the relative change in emission with respect to camera analyzer position, and subsequently analyzed to determine maximum in-plane shear strain. Image alignment plays a crucial role to obtain accurate measurements, especially when implementing an oblique excitation approach to separate the principal strains while accounting for non-strain related polarization changes due to surface inclination. Image warping methods in the image two-dimensional (2D) coordinate system provides reasonable results for 2D or simple 3D specimens; however, for complex 3D structures with moderate movement or deflection in the field-of-view, the accuracy and efficiency of these methods are not optimal. An alternative approach is to perform the analysis on a 3D grid representation of the structures. This study will research the merit of such an approach and develop the analysis procedures to separate the principal strains on 3D structures. The theoretical results will be compared to experimental data from a 2D and a 3D specimens while assessing the accuracy of the approach.Item A methodology for predicting fracture toughness of nano-graphene reinforced polymers using molecular dynamics simulations(University of Alabama Libraries, 2014) Akeptai, Avinash Reddy; Roy, Samit; University of Alabama TuscaloosaThe nano-scale interaction between polymer molecules and nanoparticle is a key factor in determining the macro-scale strength of the composite. In recent years numerous efforts have been directed towards modeling nanocomposites in order to better understand the reasons behind the enhancement of mechanical properties, even with the slight addition (a few weight percent) of nano-materials. In order to better understand the local influence of nanoparticle on the mechanical properties of the composite, it is required to perform nano-scale analysis. In this context, modeling of fracture in nano-graphene reinforced EPON 862 at the nano-scale is discussed in this dissertation. Regarding fracture in polymers, the critical value of the J-integral (JIC), where the subscript I denotes the fracture mode (I=1, 2, 3), at crack initiation could be used as a suitable metric for estimating the crack driving force as well as fracture toughness of the material as the crack begins to initiate. However, for the conventional macroscale definition of the J-integral to be valid at the nano-scale, in terms of the continuum stress and displacement fields and their spatial derivatives, requires the construction of local continuum fields from discrete atomistic data, and using these data in the conventional contour integral expression for atomistic J-integral. One such methodology is proposed by Hardy that allows for the local averaging necessary to obtain the definition of free energy, deformation gradient, and Piola-Kirchoff stress as fields (and divergence of fields) and not just as total system averages. Further, the atomistic J-integral takes into account the effect of reduction in J from continuum estimates due to the fact that the total free energy available for crack propagation is less than the internal energy at sufficiently high temperatures when entropic contributions become significant. In this research, the proposed methodology is used to compute J-integral using atomistic data obtained from LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). Further, a novel approach that circumvents the complexities of direct computation of entropic contributions is also discussed. As a case study, the feasibility of computing the dynamic atomistic J-integral over the MD domain is evaluated for a graphene nano-platelet with a central crack using OPLS (Optimized Potentials for Liquid Simulations) potential. For model verification, the values of atomistic J-integral are compared with results from linear elastic fracture mechanics (LEFM) for isothermal crack initiation at 0 K and 300 K. J-integral computations are also done using ReaxFF force field in order to simulate bond breakage during crack propagation. Good agreement is observed between the atomistic J and LEFM results at 0.1 K, with predictable discrepancies at 300 K due to entropic effects. The J-integral computation methodology was then used to computationally predict fracture toughness in nano-particle reinforced composite material at elevated temperatures using the ReaxFF force field in LAMMPS, which can simulate crack propagation more accurately by breaking and forming bonds on the fly.Item Microhardness, strength and strain field characterization of self-reacting friction stir weld and friction plug welds of dissimilar aluminum alloys(University of Alabama Libraries, 2011) Horton, Karla Renee; Barkey, Mark E.; University of Alabama TuscaloosaFriction stir welding (FSW) is a solid state welding process with potential advantages for aerospace and automotive industries dealing with light alloys. Self-reacting friction stir welding (SR-FSW) is one variation of the FSW process being developed at the National Aeronautics and Space Administration (NASA) for use in the fabrication of propellant tanks. Friction plug welding is used to seal the exit hole that remains in a circumferential SR-FSW. This work reports on material properties and strain patterns developed in a SR-FSW with a friction plug weld. Specifically, this study examines the behavior of a SR-FSW formed between an AA2014-T6 plate on the advancing side and an AA2219-T87 plate on the retreating side and a SR-FSW (AA2014-T6 to AA2219-T87) with a 2219-T87 plug weld. This study presents the results of a characterization of the micro-hardness, joint strength, and strain field characterization of SR-FSW and FPW joints tested at room temperature and cryogenic temperatures. The initial weld microstructure analysis showed a nugget region with fine grains and a displaced weld seam from the advancing side past the thermo-mechanical affected zone (TMAZ) into the nugget region. The displaced material shared the same hardness as the parent material. Dynamic recrystallization was observed in the SR-FSW zone and the displaced weld seam region. The welds revealed a fine grain structure in the SR-FSW zone with a sharp demarcation seen on the advancing side and fairly diffuse flow observed on the retreating side. The parent material hardness is 145 HV_700g with a drop in hardness starting at the HAZ to 130 HV_700g . The hardness further drops in the TMAZ to118 HV_700g with an increase representing a dispersed interface of AA2014-T6 material to 135 HV_700g . The hardness then drops significantly within the nugget region to 85 HV_700g followed by an increase through the retreating side TMAZ into the HAZ to 135 HV_700g . There was a sharp increase in the hardness value within the nugget region with the samples that were PWHT showing an increase of 58%. The welded joints were tested for ultimate strength. The testing variations included two specimen widths, two plug sizes (M3 and M5), room temperature and cryogenic testing, and post weld heat treated (PWHT) samples. Initial welds had an average ultimate strength of 370 MPa. There was a slight drop from initial weld strength to plug weld strength of approximately 13.8 MPa was observed with M3 plug strength approximately equal to M5 plug strength. The PWHT strengths at room temperature were slightly higher than non-PWHT of 13.8-20.7 MPa and PWHT strengths were equal to non-PWHT at cryogenic temperature. Non-PWHT had a cryogenic strength enhancement approximately 59.2 MPa and PWHT had a cryogenic strength enhancement of approximately 57.2 MPa in the M3 and M5 plugs. Within the subsets of data collected no major statistical significance in strength behavior was observed between the samples tested at room temperature or between the subsets tested at room temperature or between the subsets tested at cryogenic temperature. In almost all cases, failure occurred on the retreating side of the weld which corresponds to the softer material (AA2219-T87). Exceptions were characterized with flaws (weld defects) in the sample. In these cases, failure occurred on the advancing side, the side where flaws were detected. Ductile fracture was noted in most all samples. Digital image correlation using the ARAMIS system was used to define strain patterns in the weld joint. Strain accumulation was observed in the weld along the retreating side and around the plug. ARAMIS data in comparison to extensometer data shows a very reasonable comparison. The ARAMIS strain gage data showed the retreating side of the major diameter has a greater yield than the advancing side. This behavior is identical to the external electrical resistance strain gages.Item A model of thermal aging of hyper-elastic materials with an application to natural rubber(University of Alabama Libraries, 2017) Korba, Ahmed G.; Barkey, Mark E.; University of Alabama TuscaloosaUnderstanding the degradation of material properties and stress-strain behavior of rubber-like materials that has been exposed to elevated temperature is essential for rubber among components design and lifetime prediction. The complexity of the relationship between hyper-elastic materials, crosslinking density, and chemical composition present a difficult problem for the accurate prediction of mechanical properties under thermal aging. In the first part of the current research, a new and relatively simple mathematical formulation is presented to expresses the change in material properties of natural rubber subjected to various elevated temperatures and aging times. The aging temperatures ranged from 76.7 °C to 115.0 °C, and the aging times ranged from 0 to 600 hours. Based on the experimental data, the natural rubber mechanical properties under thermal aging showed a similar behavior to the rate of change of the crosslinking density (CLD) with aging time and temperature as determined as of the research. Three mechanical properties have been chosen to be studied: the ultimate tensile strength, the fracture stretch value, and the secant modulus at 11.0% strain. The proposed phenomenological model relates the mechanical properties with the rate of change of the CLD based on a form of Arrhenius equation. The proposed equations showed promising results compared to the experimental data with an acceptable error margin of less than 10% in most of the cases studied. In the second part of the current research, a closed form set of equations that was based on basic continuum mechanics assumptions has been proposed to define the material stress-strain behavior of natural rubber as an application of hyper-elastic materials. The proposed formulas include the influence of aging time and temperature. The newly proposed “Wight Function Based” (WFB) method has been verified against the historic Treloar’s test data for uni-axial, bi-axial and pure shear loadings of Treloar’s vulcanized rubber material, showing a promising level of confidence compared to the Ogden and the Yeoh methods. Tensile testing was performed on strip specimens that were thermally aged then subjected uni-axial tension and hardness tests. A non-linear least square optimization tool in Matlab (Lscurvefitt) was used for all fitting purposes.Item Modeling of delamination in carbon/epoxy composite laminates under four point bending for damage detection and sensor placement optimization(University of Alabama Libraries, 2013) Adu, Stephen A.; Roy, Samit; University of Alabama TuscaloosaLaminated carbon fiber-reinforced polymer composites (CFRPs) possess very high specific strength and stiffness and this has accounted for their wide use in structural applications, most especially in the aerospace industry, where the trade-off between weight and strength is critical. Even though they possess much larger strength ratio as compared to metals like aluminum and lithium, damage in the metals mentioned is rather localized. However, CFRPs generate complex damage zones at stress concentration, with damage progression in the form of matrix cracking, delamination and fiber fracture or fiber/matrix de-bonding. This thesis is aimed at performing; stiffness degradation analysis on composite coupons, containing embedded delamination using the Four-Point Bend Test. The Lamb wave-based approach as a structural health monitoring (SHM) technique is used for damage detection in the composite coupons. Tests were carried-out on unidirectional composite coupons, obtained from panels manufactured with pre-existing defect in the form of embedded delamination in a laminate of stacking sequence [06/904/06]T. Composite coupons were obtained from panels, fabricated using vacuum assisted resin transfer molding (VARTM), a liquid composite molding (LCM) process. The discontinuity in the laminate structure due to the de-bonding of the middle plies caused by the insertion of a 0.3 mm thick wax, in-between the middle four (4) ninety degree (90o) plies, is detected using lamb waves generated by surface mounted piezoelectric (PZT) actuators. From the surface mounted piezoelectric sensors, response for both undamaged (coupon with no defect) and damaged (delaminated coupon) is obtained. A numerical study of the embedded crack propagation in the composite coupon under four-point and three-point bending was carried out using FEM. Model validation was then carried out comparing the numerical results with the experimental. Here, surface-to-surface contact property was used to model the composite coupon under simply supported boundary conditions. Theoretically calculated bending stiffness's and maximum deflection were compared with that of the experimental case and the numerical. After the FEA model was properly benchmarked with test data and exact solution, data obtained from the FEM model were used for sensor placement optimization.