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Daniel Guggenheim School of Aerospace Engineering

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search.filters.applied.f.advisor: Hanagud, Sathyanaraya × End date: 2009 × Start date: 1980 × search.filters.applied.f.isSeriesOfPublicationTitle: Doctor of Philosophy with a Major in Aerospace Engineering ×

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Now showing 1 - 10 of 27
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    Equation of state for polytetrafluoroethylene (PTFE) and mixtures with PTFE
    (Georgia Institute of Technology, 2009-05-14) Wu, Zhibo
    The objectives of this work are to discuss multiscale models that are used to characterize the constitutive relations of the granular composite materials with dual functions. This is accomplished by the use of ab initio methods to obtain the constitutive relations of the structural energetic materials without conducting tests. First, it is necessary to study the quantum many body problem to quantitatively determine the internal energy of the material when subjected to different strain conditions. It is impossible to obtain an exact solution to the quantum many body problem that is modeled by the Schrödinger's equations with the current technology. It is possible to solve these equations approximately by the density functional theory which yields only energies at absolute 0ºK. Thus it becomes necessary to add both the lattice thermal contributions and electron thermal contribution. Then, resulting energy is used to bridge to the continuum level and obtain the constitutive equations. This is the procedure that is used in this work. The issues of the constitutive equations form the focus of this thesis. More specifically, the scope of the thesis is further restricted to analyze the constitutive equations of specific mixtures of nickel, aluminum with PTFE or Teflon as the binder. It is to be noted that the equations of state forms only a part of the complete constitutive relationships. This thesis presents solutions to the following problems: (1) Determination of the thermodynamically complete equation of state of the binder and the energetic material PTFE or Teflon, from ab initio methods based on the density functional theory. (2) Determination of the equations of state of the granular composite or the mixture of nickel, aluminum and PTFE from ab initio methods. (3) Determination of the complete constitutive equation of aluminum, from ab initio methods, under conditions of finite deformations, with principle of objectivity, material symmetry conditions and polyconvexity of the strain energy. All results are compared to test results whenever they are available.
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    Shock induced chemical reactions in energetic structural materials
    (Georgia Institute of Technology, 2009-02-03) Reding, Derek James
    Energetic structural materials (ESMs) constitute a new class of materials that provide dual functions of strength and energetic characteristics. ESMs are typically composed of micron-scale or nano-scale intermetallic mixtures or mixtures of metals and metal oxides, polymer binders, and structural reinforcements. Voids are included to produce a composite with favorable chemical reaction characteristics. In this thesis, a continuum approach is used to simulate gas-gun or explosive loading experiments where a strong shock is induced in the ESM by an impacting plate. Algorithms are developed to obtain equations of state of mixtures. It is usually assumed that the shock loading increases the energy of the ESM and causes the ESM to reach the transition state. It is also assumed that the activation energy needed to reach the transition state is a function of the temperature of the mixture. In this thesis, it is proposed that the activation energy is a function of temperature and the stress state of the mixture. The incorporation of such an activation energy is selected in this thesis. Then, a multi-scale chemical reaction model for a heterogeneous mixture is introduced. This model incorporates reaction initiation, propagation, and extent of completed reaction in spatially heterogeneous distributions of reactants. A new model is proposed for the pore collapse of mixtures. This model is formulated by modifying the Carol, Holt, and Nesterenko spherically symmetric model to include mixtures and compressibility effects. Uncertainties in the model result from assumptions in formulating the models for continuum relationships and chemical reactions in mixtures that are distributed heterogeneously in space and in numerical integration of the resulting equations. It is important to quantify these uncertainties. In this thesis, such an uncertainty quantification is investigated by systematically identifying the physical processes that occur during shock compression of ESMs which are then used to construct a hierarchical framework for uncertainty quantification.
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    Laser doppler vibrometer for efficient structural health monitoring
    (Georgia Institute of Technology, 2008-11-17) Sharma, Vinod K.
    The research effort in this thesis is devoted to develop techniques to accurately and rapidly identify the location, orientation, and magnitude of the defects by using structural health monitoring concepts that use Laser Doppler Vibrometer as a non-contact sensor with multi-point sensing capability. The first research area addresses the formulation and validation of an innovative Damage Measure that is based on the ratios of the strain energy distributions of the damaged and undamaged structure. The innovations include use of a single set of actuator/sensor pair to excite and detect the responses of a structure for low frequency vibrations as well as guided wave propagation studies. A second new capability is the estimation of the Damage Measure without requiring any knowledge of the undamaged baseline structure. This method is made possible because of the development of these new technologies: Spatial Decimation and Wavenumber/Frequency filtering. The third contribution is to develop analytical models for the structural dynamics of damaged structure and seek solutions that use perturbation methods to detect damage in a plate structure. The fourth contribution is the development of a comprehensive damage detection technique over a wide frequency dynamic range. The fifth topic of research involves automation in Structural Health Monitoring based on the comprehensive Damage Measure formulation. Under the control of software the Scanning Laser Doppler Vibrometer is used to acquire the low frequency vibration mode data for a coarse identification of all the suspect regions of damage using a threshold criterion on the Damage Measure. Each suspect region of damage is further investigated using the high frequency elastic wave propagation to clearly identify the location, orientation, and extent of the damage. The computer control of the Laser Doppler Vibrometer and a quantitative assessment of the damage provide the enabling technologies for the automation proof of concept. Finally the developed techniques of damage detection are successfully demonstrated on practical structures such as a turbine blade in the laboratory and an F-15 vertical tail in field maintenance conditions
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    An Experimental Study of Concurrent Methods for Adaptively Controlling Vertical Tail Buffet in High Performance Aircraft
    (Georgia Institute of Technology, 2007-09-10) Roberts, Patrick James
    High performance twin-tail aircraft, like the F-15 and F/A-18, encounter a condition known as tail buffet. At high angles of attack, vortices are generated at the wing fuselage interface (shoulder) or other leading edge extensions. These vortices are directed toward the twin vertical tails. When the flow interacts with the vertical tail it creates pressure variations that can oscillate the vertical tail assembly. This results in fatigue cracks in the vertical tail assembly that can decrease the fatigue life and increase maintenance costs. For many years, research has been conducted to understand this phenomenon of buffet and to reduce its adverse effects on the fatigue life of aerospace structures. Many proposed solutions to this tail buffet problem have had limited success. These include strengthening the tail, modifying the vortex flow, using an active rudder control, and leading edge extensions. Some of the proposed active controls include piezoelectric actuators. Recently, an offset piezoceramic stack actuator was used on an F-15 wind tunnel model to control buffet induced vibrations at high angles of attack. The controller was based on acceleration feedback control methods. In this thesis a procedure for designing the offset piezoceramic stack actuators is developed. This design procedure includes determining the quantity and type of piezoceramic stacks used in these actuators. The changes of stresses, in the vertical tail caused by these actuators during an active control, are investigated. In many cases, linear controllers are very effective in reducing vibrations. However, during flight, the natural frequencies of the vertical tail structural system changes as the airspeed increases. This in turn, reduces the effectiveness of a linear controller. Other causes such as the unmodeled dynamics and nonlinear effects due to debonds also reduce the effectiveness of linear controllers. In this thesis, an adaptive neural network is used to augment the linear controller to correct these effects.
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    Impact and penetration studies: simplified models and materials design from ab initio methods
    (Georgia Institute of Technology, 2006-01-13) Jiang, Tianci
    In recent impact and penetration mechanical tests, steel projectiles (AISI4340) were impacted into targets like concrete with striking velocities (1200 m/s to 1500 m/s). Results indicated a material removal from the nose of the projectile, phase changes of the projectile materials, a reduction in the length of the projectile, and a blunting of the nose shape. These observations cannot be explained by current theories and numerical integration code that are used to study impact and penetration mechanics. Thus, the objectives of the thesis research are to (a) formulate and characterize the mechanisms responsible for the material erosion of the impacting projectile and the mass loss from the nose region; and (b) to determine the physical properties of alloy steels that are important to penetration mechanics from ab initio methods. The results can be used to design new projectile materials that can provide the desired penetration characteristics. These objectives are accomplished by investigating two related problems. The first problem is to formulate simplified models that can explain the penetration mechanics. The new models include the varying cross-section nose, changes of yield stress behind the shock wave and high strain rate phase transitions. Nose erosion effects, and time-dependent penetration path can be determined by integrating ODEs. A cavity expansion theory model is used to obtain the target resistance that is responsible slowing and deforming the penetrating projectile. The second problem concerns the determination of the constitutive relations from ab initio methods. The equation of state (EOS) and magnetic moments for alloy steels are investigated by using a special quasirandom structure technique and ab initio methods. Specifically, EOS for an interstitial disordered alloy Fe1-x-yNixCy is developed. First, the EOS of iron and phase transition of iron are studied and validated. Second, Nickel is considered to investigate the substitutional disordered alloy Fe1-x-yNixCy. Third, Carbon is placed at an interstitial position in the substitutional disordered alloy. These investigations will form foundation for future work involving new projectile with steel nose and shank made of multifunctional structural energetic materials.
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    Non-equilibrium Thermomechanics of Multifunctional Energetic Structural Materials
    (Georgia Institute of Technology, 2005-11-28) Narayanan, Vindhya
    Shock waves create a unique environment of high pressure, high temperature and high strain-rates. It has been observed that chemical reactions that occur in this regime are exothermic and can lead to the synthesis of new materials that are not possible under other conditions. The exothermic reaction is used in the development of binary energetic materials. These materials are of significant interest to the energetic materials community because of its capability of releasing high heat content during a chemical reaction and the relative insensitivity of these types of energetic materials. Synthesis of these energetic materials, at nano grain sizes with structural reinforcements, provides an opportunity to develop a dual functional material with both strength and energetic characteristics. Shock-induced chemical reactions pose challenges in experiment and instrumentation. This thesis is addressed to the theoretical development of constitutive models of shock-induced chemical reactions in energetic composites, formulated in the framework of non-equilibrium thermodynamics and mixture theories, in a continuum scale. Transition state-based chemical reaction models are introduced and incorporated with the conservation equations that can be used to calculate and simulate the shock-induced reaction process. The energy that should be supplied to reach the transition state has been theoretically modeled by considering both the pore collapse mechanism and the plastic flow with increasing yield stress behind the shock wave. A non-equilibrium thermodynamics framework and the associated evolution equations are introduced to account for time delays that are observed in the experiments of shock-induced or assisted chemical reactions. An appropriate representation of the particle size effects is introduced by modifying the initial energy state of the reactants. Numerical results are presented for shock-induced reactions of mixtures of Al, Fe2O3 and Ni, Al with epoxy as the binder. The theoretical model, in the continuum scale, requires parameters that should be experimentally determined. The experimental characterization has many challenges in measurement and development of nano instrumentation. An alternate approach to determine these parameters is through ab-initio calculations. Thus, this thesis has initiated ab-initio molecular dynamics studies of shock-induced chemical reactions. Specifically, the case of thermal initiation of chemical reactions in aluminum and nickel is considered.
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