<p dir="ltr">To characterize the performance of ultrasonically absorptive flow control materials for hypersonic flight, a pressure controlled and temperature sensing bench-test was developed by the author under Office of Naval Research SBIR Contract #N6833519C0312 awarded in 2019. This testing apparatus is capable of experimentally determining the absorption power coefficient of arbitrary materials using a solid non-porous material as a reference. The testing methodology presented by the author was validated against a theoretical model. It was found that to accurately estimate the acoustic absorption coefficient of materials at high frequencies, temperature and humidity measurements are equally as critical as pressure measurements to ensure that the properties of the acoustic medium and electrical components are accounted for.</p>

<p dir="ltr">Additive manufacturing continues to show great promise for use in structural components due to the cost effectiveness and reduced complexity associated with optimized and targeted use of the method. However, before additive manufacturing can be widely accepted a more complete understanding of the material performance and microstructural features must be achieved. This thesis aims to further the understanding of cold dwell fatigue in additively manufactured Ti-6Al-4V and explore targeted microstructural control of additively manufactured Ti-6Al-4V through the use of printing parameter variations and hot isostatic pressing.</p><p dir="ltr">In the first portion of this thesis, experimental work was conducted to explore the effect of periodically applied load dwell and overloads on the stress-life relationship for additively manufactured Ti-6Al-4V. Samples printed using an optimized print parameter set, heat treated using hot isostatic pressing, machined, and longitudinally polished were tested across a variety of loading schemes including: constant amplitude, periodic dwell, periodic overload, and alternating periodic dwell and periodic overload. It was determined that, for the parameter set studied, periodic overload provided similar damage compared to constant amplitude cases, while periodic dwell provided greater damage compared to both constant amplitude and periodic overload cases. Additionally, a phenomenological failure prediction model for dwell, variable amplitude loading was created. The developed model combines the effects of plasticity and creep with an energy-based approach rooted in the fundamental behavior of the material.</p><p dir="ltr">In the second portion of this thesis a review of the literature is presented to explore the use of hot isostatic pressing in additively manufactured Ti-6Al-4V. The literature review holds the primary purpose of deepening the understanding of the relationships between hot isostatic pressing and microstructural control and how they are taken together to improve fatigue performance. The literature review explores many aspects of factors impacting fatigue life and how the additive manufacturing process impacts material microstructure. The final conclusion of the literature review is that 1 micrometer is the largest pore expected to achieve complete closure though hot isostatic pressing, that 40 micrometer is the critical pore size for fatigue failure, and the process for microstructural evolution during pore closure is dominated by creep and dynamic recrystallization. Using these facts targeted microstructural control can be explored to optimize fatigue performance through purposeful microstructural variations.</p>

<p dir="ltr">Single screw extrusion is widely used in injection molding, extrusion additive manufacturing, and material pre-compounding. A single screw extruder has three stages – the solid conveying zone, the melt-transition or compression zone, and the melt-conveying zone. As the pellets are processed, pellet rupture and fiber breakage occur in the transition and melt-conveying stages of extrusion. Existing literature focuses on modeling fiber breakage in fully molten stage, and there is a lack of understanding of fiber breakage during the partially molten – transition zone. Moreover, existing theoretical melting models apply to continuous solid melting and cannot be applied to study melting of individual pellets. As fiber length influences the thermo-mechanical properties of the manufactured composites, it is crucial to understand why and how fibers break. The goal of this thesis is to identify the mechanisms of pellet melting and fiber breakage by tracking the motion and heat transfer of an individual pellet. In the first part of this thesis, flow of long discontinuous fiber pellets through a single screw extruder is modeled using discrete element method. Results indicate a translational-conveying motion in the first half of the screw and rotational-conveying motion in the second half. In the second part, a sequentially coupled heat transfer model is developed to capture the melting of a single pellet, occurring mainly through the thermal contacts with the heated screw and barrel surfaces. Partial melting, partial crystallization, and re-melting are captured using melting and crystallization kinetics of semi-crystalline polymers. Heat transfer results indicate that the pellets melt from the outside-in, with a molten shell and a solid core. Based on the average pellet degree of melting, the region of interest for ‘melting zone’ is identified. Finally, some common modes of pellet deformation are identified for closer study.</p><p dir="ltr">Once the common pellet deformation modes are identified, analytical models based on three-point bending loading condition are developed to model pellet deformation. For a partially molten pellet with molten shell and a softer core, temperature dependent properties are used to estimate pellet deflection. The surface fibers are studied closely to identify a fiber separation mechanism. For the separated fibers, a Weibull based strength distribution is used to develop a fiber attrition algorithm for varying end loads. Results indicate that fiber attrition starts as soon as the outer layer of the pellet melts and then continues until the end of the screw. Recommendations for validation experiments and future work are provided in the end.</p>

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<div>A combined experimental and computational study has been performed in order to understand the effect of interface chemical composition on the shock induced mechanical behavior of an energetic material (EM) system consisting of Hydroxyl-Terminated Polybutadiene (HTPB) binder and an oxidizer, Ammonium Perchlorate (AP), particle embedded in the binder. The current study focuses on the effect of interface chemical composition between the HTPB binder material and the AP particles on the high strain rate mechanical behavior. The HTPB-AP interface chemical composition was changed by adding cyanoethylated polyamine (HX-878 or Tepanol) as a binding agent. A power law viscoplastic constitutive model was fitted to nanoscale impact based experimental stress-strain-strain rate data in order to obtain the constitutive behavior of the HTPBAP interfaces, AP particle, and HTPB binder matrix. An in-situ mechanical Raman spectroscopy framework was used to analyze the effect of binding agent on cohesive separation properties of the HTPB-AP interfaces, AP particle, and HTPB binder matrix. In addition, a combined mechanical Raman spectroscopy and laser impact set up was used to study the effect of strain rate, as well as the interface chemical composition on the interface shock viscosity. Finally, high velocity strain rate impact simulations were performed using an explicit cohesive finite element method framework to predict the effect of strain rate, interface strength, interface friction, and interface shock viscosity on possible strain rate dependent temperature rises at high strain rates approaching shock velocities. </div><div><br></div><div>A modified stress equation was used in the cohesive finite element framework in order to include the effect of shock viscosity on the shock wave rise time and shock pressure during impact loading with strain rates corresponding to shock impact velocities. It is shown that increasing the interface shock viscosity, which can be altered by changing the interface chemical composition, increases the shock wave rise time at the analyzed interfaces. It is shown that the interface shock viscosity also plays an important role in determining the temperature increase within the microstructure. Interface shock viscosity leads to a decrease in the overall density of the possible hot-spots which is caused by the increase in dissipation at the shock front. This increase in shock dissipation is accompanied by a decrease in the both the maximum temperature, as well as the plastic dissipation energy, within the microstructure during shock loading.</div>

<div>Textile composites have been widely used due to the excellent mechanical performance and lower manufacturing costs, but the accurate prediction of the mechanical behaviors of textile composites is still very challenging due to the complexity of the microstructures and boundary conditions. Moreover, there is an unprecedented amount of design options of different textile composites. Therefore, a highly efficient yet accurate approach, which can predict the macroscopic structural performance considering different geometries and materials at subscales, is urgently needed for the structural design using textile composites.</div><div><br></div><div>Mechanics of structure genome (MSG) is used to perform multiscale modeling to predict various performances of textile composite materials and structures. A two-step approach is proposed based on the MSG solid model to compute the elastic properties of different two-dimensional (2D) and three-dimensional (3D) woven composites. The first step computes the effective properties of yarns at the microscale based on the fiber and matric properties. The effective properties of yarns and matrix are then used at the mesoscale to compute the properties of woven composites in the second step. The MSG plate and beam models are applied to thin and slender textile composites, which predict both the structural responses and local stress field. In addition, the MSG theory is extended to consider the pointwise temperature loads by modifying the variational statement of the Helmholtz free energy. Instead of using coefficients of thermal expansions (CTEs), the plate and beam thermal stress resultants derived from the MSG plate and beam models are used to capture the thermal-induced behaviors in thin and slender textile composite structures. Moreover, the MSG theory is developed to consider the viscoelastic behaviors of textile composites based on the quasi-elastic approach. Furthermore, a meso-micro scale coupled model is proposed to study the initial failure of textile composites based on the MSG models which avoids assuming a specific failure criterion for yarns. The MSG plate model uses plate stress resultants to describe the initial failure strength that can capture the stress gradient along the thickness in the thin-ply textile composites. The above developments of MSG theory are validated using high-fidelity 3D finite element analysis (FEA) or experimental data. The results show that MSG achieves the same accuracy of 3D FEA with a significantly improved efficiency.</div><div> </div><div>Taking advantage of the advanced machine learning model, a new yarn failure criterion is constructed based on a deep neural network (DNN) model. A series of microscale failure analysis based on the MSG solid model is performed to provide the training data for the DNN model. The DNN-based failure criterion as well as other traditional failure criteria are used in the mesoscale initial failure analysis of a plain woven composite. The results show that the DNN yarn failure criterion gives a better accuracy than the traditional failure criteria. In addition, the trained model can be used to perform other computational expensive simulations such as predicting the failure envelopes and the progressive failure analysis.</div><div> </div><div>Multiple software packages (i.e., texgen4sc and MSC.Patran/Nastran-SwiftComp GUI) are developed to incorporate the above developments of the MSG models. These software tools can be freely access and download through cdmHUB.org, which provide practical tools to facilitate the design and analysis of textile composite materials and structures.</div>

This diploma thesis deals with the issue of electroerosion machining of materials used in the aerospace industry. The introductory part presents the layout of aerospace materials and the methods by which they can be machined. The next chapter describes the basic principles of EDM. The experimental part is focused on electroerosive cutting of NIMONIC 263 alloy. The greatest emphasis was placed on determining the influence of the cutting wire material on the rate of contamination of the surface layer of the workpiece. The quality of the machined surface was assessed depending on the used wire and the selected machining parameters.

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