<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>

<div> <div> <div> <p>Composite materials have become widely used in engineering applications, in order to reduce the overall weight of structures while retaining their required strength. Due to their light weight, relatively high stiffness properties, and formability into complex shapes, discontinuous fiber composites are advantageous for producing small and medium size components. However, qualifying their mechanical properties can be expensive, and therefore there is a need to improve predictive capabilities to help reduce the overall cost of large scale testing. To address this challenge, a composite material consisting of discontinuous glass fibers in a polypropylene matrix is studied at the microstructural level through coupled experiments and simulations, in order to uncover the mechanisms that cause microvoids to initiate and progress, as well as certain fiber breakage events to occur, during macroscopic tension. Specifically, this work coupled in-situ X-ray micro computed tomography (μ-CT) experiments with a finite element simulation of the exact microstructure to enable a 3D study that tracked damage initiation and propagation, and computed the local stresses and strains in the microstructure. In order to have a comprehensive 3D understanding of the evolution of the microstructure, high fidelity characterization procedures were developed and applied to the μ-CT images in order to understand the exact morphology of the microstructure. To aid in this process, ModLayer - an interactive image processing tool - was created as a MATLAB executable, and the 3D microstructural feature detection techniques were compared to traditional destructive optical microscopy techniques. For damage initiation, this work showed how high hydrostatic stresses in the matrix can be used as a metric to explain and predict the exact locations of microvoid nucleation within the composite’s microstructure. From a damage propagation standpoint, matrix cracking - a mechanism that has been notably difficult to predict because of its apparent stochastic nature - was studied during damage propagation. The analysis revealed the role of shear stress in fiber mediated flat matrix cracking, and the role of hydrostatic stress in fiber-avoidance conoidal matrix cracking. Overall, a sub-fiber simulation and an in-situ experimental analysis provided the microstructural physical phenomena that govern certain damage initiation and progression mechanisms, further enabling the strength and failure predictions of short fiber thermoplastic composites. </p></div></div></div>

<p> The Selective Laser Melting (SLM) process is a manufacturing technique that facilitates the  production of metallic parts with complex geometries and reduces both materials waste and lead  time. The high tunability of the process parameters in SLM allows the design of the as-built part’s  characteristics, such as controlled microstructure formation, residual stresses, presence of pores,  and lack of fusion. The main parameter in the SLM process that influences these parts’  characteristics is the transient temperature field resulting from the laser-matter interaction.  Nanosecond pulsed lasers in SLM have the advantage of enabling rapid and localized heating and  cooling that make the formation of ultrafine grains possible. This work shows how different pulse  durations can change the near-surface microstructure and overall mechanical properties of metallic  parts. The nanosecond pulses can melt and resolidify aluminum parts’ near-surface region to form nanograined gradient structures with yield strengths as high as 250.8 MPa and indentation  strengths as high as 725 MPa, which are comparable to some steel's mechanical properties. Knowing that the nanosecond pulsed lasers cause microstructure refinement for high-purity metals,  the microstructure variations effects were also investigated for the cast iron alloy. Cast iron was  used alone and mixed with born or boron nitride powders to induce the precipitation of  strengthening phases only enabled under high cooling rates. Although producing parts with  superior mechanical properties and controlling the precipitation of strengthening phases, the SLM  process with nanosecond pulsed lasers is still accompanied by defects formation, mainly explained  by the large thermal gradients, keyhole effect, reduced melt pool depth, and rapid cooling rates.  Ideally, a smooth heating rate able to sinter powder grains, facilitating the heat flow through the  heat-affected zone, followed by a sharper heating rate that generates a fully molten region, but  minimizes ablation at the same time are targeted to reduce the porosity and lack of fusion. Then, a  sharp cooling rate that can increase the nucleation rate, consequently refining the final  microstructure is targeted in the production of strong materials in SLM with pulsed lasers. This  work is the pioneer in controlling the transient temperature field during the heating and cooling  stages in pulsed laser processing. The temperature field control capability by shaping a nanosecond  laser pulse in the time domain affecting defects formation, residual strains, and microstructure was  achieved, opening a wide research niche in the additive manufacturing field.  </p>

<p dir="ltr">The ability to change the shape of aerodynamic surfaces is necessary for modern aircraft, both to provide control while performing maneuvers and to meet the conflicting requirements of various flight conditions such as takeoff/landing and level cruise. These shape changes have traditionally been accomplished through the use of various mechanical devices actuating discrete aerodynamic surfaces, for example ailerons and flaps. Such control surfaces and high-lift devices are generally limited to their specific functionality and create surface discontinuities which increase drag and aircraft noise. Broadly speaking, the design and study of morphing wings typically seeks to improve the performance of aircraft by completing one or more of the following objectives: reducing the drag from discontinuities in the aerodynamic surface of the wing by closing hinge gaps and creating smooth transitions, reducing weight and/or mechanical complexity by integrating mechanism functionality into compliant structures that can bear aerodynamic load and maintain shape adaptability, and providing unique or optimal functionality to the aircraft by allowing it to adjust its aerodynamic shape to meet the needs of various flight conditions with conflicting objectives and constraints.</p><p dir="ltr">The concepts proposed in this work represent potential methods for addressing these objectives. In each case, a compliant structure with multiple stable states is embedded into the wing. Exploiting elastic structural instabilities in this way provides the advantage that a structure can be made relatively stiff while still allowing for large deformations. In the first case, the development of a 3D-printable rib with an embedded bistable element creates a truss-like 2D structure that allows for modification of the airfoil. Switching states of the elements changes their local stiffness, and therefore the global stiffness of the system. By optimizing the topology of the airfoil, a passive deflection of the trailing edge can be leveraged to change the camber to leverage different lift characteristics for varying operating conditions. Primary work on this concept has included the construction of multiple experimental demonstrators for validating the concept through static structural and wind tunnel testing. In the second case, a cellular material has been investigated incorporating a bistable unit cell with a sinusoidal arch. This provides a metamaterial that can exhibit large, reversible deformations with as many stable configurations as there are rows in the honeycomb. This metamaterial is incorporated into a beam-like structure which can serve as a spar for a spanwise morphing wing, providing sufficient bending and torsional stiffness, particularly when utilized at the wing tip. Extending and retracting the wing by switching the states of the honeycomb rows provides a significant change to the wing’s induced drag and wing loading, making it ideal for optimal flight in both loitering and cruising conditions. Contributions to this concept have been the development and characterization of the bistable unit cell and honeycomb, as well as the design and analysis of the metabeam and morphing wing concept.</p>

No description available.

<p dir="ltr">Since before the Apollo era, the rocket propulsion sector has been a key player in developing standards of cleanliness and compatibility when designing, building, and operating systems with toxic propellants. The advent of hypergols and the widespread use of propellants like N₂O₄, Mixed Oxides of Nitrogen (MON), and hydrazine have forced new standards to be developed to meet the ever-growing need for safety when working with dangerous substances. These systems have only continued to grow more complex and many propellant combinations remain toxic and corrosive to most substances as the industry seeks the optimal methods for deriving the most efficient, highest performing, and generally more capable. ASTM International and other standards organizations carry on documenting standards for cleaning and passivation to ensure safe use today to meet the needs of the ever-expanding propulsion industry.</p><p dir="ltr">This thesis aims to determine the feasibility of using Raman spectroscopy as a method of characterizing interactions between metals and propellants. First, a background of knowledge regarding the spectroscopic method, propellants, and industry practices was researched and current areas of possible application were identified. The passivation and propellant storage phases of system lifecycles were determined to be the scope and target for experimentation. A multilevel passivation study consisting of exposing three metal types to different concentrations of nitric acid for various durations was conducted to begin developing a greater understanding of the applicability of and the techniques required to make Raman spectroscopy work as a complement to the ASTM passivation verification tests. Lessons learned from this and a short-duration compatibility study with MON and similar metal samples were documented and will be used for a larger scale and longer duration compatibility study in conjunction with NASA White Sands Test Facility (WSTF). The buildup of safe and adequate facilities for such a study was undertaken, completed, and documented in this work.</p><p dir="ltr">The results of testing in this thesis suggest the promising and desirable non-destructive and minimally invasive features of Raman spectroscopy have the potential to be used extensively in the propulsion sector. Suggestions for developing key techniques and methods for this application are developed and outlined as they were learned throughout the study's conduction.</p>

<div> <p>Fiber reinforced polymer composites are widely used in manufacturing advanced light weight structures for the aerospace, automotive, and energy sectors owing to their superior stiffness and strength. With the increasing use of composites, there is an increasing need to monitor the health of these structures during their lifetime. Currently, health monitoring in filament wound composites is facilitated by embedding piezoelectrics and optical fibers in the composite during the manufacturing process. However, the incorporation of these sensing elements introduces sites of stress concentration which could lead to progressive damage accumulation. In addition to introducing weak spots in the structure, they also make the manufacturing procedure difficult. </p> <p> </p> <p>Alternatively, nanofiller modification of the matrix imparts conductivity which can be leveraged for real time health monitoring with fewer changes to the manufacturing method. Well dispersed nanofillers act as an integrated sensing network. Damage or strain severs the well-connected nanofiller network thereby causing a local change in conductivity. The self-sensing capabilities of these modified composites can be combined with low cost, minimally invasive imaging modalities such as electrical impedance tomography (EIT) for damage detection. To date, however, EIT has exclusively been used for damage detection in planar coupons. These simple plate-like structures are not representative of real-world complex geometries. This thesis advances the state of the art in conductivity-based structural health monitoring (SHM) and nondestructive evaluation (NDE) by addressing this limitation of EIT. The current study will look into damage detection of a non-planar multiply connected domain – a filament-wound glass fiber/epoxy tube modified by carbon black (CB) filler. The results show that EIT is able to detect through holes as small as 7.94 mm in a tube with length-to-diameter ratio of 132.4 mm-to-66.2 mm (aspect ratio of 2:1). Further, the sensitivity of EIT to damage improved with decreasing tube aspect ratio. EIT was also successful in detecting sub-surface damage induced by low velocity impacts. These results indicate that EIT has much greater potential for composite SHM and NDE than prevailing work limited to planar geometries suggest.</p> </div> <br>

Laser cladding is a more recent approach to repair of aviation components within a damage tolerant framework, with its ability to restore not simply the geometric shape but the static and fatigue strength as well. This research analysed the fatigue performance of Ti-6Al-4V that has undergone a laser clad repair, comparing baseline specimens with laser clad repaired, and repaired and heat treated specimens. First an understanding of the microstructure was achieved by use of BSE imagery of the substrate, clad repaired region and post heat treated regions. The substrate of the material was identified with large grains which compared to a repaired clad region with a much finer grain structure that did not change with heat treatment. Next, performance of the specimens under tensile fatigue loading was conducted, with the clad specimens experiencing unexpectedly high fatigue performance when compared to baseline samples; the post heat treated specimen lasting significantly longer than all other specimens. It is theorised that the clad may have contributed to an increase in fatigue resilience due to its fine microstructure, when compared to the softer, more coarse substrate. The heat treatment is likely to have relaxed any residual stresses in the specimens leading to a reduction in any potential undesirable stresses, without impacting the microstructure. Residual stress analysis using EDD was unproductive due to the unexpected coarse microstructure and did not provide meaningful results. Fractography using the marker-band technique was explored with some success, proving a feesable method for measuring fatigue crack growth through a specimen post failure. Unfortunately fatigue crack growth throughout the entire fatigue life was not possible due to the tortuous fracture surface and potentially due to the fine micro-structure of the clad, resulting in interrupted marker-band formation. Future research shall expand on this work with a greater focus on residual stress analysis and its impact on fatigue.

Quick and accurate predictions of equivalent properties for thin-walled composite structures are required in the preliminary design process. Existing literature provides analytical solutions to some structures but is limited to particular cases. No unified approach exists to tackle homogenization of thin-walled structures such as beams, plates, or three-dimensional structures using the thin-walled approximation. In this work, a unified approach is proposed to obtain equivalent properties for beams, plates, and three-dimensional structures for thin-walled composite structures using mechanics of structure genome. The adopted homogenization technique interprets the unit cell associated with the composite structures as an assembly of plates, and the overall strain energy density of the unit cell as a summation of the plate strain energies of these individual plates. The variational asymptotic method is then applied to drop all higher-order terms and the remaining energy is minimized with respect to the unknown fluctuating functions. This has been done by discretizing the two-dimensional unit cell into one-dimensional frame elements in a finite element description. This allows the handling of structures with different levels of complexities and internal geometry within a general framework. Comparisons have been made with other works to show the advantages which the proposed model offers over other methods.

<div>Self-sensing materials are engineered to transduce mechanical effects like deformations and damages into detectable electrical changes. As such, they have received immense research attention in areas including aerospace, civil infrastructure, robotic skin, and biomedical devices. In structural health monitoring (SHM) and nondestructive evaluation (NDE) applications, damages in the material cause breakage in the conductive filler networks, resulting in changes in the material's conductivity. Most SHM and NDE applications of self-sensing materials have used direct current (DC) measurements. DC-based methods have shown advantages with regard to sensitivity to microscale damages compared to other SHM methods. Comparatively, alternating current (AC) measurement techniques have shown potential for improvement over existent DC methods. For example, using AC in conjunction with self-sensing materials has potential for benefits such as greater data density, higher sensitivity through electrodynamics effects (e.g., coupling the material with resonant circuitry), and lower power requirements. Despite these potential advantages, AC techniques have been vastly understudied compared to DC techniques. </div><div><br></div><div>To overcome this gap in the state of the art, this thesis presents two contributions: First, an experimental study is conducted to elucidate the effect of different damage types, numbers, and sizes on AC transport in a representative self-sensing composite. And second, experimental data is used to inform a computational study on using AC methods to improve damage detection via electrical impedance tomography (EIT) – a conductivity imaging modality commonly paired with self-sensing materials for damage localization. For the first contribution, uniaxial glass fiber specimens containing 0.75 wt.% of carbon nanofiber (CNF) are induced with five types of damage (varying the number of holes, size of holes, number of notches, size of notches, and number of impacts). Impedance magnitude and phase angle were measured after each permutation of damage to study the effect of the new damage on AC transport. It was observed that permutations of hole and notch damages show clear trends of increasing impedance magnitude with the increasing damage, particularly at low frequencies. These damages had little-to-no effect on phase angle, however. Increasing numbers of impacts on the specimens did not show any discernable trend in either impedance magnitude or phase angle, except at high frequencies. This shows that different AC frequencies can be more or less useful for finding particular damage types.</div><div><br></div><div>Regarding the second contribution, AC methods were also explored to improve damage detection in self-sensing materials via EIT. More specifically, the EIT technique could benefit from developing a baseline-free (i.e., not requiring a ‘healthy’ reference) formulations enabled by frequency-difference (fd) imaging. For this, AC conductivity measurements ranging from 100 Hz to 10 MHz were collected from various weight fractions of CNF-modified glass fiber/epoxy laminates. This experimental data was used to inform fdEIT simulations. In the fdEIT simulations, damage was simulated as a simple through-hole. Simulations used 16 electrodes with four equally spaced electrodes on each side of the domain. The EIT forward problem was used to predict voltage-current response on the damaged mesh, and a fdEIT inverse problem was formulated to reconstructs the damage state on an undamaged mesh. The reconstruction images showed the simulated damage clearly. Based on this preliminary study, this research shows that fdEIT does have potential to eliminate the need for a healthy baseline in NDE applications, which can potentially help proliferate the use of this technique in practice.</div>