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Over the past century, a series of large (> 6.5) magnitude earthquakes have struck along the North Anatolian Fault Zone (NAFZ) in Turkey in a roughly East to West progression. The progression of this earthquake sequence began in 1939 with the Ms 8.0 earthquake near the town of Erzincan and continued westward, with two of the most recent ruptures occurring near the Sea of Marmara in 1999. The sequential nature of ruptures along this fault zone implies that there is a connection between the location of the previous rupture and that of the future rupture zones. This study focuses on understanding how previous rupture events and tectonic influences affect the stress regime of the NAFZ and how these stress changes affect the probability of future rupture along any unbroken segments of the fault zone using a two dimensional finite element modeling program. In this study, stress changes due to an earthquake are estimated using the slip history of the event, estimations of rock and fault properties along the fault zone (elastic parameters), and the far-field tectonic influence due to plate motions. Stress changes are not measured directly. The stress regime is then used to calculate the probability of rupture along another segment of the fault zone. This study found that when improper estimates of rock properties are utilized, the stress changes may be under- or over- estimated by as much as 350% or more. Because these calculated stress changes are used in probability calculations, the estimates of probability can be off by as much as 20%. A two dimensional model was built to reflect the interpreted geophysical and geological variations in elastic parameters and the 1939 through 1999 rupture sequence was modeled. The far-field tectonic influence due to plate motions contributed between 1 and 4 bars of stress to the unbroken segments of the fault zone while earthquake events transferred up to 50 bars of stress to the adjacent portions of the fault zone. The 1999 rupture events near Izmit and Düzce have increased the probability of rupture during the next ten years along faults in the Marmara Sea to 38% while decreasing the probability of rupture along the faults near the city of Bursa by ~6%. Large amounts of strain accumulation are interpreted along faults in the Marmara Sea, further compounding the case for a large rupture event occurring in that area in the future. / Ph. D.

Digital models are essential to quantifying the behavior of structural systems. In many cases, the creation of these models involves manual measurements taken in the field, followed by a manual creation of this model using these measurements. Both of these steps are time consuming and prohibitively expensive, leading to a lack of utilization of accurate models. We propose a framework built on the processing of 3D laser scanning data to partially automate the creation of these models. We focus on steel structures, as they represent a gap in current research into this field. Previous research has focused on segmentation of the point cloud data in order to extract relevant geometries. These approaches cannot easily be extended to steel structures, so we propose a novel method of processing this data with the goal of creating a full finite element model from the information extracted. Our approach sidesteps the need for segmentation by directly extracting the centerlines of structural elements. We begin by taking "slices" of the point cloud in the three principal directions. Each of these slices is flattened into an image, which allows us to take advantage of powerful image processing techniques. Within these images we use 2d convolution as a template match to isolate structural cross sections. This gives us the centroids of cross sections in the image space, which we can map back to the point cloud space as points along the centerline of the element. By fitting lines in 3d space to these points, we can determine the equations for the centerline of each element. This information could be easily passed into a finite element modeling software where the cross sections are manually defined for each line element. / Modern buildings require a digital counterpart to the physical structure for accurate analysis. Historically, these digital counterparts would be created by hand using the measurements that the building was intended to be built to. Often this is not accurate enough and the as-built system must be measured on site to capture deviations from the original plans. In these cases, a large amount of time must be invested to send personnel out into the field and take large amounts of measurements of the structure. Additionally, these "hand measurements" are prone to user error. We propose a novel method of gathering these field measurements quickly and accurately by using a technique called "laser scanning". These laser scans essentially take a 3D snapshot of the site, which contains all the geometric information of visible elements. While it is difficult to locate items such as steel beams in the 3D data, the cross sections of these structural elements are easily defined in 2D. Our method involves taking 2D slices of this 3D scan which allows us to locate the cross sections of the structural members by searching for template cross-sectional shapes. Once the cross sections have been isolated, their centers can be mapped back from the 2D slice to the 3D space as points along the centerlines of the structural elements. These centerlines represent one of the most time consuming requirements to building digital models of modern buildings, so this method could drastically reduce the total modeling time required by automating this particular step.

To make sound decisions about the remaining life of a structure, the precise calculation of the prestress losses is very important. In post-tensioned structures, the prestress losses due to creep and shrinkage can cause serviceability issues and can reduce flexural capacity. The Varina-Enon Bridge is a cable-stayed, precast, segmental, post-tensioned box girder bridge located in Richmond, Virginia. Observation of flexural cracks in the bridge by inspectors promoted a study regarding long-term prestress losses in the structure. For understanding and sustaining the structure throughout its remaining service life, accurately quantifying prestress losses is important. Two approaches are used to predict long-term prestress losses on the Varina-Enon Bridge. The first approach involves a finite element computer model of the bridge which run a timedependent staged-construction analysis to obtain predicted prestress losses using the CEB-FIP '90 code expressions for creep and shrinkage. The second approach involves the compilation of data from instrumentation mounted in the bridge to back calculate the effective prestress force. The analysis using the computer model predicted the prestress losses as 44.6 ksi in Span 5, 47.9 ksi in Span 6, 45.3 ksi in Span 9, and 45.9 ksi in Span 11. The prestress losses estimated from field data were 50.0 ksi in Span 5, 48.0 ksi in Span 6, 46.7 ksi in Span 9, and 49.1 ksi in Span 11. It can be seen that relative to the results of field data estimations, the finite element analyses underestimated prestress loss, but given the degree of uncertainty in each form of estimation, the results are considered to fit well. / Master of Science / In order to apply a precompression force to concrete structures, post-tensioned concrete employs stressed steel strands. To construct lighter, stiffer structures, this popular building technology can be used. The steel strands undergo a reduction in force known as prestress losses over time. To make good decisions about the remaining life of a structure, the precise calculation of the prestress losses is very important. The Varina-Enon Bridge is a post-tensioned concrete box-girder bridge in Richmond Virginia. In July of 2012, observation of flexural cracks in the bridge by the inspectors promoted a study regarding long-term prestress losses in the structure. Two techniques are used to predict long-term prestress losses for this bridge. A computer model of the bridge is used in the first method to calculate losses using the design code. In order to measure prestress losses, the second technique used data from sensors mounted on the bridge. It was found that the estimation of losses closely matched those predicted at the time of the bridge construction and the computer model results. Based on this the final conclusion is made that the prestress loss in the Varina-Enon Bridge is not significantly more than expected.

Finite element modeling has become a powerful tool in orthopedic biomechanics, allowing simulations with complex geometries. Current fatigue behavior simulations are unable to accurately predict the cycles to failure, creep, and damage or modulus loss even when applied to a bending model. It is thought that the inhomogeneity of the models may be the source of the problem. It has also been suggested that the volume size of the element will affect the fatigue behavior. This is called a stressed volume effect. In this thesis non-homogeneous finite element models were used to examine the effects of "sizing factors" on damage laws in fatigue simulations. Non-homogeneous finite element models were created from micro computed tomography (CT) images of dumbbell shaped fatigue samples. An automatic voxel meshing technique was used which converted the CT data directly into mesh geometry and material properties. My results showed that including these sizing factors improved the accuracy of the fatigue simulations on the non-homogeneous models. Using the Nelder-Mead optimization routine, I optimized the sizing factors for a group of 5 models. When these optimized sizing factors were applied to other models they improved the accuracy of the simulations but not as much as for the original models, but they improved the results more than with no sizing factors at all. I found that in our fatigue simulations we could account for the effects of stressed volume and inhomogeneity by including sizing factors in the life and damaging laws. / Master of Science

The present work describes the finite element (FE) modeling and dynamic response of lightweight, deployable shelters (tent) to large external blast loads. Flexible shelters have been used as temporary storage places for housing equipments, vehicles etc. TEMPER Tents, Small Shelter System have been widely used by Air Force and Army, for various field applications. These shelters have pressurized Collective Protection System (CPS), liner, fitted to the frame structure, which can provide protection against explosives and other harmful agents. Presently, these shelter systems are being tested for the force protection standards against the explosions like air-blast. In the field tests carried out by Air Force Research Laboratory, it was revealed that the liner fitted inside the tent was damaged due to the air blast explosion at some distant from the structure, with major damage being on the back side of the tent. The damage comprised of tearing of liner and separation of zip seals. To investigate the failure, a computational approach, due to its simplicity and ability to solve the complex problems, is used. The response of any structural form to dynamic loading condition is very difficult to predict due to its dependence on multiple factors like the duration of the loading, peak load, shape of the pulse, the impulse energy, boundary conditions and material properties etc. And dynamic analysis of shell structures pose even much greater challenge. Obtaining solution analytically presents a very difficult preposition when nonlinearity is considered. Therefore, the numerical approach is sought which provide simplicity and comparable accuracy. A 3D finite element model has been developed, consisting of fabric skin supported over the frames based on two approaches. ANSYS has been used for obtaining the dynamic response of shelter against the blast loads. In the first approach, the shell is considered as a membrane away from its boundaries, in which the stress couple is neglected in its interior region. In the second approach, stress coupling is neglected over the whole region. Three models were developed using Shell 63, Shell 181 and Shell 41. Shell 63 element supports both the membrane only and membrane-bending combined options and include stress stiffening and large deflection capabilities. Shell 181 include all these options as Shell 63 does and also, accounts for the follower loads. Shell 41 is a membrane element and does not include any bending stiffness. This element also include stress stiffening and large deflection capabilities. A nonlinear static analysis is performed for a simple plate model using the elements, Shell 41 and Shell 63. The membrane dominated behavior is observed for the shell model as the pressure load is increased. It is also observed that the higher value of Young's modulus (E) increases the stresses significantly. Transient analysis is a method of determining the structural response due to time dependent loading conditions. The full method has been used for performing the nonlinear transient analysis. Its more expensive in terms of computation involved but it takes into account all types of nonlinearities such as plasticity, large deflection and large strain etc. Implicit approach has been used where Newmark method along with the Newton-Raphson method has been used for the nonlinear analysis. Dynamic response comprising of displacement-time history and dynamic stresses has been obtained. From the displacement response, it is observed that the first movement of the back wall is out of the tent in contrast to the other sides whose first movement is into the tent. Dynamic stresses showed fluctuations in the region when the blast is acting on the structure and in the initial free vibration zone. A parametric study is performed to provide insight into the design criteria. It is observed that the mass could be an effective means of reducing the peak responses. As the value of the Young's Modulus (E) is increased, the peak displacements are reduced resulting from the increase in stiffness. The increased stiffness lead to reduced transmitted peak pressure and reduced value of maximum strain. But a disproportionate increase lead to higher stresses which could result in failure. Therefore, a high modulus value should be avoided. / Master of Science

Both the identification and validation of expedient portland cement concrete (PCC) repair technologies have been the focus of the pavements research community for decades due to ever decreasing construction timelines. Precast concrete panel technology offers a potential repair alternative to conventional cast-in-place PCC because the panel is fully cured and has gained full strength prior to its use. This repaired surface may be trafficked immediately, thus eliminating the need for long curing durations required for conventional PCC. The literature reveals a number of precast PCC panel investigations in the past 50 years; however precast technology has only recently gained acceptance and increased use in the US for highway pavements. Furthermore, only limited information regarding performance of airfield applications is available. Following a review of the available technologies, an existing panel prototype was redesigned to allow for both single- and multiple-panel repairs. A series of various sized repairs were conducted in a full-scale airfield PCC test section. Results of accelerated testing indicated that precast panels were suitable for airfield repairs, withstanding between 5,000 and 10,000 passes of C-17 aircraft traffic prior to failure. Failure was due to spalling of the transverse doweled joints. The load transfer characteristics of the transverse joint were studied to determine if the joint load test could be used to predict failure. Results showed that the load transfer efficiency calculations from the joint load test data were not useful for predicting failure; however differential deflections could possibly be applied. Additionally, the practice of filling the joints with rapid-setting grout may have resulted in higher measurements of load transfer efficiency. To determine the stresses generated in the doweled joint, three-dimensional finite element analyses were conducted. Results indicated that the dowel diameter should be increased to reduce stresses and to improve repair performance. Finally, the precast repair technology was compared to other expedient repair techniques in terms of repair speed, performance, and cost. Compared to other methods, the precast panel repair alternative provided similar return-to-service timelines and traffic performance at a slightly higher cost. Costs can be minimized through modification to the panel design and by fabricating panels in a precast facility. Modifications to the system design and placement procedures are also recommended to improve the field performance of the panels. / Ph. D.

This dissertation is concerned with the active control of local sound fields, as applied to an active headrest system. Using loudspeakers and microphones, an active headrest is capable of attenuating ambient noise and providing a comfortable acoustic environment for an occupant. A finite element (FE) model of an active headrest is built and analyzed such that the expected noise reduction levels could be quantified for various geometries as well as primary sound field conditions. Both plane wave and diffuse primary sound fields are considered and it is shown that the performance deteriorates for diffuse sound fields. It is then demonstrated that virtual sensing can greatly improve the spatial extent of the quiet zones as well as the attenuation levels. A prototype of the active headrest was constructed, with characteristics similar to those of the FE model, and tested in both anechoic and reverberant sound fields. Multichannel feedforward and feedback control architectures are implemented in real-time and it is shown that adaptive feedback systems are capable of attenuating band-limited disturbances. The spatial attenuation pattern surrounding the head is also measured by shifting the head to various positions and measuring the attenuation at the ears. Two virtual sensing techniques are compared in both feedback and feedforward architectures. The virtual microphone arrangement, which assumes that the primary sound field is equivalent at the physical and virtual locations, results in the best performance when used in a feedback system attenuating broadband disturbances. The remote microphone technique, which accounts for the transfer response between the physical and virtual locations, offers the best performance for tonal primary sound fields. In broadband sound fields, a causal relationship rarely exists between the physical and virtual microphones, resulting in poor performance. / PHD / Excessive noise and vibration levels in aircraft, rotorcraft, launch vehicles, and other aerospace vehicles may create harsh acoustic environments inside the vehicle. In some extreme cases, military applications being a prime example, hearing damage can occur due to the high noise levels associated with certain vehicles. Noise canceling headsets have been proven an effective solution to this problem, although in certain instances their use may not be safe or feasible. In this work, an active noise canceling headrest, or active headrest, is explored as an alternative solution to noise canceling headphones/headsets. An active headrest uses microphones and loudspeakers, typically located non-intrusively behind the head of the seat occupant, to reduce the ambient noise levels in the vicinity of the head and create a comfortable acoustic environment. A thorough investigation of the viability of such a system in a practical vehicle is assessed through the use of theoretical analysis, finite element modeling, and real-time performance experiments. Performance predictions generated using the finite element model were verified by performing real-time experiments, thus providing a level of confidence in additional predictions for alternative headrest geometries and configurations. Factors such as loudspeaker and microphone placement, head movements away from the nominal position, primary acoustic field characteristics, and choice of control strategy are all found to heavily influence the performance of an active headrest. Real-time experiments were performed in anechoic and reverberant sound fields and it is found that the noise canceling capability of the active headrest worsens in reverberant sound fields as compared to free field conditions.

The excellent in-plane strength and stiffness to-weight ratios, as well as the ease of manufacturing have made the carbon fiber reinforced polymer composites (CFRPs) suitable structural materials for variety of applications such as aerospace, automotive, civil, sporting goods, etc. Despite the outstanding performance of the CFRPs along their fibers direction (on-axis), they lack sufficient strength and performance in the out-of-plane and off-axis directions. Various chemical and mechanical methods were reported to enhance the CFRPs' out-of-plane performance. However, there are two major drawbacks for utilizing these approaches: first, most of these methods induce damage to the carbon fibers and, therefore, deteriorate the in-plane mechanical properties of the entire CFRP, and second, the methods with minimal deteriorating effects on the in-plane mechanical performance have their own limitations resulting in very confined mechanical performance improvements. These methods include integrating nano-sized reinforcements into the CFRPs' structure to form a hybrid or hierarchical CFRPs. In lieu to all the aforementioned approaches, a relatively novel method, referred to as graphitic structures by design (GSD), has been proposed. The GSD is capable of grafting carbon nanotubes (CNTs) onto the carbon fibers surfaces, providing high concentration of CNTs where they are most needed, i.e. the immediate fiber/matrix interface, and in-between the different laminae of a CFRP. This method shows promising improvements in the in-plane and out-of-plane performance of CFRPs. Zinc oxide (ZnO) nanorods are other nano-sized reinforcing structures which can hybridize the CFRPs via their radially growth on the surface of carbon fibers. Among all the reported methods for synthesizing ZnO nanorods, hydrothermal technique is the most straightforward and least destructive route to grow ZnO nanorods over carbon fibers. In this dissertation, the GSD-CNTs growth method and the hydrothermal growth of ZnO nanorods have been utilized to fabricate hybrid CFRPs. The effect of different ZnO nanorods growth morphologies, e.g. size distribution and alignment, on the in-plane tensile performance and vibration attenuation capabilities of the hybrid CFRPs are investigated via quasi-static tension and dynamical mechanical analysis (DMA) tests, respectively. As a result, the in-plane tensile strength of the hybrid CFRPs were improved by 18% for the composite based on randomly oriented ZnO nanorods over the carbon fibers. The loss tangent of the CFRPs, which indicates the damping capability, increased by 28% and 19% via radially and randomly grown ZnO nanorods, respectively. While there are several studies detailing the effects of dispersed nanofillers on the fracture toughness of FRPs, currently, there are no literature detailing the effect of surface GSD grown CNTs and ZnO nanowire -on carbon fiber- on the fracture toughness of these hybrid composites. This dissertation probes the effects of surface grown nano-sized reinforcements on the fracture toughness via double cantilever beam (DCB) tests on hybrid ZnO nanorod or CNT grafted CFRPs. Results show that the surface grown CNTs enhanced the Mode I interlaminar fracture toughness (GIc) of the CFRPs by 22% and 32%, via uniform and patterned growth morphologies, respectively, over the reference composite based on untreated carbon fiber fabrics. The dissertation also explains the basis of the improvements of the fracture toughness via finite element method (FEM). In particular, FEM was employed to simulate the interlaminar crack growth behavior of the hybrid CFRPs under Mode I crack opening loading conditions embodied by the DCB tests. These simulations revealed that the hybrid CFRP based on fibers with uniform surface grown MWCNTs exhibited 55% higher interlaminar strength compared to the reference CFRPs. Moreover, via patterned growth of MWCNTs, the ultimate crack opening resistance of the CFRPs improved by 20%. To mimic the experimental behavior of the various CFRPs, a new methodology has been utilized to accurately simulate the unstable crack growth nature of CFRPs. Several investigations reported the effects of adding nanomaterials-including CNTs- as a filler phase inside the matrix material, on the impact energy absorption of the hybrid FRPs. However, the impact mitigation performance of CFRPs based on ZnO nanorod grafted carbon fibers has not been reported. The dynamic out-of-plane energy dissipation capabilities of different hybrid composites were investigated utilizing high velocity (~90 m/s) impact tests. Comparing the results of the hybrid MWCNT/ZnO nanorod/CFRP with those of reference CFRP, 21% and 4% improvements were observed in impact energy absorption and tensile strain to failure of the CFRPs, respectively. In addition to elevated stiffness and strength, CFRPs should possess enough tolerance not only to monotonic loadings, but also to cyclic loadings to be qualified as alternatives to traditional structural metal alloys. Therefore, the fatigue life of CFRPs is of much interest. Despite the promising potential of incorporating nano-sized reinforcements into the CFRPs structure, not many studies reported on the fatigue behavior of hybrid CFRPs so far. In particular, there are no reported investigations to the effect of surface grown CNTs on the fatigue behavior of the hybrid CFRPs, due to fact that almost all the CNT growth techniques (except for the GSD method) deteriorated the in-plane performance of the hybrid CFRPs. The hybrid ZnO nanorod grafted CFRPs have not been investigated under fatigue loading as well. In this dissertation, different hybrid CFRPs were tested under tension-tension fatigue to reveal the effects of the different nano-reinforcements growth on the fatigue behavior of the CFRPs. A remarkable fatigue damage tolerance was observed for the CFRPs based on uniform and patterned grown CNT fibers. Almost two decades of fatigue life extension was achieved for CFRPs based on surface grown MWCNTs. / Ph. D. / Carbon fiber reinforced polymer composites (CFRPs) are light-weight materials with excellent strength and stiffness along the direction of the fibers. These great mechanical properties have made CFRPs suitable structural materials for variety of applications such as aerospace, automotive, civil, sporting goods, etc. Despite the outstanding performance of the CFRPs along their fibers direction (on-axis), they lack sufficient strength and performance in the out-of-plane and off-axis directions. Various chemical and mechanical methods were reported to enhance the CFRPs’ out-of-plane performance. However, there are two major drawbacks for utilizing these approaches: first, most of these methods induce damage to the carbon fibers and, therefore, deteriorate the in-plane mechanical properties of the entire CFRP, and second, the methods with minimal deteriorating effects on the in-plane mechanical performance have their own limitations resulting in very confined mechanical performance improvements. These methods include integrating nano-sized reinforcements into the CFRPs’ structure to form a hybrid or hierarchical CFRPs. In lieu to all the aforementioned approaches, a relatively novel method, referred to as graphitic structures by design (GSD), has been proposed. The GSD is capable of grafting carbon nanotubes (CNTs) onto the carbon fibers surfaces, providing high concentration of CNTs where they are most needed, i.e. the immediate fiber/matrix interface, and in-between the different layers of a CFRP. This method shows promising improvements in the in-plane and out-of-plane performance of CFRPs. Zinc oxide (ZnO) nanorods are other nano-sized reinforcing structures which can hybridize the CFRPs via their radially growth on the surface of carbon fibers. Among all the reported methods for synthesizing ZnO nanorods, hydrothermal technique is the most straightforward and least destructive route to grow ZnO nanorods over carbon fibers. In this dissertation, the GSD-CNTs growth method and the hydrothermal growth of ZnO nanorods have been utilized to fabricate hybrid CFRPs. The effect of different ZnO nanorods growth morphologies, e.g. size distribution and alignment, on the in-plane tensile performance and vibration damping capabilities of the hybrid CFRPs are investigated via tension and dynamical mechanical analysis (DMA) tests, respectively. As a result, the in-plane tensile strength of the hybrid CFRPs were improved by 18% for the composite based on randomly oriented ZnO nanorods over the carbon fibers. The loss tangent of the CFRPs, which indicates the damping capability, increased by 28% and 19% via radially and randomly grown ZnO nanorods, respectively. Fracture toughness is a measure for the capability of a material to withstand a load in the presence of damage (i.e. crack) in the material’s structure. While there are several studies detailing the effects of dispersed nanofillers on the fracture toughness of FRPs, currently, there are no literature detailing the effect of surface GSD grown CNTs and ZnO nanowire -on carbon fiber- on the fracture toughness of these hybrid composites. This dissertation probes the effects of surface grown nano-sized reinforcements on the fracture toughness via double cantilever beam (DCB) tests on hybrid ZnO nanorod or CNT grafted CFRPs. Results show that the surface grown CNTs enhanced the Mode I interlaminar fracture toughness (G_Ic) of the CFRPs by 22% and 32%, via uniform and patterned growth morphologies, respectively, over the reference composite based on untreated carbon fiber fabrics. The dissertation also explains the basis of the improvements of the fracture toughness via finite element method (FEM). In particular, FEM was employed to simulate the interlaminar crack growth behavior of the hybrid CFRPs under Mode I crack opening loading conditions embodied by the DCB tests. These simulations revealed that the hybrid CFRP based on fibers with uniform surface grown MWCNTs exhibited 55% higher interlaminar strength compared to the reference CFRPs. Moreover, via patterned growth of MWCNTs, the ultimate crack opening resistance of the CFRPs improved by 20%. To mimic the experimental behavior of the various CFRPs, a new methodology has been utilized to accurately simulate the unstable crack growth nature of CFRPs. Several investigations reported the effects of adding nanomaterials - including CNTs - as a filler phase inside the matrix material, on the impact energy absorption of the hybrid FRPs. However, the impact mitigation performance of CFRPs based on ZnO nanorod grafted carbon fibers has not been reported. The dynamic out-of-plane energy dissipation capabilities of different hybrid composites were investigated utilizing high velocity (~90 m/s) impact tests. Comparing the results of the hybrid MWCNT/ZnO nanorod/CFRP with those of reference CFRP, 21% and 4% improvements were observed in impact energy absorption and tensile strain to failure of the CFRPs, respectively. In addition to elevated stiffness and strength, CFRPs should possess enough tolerance not only to monotonic loadings, but also to cyclic loadings to be qualified as alternatives to traditional structural metal alloys. Therefore, the fatigue life (i.e. the number of loading cycles to failure) of CFRPs is of much interest. Despite the promising potential of incorporating nano-sized reinforcements into the CFRPs structure, not many studies reported on the fatigue behavior of hybrid CFRPs so far. In particular, there are no reported investigations to the effect of surface grown CNTs on the fatigue behavior of the hybrid CFRPs, due to fact that almost all the CNT growth techniques (except for the GSD method) deteriorated the in-plane performance of the hybrid CFRPs. The hybrid ZnO nanorod grafted CFRPs have not been investigated under fatigue loading as well. In this dissertation, different hybrid CFRPs were tested under tension-tension fatigue to reveal the effects of the different nano-reinforcements growth on the fatigue behavior of the CFRPs. A remarkable fatigue damage tolerance was observed for the CFRPs based on uniform and patterned grown CNT fibers. Almost two decades of fatigue life extension was achieved for CFRPs based on surface grown MWCNTs.

Shape morphing in aerospace structures has the potential to reduce noise, improve efficiency, and increase the adaptability of aircraft. Among the many challenges in developing morphing technologies is finding suitable wing skin materials that can be both stiff to support the structural loads, while being elastic and compliant to support this shape morphing an minimize actuation energy. This remains an open challenge, but many possible solutions have been found in smart materials, namely shape memory alloys and polymers. Of these, shape memory polymers have received more attention for wing skins due to their low density and cost, and high elastic limits in excess of 100% strain, but they suffer from generally low overall moduli. Shape memory polymer composites have been considered to address this, typically in the form of particulate/nanoscale reinforcements or by using them as matrix materials in laminate composites. While these can serve to increase the stiffness of the composite, there is still a present need for reinforcement strategies that can also maintain the large changes in stiffness of shape memory polymers. An alternative shape memory composite relies on honeycomb materials with shape memory polymer infills. Previous research has shown that polymer filled honeycombs exhibit greater in-plane moduli greater than the infill or honeycomb alone, but there has been little research focused on understanding this behavior. Moreover, while most engineered cellular structures are comprised of symmetric and periodic cells, cellular structures in nature are commonly spatially varying, asymmetric networks, which have not been considered in these composites. Motivated by these challenges in designing materials for shape morphing, this work seeks to explore the use of shape memory polymer-filled honeycomb composites for use as variable stiffness materials. First, the interaction between infill and the honeycomb, and the relationship between the honeycomb geometry and the effective composite properties is not well understood. This research first investigates the mechanisms of stiffening in these composites through both unit cell finite element models and through experimental characterization. Parametric studies are completed for selected honeycomb geometry design variables, and three key mechanisms of stiffening are identified. Next, these mechanisms are further supported by experimental studies, and comparisons are made showing the limitations of the few existing analytic models. With the knowledge gained from these studies, shape memory polymer infills are considered to create variable stiffness composites. In the first study, sizing design variables are selected to parametric the honeycomb cell geometry, with the designs constrained to be symmetric in-plane. A constrained multiobjective design optimization is completed for two chosen performance objectives, and corresponding local sensitivity studies are completed as well. The results predict that these composites meet and exceed the current bounds of both shape memory polymers and their composites, but also variable stiffness materials in general. A great degree of tailorability is demonstrated, and the model predictions are validated against experimental results from fabricated honeycomb composite samples. Next, generally asymmetric cell geometries are considered by defining shape design variables for the cell geometry. These cells are constrained to be periodic but not symmetric, allowing for the possible benefits of asymmetric to be investigated. Additionally, interconnected and spatially varying multicell unit cells are considered, further allowing for the study of spatially varying cell geometries. Multiobjective optimizations are completed for two unit cell cases, and Pareto fronts are identified. The results are compared to both those from the sizing optimization study and to the current state of the art, and are similarly found to demonstrate high performance and a great degree of tailorability in effective properties. / Doctor of Philosophy / Vehicle shape morphing, the smooth, continuous change of an aircraft's external shape, can greatly improve the efficiency and reduce noise in modern and future vehicles. Among the is challenges in this field is finding suitable skin materials that can be both stiff to support the forces exerted on an aircraft, while being soft and compliant to support this shape morphing. Smart materials, namely shape memory polymers, present many attractive options for this need, but generally need to have a higher stiffness to be suitable for large scale applications. To address this, adding reinforcements to shape memory polymers has been of interest, and current work has largely been focused on using long fiber composites or particulate and nano-reinforcements. As an alternative to these strategies, inspiration can be found in nature where polygon cells are a common means of reinforcement in both plants and animals. Motivated by the current state of the art and the promise of shape morphing structures, this work seeks to investigate cellular structures in the form of hexagonal honeycombs as a means of increasing the stiffness of shape memory polymer infills. This is done by first improving the understanding of more general polymer-filled honeycomb, which exhibit effective stiffnesses greater than the honeycomb or polymer alone. With a working understanding of how the honeycomb stiffens the infill and how the cell geometry influences this behavior, variable modulus infills are next considered. First, sizing design variables (i.e. the lengths and thicknesses of the honeycomb geometry) are selected to describe cell geometries. Design optimization problems are considered and used to estimate the bounds of possible performance for these composites. Relationships between the design variables and the composite performance are investigated, and an improved understanding of these composites is developed. Next, shape design variables are selected to allow for the asymmetry and spatial variation found in natural cellular structures, and similar design optimizations are completed. The results of this work are experimentally validated, and demonstrate that these composites allow for combinations of stiffness and stiffness change that meet and exceed the current state of the art. Furthermore, tailoring the cell geometry allows for an easy means of changing the behavior of the composite. This work represents a great improvement and an important step in overcoming the challenges in developing shape morphing systems.