No / The performance of cement-based materials can be controlled and tailored by adjusting the characteristics of reinforced nano inclusions. Therefore, the lateral size effect of graphene on the nano/micro-structures of ultra-high performance concrete (UHPC) was explored, and then the mechanical properties were investigated to analyze the structure–property correlation of composites in this paper. The test results show that due to nucleation site effect and the formation of core–shell elements, incorporating graphene with lateral size of > 50 µm improves the polymerization degree and mean molecule chain length of C-S-H gel by 242.6% and 56.3%, respectively. Meanwhile, the porosity and average pore volume of composites is reduced by 41.4% and 43.4%. Furthermore, graphene can effectively inhibit the initiation and propagation of cracks by crack-bridging, crack-deflection, pinning and being pulled-out effect, and the wrinkling characteristic is conductive to the enhancement of pinning effect. These improvements on nano- and micro- structures result in that the compressive strength, compressive toughness and three-point bending modulus of UHPC are increased by 43.5%, 95.7% and 39.1%, respectively, when graphene with lateral size of > 50 µm and dosage of 0.5% is added. Compared to graphene with lateral size of > 50 µm, graphene with average lateral size of 10 µm has less folds and larger effective size, then reducing the distance between core–shell elements. Hence, the addition of graphene with average lateral size of 10 µm leads to 21.1% reduction for Ca(OH)2 crystal orientation index, as well as 30.0% increase for three-point bending strength. It can be, therefore, concluded that the lateral size of graphene obviously influences the nano/micro-structures of UHPC, thus leading to the significantly different reinforcing effects of graphene on mechanical behaviors of UHPC.

The research presented in this thesis investigates the mechanical properties of candidate materials that could be used as a skin for a morphing wing. A morphing wing is defined as a wing that changes shape. Although engineers have been designing different morphing wing configurations, there has been limited research investigating materials that could be used as a skin for a morphing wing. Specifically, after investigating the different morphing wing abilities engineers at Virginia Tech are designing, criteria were determined for candidate materials. A suitable skin material for a morphing wing will have to be elastic, flexible, have high recovery, resistant to different weather conditions, resistant to abrasions and chemicals, and have a hardness number high enough to handle the aerodynamic loads of the aircraft while in flight. Using some of the preceding criteria, different materials were selected that are readily available in the commercial market. The materials tested were a type of thermoplastic polyurethanes, copolyester elastomer, shape memory polymer, or woven materials that are made out of elastane yarns. The first study determined the required forces to strain the material in a uniaxial direction. A test stand was designed with a gripping device to hold the material. By grounding one side of the material, the other side of the material was pulled using a winch. Using a force transducer and a string potentiometer the required forces and the amount the material was strained was recorded, respectively. Utilizing the same test stand, the amount the material recovered was also acquired. Also, by measuring how much the material necked the elongation ratio was calculated. The final test determined if the forces "relaxed" after being strained to a stationary position. It was found that each material performed differently, but some materials were definitely better suited for morphing wing material. The materials that were made out of thermoplastic polyurethanes, copolyester elastomer, and shape memory polymer required less force and were able to strain more, when compared to the woven materials. The second study determined if the material could be strained in a biaxial direction. The reason for this was for a better understand how the material would perform if the material was strained to an extreme condition. A test stand was designed using the same principles and components as the uniaxial test stand. The only difference was additional sensors were required to measure the force and strain along the other axis. Although a recovery analysis was warranted for the biaxial experiments, most of the materials test failed while being strained a small amount. Also, the material strained a lot less before ripping, when compared to the straining capabilities when only being strained in the uniaxial direction. After conducting the experiments, the results were similar to the uniaxial experimental results. In terms of required forces to strain the material, the thermoplastic polyurethanes and the copolyester elastomer required less force, when compared to the woven materials. The only advantage of the woven materials was they did not break. The final study determined how much the material deflected while being subjected to a pressure load before breaking. The test stand used an air compressor to supply a pressure load to the material, while a laser vibrometer measured how much the material deflected. A regulator was used to control the amount of pressure that was applied to the material. As the pressure load was increased, the material deflected more. The test stand also determined the maximum sustained pressure load the material could handle before breaking. After conducting all the experiments and analyzing the data, it was found woven materials are not suitable as a skin material. The reason air is allowed to pass through the woven material. Therefore, woven materials could not sustain the aerodynamic loads of an aircraft while in flight. The rest of the materials performed differently. Specifically if the material strained well and required less force while conducting the uniaxial and biaxial experiments, those materials could not sustain a high pressure load. Yet, the materials that did not strain well and required more force were able to handle a larger sustained pressure load. / Master of Science

The focus of this research was to quantify the post-fire mechanical properties of 5083-H116 and 6082-T6 aluminum alloys. Post-fire exposure is considered heating the material to a particular temperature then cooling the material back to room temperature. The research included evaluating parent materials as well as welded samples. Post-fire mechanical properties of parent materials were evaluated at temperatures ranging from ambient to 500oC with isothermal and transient heating. Changes in material properties were evaluated through static tensile tests and hardness testing on cooled samples. Using this data, an assessment was performed to investigate the relationship between hardness and mechanical properties. For the alloys evaluated, empirical relationships were found between Vickers hardness and post-fire strength. Testing was also performed on butt welded samples of 6082-T6 exposed isothermally to temperatures ranging from ambient to 500oC. Vickers hardness profiles were measured across a sample to quantify the hardness of the weld, heat affected zone, and parent material. This was performed at room temperature and following different heat exposures. Static tensile tests were used to evaluate the effect of reheating on the welded samples. Post-fire strength of welded samples was strongly affected by weld geometry. Parent material hardness varied with reheating while weld hardness remained constant. At select temperatures, this resulted in the weld having a higher Vickers hardness than the parent material. Despite this tensile failure always occurred within the weld. / Master of Science

Hydroxyapatite (HAp) is an ideal bioactive material that is used in orthopedics. Chemical composition and crystal structure properties of HAp are similar to the natural bone hence it promotes bone growth. However, its mechanical properties of synthetic HAp are not sufficient for major load-bearing bone replacement. The potential of improving the mechanical properties of synthetic hydroxyapatite (HAp) by incorporating carboxyl functionalized single walled carbon nanotubes (CfSWCNT) and polymerized ɛ-caprolactam (nylon) is studied. The fracture toughness, tensile strength, Young’s modulus, stiffness and fracture energy were studied for a series of HAp samples with CfSWCNT concentrations varying from 0 to 1.5 wt. % without, and with nylon addition. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Differential Scanning Calorimetry (DSC) were used to characterize the samples. The fracture toughness and tensile test was performed under the standard protocol of ASTM D5045 and ASTM D638-02a respectively. Reproducible maximum values of (3.60 ± 0.3) MPa.m1/2 for fracture toughness and 65.38 MPa for tensile strength were measured for samples containing 1 wt. % CfSWCNT and nylon. The Young’s modulus, stiffness and fracture energy of the samples are 10.65 GPa, 1482.12 N/mm, and 644 J/m2 respectively. These values are comparable to those of the cortical bone. Further increase of the CfSWCNT content results to a decreased fracture toughness and tensile strength and formation of a secondary phase. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2016. / FAU Electronic Theses and Dissertations Collection

by Ng, Hiu Tung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1997. / Includes bibliographical references (leaves 121-123). / by Ng, Hiu Tung. / Acknowledgement --- p.i / Abstract --- p.ii / Table of Contents --- p.iv / List of Figures --- p.viii / List of Tables --- p.xi / Chapter Chapter One --- Introduction --- p.1 / Chapter 1.1 --- Non-destructive testing techniques --- p.2 / Chapter 1.1.1 --- Liquid penetration technique --- p.2 / Chapter 1.1.2 --- Electrical methods (Eddy current testing) --- p.3 / Chapter 1.1.3 --- Ultrasonic testing --- p.4 / Chapter 1.1.4 --- Radiography --- p.5 / Chapter 1.1.5 --- Magnetic methods --- p.6 / Chapter 1.2 --- The development of Barkhausen and magnetoacoustic emissions --- p.7 / Chapter 1.2.1 --- Barkhausen emission --- p.7 / Chapter 1.2.2 --- Magnetoacoustic emission --- p.8 / Chapter 1.3 --- The advantages of using Barkhausen and magnetoacoustic emission over the other nondestructive techniques --- p.10 / Chapter Chapter Two --- Fundamental concept in ferromagnetic materials --- p.12 / Chapter 2.1 --- Ferromagnetism --- p.12 / Chapter 2.1.1 --- Curie point --- p.13 / Chapter 2.1.2 --- Hysteresis loop --- p.14 / Chapter 2.2 --- Magnetic domains --- p.17 / Chapter 2.2.1 --- Magneto static energy --- p.17 / Chapter 2.2.2 --- Structure of domain wall --- p.19 / Chapter 2.2.3 --- Domain wall motion --- p.21 / Chapter 2.2.4 --- Domain nucleation --- p.22 / Chapter 2.3 --- Magnetostriction --- p.28 / Chapter 2.3.1 --- Spontaneous magnetostriction --- p.28 / Chapter 2.3.2 --- Saturation magnetostriction --- p.29 / Chapter 2.3.3 --- Field-induced magnetostriction --- p.29 / Chapter 2.3.4 --- Magnetostriction of polycrystalline --- p.30 / Chapter 2.4 --- Effect of stress on magnetic properties --- p.36 / Chapter 2.4.1 --- Stress --- p.36 / Chapter 2.4.2 --- Effect of stress on the magnetization --- p.37 / Chapter 2.4.3 --- Effect of stress on the magnetostriction --- p.38 / Chapter 2.5 --- Eddy current shielding --- p.41 / Chapter Chapter Three --- Barkhausen emission and magnetoacoustic emission --- p.42 / Chapter 3.1 --- Barkhausen emission --- p.42 / Chapter 3.1.1 --- The wall potential energy model of Barkhausen emission --- p.43 / Chapter 3.1.2 --- Typical BE profiles --- p.45 / Chapter 3.2 --- Magnetoacoustic emission --- p.48 / Chapter 3.2.1 --- Magnetoacoustic emission model --- p.48 / Chapter 3.2.2 --- Typical MAE profiles --- p.50 / Chapter Chapter Four --- Instrumentation --- p.52 / Chapter 4.1 --- Introduction --- p.52 / Chapter 4.2 --- Experimental setup for Barkhausen emission --- p.53 / Chapter 4.3 --- Experimental setup for magnetoacoustic emission --- p.56 / Chapter 4.4 --- Specimen treatment --- p.58 / Chapter 4.4.1 --- Furnace --- p.58 / Chapter 4.4.2 --- Instron loading machine --- p.60 / Chapter 4.4.3 --- Optical microscopy --- p.60 / Chapter 4.4.4 --- Vicker's hardness tester --- p.61 / Chapter Chapter Five --- Effect of field frequency and strength on Barkhausen emission in mild steel and nickel --- p.66 / Chapter 5.1 --- Introduction --- p.66 / Chapter 5.2 --- Experiments --- p.67 / Chapter 5.3 --- Results and discussions --- p.68 / Chapter 5.4 --- Conclusions --- p.76 / Chapter Chapter Six --- Effect of residual stress on Barkhausen and magnetoacoustic emissions in steel bar --- p.77 / Chapter 6.1 --- Introduction --- p.77 / Chapter 6.2 --- Experiments --- p.81 / Chapter 6.3 --- Results and discussions --- p.84 / Chapter 6.3.1 --- BE profiles --- p.84 / Chapter 6.3.2 --- MAE profiles --- p.85 / Chapter 6.3.3 --- Optical microscopy and hardness measurements --- p.85 / Chapter 6.4 --- Conclusions --- p.92 / Chapter Chapter Seven --- Effect of residual stress on Barkhausen and magnetoacoustic emissions in a nickel bar --- p.93 / Chapter 7.1 --- Introduction --- p.93 / Chapter 7.2 --- Experiments --- p.96 / Chapter 7.3 --- Results and discussions --- p.97 / Chapter 7.3.1 --- Hardness and optical microscopy measurement --- p.97 / Chapter 7.3.2 --- BE profiles --- p.98 / Chapter 7.3.3 --- MAE profiles --- p.99 / Chapter 7.4 --- Comparison of nickel and mild steel --- p.106 / Chapter 7.5 --- Conclusions --- p.108 / Chapter Chapter Eight --- Effect of dynamic stress on Barkhausen emission in mild steel --- p.109 / Chapter 8.1 --- Introduction --- p.109 / Chapter 8.2 --- Experiments --- p.110 / Chapter 8.3 --- Results and discussions --- p.112 / Chapter 8.4 --- Conclusions --- p.118 / Chapter Chapter Nine --- Conclusions --- p.119 / References --- p.121

Sex differences in bone mass and size are thought to contribute to the greater incidence of vertebral fractures in women. While these sex differences are widely recognized, the relative contributions of bone mass, bone density, and bone size to the differences in vertebral strength and fracture risk between men and women have not been fully delineated. Furthermore, it is unknown whether the roles of each of these factors in determining vertebral strength change differently with age in men and women. We studied the bone content, density and geometry as well as vertebral loading and the factor of risk of the L3 vertebra in a sample of prepubertal males and females. Our first aim was to assess differences in vertebral bone dimensions, bone density, vertebral loading patterns and fracture risk, as measured by the factor of risk, in prepubertal children. Our second aim was to determine whether pre-pubertal growth affects the geometry and density of L3 differently in boys and girls. We measured vertebral dimensions, cross-sectional area and volumetric BMD of the third lumbar vertebral body in 93 prepubertal children (54 boys and 39 girls), using dual-energy X-ray absorptiometry scans obtained in the posterior-anterior and lateral projections. We also employed basic biomechanics to estimate vertebral loading during upright standing and forward bending. Bone strength and loading data were used to assess sex differences in the factor of risk in prepubertal children. Twenty children (11 boys and 9 girls) were assessed at baseline and seven months later to examine the effects of growth on bone size and vBMD. At baseline, boys and girls were similar for age, height, weight and calcium intake. L3 width and depth were 6.7% and 5.8% greater in boys than girls, respectively (P<0.001 and P=0.01, respectively). In contrast, vertebral height was 3.5% greater in girls than boys (P= 0.04). While vertebral loading was similar between sexes, stresses on the spine were 12.2% lower in boys during upright standing and 12.0% lower in boys during forward bending at both 50° and 90°, as compared to girls (P<0.001, P<0.01 and P<0.01, respectively). The factor of risk was similar between boys and girls under each loading condition. During growth, changes in vertebral size and density were not different between boys and girls. Our results indicate that even prior to puberty, sex differences in vertebral size contribute to differences in vertebral stress during standing and forward bending. Furthermore, before the onset of puberty, growth does not result in disparate changes between sexes. / Graduation date: 2004

Polymeric composites exhibit time-dependent behavior, which raises a concern about their long term durability and leads to a viscoelastic study of these materials. Linear viscoelastic analysis has been found to be inadequate because many polymers exhibit nonlinear viscoelastic behavior. Classical laminate theory is commonly used in the study of laminated composites, but due to the plane stress/strain assumption its application has been limited to solving two dimensional, simple plate problems. A three dimensional analysis is necessary for the study of interlaminar stress and for problems involving complex geometry where certain local effects are important. The objective of this research is to develop a fully three-dimensional, nonlinear viscoelastic analysis that can be used to model the time-dependent behavior of laminated composites. To achieve this goal, a three-dimensional finite element computer program has been developed. In this program, 20-node isoparametric solid elements are used to model the individual plies. The three-dimensional constitutive equations developed for numerical calculations are based on the Lou-Schapery one-dimensional nonlinear viscoelasticity model for the uniaxial stress case. The transient creep compliance in the viscoelastic model is represented as an exponential series plus a steady-flow term, which allows for a simplification of the numerical procedure for handling hereditary effects. A cumulative damage law for three dimensional analysis was developed based on the Brinson-Dillard two-dimensional model to predict failure initiation. Calculations were performed using this program in order to evaluate its performance in applications involving complex structural response. IM7/5260-H Graphite/Bismaleimide and T300/5208 Graphite/Epoxy were the materials selected for modeling the time-dependent behavior. The cases studied include: 1) Tensile loading of unnotched laminates; 2) bending of a thick laminated plate; and 3) tensile loading of notched laminates. The analysis emphasized the study of the traction-free edge-effect of laminated composites, stress distribution around a circular hole, and stress redistribution and transformation in the layers. The results indicate that the stress redistributions over time are complicated and could have a significant effect on the long-term durability of the structure. / Graduation date: 1994

Advanced nano-structured ferritic alloys (NFAs) containing a high density of ultra-fine (2-5 nm) nanoclusters (NCs) enriched in Y, Ti, and O are considered promising candidates for structural components in future nuclear systems. The superior tensile strengths of NFAs relative to conventional oxide dispersion strengthened (ODS) ferritic alloys are attributed to the high number density of NCs, which may provide effective trapping centers for point defects and transmutation products generated during neutron irradiation. This study consists of production, irradiation, and characterization of an advanced NFA, designated 14YWT, currently being developed at Oak Ridge National Laboratory (ORNL), in Oak Ridge, Tennessee. The purpose of this study was to characterize the tensile and fracture toughness properties of 14YWT produced during this project at ORNL before and after irradiation to evaluate it's resistance to radiation-induced changes in mechanical properties. Another alloy, designated 14WT, was produced during this project using identical production parameters used for 14YWT but without the Y2O3 addition during ball milling required for NC formation. Tensile and fracture toughness specimens were produced from both alloys and irradiated in small "rabbit" capsules in the High Flux Isotope Reactor (HFIR) at ORNL. Five other structural alloys that are currently being evaluated for applications in nuclear environments were irradiated and tested during this project to serve as comparison materials. Microstructural characterization was performed using optical microscopy, scanning electron microscopy, transmission electron microscopy, and atom probe tomography. Tensile strengths for 14YWT were found to be far superior to the other alloys for both irradiated and unirradiated conditions, with yield strength for 14YWT decreasing from ~1,450 MPa at 26°C to ~700 MPa at 600°C. Moderate radiationinduced hardening (50-200 MPa) and reduction in ductility was observed for 14YWT for all irradiation conditions and test temperatures. Fracture toughness results showed 14YWT in the unirradiated condition had a fracture toughness transition temperature (FTTT) around -150°C and upper-shelf K[subscript JIc] values around 175 MPa m. Results from irradiated 14YWT fracture toughness tests were found to closely mirror the unirradiated data and no shift in FTTT or decrease in K[subscript JIc] values were observed following neutron irradiation to 1.5 dpa at 300°C. Master curve analysis of the fracture toughness data show 14YWT to have a T[subscript o] reference temperature of -188 and -176°C in the unirradiated and irradiated condition, respectively, which is unprecedented for a high-strength dispersion strengthened ferritic alloy. The results from this study show 14YWT to be resistant to radiation-induced changes in mechanical properties and a promising candidate for structural applications in future nuclear systems. / text

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

The diseases of cardiovascular system and the respiratory system have been the second and third killers causing deaths in Hong Kong. In this stressful civilized world, the prevalence and incidence of these diseases increased prominently which arouse our concern on the theories behind the pathological conditions. This report will focus on the biofluid mechanics in the large artery and in the upper airway. Thoracic aortic dissection, characterized by the tearing in the middle layer of vessel wall, is a catastrophic vascular disorder. The wall of the newly formed channel, the false lumen, is weakened and prone to aortic events. Endovascular repair is a minimally invasive technique for treating dissection patients. The biomechanical factors and the length of endograft were studied by computational fluid dynamics. Two geometrical factors showed a significant impact on the backflow in the false lumen. A larger false lumen and a larger distal tear size greatly affected the extent of thrombosis in the false lumen. It made the false lumen under a higher risk of vessel rupture. The computational prediction also demonstrated a more stable hemodynamic condition in the model with a longer endograft. These results provide important information for the clinicians to propose the surgical procedures and to improve the design of endografts. Airway obstruction is a common breathing disorder but it is always underdiagnosed. Obstructive sleep apnea (OSA) and different dentofacial deformities are two pathological conditions in which the patients have the abnormal sizes of airways. Computational fluid dynamic was employed in both conditions with patient–specific models. In the part of OSA, pre– and post–operative models were studied. The dimensions and flow resistance of the upper airway showed a significant improvement after mandibular distraction. The percentage of stenosis and the flow resistance was reduced by 27.3% and 40.7% respectively. For the patients in three facial skeletal deformity groups, the cross–sectional area and the flow resistance were compared. The patients with Class II deformity had the smallest retroglossal and retroplatal dimensions as well as the greatest flow resistance. The results confirmed the effectiveness of mandibular distraction and also provide valuable implications for the clinicians on the treatment planning, particularly for the Class II subjects. / published_or_final_version / Mechanical Engineering / Master / Master of Philosophy