Neurulation is critical for the proper development of the central nervous system during embryogenesis. This process requires coordinated morphogenetic movements driven by localized cell movements. The key morphogenetic process responsible for lengthening the neural plate is convergent extension. During convergent extension medially oriented cell polarity, protrusive activity, and motility are thought to generate forces through cell intercalation resulting in stiffer elongating tissues. My research determines that forces that help shape the neural plate arise from morphogenetic movements in the neural tissue and determines PCP signaling regulates tissue stiffness in the neural ectoderm. We have established an experimental system sensitive enough to evaluate the stiffness of Xenopus neural tissue. Stiffness is measured by gluing two fine wires onto neural explants from an early gastrula stage Xenopus laevis embryo. The wires stretch the tissue at a constant strain rate using a real-time image-based feedback system and stiffness is determined by measuring the deflection of one wire. Measurements obtained from control embryos prior to neurulation estimate tissue stiffness at approximately 12.7 ± 0.53 mN/m in both mediolateral and anteroposterior directions. Stiffness measurements double in early neurula embryos (P < 0.05). Mediolateral stiffness, 24.9 ±6.2 mN/m, is significantly greater than anteroposterior stiffness, 21.4 ±5.3 mN/m (P < 0.05). These trends are strengthened in normalized data to reduce clutch-to-clutch variation. Expressions of dominant-negative Wnt11, Fz7, and Dsh constructs successfully disrupt neurulation by interfering with the PCP pathway. Changes in stiffness of the neural plate were measured and show reduced stiffness at early neurula stage in both mediolateral and anteroposterior directions suggesting mechanical forces are generated within the neural plate.

Neurulation is critical for the proper development of the central nervous system during embryogenesis. This process requires coordinated morphogenetic movements driven by localized cell movements. The key morphogenetic process responsible for lengthening the neural plate is convergent extension. During convergent extension medially oriented cell polarity, protrusive activity, and motility are thought to generate forces through cell intercalation resulting in stiffer elongating tissues. My research determines that forces that help shape the neural plate arise from morphogenetic movements in the neural tissue and determines PCP signaling regulates tissue stiffness in the neural ectoderm. We have established an experimental system sensitive enough to evaluate the stiffness of Xenopus neural tissue. Stiffness is measured by gluing two fine wires onto neural explants from an early gastrula stage Xenopus laevis embryo. The wires stretch the tissue at a constant strain rate using a real-time image-based feedback system and stiffness is determined by measuring the deflection of one wire. Measurements obtained from control embryos prior to neurulation estimate tissue stiffness at approximately 12.7 ± 0.53 mN/m in both mediolateral and anteroposterior directions. Stiffness measurements double in early neurula embryos (P < 0.05). Mediolateral stiffness, 24.9 ±6.2 mN/m, is significantly greater than anteroposterior stiffness, 21.4 ±5.3 mN/m (P < 0.05). These trends are strengthened in normalized data to reduce clutch-to-clutch variation. Expressions of dominant-negative Wnt11, Fz7, and Dsh constructs successfully disrupt neurulation by interfering with the PCP pathway. Changes in stiffness of the neural plate were measured and show reduced stiffness at early neurula stage in both mediolateral and anteroposterior directions suggesting mechanical forces are generated within the neural plate.

The Electron Energy Corporation (EEC) along with National Aeronautics and Space Administration (NASA) in collaboration with Vibration Control and Electro mechanics Lab (VCEL), Texas A & M University, College Station, TX are researching on high temperature permanent magnet based magnetic bearings. The magnetic bearings are made of high temperature resistant permanent magnets (up to 1000 degrees F). A test rig has been developed to test these magnetic bearings. The test rig mainly consists of two radial bearings, one axial thrust bearing and two catcher bearings. The test rig that the catcher bearing is inserted in is the first ultra-high temperature rig with permanent magnet biased magnetic bearings and motor. The magnetic bearings are permanent magnet based which is a novel concept. The Graphalloy bearings represent a new approach for ultra-high temperature backup bearing applications. One of the main objectives of this research is to insure the mechanical and electrical integrity for all components of the test rig. Some assemblies and accessories required for the whole assembly need to be designed. The assembly methods need to be designed. The preliminary tests for coefficient of friction, Young's modulus and thermal expansion characteristics for catcher bearing material need to be done. A dynamic model needs to be designed for studying and simulating the rotor drop of the shaft onto the catcher bearing using a finite element approach in MATLAB. The assembly of the test rig was completed successfully by developing assembly fixtures and assembly methods. The components of the test rig were tested before assembly. Other necessary systems like Sensor holder system, Graphalloy press fit system were designed, fabricated and tested. The catcher bearing material (Graphalloy) was tested for coefficient of friction and Young's modulus at room and high temperatures. The rotor drop was simulated by deriving a dynamic model, to study the effect of system parameters like clearance, coefficient of friction, negative stiffness, initial spin speed on system behavior. Increasing the friction increases the backward whirl and decreases the rotor stoppage time. Increasing the clearance reduces the stoppage time and increases the peak bearing force. Increasing the initial spin speed increases the rotor stoppage time. The maximum stress encountered for as built conditions is more than allowable limits.

This thesis presents the development of a micro-aspirator chip using vacuum expanded microchannels for mechanical characterization of single cells. Mechanical properties of cells can offer valuable insights into the pathogenic basis of diseases and can serve as a biomarker to identify cells depending on disease state, and thus have the potential for use in human disease diagnostic applications. Micropipette aspiration and atomic force microscopy (AFM) are the most commonly used techniques for measuring mechanical properties of single cells. Though powerful and versatile, both methods have two drawbacks. First, micromanipulation of glass micropipettes and AFM tips require expertise and extensive operator skills. Second, the serial manipulation process severely limits the throughput. Although recently reported microfluidic micropipette device showed the potential of microfluidic chip type micropipette aspiration, difficulty in cell trapping and unnatural cell deformation remain to be solved. In order to address these limitations, a high-throughput micro-aspirator chip, which can deliver, trap, and deform multiple cells simultaneously with single-cell resolution without skill-dependent micromanipulation was developed. The micro-aspirator chip is composed of 20 arrays of cell traps and aspiration channels. The principle of cell trapping is based on differences in flow resistance inside the microfluidic channels. Once the first cell trap is filled with a cell, the next cell coming in passes by the trap and is captured in the next trap. After all traps are filled with cells, negative pressure can then be applied to the integrated aspiration channels using hydrostatic pressure. The aspiration channels are positioned at the center of a trapped cell both in vertical and horizontal directions to obtain a good seal just like a traditional micropipette, a design made possible through a vacuum expanded raised microfluidic channel fabrication technique. Device operation was demonstrated using HeLa cells. The cell trapping efficiency was almost 100 percent. Using this device, Young's modulus of 1.3 ± 0.8 kPa (n = 54) was obtained for HeLa cells. Device to device variation was less than 15.2 percent (n = 3), showing good repeatability of the device. No dependence of the Young's modulus on the cell diameter was found.

ABSTRACT AS-4 carbon fibers reinforced polyetheretherketone (PEEK) composite materials have been widely used in aerospace industry because of longer fatigue life, high specific stiffness and strength. The thesis is aimed to investigate the residual strength, residual stiffness and mechanical properties of thermoplastic AS-4/PEEK composite laminates subjected to tension-tension (T-T) cyclic loading at room temperature. We adopt modified diaphragm forming method by controlling temperature, pressure, vacuum and time conditions according to the obtained beast curing process to form composite laminates of low crystallinity, transcrystallinity and good fiber / matrix interfaces. Two common type of laminates are used, such as cross-ply [0/90]4S and quasi-isotropic [0/+45/90/-45]2S. Static tension test is performed to measure the elastic modulus and ultimate strength. And T-T fatigue test is conducted with maximum stress of 60% and 80% ultimate strength to find the residual strength and stiffness. Then, through the observation of failure surfaces of composite laminates we understand the failure initiation and mechanism by Scanning Electron Microscope (SEM). The results of experiment can be concluded as follows. The ultimate strength, elastic modulus and fatigue strength of cross-ply composite laminates are larger than those of quasi-isotropic. As centrally notched, the net area of the specimen is reduced, the ultimate strength and fatigue strength of composite materials are lower. The residual strength, adopted to describe the damage process, is monotonically decreasing with increasing of applied cycles. It is found that the residual strength of cross-ply laminates is larger than that of quasi-isotropic laminates. However, the residual stiffness has little change with increasing of applied cycles.

Abstract In this thesis, the effect of the orientation of trussed honeycomb core design on the stiffness of a composite honeycomb plate is presented. A commercial finite element (FEM) package ¡¥MSC-MARC¡¦ is employed in the stiffness simulation. To illustrate the feasibility and accuracy of the proposed FEM model, the measured and calculated data for two different sizes regular honeycomb plates (with regular hexagonal cell) are compared. Results show that a good agreement between the simulated and the measured static deflections and dynamic characteristics is found. Numerical results indicate that different orientations of trussed honeycomb core design may improve the stiffness/density ratio of a composite honeycomb plate significantly. The effects of other design parameters of composite honeycomb plate, e.g. width and height of plate, thickness of truss, cell wall and faces, and material of truss, on the stiffness/density ratio have also been investigated in this study.

New Zealand grown Pinus radiata is limited in its application for structural purposes by its stiffness deficiencies. This dissertation aims to estimate potential improvements in stem value through selection for improved stiffness. A new method to model and value volumes of structural wood grades within a stem was used to calculate these value improvements. Data for each stem from a stand in Kaingaroa Forest bred for improved wood quality was used to perform this analysis. This data was from a stand bred for improved wood quality and included information on the stiffness, density and width of each growth ring for each stem. The data was in the form of cores. Height and volume data was not recorded and therefore needed to be modelled. The volumes of MSG8, MSG11 and MSG13 wood were estimated by modelling the stem volume at the age when wood is produced that is stiff enough to qualify for each grade. The majority of stems had merchantable volumes between 1-2.5m3 with the largest stems containing 3.6m3. Average stiffness ranged between 5.2GPa and 11.3GPa with the stand average being 8.4GPa. There was no relationship between average stiffness and merchantable volume. Stem values were found to range between $60-$131/m3 with the stand average being $91/m3. The 10 most valuable stems had a total stem value ($318) twice that of the stand average ($157). The most valuable stem ($411) showed a 160% increase in stem value from the average. The increases in value/m3 were caused by large increases in the proportion of MSG11 and MSG13 wood held within the merchantable volume. These potential gains in stem value could help tree breeders assign an accurate economic weighting to stiffness improvements. Forest managers wanting to justify using a more expensive, improved stiffness seedlot may also find these results valuable.

Unilateral below-knee amputees commonly experience asymmetrical gait patterns and develop comorbidities in their intact (non-amputated) and residual (amputated) legs, with the mechanisms leading to these asymmetries and comorbidities being poorly understood. Prosthetic feet have been designed in an attempt to minimize walking asymmetries by utilizing elastic energy storage and return (ESAR) to help provide body support, forward propulsion and leg swing initiation. However, identifying the influence of walking speed and prosthetic foot stiffness on amputee gait mechanics is needed to develop evidence-based rationale for prosthetic foot selection and treatment of comorbidities. In this research, experimental and modeling studies were performed to identify the influence of walking speed and prosthetic foot stiffness on amputee walking mechanics. The results showed that when asymptomatic and relatively new amputees walk using clinically prescribed prosthetic feet across a wide range of speeds, loading asymmetries exist between the intact and residual knees. However, knee intersegmental joint force and moment quantities in both legs were not higher compared to non-amputees, suggesting that increased knee loads leading to joint disorders may develop in response to prolonged prosthesis usage or the onset of joint pathology over time. In addition, the results showed that decreasing ESAR foot stiffness can increase prosthesis range of motion, mid-stance energy storage, and late-stance energy return. However, the prosthetic foot contributions to forward propulsion and swing initiation were limited due to muscle compensations needed to provide body support and forward propulsion in the absence of residual leg ankle muscles. A study was also performed that integrated design optimization with forward dynamics simulations of amputee walking to identify the optimal prosthetic foot stiffness that minimized metabolic cost and intact knee joint forces. The optimal stiffness profile stiffened the toe and mid-foot while making the ankle less stiff, which decreased the intact knee joint force during mid-stance while reducing the overall metabolic cost of walking. These studies have provided new insight into the relationships between prosthetic foot stiffness and amputee walking mechanics, which provides biomechanics-based rationale for prosthetic foot prescription that can lead to improved amputee mobility and overall quality of life. / text

The research presented in this thesis focuses on the modeling, design, and experimentation of systems containing negative stiffness mechanisms for both vibration and shock isolation. The negative stiffness element studied in this research is an axially compressed beam. If a beam is axially compressed past a critical value, it becomes bistable with a region of negative stiffness in the transverse direction. By constraining a buckled beam in its metastable position through attaching a stiff linear spring in mechanical parallel, the resulting system can reach a low level of dynamic stiffness and therefore provide vibration isolation at low frequencies, while also maintaining a high load-carrying capacity. In previous research, a system containing an axially compressed beam was modeled and tested for vibration isolation [7]. In the current research, variations of this model were studied and tested for both vibration and shock isolation. Furthermore, the mathematical model used to represent the compressed beam in [7] was improved and expanded in current research. Specifically, the behavior exhibited by buckled beams of transitioning into higher-mode shapes when placed under transverse displacement was incorporated into the model of the beam. The piecewise, nonlinear transverse behavior exhibited by a first-mode buckled beam with a higher-mode transition provides the ability of a system to mimic an ideal constant-force shock isolator. Prototypes manufactured through Selective Laser Sintering were dynamically tested using a shaker table. Vibration testing confirmed the ability of a system containing a constrained negative stiffness element to provide enhanced vibration isolation results with increasing axial compression on a beam. However, the results were limited by the high sensitivity of buckled beam behavior to geometrical and boundary condition imperfections. Shock testing confirmed the ability of a system containing a buckled beam with a higher-mode transition to mimic the theoretically ideal constant-force shock isolator. / text

This work demonstrates the effectiveness of coextrusion as a method to manufacture negative stiffness inclusions for use in vibrational damping applications. The theory and mechanics of negative stiffness and coextrusion are introduced and the process of creating and extruding a feed rod with negative stiffness architecture explained. Coextrusion is shown to be a viable method to create negative stiffness inclusions / text