Maximum voluntary forces and psychophysically acceptable forces are often used to set force guidelines for exertions as a means to protect against overexertion injuries in the workplace. The focus of this dissertation was the exploration of the roles of whole body balance, shoe-floor friction and joint strength in limiting the capacity of a person to produce maximum voluntary hand forces and psychophysically acceptable hand forces. The underlying goal was to advance knowledge regarding how physical exertion capacity is biomechanically governed, then to use this information to develop models to predict capability based on these governing principles. The hypothesis underscoring this work was that maximum voluntary hand force capability is governed by whole body balance, shoe-floor friction and joint strength; and consequently, psychophysically acceptable forces would be chosen proportionally to this maximum voluntary force capability, where the magnitude of the proportionality was dependent on the limiting factor, or ‘weakest link’. To investigate this hypothesis, both experimental and mathematical modeling paradigms were used. Initially, an experimental study was used to investigate how biomechanical factors governed maximum hand force capability across a range of exertions. It revealed that each governing factor differentially limited maximum force capability. Moreover, this study identified how foot placement, handle height, distance from the handle, friction, and body posture all influence the underlying biomechanical weakest link, and ultimately force producing capability. Data gathered in the experimental study was next used to evaluate a mathematical model that was developed to predict maximum force capability, given information on posture and direction of force application. In addition, the model also predicted population variability in maximum capacity based on the inclusion of a novel approach to probabilistically represent population variability. The evaluation demonstrated that the model underestimated maximum hand force capability compared to measured hand forces by approximately 18, 26, and 41% during medial, pulling and downward exertions respectively. However, it appeared that the ‘weakest link’ principle for predicting maximum force capacity was plausible, as evidenced by significant rank ordered correlations between the measured and predicted hand forces. Further research investigated if psychophysically acceptable forces were selected as a proportion of task specific maximum voluntary force capability, where the proportionality was related to the biomechanical weakest link. Using an experimental design, psychophysically acceptable forces and corresponding maximum forces were measured. Participants chose psychophysically acceptable forces that were 4/5ths of their task specific maximum voluntary force capability when capability was limited by balance. Additionally, they choose psychophysically acceptable forces that were 2/3rds of their maximum voluntary force capability when capability was limited by joint strength. The identification and confirmation of a weakest link proportionality principle represents an important contribution to the field of occupational biomechanics. The weakest link proportionality principle was integrated into the model to allow prediction of: maximum voluntary hand force capability, the limiting factor, and psychophysically acceptable hand force capability. The updated model underestimated empirically measured psychophysically acceptable forces by 24% and 43% during downward and pulling exertions respectively. However, the original model underestimated the maximum hand force capacity by 23% and 34% during the same exertions, without the proportional relationships. This underestimation may be a result of the underlying assumption that joint strength is independent, resulting in an underestimation of maximum joint strength capacity and a corresponding underestimation of maximum hand force capacity. The underestimation may also be due to differences in strength capacities between the participants tested during this thesis compared to those tested in past research used to determine the maximum strength indices reported in the literature. This body of work supported the hypothesis that psychophysically acceptable forces are selected as a proportion of the maximum voluntary hand force, where the proportionality depends on the underlying biomechanical weakest link. The model is a promising first step towards predicting maximum and psychophysically acceptable occupational force threshold limits.

Alzheimer's Disease (AD) is a neurodegenerative disorder that is prevalent among the elderly population. Aß protein has been heavily implicated in the pathogenesis of AD. This protein in its fibrillar form is a major component in the senile plaques that form on neuronal cellular membranes during the course of AD. Despite substantial efforts the exact mechanism of Aß toxicity towards a cell membrane is not well-understood. The determination of this mechanism, however, is of utmost importance, since the membrane presents the first site of Aß interaction with neurons, which in turn maybe the origin of Aß neurotoxicity. The purpose of this study was to find a lipid composition that can be used as a model of neuronal membrane for subsequent studies of the role of topographical heterogeneity (domain formation) on Aß-membrane interaction as related to AD. The lipids used in the study were 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), cholesterol (Chol), sphingomyelin (SM) and ganglioside GM1 (GM1). These lipids were combined in different proportions and deposited on a mica substrate to form supported monolayers. They were then imaged with an atomic force microscope (AFM) to determine if any of them exhibited domain formation. Three of the studied samples: POPC/POPG/SM 40:40:20 +5%Chol, POPC/SM/Chol 75:20:5 and POPC/SM/GM1/Chol 74:2:1:23 were found to possess interesting topography, rich in structural features: pores and domains. The average height difference between the domain features for each sample was found to be 0.58±015 nm, 0.61±0.12 nm and 0.27±0:07 nm.

<p>This research presents new calibration techniques for the characterization of atomic force microscopy cantilevers. Atomic force microscopy cantilevers are sensors that detect forces on the order of pico- to nanonewtons and displacements on the order of nano- to micrometers. Several calibration techniques exist with a variety of strengths and weaknesses. This research presents techniques that enable the noncontact calibration of the output sensor voltage-to-displacement sensitivity and the cantilever stiffness through the analysis of the unscaled thermal vibration of a cantilever in a liquid environment.</p><p>A noncontact stiffness calibration method is presented that identifies cantilever characteristics by fitting a dynamic model of the cantilever reaction to a thermal bath according to the fluctuation-dissipation theorem. The fitting algorithm incorporates an assumption of heavy fluid loading, which is present in liquid environments.</p><p>The use of the Lorentzian line function and a variable-slope noise model as an alternate approach to the thermal noise method was found to reduce the difference between calibrations preformed on the same cantilever in air and in water relative to existing techniques. This alternate approach was used in combination with the new stiffness calibration technique to determine the voltage-to-displacement sensitivity without requiring contact loading of the cantilever.</p><p>Additionally, computational techniques are presented in the investigation of alternate cantilever geometries, including V-shaped cantilevers and warped cantilevers. These techniques offer opportunities for future research to further reduce the uncertainty of atomic force microscopy calibration.</p> / Dissertation

Fretting fatigue occurs when there is a small amplitude oscillatory movement between two contacting surfaces while the bodies are undergoing fatigue loading. Fretting fatigue conditions can substantially reduce the fatigue life of a component. Many engineering components such as Ti-6Al-4V gas turbine engine disks in military aircraft commonly experience fretting fatigue conditions that can potentially lead to catastrophic failure of critical components. The aim of this study is to characterize the behavior of Ti-6Al-4V under fretting fatigue conditions. Experiments are performed to analyze the influence of stress amplitude, stress ratio, and contact geometry. The effect of surface treatments such as low plasticity burnishing on the fretting fatigue life is also explored. The experimental results are being used to validate a proposed crack nucleation life prediction model. The proposed model utilizes a crack nucleation parameter H that is based on the strength of the singular stress field at the contact boundary. An advantage of this singular parameter is that neither a coefficient of friction nor the location of the stick/slip boundary needs to be determined. These two parameters are often difficult to define with certainty a priori. H is also independent of geometry making it well suited for use as a design parameter for designing structural joints and other fitted connections between components.

No description available.

Atomic force microscopy is one of the powerful instruments used to explore the mechanical properties of nanoscale materials. It not only can produce high-resolution images and surface mechanical properties, but also can make use of its probe for surface etching. In this study, we first use atomic force microscopy to measure the Adhesion Map of insulin amyloid fibers, then conduct mechanical lithography on the surface with the probe. In the end, we discuss the effect on insulin amyloid fibrils due to exert different forces and different speeds with the probe. According to Nanoindentation theory and Hertzian model, we can derive the Young's modulus of insulin amyloid fibrils from force-indentation relations. Then we cut the Insulin amyloid fibers with probe. The results showed that when we applied 3.23 nN force by the probe, the insulin amyloid fibers began to break. When we applied 7.07 nN force, insulin amyloid fibers are cut off easily. Therefore, we can bite off insulin amyloid fibers of different lengths and sections, and arrange in the desired pattern by atomic force microscope.

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Feeding biomechanics and foraging behavior are likely contributors to loggerhead sea turtle (Caretta caretta) bycatch in the pelagic longline fishery. To investigate these contributions, loggerhead bite performance was measured in several size classes of captive-reared juveniles, captive sub-adults and adults, as well as wild loggerheads. A kinematic study was conducted to investigate loggerhead interactions with modified longline hooks. Kinematic and behavioral variables were assessed in relation to five longline hooks to determine if loggerhead feeding behavior is modulated relative to hook type, size, and offset. The bite force study demonstrated that mean maximum post-hatchling bite force was 2.5N and mass was the best predictor of post-hatchling bite force. Mean maximum bite force of juveniles with mean straight carapace length (SCL) of 12, 31, 44, and 65 cm were 27, 152, 343, and 374 N, respectively. Sub-adult and adult mean maximum bite force was 575 N. Maximum bite force had a positive linear relationship with all head and body morphometrics (P<0.001). Carapace width was the best predictor of bite force throughout ontogeny. The kinematic study demonstrated no differences between hook treatments in all kinematic variables analyzed. The results of this study suggest loggerhead feeding behavior may be stereotypical. Only 33% of all interactions resulted in “hooking” events. “Hooking” was lowest in 16 gage circle hooks with no offset and the 18 gage circle hooks with 10°offset which may be indicative of a lower possibility of the turtle drowning. “Hooking” was highest in the 16 gage circle hooks with 10°offset. The proportion of turtles “hooked” in the mouth was significantly greater than those “hooked” in the throat (P=0.001). Sixteen gage circle hooks with 10° offset had the highest percentage of throat “hooking”, and the 18 gage circle hooks without offset resulted in the lowest percentage of throat hooking. When interacting with J hooks with a 25° offset (9 gage), turtles mostly oriented their head away from the hook offset; however, when interacting with the 16 and 18 gage circle hooks with 10° offset, turtles mostly oriented their heads toward the hook offset. These data suggest that turtles may distinguish between small and large offsets, and may modulate their feeding behavior accordingly. Alternatively, turtles may be detecting hook size or hook shape. A more thorough characterization of loggerhead bite performance and feeding kinematics will be useful when developing or modifying longline fishery gear aimed at reducing loggerhead bycatch.

In recent years, nanowires have become increasingly important due to their unique properties and applications. Thus, processes in the fabrication to nanostructures has come a focal point in research. In this research, a new method to fabricate nanowires has been developed. The new technique is called the Scanning Probe Alloying Nanolithography (SPAN). The SPAN was processed using an Atomic Force Microscope (AFM) in ambient environment. Firstly, an AFM probe was coated with gold (Au), and then slid on a silicon (Si) substrate. The contact-sliding motion generated a nanostructure on the substrate, instead of wear. Subsequently, careful examination was carried out at the scale relevant to an AFM probe, in terms of physical dimension and electrical conductivity. The measured conductivity value of the generated microstructures was found to be between the conductivity values of pure silicon and gold. Simple analysis indicated that the microstructures were formed due to frictional energy dispersed in the interface forming a bond to sustain mechanical wear. This research proves the feasibilities of tip-based nanomanufacturing. The SPAN process was developed to increase efficiency of the technique. This study also explored the possibility of the applications as a biosensor and a flexible device. This dissertation contains nine sections. The first section introduces backgrounds necessary to understand the subject matter. It reviews current status of the nanofabrication technologies. The basic concepts of AFM are also provided. The second section discusses the motivation and goals in detail. The third section covers the new technology, scanning probe alloying nanolithography (SPAN) to fabricate nanostructures. The fourth talks about characterization of nanostructures. Subsequently, the characterized nanostructures and their mechanical, chemical, and electrical properties are discussed in the fifth section. In the sixth section, the new process to form a nanostructure is evaluated and its mechanism is discussed. The seventh section discusses the feasibility of the nanostructures to be used in biosensors and flexible devices. The conclusion of the research is summarized in the seventh section.

This thesis presents a systematic scheme to determine the optimal propulsive/axial force ratio of a single-sided permanent magnet axial-flux motor (SPMAM) along with its operational constraints on both the winding currents and the speed induced voltages. According to the rotating magnetic field theory with combining the recoil line characteristics of permanent magnet and the equivalent operational magnetic circuits, appropriate projection of the stator currents to achieve an optimal ratio of the machine propulsive/axial forces has been confirmed through detailed three-dimensional finite element analysis (3-D FEA) and numerical studies. From these evaluations, a feasible operational guidance for SPMAM field oriented control (FOC) scheme realizations can be suitably provided. Finally, based on the proposed optimal scheme, a DSP-based drive system has been successfully implemented, and the desired operational strategy realization can be achieved.