An investigation of the possible performance improvements of thin circular cylindrical shells through the use of the variable stiffness concept is presented. The variable stiffness concept implies that the stiffness parameters change spatially throughout the structure. This situation is achieved mainly through the use of curvilinear fibers within a fiber-reinforced composite laminate, though the possibility of thickness variations and discrete stiffening elements is also allowed. These three mechanisms are incorporated into the constitutive laws for thin shells through the use of Classical Lamination Theory. The existence of stiffness variation within the structure warrants a formulation of the static equilibrium equations from the most basic principles. The governing equations include sufficient detail to correctly model several types of nonlinearity, including the formation of a nonlinear shell boundary layer as well as the Brazier effect due to nonlinear bending of long cylinders. Stress analysis and initial buckling estimates are formulated for a general variable stiffness cylinder. Results and comparisons for several simplifications of these highly complex governing equations are presented so that the ensuing numerical solutions are considered reliable and efficient enough for in-depth optimization studies. Four distinct cases of loading and stiffness variation are chosen to investigate possible areas of improvement that the variable stiffness concept may offer over traditional constant stiffness and/or stiffened structures. The initial investigation deals with the simplest solution for cylindrical shells in which all quantities are constant around the circumference of the cylinder. This axisymmetric case includes a stiffness variation exclusively in the axial direction, and the only pertinent loading scenarios include constant loads of axial compression, pressure, and torsion. The results for these cases indicate that little improvement over traditional laminates exists through the use of curvilinear fibers, mainly due to the presence of a weak link area within the stiffness variation that limits the ultimate load that the structure can withstand. Rigorous optimization studies reveal that even though slight increases in the critical loads can be produced for designs with an arbitrary variation of the fiber orientation angle, the improvements are not significant when compared to traditional design techniques that utilize ring stiffeners and frames. The second problem that is studied involves arbitrary loading of a cylinder with a stiffness variation that changes only in the circumferential direction. The end effects of the cylinder are ignored, so that the problem takes the form of an analysis of a cross-section for a short cylinder segment. Various load cases including axial compression, pressure, torsion, bending, and transverse shear forces are investigated. It is found that the most significant improvements in load-carrying capability exist for cases which involve loads that also vary around the circumference of the shell, namely bending and shear forces. The stiffness variation of the optimal designs contribute to the increased performance in two ways: lowering the stresses in the critical areas through redistribution of the stresses; and providing a relatively stiff region that alters the buckling behavior of the structure. These results led to an in-depth optimization study involving weight optimization of a fuselage structure subjected to typical design constraints. Comparisons of the curvilinear fiber format to traditional stiffened structures constructed of isotropic and composite materials are included. It is found that standard variable stiffness designs are quite comparable in terms of weight and load-carrying capability yet offer the added advantage of tailorability of distinct regions of the structure that experience drastically different loading conditions. The last two problems presented in this work involve the nonlinear phenomenon of long tubes under bending. Though this scenario is not as applicable to fuselage structures as the previous problems, the mechanisms that produce the nonlinear effect are ideally suited to be controlled by the variable stiffness concept. This is due to the fact that the dominating influence for long cylinders under bending is the ovalization of the cross-section, which is governed mainly by the stiffness parameters of the cylindrical shell. Possible improvement of the critical buckling moments for these structures is investigated using either a circumferential or axial stiffness variation. For the circumferential case involving infinite length cylinders, it is found that slight improvements can be observed by designing structures that resist the cross-sectional deformation yet do not detract from the buckling resistance at the critical location. The results also indicate that bucking behavior is extremely dependent on cylinder length. This effect is most easily seen in the solution of finite length cylinders under bending that contain an axial stiffness variation. For these structures, the only mechanism that exhibits improved response are those that effectively shorten the length of the cylinder, thus reducing the cross-sectional deformation due to the forced restraint at the ends. It was found that the use of curvilinear fibers was not able to achieve this effect in sufficient degree to resist the deformation, but that ring stiffeners produced the desired response abmirably. Thus it is shown that the variable stiffness concept is most effective at improving the bending response of long cylinders through the use of a circumferential stiffness variation. / Ph. D.

No / Visual impairment is an important risk factor for falls. However, relatively little is known about how visual impairment affects stair or step negotiation. The aim of the present study was to determine the effects of blurred vision on the mechanics of landing during stepping down by the elderly. Twelve elderly subjects (72.3±4.7 year) stepped down from three levels (7.2 cm, 14.4 cm and 21.6 cm). Step execution time, ankle and knee joint angular displacements at the instance of ground contact, and vertical landing stiffness and the amount of bodyweight supported by the contralateral (support) limb during the initial contact period were recorded. Measurements were repeated with vision blurred by light scattering lenses. With blurred vision, step execution time increased (P<0.05), knee flexion and ankle plantar-flexion increased (P<0.05), vertical stiffness decreased (P<0.01), and the amount of bodyweight being supported by the contralateral leg increased (P<0.05). These findings suggest that under conditions of blurred vision, subjects were more cautious and attempted to ‘feel’ their way to the floor rather than ‘drop’ on to it. This may have been an adaptation to increase the kinaesthetic information from the lower limb to make up for the unreliable or incomplete visual information. Correcting common visual problems such as uncorrected refractive errors and cataract may be an important intervention strategy in improving how the elderly negotiate stairs.

BACKGROUND: The cornea is the outer portion of the eye and protects the eye from infection or debris. When the cornea becomes compromised due to age and disease (specifically Diabetes Mellitus), it becomes impaired and can have profound impacts on an individual’s quality of life by leading to vision loss or blindness. The different layers of the cornea all contain many proteins and collagen, and have varying degrees of thickness and biomechanical properties. Stiffness in the cornea has either been measured via the use of AFM (Atomic Force Microscopy) which involves removing a slice of the cornea and adhering to the surface, as a function of IOP (Intraocular Pressure), or tensile testing. Previous research has also used the nanoindenter to measure the stiffness of different layers in the intact globe (eyeball) within the mouse head or by adhering to PEG submerged in PBS. However, no studies to our knowledge have used the intact globe exposed to air and placed on a 3D printed model to measure different corneal layers via the use of nanoindentation. METHODS: 6 C57BL/6J mice were obtained between 8-12 and 27 weeks of age, had the eyes extracted, and half remained with intact epithelium while the other half had the epithelium abraded with a 1.5 mm trephine. The eyes were placed in keratinocyte solution (KCM) for preservation while they were transported to the site with a nanoindenter. The globes were then placed on a 3D printed holder, cornea facing up, and irrigated with KCM solution in between indentation measurements. The PIUMA Optics 11 Nanoindenter was used to measure the Effective Young’s Modulus of the epithelium, basement membrane, and stroma. The Oliver & Pharr modeling was used as opposed to the Hertzian Model due to the biomechanical and adhesion properties of the eye. RESULTS: A comparison of control mice at 9 weeks shows an average Effective Young’s Modulus of 30.73 kPa, and an average Effective Young’s Modulus for 15 week old mice of 62.50 kPa for the epithelium. The average Effective Young’s Modulus of the basement membrane for 9 week control mice was ~6.2 kPa and for older 27 week mice was ~6 kPa. The Effective Young’s modulus for the stroma of 9 week old mice was ~68.3 kPa and for 27 week old mice was ~ 222.7 kPa. CONCLUSION: It was observed that stiffer substrates (in this instance, stiffer layers) require stiffer probes. The opposite is true of softer substrates, which require softer probes. It is beneficial in either instance to use a larger tip radius as there will be more contact and surface area measurement, so the probe has less recoil due to the adhesion from the corneal layers. The values observed in this study correlated with the values seen in the study conducted by Xu et al. However, the basement membrane values were different and could be due to probe specifications or layer thinness. Additional studies are needed to observe changes in Young’s Modulus based on probe characteristics with diseases such as Diabetes Mellitus (DM).

Anterior cruciate ligament (ACL) injuries are common orthopaedic injuries among athletes who participate in sports that involve cutting and changing directions. Many of these adolescent athletes intend to return to sports (RTS), and therefore undergo ACL reconstruction (ACLR). These athletes exhibit unfavorable landing biomechanics from muscle atrophy and asymmetrical neuromuscular control post-ACLR, putting them at a higher risk of re-injury. Thus, rehabilitation following ACLR is important to improve kinetic and kinematic outcomes and reduce re-injury risk. Biofeedback during rehabilitation is thought to be one way to potentially restore neuromuscular control deficits of athletes recovering from ACLR. Therefore, understanding the effectiveness of a biofeedback intervention on factors associated with re-injury among post-ACLR patients is essential in successful RTS. The purpose of this study is to analyze the effect of a 6-week biofeedback intervention on eccentric knee joint power (ECCKP), limb stiffness, and limb stiffness symmetry (using normalized symmetry index, NSI), in addition to secondary lower extremity outcomes that are associated with these metrics, during landing among patients following ACLR. This study used data collected from an ACL-Biofeedback Trial (ClinicalTrials.gov: AR069865) where participants were randomized into a biofeedback (BF) or control group (C). The BF group received visual and tactile feedback during a series of controlled squats while the C group participated in several online and in-person educational sessions. Participants completed 10 stop-jump tasks before (pre), after (post), and 6 weeks after (ret) the intervention. Kinetic, kinematic, and ground reaction forces (GRF) were collected from embedded force plates and 3D motion capture. Partaking in a biofeedback intervention did not improve ECCKP, limb stiffness, or limb stiffness NSI compared to controls. A group-by-time interaction was found for hip excursion (p=0.035), and a main effect of time was found for ECCKP, with this variable increasing by 18.5% from pre to ret (p=0.001). In addition, when considering surgical versus non-surgical limbs, this cohort exhibited interlimb asymmetries in stiffness, peak resultant GRF (rGRF), and time to reach peak rGRF (p<0.009). Further, a group-by-limb interaction (p=0.005) and a 7.1% reduction in peak rGRF were found from post to ret (p=0.02). Participants in this study also exhibited limb stiffness asymmetry greater than 10%, which supports existing literature that observed interlimb asymmetries in athletes following ACLR around the typical RTS time (9-12 months post-ACLR). The results from this analysis demonstrated that the current biofeedback intervention was inadequate in improving ECCKP, limb stiffness, and limb stiffness NSI, but additional biofeedback studies with larger sample sizes that investigate task dependencies are needed to better understand the effectiveness of biofeedback interventions. / Master of Science / Anterior cruciate ligament (ACL) injuries are common orthopaedic injuries among athletes who participate in sports that involve cutting and changing directions. Many of these adolescent athletes intend to return to their pre-injury level, therefore undergo a surgical procedure called ACL reconstruction (ACLR). However, following this procedure, athletes display unsafe and stiff landing patterns due to muscle weakness and asymmetrical neuromuscular, or mind-body, control post-ACLR, which increases their risk of re-injury once they return to sport (RTS) following recovery. Rehabilitation for patients following ACLR is of the utmost importance in improving unsafe movement patterns to reduce the risk of re-injury. Biofeedback training refers to receiving external signals that can be processed and transferred to the muscles in the body. This technique aims to restore the neuromuscular deficits of athletes following ACLR and could potentially be helpful during ACLR rehabilitation. Therefore, understanding the effectiveness of a biofeedback intervention on outcomes associated with an increased risk of re-injury in patients following ACLR is important to safely RTS. The purpose of this study is to determine the effect of a 6-week biofeedback intervention on the ability of the knee to absorb impact forces (quantified as eccentric knee joint power, ECCKP), limb stiffness, and limb stiffness symmetry (measured with normalized symmetry index, NSI), along with secondary outcomes related to these variables, among patients following ACLR. This study used data collected from an ACL-Biofeedback Trial (ClinicalTrials.gov: AR069865) where participants were randomized into a biofeedback (BF) or control group (C). The BF group received visual and resisted feedback during a series of controlled squats while the C group participated in several online and in-person educational sessions. Participants completed 10 stop-jump tasks before and after the intervention, and biomechanical data was obtained. The biofeedback intervention did not result in an improved ability for the knee to absorb impact from landing, and it was not able to decrease limb stiffness or limb stiffness asymmetry. It was able to improve hip excursion, which allows for a favorable, less upright posture when landing. ECCKP improved for both groups, indicating that the biofeedback did not add extra benefit to the participant's rehabilitation outside of the study. Asymmetries were observed between the surgical and non-surgical limbs in limb stiffness, peak GRF, and the time it takes to reach this peak GRF. This sample exhibited limb stiffness asymmetry greater than the recommended 10% threshold, raising concern for when these athletes RTS. The results from this analysis demonstrated that the current biofeedback intervention was inadequate in improving ECCKP, limb stiffness, and limb stiffness NSI, but biofeedback in ACLR rehabilitation can still be efficacious in improving hip biomechanics and overall neuromuscular control but may be task-dependent and call for a larger sample size.

The objective of this research is to develop the relationship between starting steps and post newels attached by different connection systems. This research focuses on analyzing solid and box posts connection performance under monotonic and reversed cyclic loads by following European standards EN 26891:1991 and EN 12512:2001, respectively. Moment carrying capacity, stiffness, energy dissipation, and ductility were calculated. Four connection systems were tested, two for solid posts (Sure-Tite™ and Fas-N-Fast™) and two for box posts (L-brackets and wood block with lag bolt). Connections had higher load capacity during the monotonic loading protocol than in the reversed cyclic loading protocol. No strength difference was observed between the solid post connection systems. However, Sure-Tite™ presented a more ductile behavior. For box posts, the L-brackets connection system was superior in strength, while the wood block with lag bolt system had a greater ductile behavior.

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Finite element simulation of contact/impact problems using the penalty method is a well-established capability. The automatic penalty stiffness provides an easy way to implement the contact analysis. However, this way in which the penalty stiffness is associated with the material property and geometry of the master surface can lead to inappropriate distributions of contact pressure at edges or mesh transitions, or even cause much numerical noise. A new method of defining the penalty stiffness, which is associated with geometry of the slave surface, the reference penetration and reference contact pressure, is developed to consistently relate forces on contacting nodes with the contact pressure. This technique is successfully applied to several examples as the clamping simulation during the punch test and the rolling process. The results of such applications of new contact stiffness model demonstrate the effectiveness of such a model in avoiding the stress edge effect and the accompanying numerical noise. As an alternative approach to define the penalty stiffness, this new model provides another option for the contact analysis and gives the users more possibilities to control the contact performance. / Thesis / Master of Engineering (ME)

Spinal stability describes the ability of the neuromuscular system to maintain equilibrium in the presence of kinematic and control variability, and may play an important role in the etiology of low-back disorders (LBDs). The primary mechanism for the neuromuscular control of spinal stability is the recruitment and control of active paraspinal muscle stiffness (i.e., trunk stiffness). The two major components of active muscle stiffness include the immediate stiffness contribution provided by the intrinsic stiffness of actively contracted muscles, and the delayed stiffness contribution provided by the reflex response. The combined behavior of these two components of active muscle stiffness is often referred to as "effective stiffness". In order to understand the neuromuscular control of spinal stability, stochastic system identification methods were utilized and nonparametric impulse response functions (IRFs) calculated in three separate studies in an effort to: 1) Quantify the effective dynamics (stiffness, damping, mass) of the trunk Nonparametric IRFs were implemented to estimate the dynamics of the trunk during active voluntary trunk extension exertions. IRFs were determined from the movement following pseudo-random stochastic force disturbances applied to the trunk. Results demonstrated a significant increase in effective stiffness and damping with voluntary exertion forces. 2) Quantify the reflex dynamics of the trunk Nonparametric IRFs were computed from the muscle electromyographic (EMG) reflex response following a similar pseudo-random force disturbance protocol. Reflexes were observed with a mean response delay of 67 msec. Reflex gain was estimated from the peak of the IRF and increased significantly with exertion effort. 3) Separate the intrinsic and reflexive components of the effective dynamics and determine the relative role of each in the control of spinal stability. Both intrinsic muscle and reflexive components of activation contribute to the effective trunk stiffness. To evaluate the relative role of these components, a nonlinear parallel-cascade system identification procedure was used to separate the intrinsic and reflexive dynamics. Results revealed that the intrinsic dynamics of the trunk alone can be insufficient to counteract the destabilizing effects of gravity. This illustrates the extreme importance of reflexive feedback in the maintenance of spinal stability and warrants the inclusion of reflexes in any comprehensive trunk model. / Ph. D.