Block stability is common and has to be studied in detail for designing tunnels. Stability of block depends upon the shape and size of the blocks, stresses around the block and factors such as clamping forces and the ratio between joint stiffness. These factors are studied in detail and are the main objective of this thesis. In this thesis influence of loading and unloading of blocks on joint stiffness and thus on ultimate pullout force are analyzed. Normal stress on the joint plane is linked with shear stiffness of the joint and relaxation of forces. Changes of forces were considered to estimate joint stiffness and ultimate pullout force using new methods in the present thesis. First method takes into account changing clamping forces considering stiffness ratio constant (Crawford and Bray). The second method was developed in which the ratio between normal and shear stiffness was taken as a function of normal stress (Bagheri and Stille). In third method, gradually pullout force is increased which changes the normal stress and joint stiffness. The lower limit of joint stiffness gives a very conservative design. So a stiffness value based on the average of lower and upper limit of normal force has also been considered. A comparison between the new methods and the previous method proposed by Crawford and Bray which considers a constant ratio of normal and shear stiffness and constant clamping forces shows that Crawford and Bray’s solution overestimates the pullout forces hence the design is unsafe. It was observed that stiffness ratio is an important factor for estimating required rock support and safety.

Femoroacetabular impingement syndrome (FAIS) is a disorder characterized by specific morphology of the femur and/or acetabulum, which may lead to hip pain during gait. Compared to individuals without pain, people with FAIS walk with more anterior pelvic tilt, and their pain may result from excessive anteriorly-directed hip joint forces. Previous approaches using musculoskeletal modelling to calculate joint forces, however, may inaccurately assume that each individual stands in an entirely neutral position when determining static joint angles. Consequently, information on parameters that affect joint forces (such as pelvic tilt) is lost in kinematic data used to estimate joint loading. To observe the effect of computationally altered pelvic tilt on joint forces, gait data of six healthy individuals were processed using Vicon and Visual3D. Each participant’s pelvic tilt was adjusted by ±5 degrees and ±10 degrees of tilt at all time points. Five analyses were performed per individual: no change in tilt, two posterior (positive) tilts, and two anterior (negative) tilts. The resulting data were imported into OpenSim to estimate forces from the femur onto the acetabulum in the anterior, superior, and medial directions. Data for each participant were normalized for gait cycle and body weight in MATLAB. Statistical parametric mapping software was used to determine if the differences in joint loads were significant. A more anterior pelvic tilt led to a reduction in anteriorly-directed joint forces, and an increase in the superior and medial directions. Based on these results, each individual’s pelvic tilt (obtained from their stationary kinematic data) was accounted for when modeling FAIS and healthy individuals. Using the same methods as above, the hip joint forces of 22 people with FAIS were compared to those of 22 healthy individuals as both groups walked at a prescribed speed. Although there were reductions in joint forces in both FAIS limbs compared to those of the control group, the differences were not significant, possibly due to the high variability of joint forces. Despite the significant effects of pelvic tilt on hip joint force, other underlying assumptions need to be addressed in musculoskeletal modeling software in order to compare different conditions, such as the use of the same generic model despite differences in sex and hip morphology. Future studies comparing pathological and healthy joint loads can inform researchers on gait alteration strategies and the design of assistive devices to manage the symptoms and onset of conditions such as FAIS. / 2022-05-15T00:00:00Z

The essay charts forty years of Zionist history to illuminate the remarkable evolution of Israel's unified, apolitical army and Israel's "democratic civil-military tradition," forged in the fires of opposing military styles, ideological rivalry, competing underground forces, war and civil war.

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The prevalence of knee osteoarthritis, a degenerative joint disease characterized by the degradation of articular cartilage, is correlated with the rise in obesity. The rising rates of obesity in children and adults highlight the need for identifying a sustainable physical activity that promotes fitness while mitigating initiation and progression of osteoarthritis. The objective of this study was to determine an effective rehabilitation and lifelong fitness sustainment exercise regimen that minimize risk of osteoarthritis in individuals with high body mass index (BMI). The aim was to examine knee medial and lateral contact forces in gait and cycling training. Gait at self-selected speeds and cycling at moderate resistance were studied using motion analysis in normal BMI and high BMI participants. Individuals with high BMI exhibited abnormal kinematics and increased kinetics in gait but neutral knee abduction-adduction angles, lower knee contact forces, and balanced mediolateral force distribution in cycling. The combination of maladaptive kinetics (excessive cartilage loading) and altered kinematics (primarily knee adduction angles) observed in gait for the high BMI cohort demonstrate the profound adverse effect of weight bearing and impact exercises on knee biomechanics. Exercise rehabilitation modalities should aim to minimize cartilage loading, correct altered knee angles, and prioritize balanced mediolateral force distributions in individuals with high BMI. Cycling, a non-weight bearing and low impact exercise, addresses all these factors because it constrains kinematic patterns with the pedals and carries significant body weight on the saddle.

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The flow properties of polydimethylsiloxane (PDMS) melts at room temperature were studied by measurement of lubrication forces using an Atomic Force Microscopy (AFM) colloidal force probe. A glass probe was driven toward a glass plate at piezo drive rates in the range of 12 – 120 μm/s, which produced shear rates up to ~10⁴ s⁻¹. The forces on the probe and the separation from the plate were measured. Two hypotheses were examined: (1) when a hydrophilic glass is immersed in a flow of polymer melt, does a thin layer of water form at the glass surface to lubricate the flow of polymer and (2) when a polymer melt is subject under a shear stress, do molecules within the melt spatially redistribute to form a lubrication layer of smaller molecules at the solid surface to enhance the flow? To examine the effect of a water lubrication layer, forces were compared in the presence and the absence of a thin water layer. The presence of the water layer was controlled by hydrophobization of the solid. In the second part, the possibility of forming a lubrication layer during shear was examined. Three polymer melts were compared: octamethyltrisiloxane (OMTS, n = 3), PDMS (n _avg = 322), and a mixture of 70 weight% PDMS and 30 weight% OMTS. We examined whether the spatial variation in the composition of the polymer melt would occur to relieve the shear stress. The prediction was that the trimer (OMTS) would become concentrated in the high shear stress region in the thin film, thereby decreasing the viscosity in that region, and mitigating the shear stress. / Master of Science

Understanding and controlling the forces between colloidal particles in solution, along with the resulting stability of a dispersion of such particles, continues to be at topic of great interest. Although most laboratory studies focus on model systems in which the number of system species is kept to a minimum, real colloidal systems can be much more complex, consisting of multiple components that can vary greatly in size, charge, shape, etc. This dissertation focused on a topic that has received very little prior study, namely synergistic effects that can arise in mixed colloidal systems in which the resulting force and stability of the system cannot be predicted using results obtained in more idealized systems consisting of fewer components. Two specific systems were studied. The first was a ternary system of particles in which micron-sized particles were in a dispersion containing both nanoparticles and submicron particles. It was shown through both computation modeling and direct force measurements that the nanoparticles can create attractive forces between the micron and submicron particles such that a halo of submicron particles is formed. This halo results in long range forces between the microparticles that cannot be predicted from measurements in systems containing only nanoparticles or only submicron particles. In addition, the forces can be large enough to alter the stability of a dispersion of these microparticles. The second system consisted of microparticles in a solution containing nanoparticles and a polyelectrolyte, specifically poly(acrylic) acid. Again, through modeling and experimentation, it was found that complexation of the nanoparticles and polyelectrolyte molecules led to depletion and structural forces between the microparticles that were substantially greater than the sum of the forces measured in systems of only nanoparticles or only polyelectrolyte. It was also found that these greater forces could lead to destabilization of a dispersion of microparticles that was stable when only nanoparticles or only polyelectrolyte was present. While these results clearly demonstrate the difficulty associated with predicting forces and stability in mixed colloidal systems, they also indicate that such systems offer new and interesting opportunities for controlling stability that clearly warrant additional study. / Ph. D.