Luge is an Olympic sport in which athletes ride feet-first on sleds down an ice-covered track. Competitors spring from the starting position and accelerate their sled by paddling with spiked gloves against the ice surface. Once the Luger leaves the starting section, their downhill motion is solely propelled by the effects of gravity. Athletes compete, one after the other, for the fastest time. Runs can differ by as little as a thousandth of a second, meaning that every minor sled adjustment, change of line choice, and shift of body position is critical. In the past, the sport of Luge has progressed through a series of steps involving trial and error, where changes to the sled and strategy rely more on intuition and race results, rather than in-depth, mathematical analysis. In an effort to try and improve track times for the US Olympic Luge team, a track and driver model is in development in order to simulate a sled going down the track. By doing this, the hope is to be able to pinpoint areas of possible improvement to the sled and see how adjustments can affect the optimum line down the track. A part of this model, which is the focus of the following paper, is the inclusion of an analysis to identify the frictional relationship between the ice surface and the steels of the sled. The model created of the ice-steel interaction was put in the form of a function file, which includes inputs of down force, ice temperature, sled velocity, and steel geometry. Creation of this model and completion of a set of parametric studies allowed for further understanding the interaction between the sled steels and ice surface, specifically applying to the sport of Luge. The model predicts for lower temperatures that at slower sled velocities the coefficient of friction is greater compared to faster sled velocities. This relationship inverts as the ice temperature moves closer to the melting temperature. A sharper steel edge radius was found to be beneficial in lowering the coefficient of friction at lower sled velocities. The sharp edge radius friction benefit decreases as the sled speed increases and is predicted to actually increase friction slightly compared to duller blades at greater velocities. A flat as possible rocker radius lowers friction at all sled velocities, as well as in banked turns where two contact patches are possible. On curves, the pressure on the steel is increased due to the effects of centripetal accelerations. A 1 g versus 5 g normal loading, experienced on the last turns of the track, increases the coefficient of friction on the blade, but also increases the allowable lateral force on the sled before side slip occurs. Understanding the relationships of these parameters, along with the information that may be gained from the driver model, may prove to be useful in choosing optimum sled characteristics and line choice.

This thesis deals with the study of the effect of the constituents of the model synovial fluid on the frictional properties of articular cartilage. The influence of constituents, concentration, speed and load is observed. Experiments were performed on a commercial tribometer at configuration pin-on-plate. Several types of lubricants containing synovial fluid constituents have been selected for the experiments. Lubricants were prepared at two concentrations, the concentration of healthy individuals and at a concentration typical of for osteoarthritic patients. Speeds 5 and 10 mm/s and 5 and 10 N loads were used for all experiments. It is shown that when using only lubricant containing proteins, no difference in the coefficient of friction is observed and the effect of concentration is also not observed. The addition of hyaluronic acid has a synergistic effect with -globulin, however in the case of lubricants containing albumin, the effect is opposite. After the addition of phospholipids, no significant effect on friction is observed in -globulin containing lubricants. No significant effect of the composition and concentration of the lubricants is observed with the load change.

In Tribology, the well-known Stribeck curve is often used to relate friction behaviour to the properties of a system. Richard Stribeck, (Jacobson, 2003), (Stribeck, 1902) developed these curves while researching various bearings and found that the Stribeck curve can be generated for all tribological contacts of the Hertzian type. These curves give a relation between the coefficient of friction and the Sommerfeld number for a lubricant and given surfaces. All his test work was done on contacts that move in a single direction, unidirectional motion. This leaves the question, could Stribeck curves be applied to reciprocating contacts and how does linear velocity affect wear in these contacts? This research project is aimed to further the knowledge on how linear velocity affects reciprocating contacts with a focus on Stribeck curves and lubricity. Tests were conducted on two reciprocating instruments using ball and disk configurations. Two parameters were varied to change the linear velocity, namely oscillating frequency, and stroke length. To shift focus away from viscosity, n-Hexadecane was used as the base fluid due to its lack of lubrication properties. To improve the base fluid lubricity 3 carboxylic acids with 3 different chain lengths were used as additives. / Dissertation (MEng (Chemical Engineering)) Univercity of Pretoria, 2021. / Chemical Engineering / MEng (Chemical Engineering) / Restricted

The Stanton gauge technique provides an indirect method for measurement of skin friction on a smooth aerodynamic surface in which a pressure tap is available. This thesis presents the design and evaluation of a new type of skin friction measurement gauge based on the Stanton gauge concept but not requiring a surface pressure tap. This new skin friction measurement gauge, called a "Flow Tab", can therefore be used on an aerodynamic model or aircraft surface without alteration of the surface. The Flow Tab is thus particularly well-suited to use with Cal Poly's Boundary Layer Data System (BLDS), a small, self-contained instrument that can be installed onto a model or aircraft surface without permanent alteration of the surface. A series of preliminary experiments conducted in a low-speed wind tunnel on a flat plate model with mild favorable pressure gradient, with both laminar and turbulent boundary layers, led to selection of three variants of the Flow Tab design. These Flow Tabs had edge heights of 0.002, 0.0035, and 0.005 inches, giving dimensionless heights h+ of 1.4 -16 over the streamwise Reynolds number range of about 0.7 to 2.2 million. Uncertainty analysis and test results demonstrated that better than 10% measurement uncertainty for the Flow Tab results could be achieved with edge heights of 0.0035 and 0.005 inches using the same calibration equations as published for the Stanton gauge. Further investigation of its performance over a wider range of Reynolds numbers, and in more complex conditions including those encountered on swept wings with a variety of pressure gradients, is recommended. Integration of the flow tab with BLDS for flight testing applications presents challenges related to its relatively small pressure signal that may require some special modifications to existing BLDS hardware and software.

L’objectif général de cette thèse était d’identifier les mécanismes moléculaires du frottement d’un fluide de polymère sur une surface solide à partir de l’écart à la condition de vitesse nulle à la paroi.La première partie de cette thèse a consisté à développer une nouvelle méthode de mesure du glissement des liquides de polymère par suivi d’un motif photolysé dans un fluide fluorescent. Cette méthode, qui est une évolution de celle utilisée précédemment dans le groupe, donne directement accès au champ de déplacement du fluide lors de l’écoulement. À l’aide de ce nouveau dispositif, nous avons étudié expérimentalement l’effet de la nature de la surface, de la viscosité du liquide et de la température sur le glissement de fondus de polymère (T>Tg). Il est apparu que l’effet de glissement est contrôlé par la viscosité des fondus et par un coefficient de frottement à l’interface qui ne dépend que des natures moléculaires du liquide et de la surface. En particulier, ce coefficient de frottement est le même pour un fondu ou pour un élastomère constitué du même polymère. La dépendance en température de cet effet est fixé par la différence des énergies d’activation des processus activés que sont la viscosité du liquide et le frottement sur la surface solide. Nous avons également étudié le cas des solutions concentrées de polymère pour lesquelles le mécanisme du frottement est différent. En effet nous avons montré que le coefficient de frottement n’est plus une grandeur locale et dépend de la concentration de la solution. Enfin nous nous sommes intéressés à l’évolution temporelle de la transition de glissement dans les fondus et les solutions causée par la présence de chaînes de polymère adsorbées sur les parois. / The goal of this work was to identify the molecular mechanisms governing the friction of a polymeric liquid on a solid surface by studying the deviation from the no-slip boundary condition.First, we developed a new method of measurement of slip of a polymeric liquid based on the observation under shear of a pattern photobleached in a fluorescent fluid. This method, which is an evolution of one already used in the group, give a direct access to the displacement field of the liquid under shear. Using this setup, we studied experimentally the effect of the chemical nature of the surface, of the liquid viscosity and of the temperature on the slip of polymer melt (T>Tg). It appeared that the slip effect is governed by the viscosity of the liquid and a friction coefficient which depends only on the chemical natures of the liquid and the surface. In particular this coefficient is the same for a melt and for an elastomer made of the same polymer. The temperature dependence of this effect is characterized by the difference of activation energy of viscosity of the liquid and of the friction on the solid surface that both are activated processes. We also studied the case of concentrated polymer solutions for which the molecular mechanism of friction differs. Indeed we showed that the friction coefficient is no longer a local quantity and depends on the concentration. Finally, we investigated the evolution over time of the slip transition of polymer melts and solutions which is due to the adsorption of polymer chains on the solid walls.

AN ABSTRACT OF THE THESIS OFChristoph Michael Goettler, for the Master of Science degree in Mechanical Engineering, presented on Nov 6, 2020, at Southern Illinois University Carbondale. TITLE: EFFECT OF DENSITY ON FRICTION AND WEAR PERFORMANCE OF CARBON-CARBON COMPOSITE MATERIALSMAJOR PROFESSOR: Dr. Peter FilipCarbon-carbon (C/C) composite materials exhibit high thermal conductivity, high thermal stability, low density, and high mechanical strength. Due to these properties, C/C composites are ideal for use in high performance braking systems. However, C/C composites are incredibly expensive to manufacture, and thus improving the longevity of these materials is vital. C/C composite materials inherently have a density gradient due to manufacturing limitations. By determining the effect of density on friction and wear performance of C/C composite materials, manufacturers could use that data to alter manufacturing methods to improve the lifespan of C/C composites. In this study, the effect of density on friction and wear performance of C/C composite materials was studied. Friction tests were conducted through use of a universal mechanical tester (UMT) manufactured by Bruker and subsequent analysis was done through use of scanning electron microscopy, energy dispersive x-ray spectroscopy, and polarized light microscopy. Numerous samples from depths of 0 mm and 5 mm were taken from two C/C composite materials with varying matrices and friction tested at varying conditions to determine friction properties, friction surface characteristics, microstructure just below the friction surface characteristics, friction layer characteristics, and wear characteristics. Density, apparent density, and apparent porosity gradients were also measured to be able to correlate observations to density differences. It was observed that while density does not seem to be the main cause in differences in friction and wear performance of C/C materials at depths of 0 mm and 5 mm, there still existed significant differences in friction performance, wear performance, and post friction test material characteristics when comparing 0 mm samples to 5 mm samples. In conclusion, density was not found to be a significant cause in variations in friction performance. However, friction surface depth was found to have a significant effect on friction performance, wear performance, and the friction surface. Further research is needed to be able to determine the exact cause of the variations in performance at depths of 0 mm and 5 mm. Keywords: carbon-carbon, composite, C/C, density, friction, wear, brake

Systematic investigation of the Friction Stir Welding (FSW) process shows that a fixed rotational velocity and feed rate may not yield uniform mechanical properties along the length of a weldment. Nevertheless, correlations between process parameters and post-weld material properties have successfully demonstrated that peak temperature and cooling rate drive post-weld properties. There have been many reported methodologies for controlling friction stir welding, with varying degrees of cost to implement and effectiveness. However, comparing data from uncontrolled FSW of AA 6111-T4 sheet with controlled FSW at temperatures ranging from 375 °C to 450 °C demonstrates that a simplified methodology of a single-loop PID controlling with spindle speed may be used to effectively control temperature. This methodology can be simply used with any machine that already has the ability to actively control spindle speed, and has been previously shown to be able to be auto-tuned with a single weld. Additionally, implementation of this method compared to uncontrolled FSW in AA6111 at linear weld speeds of 1-2 meters per minute showed improved mechanical properties and greater consistency in properties along the length of the weld under temperature control. Further results indicate that a minimum spindle rpm may exist above which tensile specimens did not fracture within the weld centerline, regardless of temperature. This work demonstrates that a straight-forward, PID-based implementation of temperature control at high weld rates can produce high quality welds with auto-tuned gains. This method also shows promise in application to other processes in the Friction Stir family, and preliminary results in an application to the Additive Friction Stir Deposition (AFSD) process are also presented.

Temperature control in friction stir welding (FSW) is of interest because of the potential to improve the mechanical and microstructure characteristics of a weld. Two types of active temperature control have been previously implemented for steady-state friction stir welding conditions: PID Feedback Control and Model Predictive Control. The start-up portion of a weld is an obstacle for these types of active control.To date, only minimal exploratory research has been done to develop an active temperature controller for the start-up portion of the weld. The FSW temperature controller presented in this thesis, a Position-Velocity-Acceleration (PVA) controller implemented with gain-scheduling, is capable of active control during the start-up portion of a weld. The objectives of the controller are (1) to facilitate fully-automated active temperature control during the entire welding process, (2) to minimize the rise time, the settling time, the percentage maximum post-rise error (overshoot calculated as a percentage of the settling band half-width), and the post-settled root-mean-square (RMS) of the temperature error, and (3) to maintain the steady state performance of previous control methods.For welds performed in 6.35 mm plates of 7075-T651 Aluminum with controller gains identified through a manual tuning process, the mean controller performance is a rise time of 10.82 seconds, a settling time of 11.35 seconds, a percentage maximum post-rise error of 69.86% (as a percentage of the 3◦C settling band half-width), and a post-settled RMS error of 0.92◦C.Tuning of the start-up controller for operator-specified behavior can be guided through construction of regression models of the weld settling time, rise time, percent maximum post-rise error, and post-settled RMS error. Characterization of the tuning design space is performed through regression modeling. The effects of the primary controller tuning parameters and their interactions are included. With the exception of the post-settled RMS error model, these models are inadequate to provide useful guidance of the controller tuning, as significant curvature is present in the design space. Exploration of higher-order models is performed and suggests that regression models including quadratic terms can adequately characterize the design space to guide controller tuning for operator-specified behavior.

Friction stir welding (FSW) is an advantageous solid-state joining process, suitable for many materials in the energy, aerospace, naval and automotive industries. Like all other welding processes, friction stir welding requires non-destructive evaluation (NDE). The time and resources to preform NDE is expensive. To reduce these costs, nontraditional NDE methods are being developed for FSW. Spectral based defect recognition uses the forces during the welding process to validate weld quality. Although spectral NDE methods have shown promise as an alternative NDE processes, many research welding speeds do not correspond to manufacturing speeds, nor do they explain the relationship between the spectral data and the process. The purpose of this work is to explore the possibility of acquiring additional information about the defect. Namely the defect’s type, location, and magnitude. In this study, welds with “wormhole” defects were produced at 2000, 2500 and 3000 mmpm in 5754 aluminum. The welding process forces and torque were measured and analyzed spectrally. The welded plates were then imaged with x-ray photography, a validated NDE method. It was found that low frequencies (0 – 4 Hz) in the y & z force signals correlate with defect presence in high speed FSW. In addition, the strong correlation between the spectral data and the presence of a defect allowed for defect magnitude predictions. Linear fits were applied to the defect measurements and the spectral data. Large error inhibits the wide use of this prediction method.

During braking, both the rotor and the pads in disc brakes are worn. Since disc brakes are not sealed, some of the wear particles generated can become airborne. Several studies have found an association between adverse health effects and the concentration of particles in the atmosphere, so it is of interest to improve our knowledge of the airborne wear particles generated by disc brakes. However, in field tests it is difficult to distinguish these particles from others in the surrounding environment, so it may be preferable to use laboratory test stands and/or simulation models to study the amount of airborne wear particles generated. This thesis deals with a simulation methodology for prediction of airborne wear particles from disc brakes and three experimental methods for testing disc brake materials with focus on airborne wear particles. The four appended papers discuss the possibility to both measure and predict the number and size distribution of airborne wear particles that originate from the pad to rotor contact. The objective is to develop a simulation methodology that predicts the number and size distribution of airborne wear particles from disc brakes. Paper A describes how a modified pin-on-disc machine was used to study airborne wear particles originating from different disc brake materials. The results indicate that the test setup can be used to measure and rank the number concentration and size distribution of the airborne wear particles generated. Paper B describes a disc brake assembly test stand for measurements of airborne wear particles from disc brakes. The results indicate that the test setup can be used to measure the number concentration and size distribution of airborne wear particles generated from disc brake materials. The results also indicate a promising ability to rank different pad/rotor material combinations with respect to the number concentration of airborne wear particles. Paper C compares measurements made in passenger car field tests with measurements made in a disc brake assembly test stand and in a pin-on-disc machine. A promising correlation between the three different test methods is found. Paper D presents a simulation methodology for predicting the number and size distribution of airborne wear particles using finite element analysis (FEA). The simulated number distribution is compared with experimental measurements at component level. The result indicates that the proposed methodology may be used to predict the number concentration and size distribution of airborne particles generated in the pad-to-rotor contact.