This paper will present an online approach for friction approximation to be utilized in con- cert with whole body control on humanoid robots. This approach allows humanoid robots with ankle mounted force-torque sensors to extrapolate information about the friction constraints at the hands during multi-contact poses without the addition of hardware to the platform. This is achieved by utilizing disturbance detection as a method of monitoring active forces at a single external point and deriving available friction force at said contact point in accordance with Coulomb's Law of Friction. First, the rigid body dynamics and required compliant humanoid model optimization are established which allow incorporation of friction constraints. These friction constraints are then informed by monitoring of external forces, which can be used as an indicator of slip based on tangential force. In practice, the robot with operational multi-contact whole body control is navigated to the desired contact surface and normal force only contact is initiated. Using an iterative coefficient estimation based on the achieved system forces, the robot tests the boundaries of its operable force range by inducing slip. Slip detection is utilized as the basis for coefficient estimation, which allows the robot to further understand its environment and apply appropriate forces to its contact points. This approach was implemented on a simple 3 link model to verify expected performance, and then on both the simulated model of Virginia Tech's ESCHER robot and in practice on the actual ESCHER platform. The proposed approach was able to achieve estimation of slip parameters, based largely on time spent measuring, actual friction coefficient, and the available contact force. Though the performance of the proposed approach is dependent on a number of variables, it was able to provide an operational parameter for the robot's whole body controller, allowing expansion of the support region without risking multi-contact slip. / Master of Science

The goal to design and build a reliable instrument for the direct measurement of skin friction on the surface of Magnetically Levitated (MagLev) trains was successfully achieved. A wall mounted, cantilevered beam device was used to measure the small tangential flow shear force that passes over the non-intrusive floating element. Piezoresistive strain gage units measure the relatively small strain that is generated by the wall shear. By adapting the geometry of the sensing unit, this design can be adapted for a variety of test flows. Measurements were made on two different vehicle geometries provided by Northrop/Grumman and Lockheed/Martin as well as on a special model designed to study the influence of propulsion rails on the Lockheed/Martin model aerodynamics. The obtained values of the skin friction coefficient C_t are deemed reasonable for the type of flow studied. The estimated uncertainty of the gage is ± 5.2%. The data agreed to within 10% with estimated values from an idealized Couette flow analysis. / Master of Science

In an effort to save weight, transportation and aerospace industries have increasing investigated magnesium alloys because of their high strength-to-weight ratio. Further efforts to save on material use and machining time have focused on the use of additive manufacturing. However, anisotropic properties can be caused by both the HCP structure of magnesium alloys as well as by layered effects left by typical additive manufacturing processes. Additive Friction Stir (AFS) is a relatively new additive manufacturing technology that yields wrought microstructure with isotropic properties. In this study, Additive Friction Stir (AFS) fabrication was used to fabricate WE43 magnesium alloy, with both atomized powder and rolled plate as filler material, into multilayered structures. It was found that the WE43 alloy made by AFS exhibited nearly isotropic tensile properties. With aging these properties exceeded the base material in the T5 condition. The toughness measured by Charpy impact testing also showed an increase over the base material. The relationships among tensile properties, Vickers microhardness, impact toughness, microstructure and thermal history are developed and discussed. / Master of Science

A device was developed to directly measure skin friction on an external test plate in transonic flight conditions. The tests would take place on the FTF-II flight test plate mounted underneath a NASA F-15 aircraft flying at altitudes ranging from 15,000 to 45,000 ft. at Mach numbers ranging from 0.70 to 0.99. These conditions lead to predicted shear levels ranging from 0.3 to 1.5 psf. The gage consisted of a floating element cantilevered beam configuration that was mounted into the surface of the test plate in a manner non-intrusive to the flow it was measuring. Strain gages mounted at the base of the beam measured the small strains that were generated from the shear forces of the flow. A non-nulling configuration was designed such that the deflection of the floating head due to the shear force from the flow was negligible. Due to the large vibration levels of up to 8 grms that the gage would experience during transonic flight, a vibration damping mechanism needed to be implemented. Viscous damping had been used in previous attempts to passively dampen the vibrations of skin friction gages in other applications, yet viscous damping proved to be an undesirable solution due to its leakage problems and maintenance issues. Three methods of damping the gage without a fluid filled damper were tested. Each gage was built of aluminum in order to maintain constant material properties with the test plate. The first prototype used a small internal gap and damping properties of air to reduce the vibration levels. This damping method proved to be too weak. The second prototype utilized eddy current damping from permanent magnets to dampen the motion of the gage. This mechanism provided better damping then the first prototype, yet greater damping was desired. The third method utilized eddy current damping from an electromagnet to dampen the motion of the gage. The eddy current damper achieved a much larger reduction in the vibration characteristics of the gage than the previous designs. In addition, the gage was capable of operating at various levels of damping. A maximum peak amplitude reduction of 33 % was calculated, which was less than theoretical predictions. The damping results from the electromagnetic gage provided an adequate level of damping for wind tunnel tests, yet increased levels of damping need to be pursued to improve the skin friction measurement capabilities of these gages in environments with extremely high levels of vibration. The damping provided by the electromagnet decreased the deflections of the head during 8 grms and 2 grms random noise vibrations bench tests. This allowed for a greater survivability of the gage. In addition, the reduction of the peak amplitude provided output with vibration induced noise levels ranging from 24 % to 5.9 % of the desired output of the gage. The gage was tested in a supersonic wind tunnel at shear levels of tw=3.9 to 5.3 psf. The shear levels encountered during wind tunnel verification tests were slightly larger than the shear levels encountered on the F-15 flight test plate during the flight tests, but the wind tunnel shear levels were considered adequate for verification purposes. The experimentally determined shear level results compared well with theoretical calculations / Master of Science

A non-intrusive direct-measuring skin friction device with a rubber RTV sheet over the surface of the floating head, gap and housing was developed for application in 3D, unsteady, transonic flight conditions. Design conditions required optimum gage performance at altitudes ranging from 15,000 to 45,000 feet, Mach numbers ranging from 0.6 to 0.99 resulting in shear values of 0.3 to 1.5 psf. under vibration conditions up to 8.0 grms over a 15 - 2,000 Hz frequency range. The gage consisted of a rubber RTV sheet-coated floating element attached to an aluminum cantilevered beam. A dual-axis, full bridge strain gage configuration was used with the application of semi-conductor strain gages to increase instrument sensitivity. The gage was studied with and without a viscous liquid (glycerin) fill in the housing. Vibration verification testing was performed at 1.0 grms in the Virginia Tech modal analysis lab to ensure adequate damping performance over a 0−3200 Hz frequency range. Tests revealed that the rubber RTV compound sheet provided adequate viscoelastic damping, with or without viscous liquid fill. Gage performance verification testing was performed on in the Virginia Tech supersonic wind tunnel at shear levels of tw = 3.9 to 5.3 psf in a Mach 2.4 flow. Skin friction values in good agreement with previous testing and analytical predictions were obtained from the tests with adequate damping in the low vibration environment of the Virginia Tech supersonic wind tunnel. The gage proved robust as it survived repeated runs including the violent start and unstart processes typical of a supersonic, blowdown wind tunnel. Flight tests were performed at NASA Dryden Flight Research Center, with the gage mounted in a plate suspended below an F-15 aircraft. This provided a mildly 3D, turbulent boundary layer on a vibrating surface. The gage was tested without liquid fill in the gage cavity, and it performed satisfactorily in this high vibration environment. The gage demonstrated adequate damping and good robustness, surviving the complete flight test intact and remained fully operational. The sensor measured skin friction values 30%-50% higher than those predicted by indirect methods and analogies generally valid for 2D, steady flows. The gage indicated trends in skin friction values for different flight conditions in good agreement with the other methods. Possible reasons for the differences in numerical values are discussed in detail, including potential uncertainties in the gage output and limitations and uncertainties in the methods used for comparison. Finally, suggestions for further development of such gages are provided for flight test applications. / Master of Science

These studies developed from Scanning Electron Microscope (SEM) observations of abrasive wear of a polymer disk sliding against metal asperity models. The investigator was unable to observe actual particle formation but did identify elastic and plastic deformation of the polymer, and a debris buildup and extrusion process occurring at the leading edge of the asperity. On the assumption that this process could lead to a surface fatigue condition, pin-on-disk wear trials were completed using a spherical steel ball sliding on polycarbonate, rigid PVC, and ultra-high molecular weight polyethylene specimens in dry and lubricated conditions. A delay in debris formation was observed in the rigid PVC and polycarbonate dry sliding trials. In each case a higher rate of friction force increase coincided with debris formation. No debris was produced in the ultra-high molecular weight polyethylene dry sliding trials, and the friction force trace was flat. An SEM analysis of the polycarbonate and rigid PVC wear tracks revealed pitting consistent with the Delamination Theory of wear. The effect of the lubricants was to significantly alter the form of the friction force traces, but not to eliminate wear in rigid PVC and polycarbonate. The results of the investigation, particularly the delay in wear debris generation, indicated that a fatigue wear mechanism appeared to exist in dry metal pin-on-polymer disk sliding systems. A qualitative wear model was developed to relate the in-situ SEM observations and the results of the pin-on-disk trials. / M.S.

The key to a successful wetland design is duplicating the hydroperiod of the desired wetland type. Dense wetland vegetation affects surface water flow rates by increasing flow resistance. Prior research represented the vegetation as individual stems; however, many wetland species grow in clumps. Therefore, the objectives of this study were to investigate the effect of clumping vegetation on flow resistance and to develop a prediction equation for use in wetland design. A 6-m by 1-m by 0.4-m recirculating flume was planted with mature common rush, Juncus effusus, a common emergent wetland plant. Three different flow rates (3, 4, and 5 L/s) and three different tailgate heights (0, 2.5, and 5 cm) were used to simulate a variety of wetland conditions. Plant spacing and clump diameter were varied (20 and 25 cm, 8 and 12 cm, respectively). Friction factors ranged from 9 to 40 and decreased with increasing plant density. Non-dimensional parameters determined through Buckingham Pi analysis were used in a regression analysis to develop a prediction model. Results of the regression analysis showed that the fraction of vegetated occupied area (P) was most significant factor in determining friction factor. / Master of Science

Although contact problems with friction have received considerable attention in recent years, analytical as well as experimental limitations have tended to obscure some of their essential features. All the experimental techniques employed in the past lacked either required sensitivity or adequate spatial resolution for local measurements of deformation near the contact surface. Further, most techniques also did not allow the use of prototype material which is crucial for investigation of contact stress problems with friction. In the present study, a relatively new experimental method, which has been developed at VPI & SU, is employed: high-sensitivity moiré interferometry. Using a clearance pin-joint model made of prototype structural material, Aluminum 7071-T6, studies on frictional phenomena between the pin and plate are conducted to provide a comprehensive treatment of the following subjects: slip-stick phenomena, variation of contact zone, distribution of frictional force at the contact region, effects of frictional shear stress on stress concentration, and identification of slip amplitude. Experimental techniques and algorithms of analysis necessary for the research are further developed. The results showed a strong influence of friction, including significant differences in the load-increasing and load-decreasing phases. / Ph. D.

L'objectif de cette thèse est de contribuer à la compréhension de l'interaction entre des sollicitations de haute fréquence tangentielles et le déclenchement du glissement macroscopique, ainsi que la dynamique frictionnelle subséquente. Le travail est basé sur une approche expérimentale permettant d'étudier, à des fréquences ultrasonores, les propriétés élastiques et dissipatives de contacts sphère-plan en régime linéaire et non-linéaire. La méthode consiste à mesurer les caractéristiques de la résonance d'un piezoélectrique (fréquence de résonance et facteur de qualité) modifiées par l'établissement d'un contact mécanique. Le seuil de glissement est ajusté par le contrôle de la nature moléculaire de l'interface frictionnelle, constituée de films nanométriques greffés ou déposés par spin-coating. Nous commençons par étudier les propriétés physiques et mécaniques en régime linéaire de ces interfaces contrôlées. Ensuite, nous évaluons l'influence de l'amplitude des oscillations sur le l'angle de repos d'une sonde placée sur un plan incliné. Lorsque l'amplitude augmente, le seuil de glissement est abaissé quelle que soit la polarisation des oscillations. L'effet des vibrations est de diminuer la surface effective sur laquelle est appliquée la contrainte seuil statique, ce qui diminue la force seuil. Nous discutons de ces résultats à l'aide interprétation basée sur l'énergie vibratoire, qui jouerait un rôle proche d'une température effective dans un diagramme de « jamming ».Ensuite, nous étudions le comportement de contacts sphère-plan à de très hautes amplitudes oscillatoires. Nous observons un régime micro-glissement partiel au cours duquel le comportement du contact oscille entre micro-glissement partiel et total. Aux plus hautes amplitudes, les oscillations provoquent une fracture interfaciale et le rayon de contact diminue. Enfin, l'application d'oscillations tangentielles augmente la vitesse de glissement d'un patin soumis à une force constante. Dans le cas d'un film interfacial macromoléculaire, nous discutons des diverses interprétations possibles aux échelles microscopiques / The aim of this work is to bring new insights to the understanding of the interaction between high frequency tangential vibrations and the macroscopic triggering of sliding, as well as the subsequent frictional dynamics. This experimental work is based on a method that allows studying, at ultrasonic frequencies, the elastic and dissipative characteristics of sphere-plane contacts, in linear and non-linear regimes. The method relies on the measurement of the resonance frequency and the quality factor of a quartz, which are modified by the mechanical contact with a probe. The sliding threshold is adjusted by the control of the molecular nature of the frictional interface, which can be either grafted or spin-coated onto the quartz. We first characterize the physical and mechanical properties of the various interfaces in the linear regime. Then, we study the effects of the oscillation amplitude on the angle of repose of the probe in contact with the quartz which is grafted with monolayers of high and low adhesion energy. When the oscillation amplitude increases, the angle of repose decreases whatever the polarisation of the vibrations. The vibrations shall decrease the effective surface area on which the static stress applies, which decreases the static force at the threshold. We discuss the results within the framework of the Mindlin model, which suggests that the oscillatory energy could play the role of an effective temperature in a jamming diagram. In the last part of the work, we study the response of the mechanical contact at very high amplitudes. We observe a micro-slip regime beyond the Mindlin limit, where the behaviour of the contact oscillates between partial and full microslip, leading eventually to interfacial fracture. When the probe is sliding on the incline at constant velocity, the high frequency vibrations increase the sliding velocity. In the case of a macromolecular interfacial film, we propose a possible explanation for this effect, based on a mechanism at the local scale

This project was performed together with ÅF Industry AB in Trollhättan, Sweden. ÅF’s expertise in Trollhättan is oriented towards the automotive industry. It was conducted within the section of CAE and safety where they, for instance, dimension bolt joints in the cars. Bolt joints play an important role in the automotive industry. Slip critical bolt joints are used widely throughout the vehicles. With lack of good test data, the bolt joints need to be dimensioned conservatively. This may lead to that bolt joints are over-dimensioned, adding more mass to the car. On the contrary, the availability of reliable test data enables designers to optimize joint dimensions to achieve a safe design with minimized mass. A mechanical testing configuration has been designed as well as a testing procedure for a test to determine the static friction value between mating surfaces in bolt joints. The testing configuration has been used to perform tests to find the static friction coefficient in different materials. The study contains varied combinations of steel, aluminium and ED-Coated steel. The study resulted in tables with levels of probability. The developed test configuration is robust and relatively simple to use and is recommended for further use. For improved statistical significance, it was noted that more samples should be used than was used in this study. The aluminium has a smoother surface finish and that could be the reason why its coefficient of friction is lower than steel. It is therefore considered important to also include surface roughness when presenting coefficient of friction results.