This study concerns the design, analysis, and initial testing of a novel skin friction gage for applications in three-dimensional, high-speed, high-enthalpy flows. Design conditions required favorable gage performance in the Arc-Heated Facilities at Arnold Engineering Development Center. Flow conditions are expected to be at Mach 3.4, with convective heat properties of h= 1,500 W/(m°·K) (264 Btu/(hr·ft°·°R)) and T_aw= 3,900 K (7,000 °R). The wall shear stress is expected to be as high as τ_w= 2,750 Pa (0.40 psi) with a correlating coefficient of skin friction value around C_f= 0.0035. Through finite element model and analytical analyses, a generic gage design is predicted to remain fully functional and within reasonable factors of safety for short duration tests. The deflection of the sensing head does not exceed 0.025 mm (0.0001 in). Surfaces exposed to the flow reach a maximum temperatures of 960 K (1,720 °R) and the region near the sensitive electronic components experience a negligible rise in temperature after a one second test run. The gage is a direct-measuring, non-nulling design in a cantilever beam arrangement. The sensing head is flush with the surrounding surface of the wall and is separated by a small gap, approximately 0.127 mm (0.005 in). A dual-axis, semi-conductor strain gage unit measures the strain in the beam resulting from the shear stress experienced by the head due to the flow. The gage design incorporates a unique bellows system as a shroud to contain the oil filling and protect the strain gages. Oil filling provides dynamic and thermal damping while eliminating uniform pressure loading. An active water-cooling system is routed externally around the housing in order to control the temperature of the gage system and electronic components. Each gage is wired in a full-bridge Wheatstone configuration and is calibrated for temperature compensation to minimize temperature effects. Design verification was conducted in the Virginia Tech Hypersonic Tunnel. The gage was tested in well-documented Mach 3.0, cold and hot flow environments. The tunnel provided stagnation temperatures and pressures of up to T₀= 655 K (1,180 °R) and P₀= 1,020 kPa (148 psi) respectively. The local wall temperatures ranged from T_w= 292 to 320 K (525 to 576 °R). The skin friction coefficient measurements were between 0.00118 and 0.00134 with an uncertainty of less than 5%. Results were shown to be repeatable and in good concurrence with analytical predictions. The design concept of the gage proved to be very sound in heated, supersonic flow. When it worked, it did so very effectively. Unfortunately, the implementation of the concept is still not robust enough for routine use. The strain gage units in general were often unstable and proved to be insufficiently reliable. The detailed gage design as built was subject to many potential sources of assembly misalignment and machining tolerances, and was susceptible to pre-loading. Further recommendations are provided for a better implementation of this design concept to make a fully functional gage test ready for Arnold Engineering Development Center. / Master of Science

An experimental investigation of surface temperatures generated by dry sliding friction was carried out with the use of an infrared radiometric microscope system that was developed at Virginia Polytechnic Institute and State University. The objective of this study was to investigate the effect of subdividing the apparent contact area on the surface temperature generated by friction. Subdividing the geometric contact resulted in lowering the surface temperatures and followed the trend predicted by theory quite well. The study concluded that the surface temperatures predicted by the Archard and Jaeger theories correctly described the influence of nominal load and sliding velocity on the experimental surface temperatures. A detailing description of the experimental apparatus, the radiation analysis used to convert the radiance output of the microscope to temperature, and the experimental procedure is included. / Master of Science

An investigation was conducted to determine the effect of nitrogen concentration between 0.27 percent and 1.30 percent on the internal friction peaks in 304L stainless steel. The amplitude of the internal friction peak associated with the stress-induced diffusion of interstitial nitrogen increased as a linear function of the nitrogen content. The activation energy of diffusion was found to decrease with an increase in nitrogen content. The presence of another internal friction peak was observed in the spectrum of the sar.iple containing 1.30 percent nitrogen. A metallographic investigation and a change in the magnetic properties of the specimens after testing along with the disappearance of the internal friction peak caused by nitrogen diffusion when the specimen was rerun indicated that the second peak probably resulted form a chromium-nitrogen interaction. / M.S.

This thesis presents a simulation model of sliding process of friction, which combines Newtonian particle dynamics and finite element method to study friction mechanisms that bridging micro and macro scales. In the thesis, it first reviews the importance of studying pavement friction that is associated to safety of drivers, society economics and environmental impact. Then, the hybrid numerical methods of Newtonian particle dynamics and finite element method have been introduced, and the rules to bridge these two methods also have been discussed for solid material that assumes the forces and displacements are continuous at the interface of these two methods. The fundamental theories of friction mechanisms are built upon the surface roughness, adhesion and deformation at the contact between two surfaces. At last, the simulation model of sliding process is presented with the hybrid method, and its visualization and result analysis has been given. At the same time, this thesis also includes the procedures of establishing the simulation of the hybrid methods with C++ programming like the program framework, structure and the major pieces of the program. / Master of Science

This study was conducted to investigate whether hamstring muscle activation rate could potentially serve as an indicator for slip-induced falls, particularly for older adults. Kinematics (heel contact velocity, walking velocity, slip distance, and step length), kinetics (friction demand), and electromyography (EMG) while walking over a slippery surface were collected and examined in the study. Normalized EMG data were examined in term of activation rate and compared to heel contact velocity. Twenty-eight subjects from two age groups (14 young and 14 elderly) walked across a track with embedded force platforms while wearing a fall arresting harness attached to an arresting rig for safety. In order to obtain realistic unexpected slip-induced fall data, the slippery surface was hidden from the subjects and unexpectedly introduced. The primary objective of the study was to evaluate if hamstring activation rate could be a valid indicator for the initiation of slip-induced falls. The results suggested that hamstring activation rate in younger adults was higher than older adults, whereas, younger adults’ heel contact velocity was not different from older adults. These results suggested that heel contact velocity in younger adults was sufficiently reduced before the heel contact phase of the gait cycle. This could be due to the outcome of higher hamstring activation rate in younger adults in comparison to older adults. However, an equal number of falls in two age groups, in spite of older adults’ slower walking velocity, lower RCOF, shorter slip distance, and slower peak sliding heel velocity, suggested that the recovery phase of the slip-induced fall accidents should be studied further. / Master of Science

An infrared microscope system was used to measure the temperatures at the interfaces of graphite-epoxy and carbon-PEEK composites in unidirectional sliding contact with sapphire. Effects of fiber orientation and velocity on tribological parameters were examined. Oscillating contact conditions with graphite-epoxy were also examined. Surface temperatures on the order of 100-160°C were measured at relatively low rates of frictional heat generation. The corresponding coefficients of friction were on the order of 0.45-0.65. In graphite-epoxy, fiber orientation was seen to affect coefficient of friction and wear significantly; but surface temperature was very little affected by fiber orientation. In carbon-PEEK, fiber orientation affected the coefficient of friction, wear and surface temperatures significantly. Surface temperatures in both materials initially increased with velocity, but stayed constant as the glass transition temperature of the matrix material was reached. The total wear is believed to be due to a combination of adhesive wear and fatigue wear. Comparison of the measured surface temperatures with theoretical predictions is done. A “two-velocity-regime” tribological model is proposed to explain the tribological behavior of polymer composites. / Master of Science

Research has shown that older adults who have experienced a previous fall are 60-70% more likely to suffer future falls. A study was conducted to investigate if stress and anxiety associated with a fear of falling contributes to the increased incidents of falls among older adults. The investigation compared physiological parameters, with biomechanical parameters of walking for twenty-eight participants in two age groups: (18-35) and (65 or older). Both age groups were evaluated while walking over dry and slippery floor surfaces. Biomechanical parameters included: step length, required coefficient of friction (RCOF), slip distance, and heel contact velocity. Physiological parameters included: stress and anxiety. Overall, the results indicated that there were differences between older and younger adult's biomechanical parameters of walking, and their physiological stress and anxiety associated with an inadvertent slip. Younger adult's normal RCOF was higher and their normal step length was longer compared to older adults. Older adult's stress level after a slip was significantly higher than younger adults. However, younger and older adult's anxiety scores were not significantly different. Furthermore, younger and older adults modified their step length differently to avoid slipping, when walking over the slippery floor surface. It was concluded that some anxiety and stress may be beneficial in reducing the occurrence of inadvertent slips and falls due to an increased awareness of one's external environment. / Master of Science

Experimental measurements of surface temperatures produced during dry sliding contact were made by using an advanced infrared microscope which receives radiation from a very small target area e.g., 1.78 x 10⁻⁵m in diameter for a 36X objective, allowing temperature measurements within a general region of contact. The sliding system consisted of an Armco iron pin, with a hemispherical end loaded against a rotating sapphire disk. A statistical study was made concerning the effect of environment, load, and velocity on temperature, wear, and coefficient of friction. The formation of iron oxides and its influence on emissivity and possible correlation with wear and friction is discussed. Comparison between the experimental results and the flash temperature theories by both Jaeger and Archard is made. / Master of Science

Friction induced vibration of an epoxy coated shaft rotating in an elastomeric bushing is investigated. This study investigates the manner in which system dynamics and friction mechanisms are responsible for friction induced vibration and noise generation. A test method was developed to measure the friction torque and the system and acoustic response of the sliding system. Several materials including a fluorocarbon elastomer, a polydimethylsiloxane, and a natural rubber were tested. Three friction regimes were observed which were stick-slip oscillations, quasi-harmonic oscillations, and steady sliding. System stiffness and load were varied to observe changes in the critical velocities bounding each regime. System parameters were varied to determine sliding conditions leading to self-induced vibration, to establish how the character of vibration is affected, and to correlate friction torque with system and acoustic vibration for each elastomeric material. A two degree-of-freedom, lumped parameter model was developed to simulate the effect of system dynamics on the sliding behavior of the elastomeric bushing. The comparison of simulated and experimental response using analyses in the time and frequency domain indicate the predictive model provides an excellent representation of stick-slip behavior at various operating conditions. / Master of Science

A cantilever beam type, capacitive sensing, skin friction gage has been developed. A prototype along with supporting electronics has been constructed. The cantilever beam gage is a change of area variable capacitive transducer. It is designed to measure the wall shear stress in a short duration, supersonic flow. The supporting electronics consists of an electrical oscillator for frequency modulation, and a frequency demodulator. The change in capacitance due to the shear stress in the flow modulates the output signal of the oscillator, which is then demodulated to extract a voltage signal which corresponds to the change in capacitance of the gage. The gage and the electronics were constructed from simple, inexpensive components for the purpose of proving the concept of a capacitive sensing transducer. static calibrations have been completed and statistical analysis has been done to test the performance of the gage. A 0.12 mV response due to the expected 98.1 g m/s2 force input of the skin friction of the Mach 2.9 design flow, over the 0.49 in2 (316.1 mm) area of the gage's sensing head, was measured as the average output of the skin friction gage instrumented with stainless steel strips. / Master of Science