Fretting fatigue occurs when there is a small amplitude oscillatory movement between two contacting surfaces while the bodies are undergoing fatigue loading. Fretting fatigue conditions can substantially reduce the fatigue life of a component. Many engineering components such as Ti-6Al-4V gas turbine engine disks in military aircraft commonly experience fretting fatigue conditions that can potentially lead to catastrophic failure of critical components. The aim of this study is to characterize the behavior of Ti-6Al-4V under fretting fatigue conditions. Experiments are performed to analyze the influence of stress amplitude, stress ratio, and contact geometry. The effect of surface treatments such as low plasticity burnishing on the fretting fatigue life is also explored. The experimental results are being used to validate a proposed crack nucleation life prediction model. The proposed model utilizes a crack nucleation parameter H that is based on the strength of the singular stress field at the contact boundary. An advantage of this singular parameter is that neither a coefficient of friction nor the location of the stick/slip boundary needs to be determined. These two parameters are often difficult to define with certainty a priori. H is also independent of geometry making it well suited for use as a design parameter for designing structural joints and other fitted connections between components.

Thesis (M.S.)--Brigham Young University. Dept. of Mechanical Engineering, 2006. / Includes bibliographical references (p. 105-106).

No description available.

This thesis is divided in two parts. The first part is devoted to the micro-macro modeling of the behavior of concrete under cyclic loading. Correlations between microscopic (frictional sliding of mesocracks) and macroscopic (energy dissipation) properties are investigated. The second part focuses on the description of a series of uniaxial cyclic tests on concrete and concrete-like materials performed in the "Rock Mechanics Laboratory" of Virginia Polytechnic Institute and State University, Virginia, USA. Data deriving from these experiments are analyzed on the basis of the micro-macro identification theory. Interesting predictions of the real mechanical behavior of concrete can be found by means of the aforementioned theory and are presented in the last part of the thesis. / Master of Science

The friction of rubber on ice and snow surfaces is complex. Deeper scientific understanding is important for optimising performance of tyres in winter. Rubber, ice and snow systems exhibit frictional behaviour which depends on their material properties. The viscoelastic nature of rubber results in a higher real contact area compared to most other solids. At temperatures close or below the glass transition temperature, the frictional behaviour of rubber changes and its hardness increases. Thus, the real area of contact decreases, while the dissipation in the bulk of the rubber increases. Sliding of rubber on ice or snow leads to a temperature increase at the interface because of frictional heating, this can cause the surface to melt which decreases friction significantly. In this study we measured the friction of rubber on ice and snow and related the behaviour to mechanisms that occur. Key parameters affecting friction were examined and quantified. For this work a cold room and a new linear tribometer were specially designed and constructed. The rubber samples were made from various compounds and had different geometries. Typically they were the size of a “tread block element”. The geometries were chosen systematically to investigate the effects of surface area, sharp/rounded edges and sipes (small slits in the tread block that are used on snow tyres). A significant part of the work was developing consistent and reproducible ice and snow surfaces. New protocols were devised for these. The ice surfaces were made of de-ionised water, tap water and de-ionised water with salt. For the snow surface production: artificial snow was made and then compacted in a specially manufactured press, resulting in hard packed snow tracks for testing. Static and dynamic friction were investigated. Both were affected by speed, load, temperature and ice composition. The dynamic friction behaviour on ice was explained in terms of melt-water formation and the real area of contact of the rubber. The static friction was significantly affected by the losses inside the rubber bulk, the adhesive forces at the interface, and the time of stationary contact before the test. The investigation of rubber sliding on snow showed some similarities with sliding on ice; the surface of the rubber block slides over snow particles resulting in similar mechanisms as are seen on ice. However with snow there can also be a “ploughing” effect, where snow is cut by the leading edge of a sharp tread block. This effect contributes to friction. Experiments were made with simple rounded edged samples to avoid ploughing; the results showed the same trends as seen on ice, i.e. lower friction with increased speed, load and temperature. Investigations of siped tread blocks showed the same friction at low speeds as tread blocks without sipes. At higher speeds siped blocks exhibited less, or no, decrease in friction; more sipes gave less friction decrease. Our industrial collaborator, Michelin, made vehicle tests on snow using whole tyres with similar tread blocks. The trends they found were identical to our tests despite the dynamics of the system being more complex. This indicates how powerful the approach of using simple systematic experiments is for generating deeper understanding of the processes involved in sliding on ice and snow.

Continuous design and improvements in performance of roller bearings have resulted in an increased power transmission whilst size and spatial requirements have been reduced. Radial cylindrical roller bearings have followed this trend and due to their high performance characteristics (such as high loading capacity and high rotational speed capabilities), they have obtained a firm place as high performance machine elements. An essential characteristic of a radial cylindrical roller bearing is the minimisation of frictional torque for particular operating conditions. In spite of these improvements in design and operation the bearing frictional torque is still usually calculated according to the Palmgren method first published in 1957. However, as a result of the increased performance of radial cylindrical roller bearings and a corresponding wider range of application this Palmgren method is insufficiently accurate for the prediction of frictional torques in modern bearings. Whilst the literature review in this thesis identified a variety of bearing frictional torque calculation methods, most of these methods are based on various ball bearing designs and are not necessarily applicable to cylindrical roller bearings types. As a result an accurate frictional torque prediction method is required for radial cylindrical roller bearings because of customer's demands. Consequently, the current project is combined with the development of a method of frictional torque prediction (FTP - Method), which enables the frictional torque of purely radially and radially and axially loaded radial cylindrical roller bearings to be calculated accurately. The FTP - Method is based on the physical effects producing the frictional torque in a bearing, such as the EHL - theory and a contact analysis to determine raceway rolling resistance, and in addition takes into account the frictional forces of the rib / rolling element end face rolling and sliding contact. Comprehensive experimental tests have been undertaken on different radial cylindrical roller bearing designs (including cage guided and full complement types) for a minimum of three different sizes to validate the derived equations. Good agreement was obtained between the predictions according to the FTP - Method and the measured test data for the frictional torqye of both radially and combined loaded bearings. Moreover, the thermal reference speed and the thermal limiting speed of a radial cylindrical roller bearing can be readily calculated for any operating condition using an explicit equation in the radial part of the FTP - Method. The thesis also presents a calculation program to illustrate a method of bearing design based on the thermal balance within a bearing. The bearing is initially pre-selected according to its life time requirements using a life time calculation. After the lubrication method has been defined a thermal balance can be used to design the bearing.

Thesis (MScEng)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: This investigation arose out of the pressing need for alternatives to ineffective existing methodologies for low flow measurement in boulder bed rivers, with particular focus on the mountain streams in the Western Cape. Both empirical and mathematical verification were regarded as important if progress was to be made towards identifying a suitable approach. Historically the inability to determine the frictional effect of the streambed on the flow rate has been a major obstacle limiting the accuracy of flow calculations. From literature, the most likely relationship appears to be a power function, utilising hydraulic variables derived from physical parameters characteristic of a section of stream. Local Western Cape data was thus collected from various typical mountain streams, so that relevant analysis could be done. Testing of existing equations from literature sources on this set of data revealed limited applicability. Subsequent empirical experimentation has shown that particle size is a dominant variable in determining boulder bed flow resistance under low flow conditions. A mathematical approach was sought to provide a more suitable base for a locally applicable formula. Sediment transport theory, based on simple power conservation laws, was successfully implemented, partially bridging the gap between the applications for sand bed and boulder bed flow conditions respectively. After a certain degree of empirical adjustment, an equation form was finalised that is believed to be the most suitable for Western Cape mountain streams, with definite potential for wider application, provided further research is done. / AFRIKAANSE OPSOMMING: Hierdie ondersoek spruit uit die dringende behoefte aan bruikbare alternatiewe vir bestaande laagvloei meetmetodes in riviere met bodems bestaande uit spoelklip, met spesifieke klem op die bergstrome van die Wes Kaap. Dit is as belangrik beskou om beide empiriese en wiskundige bevestiging te vind vir 'n verbeterde metode. Histories is die akkuraatheid van vloeitempo berekeninge hoofsaaklik beperk deur die onvoorspelbaarheid van die ruheidseffek van die rivierbodem op die vloei. Uit bestaande literatuur blyk dit dat die mees geskikte verwantskap waarskynlik 'n magsfunksie is, wat saamgestel is uit hidrouliese veranderlikes verkry vanaf fisiese parameters kenmerkend aan die spesifieke riviersnit. Plaaslike Wes-Kaapse data is dus versamelop verskeie tipiese bergstrome in die gebied sodat geskikte ontledings gedoen kon word. Verskeie bestaande formules is getoets teen hierdie stel data en ongeskik gevind vir direkte aanwending. Empiriese toetse het gevolg en getoon dat partikel grootte 'n dominante invloed het op die vloeiweerstand van spoelklip bodems onder laagvloei omstandighede. 'n Wiskundige benadering is daarna gevolg om 'n beter basis te verskaf waaruit 'n plaaslik bruikbare vergelyking kon volg. Sedimentvervoer beginsels, wat gebaseer is op basiese drywingsteorie, is suksesvol aangewend vir hierdie doel, en het in 'n mate die gaping tussen aanwending op sand en klip bodems oorbrug. Na afloop van empiriese verstelling is 'n formule gefinaliseer wat beskou word as die mees geskikte vir Wes-Kaapse bergstrome, maar wat ook die potensiaal besit vir wyer aanwending, mits verdere navorsing gedoen sou word.

Qin, Jie. / "November 2010." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 162-173). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Flow-vegetation interactions are critically important for most hydraulic and sediment processes in streams and rivers and thus need to be accounted for in their management. The central goal of this project therefore was to improve the understanding of flow-vegetation interactions in patchy-vegetated river beds, which are typical in rivers. Based on laboratory experiments covering a range of selected hydraulic and patch mosaic scenarios, the hydraulic resistance mechanisms, turbulence structure, and transport mechanisms were studied. The effects of regular patch mosaic patterns (aligned and staggered) on the bulk hydraulic resistance were investigated first. For the cases in which the relative vegetation coverage BSA in respect to the total flume bed is low (BSA = 0.1), the patches mutual positions do not affect values of the friction factor. When the parameter BSA increases to intermediate values (BSA = 0.3), the spatial distribution of the vegetation patches and their interactions become crucial and lead to a significant increase in the bulk hydraulic resistance. When further increase of the vegetation cover occurs (BSA = 0.6), the effects on hydraulic resistance of patch patterns vanish. To clarify the mechanisms of the revealed patch effects on the overall hydraulic resistance, flow structure was assessed at both scales: individual patch and patch mosaic. The presence of a submerged isolated vegetation patch on the bed introduces a flow diversion which strongly alters the velocity field and turbulence parameters around the patch. Coherent structures, generated at the canopy top due to velocity shear, control the mass and momentum transfer between the layers below and above the vegetation patch. At the patch mosaic scale, a complex three-dimensional flow structure is formed around the patches which depends on the patch spacing and spatial arrangements. For the low surface area blockage factor (BSA = 0.1), the patches are sparsely distributed and the wakes are (nearly) fully developed before they are interrupted by the effects of the downstream patches. At the intermediate surface area blockage factor (BSA = 0.3), significant differences in flow structure between the aligned and staggered patches were observed. For the highest surface area blockage factor investigated (BSA = 0.6) both aligned and staggered patch mosaic configurations showed a similar behaviour. The results on the flow structure are used to provide mechanistic explanation of the observed patch mosaic effects on the bulk hydraulic resistance.

Leung, Chung Yin = 聚合物濃度對湍流減阻的影響 / 梁頌賢. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 106-108). / Abstracts in English and Chinese. / Leung, Chung Yin = Ju he wu nong du dui tuan liu jian zu de ying xiang / Liang Songxian. / Abstract --- p.i / 概要 --- p.iii / Acknowledgements --- p.iv / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- The phenomenon of polymer drag reduction --- p.1 / Chapter 1.2 --- The Phenomenology of drag reduction --- p.9 / Chapter 1.3 --- A recent theory --- p.19 / Chapter 2 --- Review of a recent theory of drag reduction --- p.21 / Chapter 2.1 --- The balance equations --- p.21 / Chapter 2.2 --- Estimating the average terms --- p.26 / Chapter 2.2.1 --- Average terms for flexible polymers --- p.27 / Chapter 2.2.2 --- Average terms for rodlike polymers --- p.32 / Chapter 2.3 --- Solving the balance equations --- p.34 / Chapter 3 --- Behavior near the maximum drag reduction asymptote --- p.42 / Chapter 4 --- Reduction of drag at small concentrations --- p.57 / Chapter 5 --- Velocity profiles and friction factor relations --- p.61 / Chapter 5.1 --- Newtonian flow without polymer --- p.61 / Chapter 5.2 --- Flow at high polymer concentration --- p.63 / Chapter 5.3 --- Flow at intermediate concentration --- p.68 / Chapter 6 --- The Reynolds stress profile --- p.88 / Chapter 7 --- Percentage drag reduction --- p.95 / Chapter 8 --- Conclusion and future work --- p.104 / Bibliography --- p.106