Browsing by Subject "Strength of materials"
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Item A critical study of Vlasov model for beams on elastic foundations(1987-12) Sim, Jae Hwang; Mehta, Kishor; Wray, Warren K.Not AvailableItem Load resistance charts for heat-treated window glass(Texas Tech University, 2000-08) Rodriguez-Mendieta, RobertoMajor U.S. glass manufacturers such as LOF/Pilkington and PPG Industries, Inc. once supplied design recommendations in the form of glass thickness selection charts. These charts enabled users to select an appropriate glass thickness based on required design pressures and glass area. ASTM standardized these charts into a set of nominal thickness selection charts for Annealed (AN) window glass. At the same time ASTM provided strength factors to accommodate for the design of Heat Strengthened (HS) and Fully Tempered (FT) window glass. ASTM El 300 (1997) assigns HS glass a strength twice that of AN window glass having the same geometry. Similarly, ASTM assigns FT glass a strength four times that of AN window glass with the same geometry. Even though it is not clear how these factors are selected to represent the two glass types, they are widely accepted by glass designers and manufacturers over the world. While this thesis takes a more theoretical approach, it does not alter the traditional factors radically for the minimum values of residual compressive surface stresses (RCSS). However, the development of the new load resistance charts for HS and FT window glass is based on specific criteria through mainly theoretical veins, due to the paucity of experimental data for HS and FT window glass lites.Item Scale dependence in friction: the transition from intimate contact to monolayer lubricated contact(2007-12) Xu, Dewei, 1974-; Liechti, Kenneth M.; Ravi-Chandar, KrishnaswamyOver the years, nonwear friction with single asperity contact has been examined through experiments using the Surface Force Apparatus (SFA) and the Atomic Force Microscope (AFM). The contact radii in SFA and AFM friction experiments ranged in the order of tens of [mu]m (>10⁵ m) and several nm (< 10⁻⁸ m), respectively. In spite of the fact that the contact radii in these experiments differ by several orders of magnitude, the data from both experiments obey Bowden and Tabor's friction law F = [tau]A , where F is the friction force, [tau] is the frictional shear strength and A is the real contact area. However, there is a crucial difference between the results obtained with the two instruments. The shear strength from the SFA experiments in dry environment is in the tens of MPa, while the shear strength from the AFM measurement is several hundreds of MPa. In the intervening mesoscales, with contact radii ranging from 10⁻⁸ < a < 10⁻⁵, the frictional shear strength must be dependent on contact area in order to link these two extremes. Some models based on dislocation motions have recently been developed to bridge the gap (Hurtado and Kim, 1999a; b). Hitherto, no systematic mesoscale friction experiments to bridge the shear strengths obtained from AFM and SFA have been provided. In addition, this is precisely the range in which MEMS and potential NEMS devices are expected to operate. Therefore, apart from the fundamental challenges involved in resolving the scale dependence of friction, there is a strong technological motivation for studying friction at this scale. In the present work, this transition in shear strength is bridged using a newlydeveloped Mesoscale Friction Tester (MFT) over a wide range of contact radii and relative humidity levels. Since a nonwear and single asperity contact is of interest, novel procedures to fabricate tungsten probes with subnanometer (<0.3nm) surface roughness are initiated. In order to choose an appropriate contact mechanics theory in an ambient environment to obtain the true contact area, a modified Tabor parameter for JKR-DMT transition for capillary force dominant contact is employed. Results from friction experiments show that the transition in shear strength occurred over contact radii of only 20~30nm in both ambient and dry environments. It is hypothesized that shear strengths in the tens of MPa resulted from contact separated by a monolayer of interfacial molecules and shear strengths in hundreds of MPa resulted from intimate contact (no interfacial molecules inside the contact zone). It was the interfacial condition inside the contact zone that governed the transition. Furthermore, there is no continuous spectrum of shear strength, but a "quantized" behavior. A continuum analysis based on Lifshitz theory, which related the shear strength to the estimated strength of van der Waals bonds is proposed to explain the quantized shear strengths obtained from current experiments and both previous AFM and SFA friction experiments.Item Strain-rate sensitivity of strength in macro-to micro-to nano crystalline nickel(2010-12) Humphrey, Ryan T.; Jankowski, Alan F.; Rasty, Jahan; Rivero, Iris V.The strain-rate sensitivity of strength is a key parameter to evaluate the deformation mechanism in crystalline materials. It is widely reported that many metals strengthen with increasing strain rate, wherein an increase in the strain-rate exponent occurs as the grain size decreases. The strain-rate sensitivity exponent as evaluated from a power-law relationship between yield strength and strain rate is thought to increase when the deformation mechanisms change. As increase in the strain-rate occurs, strengthening is attributable to alloy content, then to dislocation activity, and finally to an increase in effective mass – also known as the phonon drag regime. We will evaluate the behavior of nickel over eight-orders of magnitude change in strain rate to see if the change in strain-rate exponent is affected by the scale of the grain size from the macro- to micro- to nano- scale range as the strain rate increases. In this study, tensile testing is used to measure the strain-rate dependence of the tensile strength on the grain size in crystalline nickel foils. Similarly, micro-scratch testing is used to determine the strain-rate dependence of the scratch hardness variation with scratch velocity. Results for these two tests methods are compiled for strain rates that range from 0.00001 to 1000 sec-1. It is found that these mechanical test results can be directly compared, and the increase in strain rate sensitivity exponent with increasing strain rate is slower for nanocrystalline than for microcrystalline nickel.Item The effect of the static fatigue constant upon window glass strength characterization(Texas Tech University, 1986-12) Hwang, Yuh-shingNot availableItem The experimental verification of the state of stress on a square glass plate loaded by concentric rings(Texas Tech University, 1986-12) Knox, John DennisNot availableItem The strength of new window glass using surface characteristics(Texas Tech University, 1984-08) Kanabolo, Diboro CelestineNot available