Browsing by Subject "Residual stress"
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Item An investigation of variability among residual stress measurement techniques and prediction of machining induced distortion(2007-12) Poerner, Nathan W.; Rasty, Jahan; Ekwaro-Osire, Stephen; Ertas, AtilaThe primary objective of this research was to compare and contrast the effectiveness of several Residual Stress (RS) measurement techniques. This was achieved by preparing a series of samples with known RS distribution and comparing the results from different measurement techniques. This round robin study verified the RS distribution of the samples relative to predicted distributions obtained via Finite Element Analysis (FEA). Among the methods examined, all but X-ray diffraction showed a consistently accurate measurement of the RS profile. The secondary objective of this study was to investigate the sensitivity of the slitting technique for residual stress measurement to the type of machining method used for creating the slit. Slitting RS measurements were made on samples with known RS distributions, while employing two different machining methods for creating the slit, namely, electrical discharge machining (EDM) versus computer numerically controlled (CNC) milling. It was shown that CNC milling could be utilized with a reasonable degree of accuracy in place of wire EDM to obtain data for determination of RS distribution. The final objective of this study was to determine whether FEA could accurately predict machining distortion caused by removal of material from a component having a known RS distribution. This was done by comparing FEA results with actual experimental measurements of machining distortion, conducted on specimens with known RS distributions. Comparison of the experimental machining distortion values with the predicted values showed a deviation between the results deep into the cut. Thus, it was concluded that there needs to be improvements made in the modeling of the machining process.Item An analysis of the feasibility of predictive process control of welding applications using infrared pyrometers and thermal metamodels(2010-05) Ely, George Ray; Seepersad, Carolyn C.; Taleff, EricPredictive process control (PPC) is the use of predictive, physical models as the basis for process control [1]. In contrast, conventional control algorithms utilize statistical models that are derived from repetitive process trials. PPC employs in-process monitoring and control of manufacturing processes. PPC algorithms are very promising approaches for welding of small lots or customized products with rapid changes in materials, geometry, or processing conditions. They may also be valuable for welding high value products for which repeated trials and waste are not acceptable. In this research, small-lot braze-welding of UNS C22000 commercial bronze with gas metal arc welding (GMAW) technology is selected as a representative application of PPC. Thermal models of the welding process are constructed to predict the effects of changes in process parameters on the response of temperature measurements. Because accurate thermal models are too computationally expensive for direct use in a control algorithm, metamodels are constructed to drastically reduce computational expense while retaining a high degree of accuracy. Then, the feasibility of PPC of welding applications is analyzed with regard to uncertainties and time delays in an existing welding station and thermal metamodels of the welding process. Lastly, a qualitative residual stress model is developed to nondestructively assess weld quality in end-user parts.Item Residual stresses in thin polymer films(2008-05) Boit, Kipchirchir A.; McKenna, Gregory B.; Dai, Lenore L.; Khare, RajeshPolymeric glass forming liquid films have increasingly become materials of great interest to the evolution of a wide array of technologies ranging from membranes and adhesives to coatings and microelectronics. For many of these prospective applications, a homogeneous defect-free film topology is essential. As such, a fundamental understanding of both the mechanical properties and thermodynamic stability of these films have become important topics of research. A majority of the studies conducted on thin polymer films have shown a conclusive departure of a variety of physical attributes from normal bulk behavior, mainly for film thicknesses below 100 nm. From a broader perspective, research work on thin polymer films has effectively become a study on the relation between 2 dimensional (2D) nanoconfinement and macromolecular mobility. 2D nanoconfinement does not, however, uniquely define polymeric chain behavior. Considering the well documented effects of thermal and mechanical history on the viscoelastic response of polymer chains in the bulk, the same is considered true for thin polymer films. In particular, recently published studies highlight the effects of residual stresses on thin film stability through dewetting. Residual stresses develop during preparation of thin polymer films - the magnitudes of these stresses depend on the method of preparation and subsequent heat treatments. For the case of thin polymer films, it has been postulated by McKenna that the magnitude of these residual stresses could possibly surpass the yield stress. A survey of published literature shows that studies on the measure of residual stress magnitudes in thin polymer films are few. The direct measurement of residual stress magnitudes from the curvature of metal and ceramic polymer coated cantilevers through the bending beam method is however well documented in published literature. The bending beam method has also been used to determine the thermal stresses for thin polystyrene films coating silicon beams. The bending beam method thus presents an opportunity to directly measure the magnitude of residual stresses in thin polymer films. The objective of this thesis is to measure the magnitude of residual stress in supported thin polymer films prepared through the spin coating process.