The implementation of new materials for light-weighting purposes in the automotive industry has often been hindered due to the low ductility of these materials, as well as inadequate empirical ...knowledge about their fracture behavior and inadequate material modeling techniques. This thesis addresses these issues through extensive experimental and numerical study of plastic anisotropy and ductile fracture of several aluminum alloys and a stainless-steel. The test materials used for this study include AA365 die-casting, AA6013 and AA6111 aluminum sheets, AA6260 aluminum tube and SS304L stainless-steel microtube. The plastic anisotropy is assessed using uniaxial tension, plane-strain tension and disc compression experiments for the die-casting and the sheets; and using biaxial experiments for the tubes. These experiments are then used to model the anisotropic plastic behavior of the test materials using advanced non-quadratic anisotropic yield criteria including Yld2000-2D and Yld2004-3D. The fracture behavior of the casting and sheets is investigated using conventional notched tension and central hole specimens, as well as novel specimen designs for shear and biaxial stress states. These improved specimen designs exhibit stress states that develop at the neighborhood of the fracture initiation point to remain proportional throughout the loading history. Likewise, the fracture behavior of the tubes is assessed by loading them under axial force and internal pressure along different stress paths. The ability to control the force/pressure ratio enables probing the fracture behavior under proportional and non-proportional loading paths. Fracture oftentimes initiate at the interior (for example through-thickness mid-plane) of the specimens and thus direct measurement of fracture parameters i.e., stress triaxiality, Lode angle parameter and equivalent plastic strain is not possible from experiments alone. Instead, these parameters at the onset of fracture are obtained in this work using finite element modeling with the material modeling framework using anisotropic yield criteria described above. The loading path and the resulting fracture locus are found to be sensitive to the constitutive model employed, which underscores the importance of an appropriate modeling of plastic anisotropy in ductile fracture studies. Based on the finite element results, the fracture locus is represented by numerous fracture initiation criteria common in literature (e.g., Oyane, Johnson-Cook, Hosford-Coulomb and DF2015), as well as a newly proposed one, created during the course of this research, that is shown to offer better agreement with the experiments, without additional calibration or implementation cost.
The thermal cycle of paint baking for auto bodies alters the mechanical properties of aluminum alloys including, possibly, their fracture behavior. In this study, the plasticity and fracture of ...AA6111 sheet after a 30 min heating cycle at 180 °C is investigated. Uniaxial tension, plane-strain tension and disk compression experiments are performed to assess the plasticity of the material. The results are used to calibrate Yld20004-18p anisotropic yield function. An evolution of the yield locus with plastic deformation is observed. This is represented by evolving the exponent of the yield function, rather than the coefficients themselves. The post-necking hardening curve is identified by numerically simulating a notched-tensile test. The fracture locus is probed using notched-tension, center-hole, and shear experiments. In every case, digital image correlation (DIC) is used to acquire the surface strain fields. In parallel, finite element simulations of the fracture specimens are used to approximate the fracture strains, triaxiality, and Lode angle parameter. Yld2004-18p model shows good overall agreement with the experiments; thus, the fracture locus is probed with this model.
This paper presents a constitutive model, which combines the models proposed by Stewart and Cazacu (2011) and Zhou et al. (2014), to describe the ductile damage process in a commercially pure ...titanium (CP Ti) and to simulate its mechanical response. In particular, a Gurson-type porous material model is modified by coupling two damage parameters, accounting for the void damage and the shear damage respectively, into the yield function and the flow potential. The plastic anisotropy and tension–compression asymmetry exhibited by CP Ti are accounted for by a plasticity model based on the linear transformation of the stress deviator. The theoretical model is implemented in the general purpose finite element software ABAQUS via a user defined subroutine and calibrated using experimental data. Good comparisons are observed between model predictions and experimental results for a series of specimens in different orientations and experiencing a wide range of stress states.
The anisotropic plastic flow and ductile fracture of Al-Si-Mg die-cast aluminum alloy (trade name, Aural2) is investigated using a combination of experiments and analysis. The material is in the form ...of a 2 mm thick sheet and was received in the –T7 temper. The plasticity of the material is probed using uniaxial tension and plane-strain tension experiments at 3 angles to the material direction, and disc-compression experiments. Good repeatability is obtained in every case. These experiments are used to calibrate the Yld2004-18p anisotropic yield function. This function is flexible enough so that very good fitting of the experiments is obtained. The ductile fracture of the alloy is then probed using notched-tension (with 6 mm and 20 mm notch radii) and central-hole experiments. Each of these specimens is designed so that it provides a different stress triaxiality in the region of rupture, while still requiring only a universal testing machine. Digital Image Correlation is used throughout the experimental work to assess the surface strain fields. However, since the sheet is relatively thick, rupture did not necessarily initiate at the surface. For this reason, fully-3D finite element simulations, using the Yld2004-18p yield function and 3 post-necking hardening models (Swift, Voce and combined) are performed. The surface strains predicted by the simulations depend strongly on the material model adopted. Hence the predicted fracture strains reported here underscore the fact that the plasticity of the material needs to be carefully established before any fracture study is undertaken.
The plastic anisotropy of Commercially-Pure Titanium (CP-Ti, or Grade 2 Ti) is probed with an extensive set of experiments, performed on a hot-rolled CP-Ti plate of 12.7 mm thickness. The experiments ...reported here are: 1) uniaxial tension at 15o angles to the rolling direction (RD) of the plate, 2) uniaxial tension in the thickness, or normal direction (ND) of the plate, 3) uniaxial compression at 15° angles to the RD, as well as in ND, 4) plane-strain tension at 15° angles to the RD, 5) plane-strain compression at 15° angles to the RD. A total of 30 experiments (and two repetitions for each) were performed for this research. The uniaxial tension and compression experiments involve standard specimen geometries, except for the uniaxial tension in the ND, which required the creation of a custom, miniature tensile specimen. A novel method for determining the stresses in a plane-strain tension experiment was devised using finite element analysis. The experiments reveal the plastic anisotropy and tension-compression asymmetry of CP-Ti. Furthermore, they are used to calibrate 3 commonly used constitutive models (yield functions). While not pursued here, it is expected that this improved understanding and representation of the behavior of CP-Ti can lead to improved numerical simulations of material response during manufacturing or service.
In Nepal, we tested 300 males for glucose-6-phosphate dehydrogenase (G6PD) activity. Two subjects were G6PD deficient (0.67%). Compared with normal controls, G6PD activity was 12% and 26%, ...respectively. The hemoglobin concentration of these two subjects was normal. We extracted genomic DNA from whole blood and read all sequences of G6PD. Both subjects had the same replacement of 563C>T, which was classified as G6PD Mediterranean. The amino acid might change from Ser to Phe at codon 188. These subjects also had a replacement of 1311C>T, which caused no replacement of an amino acid. A similar replacement pattern of G6PD Mediterranean is described from persons living in Mediterranean countries and Middle East countries. However, G6PD Mediterranean found in India and Pakistan has no replacement at nucleotide 1311. Thus, these two subjects in Kathmandu, Nepal, would be closer to people in Middle East countries than people in India. This is the first study of molecular analysis for G6PD deficiency in Nepal.
This research addresses water quality issues in water distribution systems in general, and more specifically, pipe corrosion which can entail many adverse effects including reduction of hydraulic ...capacity, compromised structural integrity of conduits and water quality deterioration. Yet, for all its familiarity, corrosion is a complex phenomenon due to the inter-dependent set of reactions that can take place at the pipe wall with chemicals in the bulk water. The principle objective of this study is to extend the existing, but basic, 1-D corrosion-water quality model and to explore the impact of various chemical species on the formation of ferrous and ferric ions during transient conditions. In addition, the associated formation of various iron precipitates is numerically explored. Each chemical species is tracked using the ADRE which is coupled with a hydraulic model. The numerical outcome of the model is compared with the results of the existing model, with both exhibiting good agreement.
In Nepal, we tested 300 males for glucose-6-phosphate dehydrogenase (G6PD) activity. Two subjects were G6PD deficient (0.67%). Compared with normal controls, G6PD activity was 12% and 26%, ...respectively. The hemoglobin concentration of these two subjects was normal. We extracted genomic DNA from whole blood and read all sequences of G6PD. Both subjects had the same replacement of 563C>T, which was classified as G6PD Mediterranean. The amino acid might change from Ser to Phe at codon 188. These subjects also had a replacement of 1311C>T, which caused no replacement of an amino acid. A similar replacement pattern of G6PD Mediterranean is described from persons living in Mediterranean countries and Middle East countries. However, G6PD Mediterranean found in India and Pakistan has no replacement at nucleotide 1311. Thus, these two subjects in Kathmandu, Nepal, would be closer to people in Middle East countries than people in India. This is the first study of molecular analysis for G6PD deficiency in Nepal.
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