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You searched for +publisher:"University of North Texas" +contributor:("Needleman, Alan"). Showing records 1 – 3 of 3 total matches.

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University of North Texas

1. Srivastava, Ankit. Void Growth and Collapse in a Creeping Single Crystal.

Degree: 2011, University of North Texas

Aircraft engine components can be subjected to a large number of thermo-mechanical loading cycles and to long dwell times at high temperatures. In particular, the understanding of creep in single crystal superalloy turbine blades is of importance for designing more reliable and fuel efficient aircraft engines. Creep tests on single crystal superalloy specimens have shown greater creep strain rates for thinner specimens than predicted by current theories. Therefore, it is necessary to develop a more predictive description of creep processes in these materials for them to be used effectively. Experimental observations have shown that the crystals have an initial porosity and that the progressive growth of these voids plays a major role in limiting creep life. In order to understand void growth under creep in single crystals, we have analyzed the creep response of three dimensional unit cells with a single spherical void under different types of isothermal creep loading. The growth behavior of the void is simulated using a three dimensional rate dependent crystal plasticity constitutive relation in a quasi-static finite element analysis. The aim of the present work is to analyze the effect of stress traixiality and Lode parameter on void growth under both constant true stress and constant engineering stress isothermal creep loading. Advisors/Committee Members: Needleman, Alan, Srivilliputhur, Srinivasan G., Wang, Zhiqiang.

Subjects/Keywords: creep; single crystal; finite element; superalloy; crystal plasticity

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Srivastava, A. (2011). Void Growth and Collapse in a Creeping Single Crystal. (Thesis). University of North Texas. Retrieved from https://digital.library.unt.edu/ark:/67531/metadc84281/

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Chicago Manual of Style (16th Edition):

Srivastava, Ankit. “Void Growth and Collapse in a Creeping Single Crystal.” 2011. Thesis, University of North Texas. Accessed November 27, 2020. https://digital.library.unt.edu/ark:/67531/metadc84281/.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

MLA Handbook (7th Edition):

Srivastava, Ankit. “Void Growth and Collapse in a Creeping Single Crystal.” 2011. Web. 27 Nov 2020.

Vancouver:

Srivastava A. Void Growth and Collapse in a Creeping Single Crystal. [Internet] [Thesis]. University of North Texas; 2011. [cited 2020 Nov 27]. Available from: https://digital.library.unt.edu/ark:/67531/metadc84281/.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Council of Science Editors:

Srivastava A. Void Growth and Collapse in a Creeping Single Crystal. [Thesis]. University of North Texas; 2011. Available from: https://digital.library.unt.edu/ark:/67531/metadc84281/

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation


University of North Texas

2. Devaraj, Arun. Phase Separation and Second Phase Precipitation in Beta Titanium Alloys.

Degree: 2011, University of North Texas

The current understanding of the atomic scale phenomenon associated with the influence of beta phase instabilities on the evolution of microstructure in titanium alloys is limited due to their complex nature. Such beta phase instabilities include phase separation and precipitation of nano-scale omega and alpha phases in the beta matrix. The initial part of the present study focuses on omega precipitation within the beta matrix of model binary titanium molybdenum (Ti-Mo) alloys. Direct atomic scale observation of pre-transition omega-like embryos in quenched alloys, using aberration-corrected high resolution scanning transmission electron microscopy and atom probe tomography (APT) was compared and contrasted with the results of first principles computations performed using the Vienna ab initio simulation package (VASP) to present a novel mechanism of these special class of phase transformation. Thereafter the beta phase separation and subsequent alpha phase nucleation in a Ti-Mo-Al ternary alloy was investigated by coupling in-situ high energy synchrotron x-ray diffraction with ex-situ characterization studies performed using aberration corrected transmission electron microscopy and APT to develop a deeper understanding of the mechanism of transformation. Subsequently the formation of the omega phase in the presence of simultaneous development of compositional phase separation within the beta matrix phase of a Ti-10V-6Cu (wt%) alloy during continuous cooling has been investigated using a combination of transmission electron microscopy and atom probe tomography. The results of these investigations provided novel insights into the mechanisms of solid-state transformations in metallic systems by capturing the earliest stages of nucleation at atomic to near atomic spatial and compositional resolution. Advisors/Committee Members: Banerjee, Rajarshi, Srivilliputhur, Srinivasan, Needleman, Alan, Nag, SoumyA, Fraser, Hamish.

Subjects/Keywords: Titanium alloys; phase transformation; phase separation

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Devaraj, A. (2011). Phase Separation and Second Phase Precipitation in Beta Titanium Alloys. (Thesis). University of North Texas. Retrieved from https://digital.library.unt.edu/ark:/67531/metadc67975/

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Chicago Manual of Style (16th Edition):

Devaraj, Arun. “Phase Separation and Second Phase Precipitation in Beta Titanium Alloys.” 2011. Thesis, University of North Texas. Accessed November 27, 2020. https://digital.library.unt.edu/ark:/67531/metadc67975/.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

MLA Handbook (7th Edition):

Devaraj, Arun. “Phase Separation and Second Phase Precipitation in Beta Titanium Alloys.” 2011. Web. 27 Nov 2020.

Vancouver:

Devaraj A. Phase Separation and Second Phase Precipitation in Beta Titanium Alloys. [Internet] [Thesis]. University of North Texas; 2011. [cited 2020 Nov 27]. Available from: https://digital.library.unt.edu/ark:/67531/metadc67975/.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Council of Science Editors:

Devaraj A. Phase Separation and Second Phase Precipitation in Beta Titanium Alloys. [Thesis]. University of North Texas; 2011. Available from: https://digital.library.unt.edu/ark:/67531/metadc67975/

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation


University of North Texas

3. Srivastava, Ankit. Mechanics and Mechanisms of Creep and Ductile Fracture.

Degree: 2013, University of North Texas

The main aim of this dissertation is to relate measurable and hopefully controllable features of a material's microstructure to its observed failure modes to provide a basis for designing better materials. The understanding of creep in materials used at high temperatures is of prime engineering importance. Single crystal Ni-based superalloys used in turbine aerofoils of jet engines are exposed to long dwell times at very high temperatures. In contrast to current theories, creep tests on Ni-based superalloy specimens have shown size dependent creep response termed as the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal creep tests were performed on uncoated Ni-based single crystal superalloy sheet specimens with two thicknesses and under two test conditions: a low temperature high stress condition and a high temperature low stress condition. At the high temperature, surface oxidation induced microstructural changes near the free surface forming a layered microstructure. Finite element calculations showed that this layered microstructure gave rise to local changes in the stress state. The specimens also contained nonuniform distribution of initial voids formed during the solidification and homogenization processes. The experiments showed that porosity evolution could play a significant role in the thickness debit effect. This motivated a basic mechanics study of porosity evolution in single crystals subjected to creep for a range of stress states. The study was performed using three-dimensional finite deformation finite element analysis of unit cells containing a single initially spherical void in a single crystal matrix. The materials are characterized by a rate-dependent crystal plasticity constitutive relation accounting for both primary and secondary creep. The effect of initial void spacing and creep exponent was also explored. Based on the experimental observations and results of finite element calculations a quantitative mechanistic model is proposed that can account for both bulk and surface damage effects and assess their relative roles in the observed thickness debit effect. Another set of calculations aim at relating the crack growth resistance and fracture surface morphology to material microstructure for ductile structural metals. The process that governs the ductile fracture of structural materials at room temperature is one of nucleation, growth and coalescence of micron scale voids, and involves large plastic deformations. Experimental studies have shown that fracture surfaces in a wide variety of materials and under a wide variety of loading conditions have remarkable scaling properties. For thirty years, the hope to relate the statistical characterization of fracture surfaces to a measure of a material's crack growth resistance has remained unfulfilled. Only recently has the capability been developed to calculate sufficient amounts of three dimensional ductile crack growth in heterogeneous microstructures to obtain a statistical characterization of the… Advisors/Committee Members: Needleman, Alan, Srivilliputhur, Srinivasan G., Scharf, Thomas, Xia, Zhenhai, Wang, Zhiqiang.

Subjects/Keywords: Solid mechanisms; creep; ductile fracture

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Srivastava, A. (2013). Mechanics and Mechanisms of Creep and Ductile Fracture. (Thesis). University of North Texas. Retrieved from https://digital.library.unt.edu/ark:/67531/metadc283799/

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Chicago Manual of Style (16th Edition):

Srivastava, Ankit. “Mechanics and Mechanisms of Creep and Ductile Fracture.” 2013. Thesis, University of North Texas. Accessed November 27, 2020. https://digital.library.unt.edu/ark:/67531/metadc283799/.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

MLA Handbook (7th Edition):

Srivastava, Ankit. “Mechanics and Mechanisms of Creep and Ductile Fracture.” 2013. Web. 27 Nov 2020.

Vancouver:

Srivastava A. Mechanics and Mechanisms of Creep and Ductile Fracture. [Internet] [Thesis]. University of North Texas; 2013. [cited 2020 Nov 27]. Available from: https://digital.library.unt.edu/ark:/67531/metadc283799/.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Council of Science Editors:

Srivastava A. Mechanics and Mechanisms of Creep and Ductile Fracture. [Thesis]. University of North Texas; 2013. Available from: https://digital.library.unt.edu/ark:/67531/metadc283799/

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

.