Coke drums are vertical pressure vessels used in the delayed coking process in petroleum refineries. Significant temperature variation during the delayed coking process causes the useful life of coke drums to be shortened. In order to better understand the failure mechanisms, a experimental study of elastic/plastic mechanical properties and deformation behaviors of typical coke drum materials was performed. A new biaxial thermal-mechanical material testing system has been successfully developed. Basic characterization of mechanical properties of coke drum materials is achieved through uniaxial monotonic and cyclic loading tests. In addition, strain-rate dependence and creep of coke drum materials were further experimentally investigated. Complex thermal-mechanical cyclic tests were conducted. The experimental findings help us to understand the damage mechanisms of coke drums such as bulging. In addition, experimental data serve as benchmark data to verify the predictions of the temperature dependent elastoplastic constitutive model.

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The generalized theory of thermoelasticity was employed to characterize the coupled thermal and mechanical wave propagation in high performance microelectronic packages. Application of a Gaussian heat source of spectral profile similar to high performance devices was shown to induce rapid thermal and mechanical transient phenomena. The stresses and temporal gradient of stresses (power density) induced by the thermal and mechanical disturbances were analyzed using the Gabor Wavelet Transform (GWT). The arrival time of frequency components and their magnitude was studied at various locations in the package. Comparison of the results from the classical thermoelasticity theory and generalized theory was also conducted. It was found that the two theories predict vastly different results in the vicinity of the heat source but that the differences diminish within a larger time window. Results from both theories indicate that the rapid thermal-mechanical waves cause high frequency, broadband stress waves to propagate through the package for a very short period of time. The power density associated with these stress waves was found to be of significant magnitude indicating that even though the effect, titled short time scale effect, is short lived, it could have significant impact on package reliability. The high frequency and high power density associated with the stress waves indicate that the probability of sub-micron cracking and/or delamination due to short time scale effect is high. The findings demonstrate that in processes involving rapid thermal transients, there is a non-negligible transient phenomenon worthy of further investigation.

The generalized theory of thermoelasticity was employed to characterize the coupled thermal and mechanical wave propagation in high performance microelectronic packages. Application of a Gaussian heat source of spectral profile similar to high performance devices was shown to induce rapid thermal and mechanical transient phenomena. The stresses and temporal gradient of stresses (power density) induced by the thermal and mechanical disturbances were analyzed using the Gabor Wavelet Transform (GWT). The arrival time of frequency components and their magnitude was studied at various locations in the package. Comparison of the results from the classical thermoelasticity theory and generalized theory was also conducted. It was found that the two theories predict vastly different results in the vicinity of the heat source but that the differences diminish within a larger time window. Results from both theories indicate that the rapid thermal-mechanical waves cause high frequency, broadband stress waves to propagate through the package for a very short period of time. The power density associated with these stress waves was found to be of significant magnitude indicating that even though the effect, titled short time scale effect, is short lived, it could have significant impact on package reliability. The high frequency and high power density associated with the stress waves indicate that the probability of sub-micron cracking and/or delamination due to short time scale effect is high. The findings demonstrate that in processes involving rapid thermal transients, there is a non-negligible transient phenomenon worthy of further investigation.

Accurate mechanical analysis of composite structures is necessary for the prediction of laminate behavior. Cylindrical composite tubes are a mainstay in many structural applications. The fundamental design of circular composite cross-sections necessitates the development of a comprehensive composite lamination theory. A new analytical method is developed to characterize the behavior of thin-walled composite cylindrical tubes using a modified plate theory. A generated numerical solver can predict properties such as axial stiffness, bending stiffness, layer stresses, and layer strains in composite tubes subjected to combined mechanical loading and thermal effects. The model accounts for the curvature by transforming and translating the material in-plane lamina properties over a global reference coordinate system. A MATLAB-based solver is used to determine the lamina stiffness and stress outcomes with adjustable parameters, including elastic material properties, thermal coefficients, tubing radius, the orientation of fibers, and the ply stacking sequence. The results are then validated using a FE model developed in ABAQUS using a simple quadrature S4R element type. Parametric case studies confirm the validity of the analytical model by accounting for different ply orientations, stacking sequence, and thermal, mechanical loading.

A nonlinear finite element model of the hot and cold rolling processes has been developed for flat rolling stock with rectangular cross section. This model can be used to analyze the flat rolling of cold and hot steel rectangular strips under a series of different parameters, providing the rolling designer with a tool that he can use to understand the behavior of the steel as it flows through the different passes. The models developed, take into account all of the non-linearities present in the rolling problem: material, geometric, boundary, and heat transfer. A coupled thermal-mechanical analysis approach is used to account for the coupling between the mechanical and thermal phenomena resulting from the pressure-dependent thermal contact resistance between the steel slab and the steel rolls. The model predicts the equivalent stress, equivalent plastic strain, maximum strain rate, equivalent total strain, slab temperature increase, increase in roll temperature, strip length increase, slab thickness % reduction (draft), and strip's velocity increase, for both the cold and hot rolling processes. The FE model results are an improvement over the results obtained through the classical theory of rolling. The model also demonstrates the role that contact, plastic heat generation and friction generated heat plays in the rolling process. The analysis performed shows that the steel in cold rolling can be accurately modeled using the elastic-plastic (solid Prandtl-Reuss) formulation, with a von Mises yield surface, the Praguer kinematic hardening rule, and the Ramberg-Osgood hardening material model. The FE models also demonstrate that the steel in hot rolling can be modeled using the rigid-viscoplastic (flow Levy-Mises) formulation, with a von Mises yield surface, and Shida's material model for high temperature steel where the flow stress is a function of the strain, strain rate, and the temperature. Other important contributions of this work are the demonstration that in cold rolling, plane sections do not remain plane as the classic theory of rolling assumes. As a consequence, the actual displacements, velocity, and stress distributions in the workpiece are compared to and shown to be an improvement over the distributions derived from the classical theory. Finally, the stress distribution in the rolls during the cold rolling process is found, and shown to be analogous to the stress distribution of the Hertz contact problem. / Master of Science

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A vast array of high value parts in land- and air-based turbomachinery are subjected to non-isothermal cycling in the presence of mechanical loading. Crack initiation, growth and eventual failure more significantly reduce life in these components compared to isothermal conditions. More accurate simulation of the stress and strain evolution at critical locations of components, as well as test specimens, can lead to a more accurate prediction of remaining life to a structural integrity specialists. The focus of this thesis is to characterize the effects of thermomechanical fatigue (TMF) on generic turbomachinery alloy. An expression that can be used to estimate the maximum and minimum stress under a variety of loading conditions is formulated. Analytical expressions developed here are modifications of classic mechanics of materials methods (e.g. Neuber's Rule and Ramberg-Osgood). The novel models are developed from a collection of data based on parametric finite element analysis to encompass the complex load history present in turbine service conditions. Relevance of the observations and formulated solutions are also explored for the case of a tensile specimen containing a v-shaped notch. Accurate estimations of non-isothermal fatigue presented here endeavor to improve component lifing and decrease maintenance costs.