<p>An advanced bulk constitutive model is used with a new cohesive zone model that is stress state and rate-dependent in order to simulate the ductile to brittle failure transition in polycarbonate. The cohesive zone model is motivated by experimental evidence that two different critical energies per unit area of crack growth exist in glassy polymers. A higher energy state is associated with ductile failure (slow crack growth), while a lower energy state is associated with brittle failure (fast crack growth). The model is formulated so that as rate or stress state changes within a simulation, the fracture energy and thus fracture mode may also change appropriately. The ductile to brittle transition occurs when the cohesive opening rate is over a threshold opening rate and when the stress state is close to plane strain in a fracture specimen. These effects are coupled. The principal contribution of this work is that this is the first time a single set of material input parameters can predict the transition from slow to fast crack growth as test loading rate and sample thickness are varied. This result enlisted the use of an advanced constitutive model and the new cohesive zone model with rate and stress-state dependencies in three-dimensional finite element analysis.</p> / Dissertation

At micrometer length scales of material removal, size effect is observed in mechanical micro-cutting where the energy per unit volume i.e. specific cutting energy increases nonlinearly as the uncut chip thickness is reduced from several hundred microns to a few microns (or less). There is no consensus in the literature on the cutting mechanism that causes this size effect. Noticeable discrepancy is also observed in the surface roughness produced at small feeds in micro-turning between the theoretical and the measured roughness. To date, there has been little effort made to develop a detailed process model for micro-cutting to accurately predict the size effect in specific cutting energy, and to develop a fundamental understanding of surface generation at the low feeds typical of micro-cutting processes. The main objective of this thesis is therefore to develop a predictive process model of micro-cutting of ductile metals that is capable of accurately predicting the size effect in specific cutting energy based on strain gradient based material strengthening considerations. In addition, this thesis attempts to explain the discrepancy between the theoretical and measured surface roughness at small feeds in micro-turning via a model that accounts for the size effect due to material strengthening. A coupled thermo-mechanical finite element model formulation incorporating strain gradient plasticity is developed to simulate orthogonal micro-cutting process. The thermo-mechanical model is experimentally validated in orthogonal micro-cutting of a strain rate insensitive aluminum alloy Al5083-H116. The model is then used to analyze the contributions of two major material strengthening factors to the size effect in specific cutting energy: strain gradient and temperature. The effects of cutting edge radius on the specific cutting energy and its role relative to the material length scale arising from strain gradient plasticity are also examined. A surface roughness model for micro-turning that incorporates the effects of kinematic roughness, cutting edge roughness and surface roughening due to plastic side flow is developed and shown to explain the observed discrepancy between the theoretical and measured surface roughness in micro-cutting. In addition, the model is found to accurately capture the increase in surface roughness at very low feeds.

In a typical printed circuit board assembly, over 70 percent of the electronic components are passives such as resistors, inductors, and capacitors, and these passives could take up to 50 percent of the entire printed circuit board area. By embedding the passive components within the substrate instead of being mounted on the surface, the embedded passives could reduce the system real estate, eliminate the need for surface-mounted discrete components, eliminate lead based interconnects, enhance electrical performance and reliability, and potentially reduce the overall cost. Even with these advantages, embedded passive technology, especially for organic substrates, is at an early stage of development, and thus a comprehensive experimental and theoretical modeling study is needed to understand the fabrication and reliability of embedded passives before they can be widely used. This thesis aims to fabricate embedded passives in a multilayered organic substrate, perform extensive electrical and mechanical reliability tests, and develop physics-based models to predict the thermo-mechanical reliability of embedded capacitors. Embedded capacitors and resistors with different geometric shapes, planar dimensions, and thus different electrical characteristics have been fabricated on two different test vehicles. Capacitors are made with polymer/ceramic nanocomposite materials and have a capacitance in the range of 50 pF to 1.5 nF. Resistors are carbon ink based Polymer Thick Film (PTF) and NiCrAlSi and have a resistance in the range of 25 to 400 k. High frequency measurements have been done using Vector Network Analyzer (VNA) with 2 port signal-ground (S-G) probes. Accelerated thermal cycling (-55 to 125oC) and constant temperature and humidity tests (85oC/85RH) based on JEDEC and MIL standards have been performed. Furthermore, physics-based numerical models have been developed and validated using the experimental data. By focusing on the design and fabrication as well as the experimental and theoretical reliability assessments, this thesis aims to contribute to the overall development of embedded passive technology for Digital and Radio Frequency (RF) applications.

This study addresses the development of a prototype software system for analysis of horizontally curved steel I-girder bridges using open-section thin-walled beam theory. Recommendations are provided for the use of three-dimensional (3D) grid idealizations in analyzing curved I-girder bridge structural systems. The 3D grid idealizations account for the general displacements and rotations common within complex curved I-girder bridge structures, i.e., none of the displacement and rotational degrees-of-freedom are arbitrarily assumed to be equal to zero. Also, these idealizations account for the warping (or cross-bending) deformations of the I-girder flanges that dominate typical girder torsional responses. An approximate approach is investigated for capturing the influence of girder web distortion on composite I-girder responses. A key focus of this research is the development of prototype methods for simulating the construction of curved steel I-girder bridges, including erection of the steel and staged casting of the slab. The resulting capabilities allow engineers to evaluate the deflections, reactions and/or stresses at different stages of the steel erection or concrete slab construction, determine required crane capacities, tie-down, jacking or come-along forces, and calculate incremental displacements due to removal of temporary supports. Also, the capabilities can be used to determine the influence of different steel detailing methods on the bridge geometry, such as the web plumbness under the steel or total dead load. Key requirements necessary to ensure accuracy of the analysis results are addressed.

Each year in the United States over a million people experience a myocardial infarction. The majority of these attacks are caused by coronary artery plaque cap rupture with subsequent thrombus formation. Because rupture is a mechanical event and the tendency of a plaque to rupture is due in part to increases in the mechanical stresses in the fibrous cap, mechanical analyses are important to understanding plaque stability. Histology is the only method capable of identifying plaque features that are associated with vulnerability. Therefore, minimally distorted histologic sections should serve as a basis for constructing the models used in mechanical analyses. Further, because substantial longitudinal variations in geometry and mechanical properties often exist, models should be three-dimensional (3-D). Finally, given the complex geometries of atherosclerotic plaques and the fact that they are composed of different materials, the finite element (FE) method should be used to determine the distribution of stress under physiological loading. Until now, a critical need has existed to determine the distribution of stress in 3-D FE models of human coronary atherosclerotic plaques based on minimally distorted histologic sections. In this research study, a method to measure and correct for distortions caused by acrylic histologic processing was first created. The devised strain-based method yields a limited set of parameters needed for a first order correction. Thus, corrections can be easily implemented using FE methods. Next, a methodology to create 3-D finite FE models of human coronary atherosclerotic plaques based on stable acrylic histologic sections was developed. Models of plaques, ranging in disease severity, were generated using the developed methodology. Lastly, the distributions of stress in these models were obtained and the effects of some plaque features on stresses were determined. Results from this study confirm that morphological description of a plaque is not sufficient to predict plaque rupture. The findings suggest that in many cases the 3-D stress field within a plaque must be known in order to assess plaque stability. Finally, the results show that patient specific models must be developed if the 3-D stress field within a plaque is to be determined.

During foil rolling, back-up and work rolls undergo elastic deformation resulted from the rolling reaction force, which results in non-uniform thickness distribution in the width direction, even causes waves and fracture in the rolled foils. This paper aims to propose a mathematical model for a four-high mill to analyze the elastic deformation of the rolls and discuss the relationship between axial defection of the back-up and work rolls and the rolling conditions, from which the thickness distribution of the product is then predicted. The finite element simulation is also used to analyze the rolling force and roll¡¦s elastic deformation of a four-high mill. From the predicted foil shape, the roll profiles are designed. The mathematical model is validated by comparing the analytical thickness distribution with experiment values. Rolling pass schedules are also designed. From the arrangerement of reductions and heat treatment, experimental results of stainless steels foils with 80£gm thick and 2£gm variation, pure copper foils with 20£gm thick and 2£gm variation, and aluminum foils with 15£gm thick and 3£gm variation are successfully obtained. A rolling technology for foil rolling is developed.

The interaction between the operating speed and the creep behavior of mini plastic fan has investigated in this study. The thermal-elastic-creep coupling model in Marc finite element method package are employed to simulate the stress distribution and creep deformation of a plastic fan operated in different operating temperature are simulated in this study. Results indicate that operating temperature affect the creep deformation significantly for a plastic fan or impeller. A comparison between the simulated data and measured data of PA66+ GF30 plastic fan was provided. A good agreement has been observed in this study. A comparison between the creep deformation of PET+GF30 and PBT+GF30 fan sets has also presented. Results indicate that PA66+GF30 plastic fan has a much better creep resistance a high temperature operating.

In one of the non-destructive testing techniques, Lamb waves, because of its ability to propagate a long distance and being hard to attenuate, can detect a wide range of area. However, due to its multimodal and dispersive characteristics, identifying the signals of defects during the test is often difficult. Time reversal method, a self-focusing technique, can offset the dispersion of Lamb waves and effectively focus on the spatial and temporal domain. This study applies the finite element method to stimulate the propagation of Lamb waves on an aluminum plate, selecting four sets of frequency-thickness products and two excitation types to excite the single-mode or multimode Lamb waves. This study aims to discuss the effects of modal and dispersion on the focus of the time reversal methods. The results show that 2 MHz-mm and in-plane excitation can produce numerous, more dispersive modals with the best focus effect. If we applied the time reversal method to testing the defects of Lamb waves, and the defects are circular and longitudinal notches, then, according to the results, the reflection signal amplitude of the circular defects can be highly increased. According to the test results of small-sized notches, the time reversal method cannot effectively improve the detecting ability of this defect.

Die surface shape may improve the flow of materials, reduce stress concentration of the products, and decrease the processing load to extend the life of die. The objective of this paper is to show that how to design the die surface shape of T-shape protrusion hydroforming with different diameters. A finite element code DEFORM 3D is used to simulate the process of THF, including adaptive simulation to predict the internal pressurization in the tube, and utilize flow net distribution to predict the axial feeding stroke and counter punch (CP) movement. After the amendment to the loading path, the flowability and appearance of the product quality will achieve the best results. Experiments of T-shape warm hydroforming of magnesium alloy AZ61 tubes are. The forming temperature is set as 250¢J. The simulated loading paths are used. From the comparisons of product shape, thickness distribution between analytical and experimental values, the validity of this analytical model is verified.. Keywords: Tube hydroforming, Finite element simulation, Die surface design.

In last decade, white light emitting diodes(LEDs) have become used widely from traditional indicator to general illumination. The increase of its power is the key improving issue. The current light efficiency of white LED about 30%. In other words ,more than 70% of the input electrical energy will be generated in the form of heat. So, how to get rid of the heat damage in high power LED is a severe problem. The finite element analysis is employed to simulate high power white LEDs temperature distribution and thermal stress distributions caused by the dissipated heat. The effects of package parameters, i.e. die attach, solder material, solder thickness, and chip substrate, on the temperature and thermal stress distributions on high power LED packages are simulated and studied in this thesis. A comparison between the 40mil single chip package and the chip on board(CoB) package has also been executed in this study. Simulated results indicate that the highest power of a single 40mil chip package is 7watt. The thermal stress distribution , i.e. the peak value of local thermal stress is over its yield strength, is occurred as the power up to 7watt. Numerical results also reveal that the appropriate fin design can improve the heat dissipation significantly in high power LED package.