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Pure aluminum deformed in torsion (shear) at elevated temperatures reaches a broad "peak" stress and then undergoes about a 17% decrease in flow stress with deformation to roughly 1-2 equivalent uniaxial strain. Beyond this strain the flow stress is approximately constant. The sources for this softening are unclear. The suggested basis includes texture softening, microstructural softening, and enhanced dynamic recovery. Experiments were performed where specimens were deformed in torsion to various strains within the softening regime followed by compression tests at ambient and elevated temperature. Analysis of the compressive yield strengths indicate that the softening is at least substantially explained by a decrease in the average Taylor factor due to the development of texture. / Graduation date: 2002

Chromium martensitic hot-work tool steel (AISI H13) is commonly used as die material in hot forming techniques such as die casting, hot rolling, extrusion and hot forging. They are developed to endure the severe conditions by high mechanical properties attained by a complex microstructure. Even though the hot-work tool steel has been improved over the years by alloying and heat treatment, damages still occur. Thermal fatigue is believed to be one of the most common failure mechanisms in hot forming tools. In this thesis tools used in hot forging and die casting were examined to determine damage, material response, thermal fatigue crack initiation and propagation. Different chromium martensitic hot-work tool steels, heat treated at four different austenitizing temperatures were experimentally tested in thermal fatigue and isothermal fatigue. The materials were then evaluated using X-ray line broadening analysis and transmission electron microscopy to explore the relation between fatigue softening and the change in microstructure. The high temperature fatigue softening was also simulated using an elasto-plastic, non-linear kinematic and isotropic model. The model was implemented in a numerical simulation to support the integration of die design, tool steel properties and its use. It was found that the dominant damage mechanisms in the investigated tools were thermal fatigue and that tool material experiences a three stage softening at high temperature loading. The primary stage was concluded to be influenced by the dislocation density and the second stage by the temper resistance i.e. carbide morphology. The microstructural changes during the softening stages were also connected to the non-linear kinematic and isotropic model. The general aim of this thesis is to increase the knowledge of the chromium martensitic hot-work tool steel damage, performance and microstructure.

Inspection of solder joint interconnection has been a crucial process in the electronics manufacturing industry to reduce manufacturing cost, improve yield, and ensure product quality and reliability. New inspection techniques are urgently needed to fill in the gap between available inspection capabilities and industry requirement of low-cost, fast-speed, and highly reliable inspection systems. The laser ultrasound inspection system under development aims to provide a solution that can overcome some of the limitations of current inspection techniques. Specifically, the fully developed system will be an automated system that is capable of inspecting hidden solder joints with multiple defect types. This research work includes the following aspects: 1) Inspection system integration and automation to improve system throughput and capability, system performance characterization by stability study and gage repeatability and reproducibility study , 2) Development and implementation of signal processing methods, including time-domain correlation coefficient analysis, auto-comparison method, and frequency-domain spectral estimation, to allow for fast and accurate interpretation of vibration signals, 3) Development of a finite element modal model followed by experimental validation. The modal analysis results indicate there are unique mode frequencies and mode shapes associated with certain solder joint defects, and 4) Study of the systems unique capability in detecting solder joint fatigue cracks.

<div>High temperature alloys have been extensively used in many applications, such as furnace muffles, fuel nozzles, heat treating fixtures and fuel nozzles. Due to such conditions these materials should have resistance to cyclic loading, oxidation and high heat. Although there are numerous prior experimental and theoretical studies, there is insufficient understanding of application of the unified viscoplasticity theory to finite element software for fatigue life</div><div>prediction.</div><div><br></div><div>Therefore, the goal of this research is to develop a procedure to implement unified viscoplasticity</div><div>theory in finite element (FE) model to model the complex material deformation pertaining to thermomechanical load and implement an incremental damage lifetime rule to</div><div>predict thermomechanical fatigue life of high temperature alloys.</div><div><br></div><div>The objectives of the thesis are:</div><div>1. Develop a simplified integrated approach to model the fatigue creep deformation</div><div>under the framework of ‘unified viscoplasticity theory’</div><div><br></div><div>2. Implement a physics - based crack growth damage model into the framework</div><div><br></div><div>3. Predict the deformation using the unified viscoplastic material model for ferritic</div><div>cast iron (Fe-3.2C-4.0Si-0.6Mo) SiMo4.06</div><div><br></div><div>4. Predict the isothermal low cycle fatigue (LCF) and LCF Creep life using the damage model</div><div><br></div><div>In this work, a unified viscoplastic material model is applied in a FE model with a combination of Chaboche non-linear kinematic hardening, Perzyna rate model and static recovery</div><div>model to model rate dependent plasticity, stress relaxation, and creep-fatigue interaction.</div><div><br></div><div>Also, an incremental damage rule has been successfully implemented in a FE model. The calibrated viscoplastic model is able to correlate deformations pertaining to isothermal LCF, LCF-Creep and thermal-mechanical fatigue (TMF) experimental deformations. The life predictions</div><div>from the FE model have been fairly good at room temperature (20°C), 400°C and 550°C under Isothermal LCF (0.00001/s and 0.003/s) and LCF-Creep tests.</div><div><div><br></div><div>The material calibration techniques proposed for calibrating the model parameters resulted in a fairly good correlation of FE model derived hysteresis loops with experimental hysteresis, pertaining to Isothermal LCF (ranging from 0.00001/s to 0.003/s), Isothermal LCF-Creep tests (with hold time) and TMF responses. In summary, the method and models developed in this work are capable of simulating material deformation dependency on temperature, strain-rates, hold time, therefore, they are capable to modeling creep-stress relaxation and fatigue interaction in high-temperature alloy design.</div></div>

Titanium matrix composites have been extensively evaluated for their potential to replace conventional superalloys in high temperature structural applications, with significant weight-savings while maintaining comparable mechanical properties. The purpose of this investigation is the evaluation of a gamma titanium aluminide alloy with nominal composition Ti-46.5Al-4(Cr,Nb,Ta,B)at.% as a matrix material for use in intermediate temperature applications (400-800㩠in future aerospace transportation systems, as very light-weight structures are needed for cost and weight reduction goals. Mechanical characterization testing was performed over the potential usable temperature range (21-800㩮 Thermal expansion behavior was evaluated, as thermal mismatch of the constituents is an expected problem in composites employing this matrix material. Monotonic testing was conducted on rolled sheet material samples to obtain material properties. The alloy exhibited good strength and stiffness retention at elevated temperatures, as well as improved toughness. Monotonic testing was also conducted on specimens exposed to elevated temperatures to determine the degradation effects of high temperature exposure and oxidation. The exposure did not significantly degrade the alloy properties at elevated temperatures; however, room temperature ductility decreased. Analytical modeling using AGLPLY software was conducted to predict the residual stress state after composite consolidation as well as the potential mechanical behavior of [0]4 laminates with a 㭍ET matrix. Silicon carbide (Ultra-SCS) and alumina (Nextel 610) fibers were selected as potential reinforcing materials for the analysis. High residual stresses were predicted due to the thermal mismatch in the materials. Laminates with Nextel 610 fibers were found to offer the better potential for a composite in this comparison as they provide a better thermal match. Coupons of SCS-6/㭍ET were manufactured with different volume fractions (10% and 20%). Both manufacturing attempts resulted in transverse cracking in the matrix from the residual thermal stress.

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Thermal cracking of asphalt concrete pavements is responsible for millions of dollars in annual maintenance and rehabilitation costs in the United States and Canada. Thermal cracking is typically associated with low temperatures in northern climates and at high elevations. However, another form of thermal cracking, known as thermal fatigue cracking, has been proposed by several researchers as a potential mode of distress in regions with relatively moderate climates but significant differences in high and low daily temperatures. The primary purpose of the research reported herein was to evaluate the possibility of occurrence of the thermal fatigue cracking mode of distress. A secondary objective was to identify a suitable laboratory test procedure to facilitate a mechanistic analysis of the thermal fatigue mode of distress. In light of these objectives, several laboratory test procedures were evaluated in the bituminous materials laboratory at Oregon State University (OSU). The test procedures evaluated included the phenomenological Thermal Stress Restrained Specimen Test (TSRST), the Energy Rate Integral Test (ERIT), the Direct Tension Test under constant rate of extension (DTT), and the Direct Tensile Creep Test (DTCT). The TSRST results were used to evaluate the possibility of occurrence of the thermal fatigue mode of distress. The ERIT, DTT, and DTCT procedures were evaluated with respect to the identification of a suitable laboratory test procedure to facilitate a mechanistic analysis of thermal fatigue. The results from the laboratory test program indicate that thermal fatigue distress in asphalt concrete mixtures is not a viable mode of distress in the absence of environmental aging. Based on the data presented herein and the results of previous researchers, it is evident that distress often attributed to thermal fatigue cracking is more likely the result of low temperature cracking of environmentally aged mixtures, and/or subgrade-related distress; fatigue distress due to thermal loading of semi-restrained pavements does not occur. / Graduation date: 1993

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