<p> Quasi static and intermediate strain rate (5 s^&ndash;1 and 500 s^&ndash;1) tests are conducted on various aluminum and steel ASTM E8 subsize tensile specimens to validate a newly developed testing method which combines a previously developed serpentine bar for load monitoring and a newly described high-speed actuator. This new actuator is controlled by a semi-passive piezoelectrically actuated brake system mounted to a standard actuator, which allows for the actuator to produce high loads and quick response times (&ap; 100 &micro;s). Limitations of this experimental method are that tests must be monotonic (tension or compression but not cyclic loading) and strain rate rise times limit this method to the intermediate strain rate regime (below 500 s^&ndash;1).</p><p>

<p> In today&rsquo;s industry, engineering materials are continuously pushed to the limits. Often, the application only demands high-specification properties in a narrowly-defined region of the material, such as the outermost surface. This, in combination with the economic benefits, makes case hardening an attractive solution to meet industry demands. While case hardening has been in use for decades, applications demanding high hardness, deep case depth, and high corrosion resistance are often under-served by this process. Instead, new solutions are required.</p><p> The goal of this study is to develop and characterize a new borochromizing process applied to a pre-carburized AISI 8620 alloy steel. The process was successfully developed using a combination of computational simulations, calculations, and experimental testing. Process kinetics were studied by fitting case depth measurement data to Fick&rsquo;s Second Law of Diffusion and an Arrhenius equation. Results indicate that the kinetics of the co-diffusion method are unaffected by the addition of chromium to the powder pack. The results also show that significant structural degradation of the case occurs when chromizing is applied sequentially to an existing boronized case. The amount of degradation is proportional to the chromizing parameters.</p><p> Microstructural evolution was studied using metallographic methods, simulation and computational calculations, and analytical techniques. While the co-diffusion process failed to enrich the substrate with chromium, significant enrichment is obtained with the sequential diffusion process. The amount of enrichment is directly proportional to the chromizing parameters with higher parameters resulting in more enrichment. The case consists of M₇C₃ and M₂₃C₆ carbides nearest the surface, minor amounts of CrB, and a balance of M₂B.</p><p> Corrosion resistance was measured with salt spray and electrochemical methods. These methods confirm the benefit of surface enrichment by chromium in the sequential diffusion method with corrosion resistance increasing directly with chromium concentration. The results also confirm the deleterious effect of surface-breaking case defects and the need to reduce or eliminate them. </p><p> The best combination of microstructural integrity, mean surface hardness, effective case depth, and corrosion resistance is obtained in samples sequentially boronized and chromized at 870&deg;C for 6hrs. Additional work is required to further optimize process parameters and case properties.</p><p>

<p> Electron Beam Additive Manufacturing (EBAM) is a relatively new additive manufacturing (AM) technology that uses a high-energy electron beam to melt and fuse powders to build full-density parts in a layer by layer fashion. EBAM can fabricate metallic components, particularly, of complex shapes, in an efficient and cost-effective manner compared to conventional manufacturing means. EBAM is an enabling technology for rapid manufacturing (RM) of metallic components, and thus, can efficiently integrate the design and manufacturing of aerospace components. However, EBAM for aerospace-related applications remain limited because the effect of the EBAM process on part characteristics is not fully understood. In this study, various techniques including microhardness, optical microscopy (OM), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and electron backscatter diffraction (EBSD) were used to characterize Ti-6Al-4V components processed using EBAM. The results were compared to Ti-6Al-4V components processed using conventional techniques. In this study it is shown that EBAM built Ti-64 components have increased hardness, elastic modulus, and yield strength compared to wrought Ti-6Al-4V. Further, it is also shown in this study that the horizontal build EBAM Ti-6Al-4V has increased hardness, elastic modulus, and yield strength compared to vertical build EBAM due to a preferential growth of the &beta; phase.</p><p>

Continuing pressures for higher performance and efficiency in energy conversion and propulsion systems are driving ever more demanding needs for new materials which can survive high stresses at the elevated temperatures. In such severe environments, the characterization of creep properties becomes indispensable. Conventional techniques for the measurement of creep are limited to about 1,700°C. A new method which can be applied at temperatures higher than 2,000°C is strongly demanded. This research presents a non-contact method for the measurements of creep resistance of ultra-high-temperature materials. Using the electrostatic levitation (ESL) facility at NASA MSFC, a spherical sample was rotated quickly enough to cause creep deformation due to the centripetal acceleration. The deformation of the sample was captured with a digital camera, and the images were then analyzed to measure creep deformation and to estimate the stress exponent in the constitutive equation of the power-law creep. To compare experimental results, numerical and analytical analyses on creep deformation of a rotating sphere have been conducted. The experimental, numerical, and analytical results showed a good agreement with one another.

High temperature studies of reactive melts can be difficult due to contamination issues associated with process container and measuring apparatus transferring material to the melts. Even minor amounts of contaminants can drastically affect thermophysical properties such as surface tension. To overcome this problem, containerless processing techniques that levitate small samples, preventing contact, can be used. In using levitation techniques, available heterogeneous nucleation sites are reduced allowing molten samples to undercool below their equilibrium melting temperatures. This capability is extremely attractive to quasicrystal research as the quasicrystal structure is similar to icosahedral ordering that forms in undercooled liquids. As the degree of icosahedral ordering changes with undercooling, changes in thermophysical properties should be observed. By measuring thermophysical properties in the undercooled state valuable insight can be gained into what parameters affect icosahedral ordering, ultimately leading to better understanding of quasicrystal formation, and possible process control for manufacturing. The goal of this research is to measure surface tension, viscosity and density of several titanium, zirconium and nickel (TiZrNi) based quasicrystal forming and related alloys. These measurements are performed using containerless processing combined with optical non-contact measuring methods. In addition to these measurements, the Transient Calorimetric Technique used to measure specific heat in electrostatic levitation based containerless processing is assessed. This research is done as part the NASA-funded microgravity flight project Quasicrystalline Undercooled Alloys for Space Investigation (QUASI). These 1-g measurements will be part of a thermophysical property database which will be used to help plan and compare against, microgravity experiments.* *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation).

Interfacial cracks in bimaterial and sandwich specimens in 4-point flexure were analyzed using the finite element method. The energy release rate(G) and the loading phase angle($\Psi$)were calculated in a variety of bimaterial and sandwich specimens. In the sandwich specimens, different crack configurations were analyzed. For sandwich specimens with fully formed cracks on both interface #s 1 and 2, the analysis predicted crack arrest on interface #1 and growth on interface #2. This phenomenon was observed in previous studies with glass/epoxy/glass sandwich specimens. The analysis also showed that the G value for the crack on interface #2 can be calculated by multiplying the G value for a crack on interface #1 by an appropriate correction factor. This correction factor depends on whether the precrack penetrated the epoxy layer or was arrested at interface #1. The loading phase angle for the crack on interface #2 was found to be greater when the precrack penetrated the intermediate layer than when it was arrested at interface #1. Subcritical crack growth and fracture energy(G$\sb{\rm c})$ measurements were made on glass/PMMA and glass/epoxy/PMMA specimens under high humidity. Threshold values for subcritical crack growth and G,values were lowest for the glass/PMMA interface and highest for the epoxy/PMMA interface with the precrack penetrating the epoxy layer. The Van der Waals bonds between glass and PMMA appear to be weaker than those between epoxy and PMMA. For a crack at the epoxy/PMMA interface in the glass/epoxy/PMMA sandwich specimen, threshold values for subcritical crack growth and G$\sb{\rm c}$ values were lower when the precrack was arrestedon interface #1 than when it penetrated the epoxy layer. The lower phase angle ($\Psi = 5\sp\circ)$ for the crack on interface #2 with the precrack arrested at interface #1 results in a large opening mode stress state on interface #2 which facilitates subcritical crack growth at lower threshold values and also lower G$\sb{\rm c}$ values as compared to the case when the precrack penetrates the epoxy layer and grows along interface #2($\Psi = 66\sp\circ).$

The fracture behavior of 2-D SiC/SiC woven composite was investigated at both ambient and elevated temperatures in this research. At ambient temperature, the fracture initiation toughness and R-curve behavior for composite were characterized and related to in-situ microscopic obseniations of damage accumulation and crack advance. Matrix cracking, crack deflection and branching were observed and dominated fracture behavior in the early loading stage. The key to toughening appeared to be associated with the mechanics of crack arrest at fiber bundles in the woven architecture. After the primary crack extension, the composite behaved non-linearly and a J-integral technique was applied to investigate the R-curve behavior. Substantial fibrous pull-out was observed in this regime of crack advance. An insight into the origin of the $J\sb{R}$-curve of a SiC/SiC woven composite was obtained by experimental characterization of the closure stress-crack opening displacement, $\sigma(u)$, relationship in the process zone of the crack. Application of a previously derived theoretical function, $\sigma\sb{b}(u)$, solely based on fiber bridging, showed consistent results with experimental data. The slow crack growth (SCG) behaviors of three different 2-D SiC/SiC composites were investigated at elevated temperatures from 700$\sp\circ$C to 1200$\sp\circ$C. In Dupont composite, crack length was measured in-situ at temperatures by an optical telescope allowing crack growth rate, V, as a function of applied stress intensity, K, to be obtained directly. Catastrophic failure followed after a limited crack extension suggesting limited SCG behavior at the intermediate temperatures. In contrast, an extensive SCG behavior was observed at 1200$\sp\circ$C. Microstructure observation indicated that crack bridging was absent at the intermediate temperatures but was present at 1200$\sp\circ$C. These results were consistent with the role of temperature on the oxidation and mechanical properties of the fiber bundles. In Goodrich composite, an introduction of boron as well as an increase of carbon interface thickness were found to enhance the time dependent deformation at the crack frontal region. Consequently, instead of the stress intensity factor, K, the energy rate C parameter exhibited a better correlation with the crack velocity.

Theoretical models are developed to predict the nature of the elevated temperature failure behavior in composites containing bridged cracks under small-scale creep conditions both for intermittently (or quasi-statically) and continuously growing matrix cracks that are fully bridged by continuous ceramic fibers. The time-dependence (or rate-dependence) in these models arises as a result of the presence of a viscous fiber/matrix interfacial layer. Under load this layer undergoes shear flow causing time-dependent pull-out of bridging fibers from the crack surfaces. The mechanics of time-dependent bridging is combined with a failure criterion based on secondary failure in a crack-tip creep process zone. The dependence of the matrix creep crack growth rates on flaw size and crack wake parameters as well as on composite microstructure is derived. It is shown that the crack wake plays a predominant role in influencing not only the magnitude of creep crack growth rates but also the relationship of growth rates to the crack sixes. The implications of the results for elevated temperature composite component design are discussed.

<p> Different positions of a material used for structures experience different stresses, sometimes at both extremes, when undergoing processing, manufacturing, and serving. Taking the three-point bending as an example, the plate experiences higher stress in the middle span area and lower stress in both sides of the plate. In order to ensure the performance and reduce the cost of the composite, placement of different composite material with different mechanical properties, i.e. selective reinforcement, is proposed. </p><p> Very few study has been conducted on selective reinforcement. Therefore, basic understanding on the relationship between the selective reinforcing variables and the overall properties of composite material is still unclear and there is still no clear methodology to design composite materials under different types of loads. </p><p> This study started from the analysis of composite laminate under three point bending test. From the mechanical analysis and simulation result of homogeneously reinforced composite materials, it is found that the stress is not evenly distributed on the plate based on through-thickness direction and longitudinal direction. Based on these results, a map for the stress distribution under three point bending was developed. Next, the composite plate was selectively designed using two types of configurations. Mathematical and finite element analysis (FEA) models were built based on these designs. Experimental data from tests of hybrid composite materials was used to verify the mathematical and FEA models. Analysis of the mathematical model indicates that the increase in stiffness of the material at the top and bottom surfaces and middle-span area is the most effective way to improve the flexural modulus in three point bending test. At the end of this study, a complete methodology to perform the selective design was developed.</p>

Thesis (M.App.Sc.) - Carleton University, 2009. / Includes bibliographical references (p.117-126). Also available in electronic format on the Internet.