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Ballistic-electron-emission microscopy has been used to investigate the effects of strain on Si l-x Ge x alloys. Lifting of the degeneracy of the conduction-band minimum of SiGe due to lattice deformation has been directly measured by application of BEEM spectroscopy to Ag/Si structures.
Protein crystals are so fragile that they often can not be handled by tweezers. Indeed, measurements of the Young modulus, E, of lysozyme crystals resulted in E approx. equals 0.1 - 1 GPa, the lower figures, 0.1 - 0.5 GPa, being obtained from triple point bending of as-grown and not cross-linked crystals sitting in solution. The bending strength was found to be approx.10(exp -2) E. On the other hand, ultrasound speed and Mandelstam-Raman-Brilloin light scattering experiments led to much higher figures, E approx. equals 2.7 GPa. The lower figures for E were found from static or low frequency crystal deformations measurements, while the higher moduli are based on high frequency lattice vibrations, 10(exp 7) - 10(exp 10) 1/s. The physical reason for the about an order of magnitude discrepancy is in different behavior of water filling space between protein molecules. At slow lattice deformation, the not-bound intermolecular water has enough time to flow from the compressed to expanded regions of the deformed crystal. At high deformation frequencies in the ultra- and hypersound waves, the water is confined in the intermolecular space and, on that scale, behaves like a solid, thus contributing to the elastic crystal moduli. In this case, the reciprocal crystal modulus is expected to be an average of the water protein and water compressibilities (reciprocal compressibilities): the bulk modulus for lysozyme is 26 GPa, for water it is 7 GPa. Anisotropy of the crystal moduli comes from intermolecular contacts within the lattice while the high frequency hardness comes from the bulk of protein molecules and water bulk moduli. These conclusions are based on the analysis of liquid flow in porous medium to be presented.
Particulate samples of experimentally shocked olivine, enstatite, and apatite and their unshocked equivalents were thinned by ion bombardment and examined by transmission methods in a conventional (100 kV) electron microscope. Enstatite shock-loaded at a peak pressure of 250 kb was partly transformed to relatively dense inverse defect spinel. Magnesian olivine shocked at a peak pressure of 200 kb shows intense lattice deformation. Magnesian olivine shock-loaded at a peak pressure of 400 kb was recrystallized to very fine polycrystalline aggregates. Apatite shock-loaded at a peak pressure of 250 kb shows planar deformation elements and shock-induced imperfections.
The role of grain boundary sliding in copper and Cu-30 pct Zn in the temperature range 0.50-0.72 Tm, where Tm is the absolute melting point of the material, is examined. First, sliding data obtained on these materials are presented. The results indicate that the stress exponent for sliding is similar to that for lattice deformation, while the activation energy for sliding varies between 0.5 and 1.6 of the activation energy for creep. Several models proposed for grain boundary sliding are discussed, and it is shown that they do not account for the observed results on copper and alpha brass. A phenomenological model is proposed, where it is assumed that grain boundary sliding results from the glide of dislocations on secondary slip planes.
Ballistic-electron-emission microscopy (BEEM) has been used to investigate the effects of strain on Si(sub 1-x)Ge(sub x) alloys. Lifting of the degeneracy of the conduction-band minimum of Si(sub 1-x)Ge(sub x), due to lattice deformation has been directly measured by application of BEEM spectroscopy to Ag/Si structures. Experimental values for this conduction-band splitting agree well with calculations. In addition, an unexpected heterogeneity in the strain of the Si(sub 1-x)Ge(sub x) layer is introduced by deposition of Au. This effect, not observed with Ag, is attributed to species interdiffusion and has important implications for metal-semiconductor devices based oil pseudomorphic Si(sub 1-x)Ge(sub x)/Si material systems.
We present a versatile discrete geometry manipulation platform for aerospace vehicle shape optimization. The platform is based on the geometry kernel of an open-source modeling tool called Blender and offers access to four parametric deformation techniques: lattice, cage-based, skeletal, and direct manipulation. Custom deformation methods are implemented as plugins, and the kernel is controlled through a scripting interface. Surface sensitivities are provided to support gradient-based optimization. The platform architecture allows the use of geometry pipelines, where multiple modelers are used in sequence, enabling manipulation difficult or impossible to achieve with a constructive modeler or deformer alone. We implement an intuitive custom deformation method in which a set of surface points serve as the design variables and user-specified constraints are intrinsically satisfied. We test our geometry platform on several design examples using an aerodynamic design framework based on Cartesian grids. We examine inverse airfoil design and shape matching and perform lift-constrained drag minimization on an airfoil with thickness constraints. A transport wing-fuselage integration problem demonstrates the approach in 3D. In a final example, our platform is pipelined with a constructive modeler to parabolically sweep a wingtip while applying a 1-G loading deformation across the wingspan. This work is an important first step towards the larger goal of leveraging the investment of the graphics industry to improve the state-of-the-art in aerospace geometry tools.
The results of an atomistic model for the simulation of the early stages of crack initiation in a two-dimensional triangular lattice are presented. In the current model, each particle in the system is treated discretely and assumed to be interacting with the surrounding particles via Lennard-Jones potentials. A uniaxial load (in incremental elongations) is applied to the rectangular two-dimensional slab in either the x or the y direction. After each incremental elongation the system is equilibrated using a static method. Initially, elastic behavior in the x and y directions is observed. Continued elongation results in plastic deformation. In lattices with point defects, the defects first move to the surface, creating vacancies which trigger plastic deformation.
In V3Si, the V atoms form an array of dense linear chains; a tight-binding approximation in one dimension was used to describe the d electrons. The electronic energy calculated by this method was reduced when the lattice is deformed. This lead to a band type of the Jahn Teller effect, which may explain the cubic to tetragonal transition which was observed at low temperatures. The theory can be extended to other superconductors of the V3X type when X=Ga, Ge, Sn, etcetera, or NB3SN.
Mechanical twins form by the simple shear of the crystal lattice during deformation In order to test the potential of narrow twins in monazite to record the timing of their formation, a ~1,700 Ma monazite grain deformed at ~980 Ma was investigated by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and atom probe tomography (APT). APT 208Pb/232Th ages indicate that the twin was entirely reset by radiogenic Pb-loss during its formation at conditions far below the monazite closure temperature. The results are consistent with a model where Pb is liberated during rupture of REE-O bonds in the large REEO9 polyhedra during twinning. Liberated Pb is likely to migrate along fast diffusion pathways such as crystal defects. The combination of a quantitative microstructural investigation and nanogeochronology provides a new approach for understanding the history of accessory phases.
Homogeneous static deformation method crystals in NaCl and spinel structures using homogeneous static deformation method
This research effort explored lightweight structural concepts married with advanced smart materials to achieve a wide variety of benefits in airframe and engine components. Lattice block structures were cast from an aerospace structural titanium alloy Ti-6Al-4V and a NiTi shape memory alloy (SMA), and preliminary properties have been measured. A finite element-based modeling approach that can rapidly and accurately capture the deformation response of lattice architectures was developed. The Ti-6-4 and SMA material behavior was calibrated via experimental tests of ligaments machined from the lattice. Benchmark testing of complete lattice structures verified the main aspects of the model as well as demonstrated the advantages of the lattice structure. Shape memory behavior of a sample machined from a lattice block was also demonstrated.
In high temperature superconductor (HTSC) materials the vortices are highly mobile and flexible. This is reflected in different models of melt of a vortex lattice. Another aspect of the problem is stressed: an easy nucleation and high mobility of dislocations in the vortex lattice. Some models of plastic deformation of vortex lattice are considered as a result of its interaction with a real crystal structure. Depinning is interpreted as yield of plastic flow is vortex medium. Effect of macroscopic defects in crystal structures (pores, inclusions, grain and domain boundaries) is being considered in detail. Available experimental facts on magnetization and a critical current in HTSC and conventional superconductors are discussed from the points of view of depinning to vortices vs. plastic flow of vortices vs. plastic flow of vortices medium.
The aim of the ARAMADAS project is to automate the construction of cuboctahedral lattice structures. Lattice materials are appealing for aerospace applications due to their strength and stiffness at ultra-light densities. However, in order for any material to be realistically considered for such environments, it must also be damage tolerant. The ability of a material to absorb damage is characterized by its fracture toughness, which remains poorly characterized for lattice materials. Consequently, the objective of this research is to develop an optical measurement system to experimentally validate the strain field ahead of a crack tip in architecture lattice materials. Although the ability to predict the strain field ahead of a crack tip has been investigated for continuum materials, such behaviour of three-dimensional architectures is under-investigated. As such, we will use a custom optical measurement system to track deformation of the voxels in a side-cracked plate fracture specimen. The system shall use 3D pose estimation, stereo imaging, and possibly color tracking, in combination with optical flow algorithms, to compute information regarding the three-dimensional movement movement of the lattice nodes during mechanical testing. Bench top experiments will validate the optical measurement system and characterize precision. Additionally, the effect of multiple cameras on precision, as well as system scalability will be investigated. Final results will compare measured lattice deformation with finite element predictions.
Reports of shock-deformed phenocrysts from the Toba ignimbrite deposits, Sumatra, have prompted considerable debate over whether shock-deformation products are clear evidence of a meteorite impact origin for the K/T boundary deposits as well as terresrial 'cryptoexplosion' structures. Evidence presented in favor of volcanically induced shock at Toba includes kinked biotites and rare occurrences of single set of lamellae in quartz grains but rests most heavily upon occurrences of mosaic extinction patterns in plagioclase phenocrysts. The present analysis of several of the same Toba samples reveals that these mosaic patterns ae related to distinct compositional zoning and cannot be attributed to deformation of the crystal lattice that shock would produce. Additionally, in more than 200 quartz grains examined, no occurrences of microdeformation features or mosaic textures similar to those associated with known impact structures and the K/T boundary are detected. It is concluded that evidence of shock deformation in the Toba deposits has not been demonstrated.
Initial investigations of investment cast superalloy lattice block suggest that this technology will yield a low cost approach to utilize the high temperature strength and environmental resistance of superalloys in lightweight, damage tolerant structural configurations. Work to date has demonstrated that relatively large superalloy lattice block panels can be successfully investment cast from both IN-718 and Mar-M247. These castings exhibited mechanical properties consistent with the strength of the same superalloys measured from more conventional castings. The lattice block structure also accommodates significant deformation without failure, and is defect tolerant in fatigue. The potential of lattice block structures opens new opportunities for the use of superalloys in future generations of aircraft applications that demand strength and environmental resistance at elevated temperatures along with low weight.
My fall 2015 semester was spent at the Lyndon B. Johnson Space Center working in the Integrated Graphics, Operations, and Analysis Laboratory (IGOAL). My first project was to create a video animation that could tell the story of OMICS. OMICS is a term being used in the field of biomedical science to describe the collective technologies that study biological systems, such as what makes up a cell and how it functions with other systems. In the IGOAL I used a large 23 inch Wacom monitor to draw storyboards, graphics, and line art animations. I used Blender as the 3D environment to sculpt, shape, cut or modify the several scenes and models for the video. A challenge creating this video was to take a term used in biomedical science and describe it in such a way that an 8th grade student can understand. I used a line art style because it would visually set the tone for what we thought was an educational style. In order to get a handle on the perspective and overall feel for the animation without overloading my workspace, I split up the 2 minute animation into several scenes. I used Blender's python scripting capabilities which allowed for the addition of plugins to add or modify tools. The scripts can also directly interact with the objects to create naturalistic patterns or movements. After collecting the rendered scenes, I used Blender's built-in video editing workspace to output the animation. My second project was to write software that emulates a physical system's interface. The interface was to simulate a boat, ROV, and winch system. Simulations are a time and cost effective way to test complicated data and provide training for operators without having to use expensive hardware. We created the virtual controls with 3-D Blender models and 2-D graphics, and then add functionality in C# using the Unity game engine. The Unity engine provides several essential behaviors of a simulator, such as the start and update functions. A framework for Unity, which was developed in the lab, provided a way to place the different widgets on the virtual console dock and have them resize correctly based on the window dimensions.. My task in this project was to create the controls and visualizations for the data coming in from the simulator for the boat portion of the project. I wrote a class for each control window to handle the functionality of that widget. I implemented 11 widgets that make up the ship portion of the simulator. The members of the lab were each masters of their craft and I'm glad I had the opportunity to learn from them. I learned to plan strategically so I could finish this project on time. I allotted time for storyboarding, development, and refinement. In regards to animating I learned to use modifiers like lattice, boolean and build deformers. I also learned how to animate with drivers, how to use the dope sheet, and how to use the graph editor. In coding I learned to limit the chances for bugs by privatizing functions that should be exclusive to their class. I learned how to use the GIT repository to commit, stash and pull the latest build. I learned a bit of everything because I had the chance to see the entire application development process from the artwork, to the implementation.
The influence of orientation of the stress rupture behavior of Mar-M247 single crystals was studied. Stress rupture tests were performed at 724 MPa and 774 C where the effect of anisotropy is prominent. The mechanical behavior of the single crystals was rationalized on the basis of the Schmid factors for the operative slip systems and the lattice rotations which the crystals underwent during deformation. The stress rupture lives were found to be greatly influenced by the lattice rotations required to produce intersecting slip, because steady-state creep does not begin until after the onset of intersecting slip. Crystals which required large rotations to become oriented for intersecting slip exhibited a large primary creep strain, a large effective stress level at the onset of steady-state creep, and consequently a short stress rupture life. A unified analysis was attained for the stress rupture behavior of the Mar-M247 single crystals tested in this study at 774 C and that of the Mar-M200 single crystals tested in a prior study at 760 C. In this analysis, the standard 001-011-111 stereographic triangle was divided into several regions of crystallographic orientation which were rank ordered according to stress rupture life for this temperature regime. This plot indicates that those crystals having orientations within about 25 deg of the 001 exhibited significantly longer lives when their orientations were closer to the 001-011 boundary of the stereographic triangle than to the 001-111 boundary.
The influence of orientation on the tensile and stress rupture behavior of 52 Mar-M247 single crystals was studied. Tensile tests were performed at temperatures between 23 and 1093 C; stress rupture behavior was examined between 760 and 1038 C. The mechanical behavior of the single crystals was rationalized on the basis of the Schmid factor contours for the operative slip systems and the lattice rotations which the crystals underwent during deformation. The tensile properties correlated well with the appropriate Schmid factor contours. The stress rupture lives at lower testing temperatures were greatly influenced by the lattice rotations required to produce cross slip. A unified analysis was attained for the stress rupture life data generated for the Mar-M247 single crystals at 760 and 774 C under a stress of 724 MPa and the data reported for Mar-M200 single crystals tested at 760 C under a stress of 689 MPa. Based on this analysis, the stereographic triangle was divided into several regions which were rank ordered according to stress rupture life for this temperature regime.