Molecular Dynamics Study of Crystal Plasticity during Nanoindentation in Ni Nanowires

Molecular dynamics simulations were performed to gain fundamental insight into crystal plasticity, and its size effects in nanowires deformed by spherical indentation. This work focused on-oriented single-crystal, defect-free Ni nanowires of cylindrical shape with diameters of 12 and 30 nm. The indentation of thin films was also comparatively studied to characterize the influence of free surfaces in the emission and absorption of lattice dislocations in single-crystal Ni. All of the simulations were conducted at 300 K by using a virtual spherical indenter of 18 nm in diameter with a displacement rate of1 ms1. No significant effect of sample size was observed on the elastic response and mean contact pressure at yield point in both thin films and nanowires. In the plastic regime, a constant hardness of 21 GPa was found in thin films for penetration depths larger than 0.8 nm, irrespective of variations in film thickness. The major finding of this work is that the hardness of the nanowires decreases as the sample diameter decreases, causing important softening effects in the smaller nanowire during indentation. The interactions of prismatic loops and dislocations, which are emitted beneath the contact tip, with free boundaries are shown to be the main factor for the size dependence of hardness in single-crystal Ni nanowires during indentation

Strain Rate and Orientation Dependencies of the Strength of Single Crystalline Copper under Compression

Molecular dynamics (MD) simulations are used to model the compression under uniaxial strain of copper single crystals of different orientations at various temperatures and strain rates. Uniaxial strain is used because of the close resemblance of the resulting stress state with the one behind a shock front, while allowing a control of parameters such as strain rate and temperature to better understand the behavior under complex dynamic shock conditions. Our simulations show that for most orientations, the yield strength of the sample is increased with increasing strain rate. This yield strength is also dependent on the orientation of the sample, but less dependent on temperature. We find three regimes for the atomistic behavior around the yield: homogeneous dislocation nucleation, appearance of disordered atoms followed by dislocation nucleation, and amorphization. Finally, we show that a criterion solely based on a critical resolved shear and normal stress is insufficient at these strain rates to determine slip on a system

Use of the Richtmyer-Meshkov Instability to Infer Yield Stress at High-Energy Densities

We use the Richtmyer-Meshkov instability (RMI) at a metal-gas interface to infer the metal’s yield stress (Y) under shock loading and release. We first model how Y stabilizes the RMI using hydrodynamics simulations with a perfectly plastic constitutive relation for copper (Cu). The model is then tested with molecular dynamics (MD) of crystalline Cu by comparing the inferred Y from RMI simulations with direct stress-strain calculations, both with MD at the same conditions. Finally, new RMI experiments with solid Cu validate our simulation-based model and infer Y~0.47 GPa for a 36 GPa shock

Aerospace engineering educational program

The principle goal of the educational component of NASA CORE is the creation of aerospace engineering options in the mechanical engineering program at both the undergraduate and graduate levels. To accomplish this goal, a concerted effort during the past year has resulted in detailed plans for the initiation of aerospace options in both the BSME and MSME programs in the fall of 1993. All proposed new courses and the BSME aerospace option curriculum must undergo a lengthy approval process involving two cirriculum oversight committees (School of Engineering and University level) and three levels of general faculty approval. Assuming approval is obtained from all levels, the options will officially take effect in Fall '93. In anticipation of this, certain courses in the proposed curriculum are being offered during the current academic year under special topics headings so that current junior level students may graduate in May '94 under the BSME aerospace option. The proposed undergraduate aerospace option curriculum (along with the regular mechanical engineering curriculum for reference) is attached at the end of this report, and course outlines for the new courses are included in the appendix

Collaborative Opportunistic Navigation

Aerospace Engineering and Engineering Mechanic

GPS-assisted Femtocell Synchronization and Localization Through Tightly-Coupled OpportunisticNavigation

Aerospace Engineering and Engineering Mechanic

Soldering device Patent

Soldering device particularly suited to making high quality wiring joints for aerospace engineering utilizing capillary attraction to regulate flow of solde

NASA-universities relationships in aero/space engineering: A review of NASA's program

NASA is concerned about the health of aerospace engineering departments at U.S. universities. The number of advanced degrees in aerospace engineering has declined. There is concern that universities' facilities, research equipment, and instrumentation may be aging or outmoded and therefore affect the quality of research and education. NASA requested that the National Research Council's Aeronautics and Space Engineering Board (ASEB) review NASA's support of universities and make recommendations to improve the program's effectiveness

Grain Growth Behavior at Absolute Zero during Nanocrystalline Metal Indentation

The authors show using atomistic simulations that stress-driven grain growth can be obtained in the athermal limit during nanocrystalline aluminum indentation. They find that the grain growth results from rotation of nanograins and propagation of shear bands. Together, these mechanisms are shown to lead to the unstable migration of grain boundaries via process of coupled motion. An analytical model is used to explain this behavior based on the atomic-level shear stress acting on the interfaces during the shear band propagation. This study sheds light on the atomic mechanism at play during the abnormal grain coarsening observed at low temperature in nanocrystalline metal

Aerospace engineering projects

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