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A Molecular Dynamics Study of Single Crystal and Intergranular Crack Growth Behavior in W\u3csub\u3e\u3ci\u3ex\u3c/i\u3e\u3c/sub\u3e M\u3csub\u3e1−\u3ci\u3ex\u3c/i\u3e\u3c/sub\u3e Binary Aloys (M = V, Mo, Ta, Re)
Get PDFThis study employs molecular dynamics simulations to investigate the fracture behavior of four binary refractory alloys WxM1−x (M = V, Mo, Ta, Re) and their dependence on crystallographic orientation, composition, and grain boundary (GB) structure, focusing on six distinct low-sigma grain boundaries. The simulations reveal that the effect of composition is complex with the most pronounced effect, accompanied by the maximum or minimum stress intensity factor, generally occurring at intermediate compositions. All compositions showed a higher fracture resistance in the [110] orientation compared to the [100] orientation. There was a strong thermodynamic tendency for Mo and V, and Ta to a lesser extent, to segregate to GBs specifically at the low temperatures. The segregation behavior was more striking in tilt compared to twist GBs and was generally associated with GB embrittlement. A strengthening effect was, however, also observed for specific grain boundaries and segregating elements, demonstrating the significance of the effect of GB structure on overall behavior. Finally, twist GBs typically had higher strength and showed a stronger dependence on strain rate in most cases when compared to tilt GBs. These results may help inform the design of next generation structural materials for extreme environments
Residential Factors Associated with Mental Health in United States Veterans, Air Force Military, and Air Force Employees
Get PDFIndividuals in Westernized countries spend most of their time indoors. However, exploration of residential building factors that may influence occupants’ mental health is limited in scientific literature. The purpose of this study was to explore investigator\u27s perceived areas of importance in residences to mental health via survey methods. To that end, we administered the Housing, Occupancy, Materials, and Environment (HOME) survey to assess factors that may influence mental health to those working in the United States (US) Air Force (n = 230) or past military members, US Veterans (n = 180). Self-reported mental health surveys were also administered to the Air Force (RAND 36-Item Short-Form) and Veterans (36-Item Short-Form survey version 2, Patient Health Questionnaire-9). The residential question that correlated to the most mental health measures for both groups was an ability to adjust indoor climate, with positive correlations. Other correlations between residential questions and health scores across the two groups were dissimilar, indicating the residential factors of importance to mental health may be variable across an individual\u27s life. For example, multiple positive correlations between mental health measures and nature in the older Veteran group support robust and support previous results on the importance of nature to older adults. Overall, this study provides a basis for future research and targeted clinical interventions that can quantify and positively impact the home environment and improve mental health outcomes
Angle-independent spectropolarimetric target classification using machine learning
No full textA method for characterizing unknown targets using a hyper-spectral polarimetric light detection and ranging (LiDAR) system is presented. Light reflected from manmade objects tends to be polarized differently than light reflected from objects in the natural world. As such, polarization measurements can be used in remote sensing applications to differentiate artificial and natural objects. Here, a k -nearest neighbors (KNN) algorithm is provided for classifying materials using hyper-spectral LiDAR with incident angles ranging from −10° to 60°, without assuming any further information about the orientation of the target relative to the LiDAR system, achieving a balanced accuracy of 86.9%
Intrinsic Defects (Vacancies and Antisites) in Neutron Irradiated CdSiP\u3csub\u3e2\u3c/sub\u3e Crystals
No full textCadmium silicon phosphide (CdSiP2) is a nonlinear optical material widely used in optical parametric oscillators. Intrinsic defects (vacancies and antisites) are responsible for unwanted broad optical absorption bands in these crystals that degrade the performance of the devices. In the present work, optical absorption and electron paramagnetic resonance (EPR) spectra are acquired (at room temperature and 12 K, respectively) from a neutron-irradiated CdSiP2 crystal. After the irradiation, the crystal is highly absorbing from the band edge near 600 nm to beyond 1.3 μm because of overlapping defect-related absorption bands. Heating to 550 °C removes nearly all the absorption induced by the high-energy neutrons. EPR spectra show that a primary effect of the neutrons is the production of phosphorous vacancies. A broad EPR signal attributed to perturbed intrinsic defects is seen before heating the irradiated crystal. After warming to 550 °C, an EPR spectrum representing isolated neutral phosphorous vacancies (VP0) is observed without light and an EPR spectrum from neutral phosphorous-on-silicon antisites (PSi0) can be photoinduced with 633 nm light. The formation of the PSi0 antisites (occurring when phosphorous interstitials created by the neutrons are trapped at silicon vacancies) provides direct evidence that the neutrons displaced phosphorous ions. The VP0 and PSi0 donors were not detected in EPR spectra taken from the as-grown CdSiP2 crystal (i.e., before the neutron irradiation)
Towards a Predictive Model of Sporadic E using Planetary Wave Signatures Quantified with the N Dimensional Lomb Scargle Peridogram
No full textSporadic E is a strong plasma density enhancement in the E region of the ionosphere that can severely degrade the performance of Over The Horizon (OTH) radar and other High Frequency (HF) technologies. Sporadic E is known to vary on tidal (1,2,3, … times per day) as well as planetary wave (2 - 30 day periods) timescales. We use a novel dataset and method to study the connections between planetary waves and sporadic E, and show progress towards a rudimentary predictor. The dataset consists of retrieved sporadic E parameters (foEs, hmEs) from Radio Occultation (RO) measurements. The method is the N-Dimensional Lomb-Scargle Periodogram (ND LSP) which allows planetary waves and tides to be quantified without the need for averaging or interpolation of the raw data. This is especially valuable for the irregular temporal and spatial sampling characteristics of RO measurements. This technique shows expected seasonal trends and evidence of non-linear interactions between tides and planetary waves
Assessment of Multiphysics Computations of Flow over a Film-Cooled Plate
No full textThis paper summarizes findings from a collection of multiphysics analyses of a heated exhaust passing over a film-cooled plate obtained for the fifth AIAA Propulsion Aerodynamics Workshop. The experimental configuration examined was a subsonic convergent nozzle with a square exit blowing a heated exhaust over a plate with three film-cooling sections. Computational fluid dynamics solutions were obtained and compared to experimental measurements of flow velocities and temperatures, as well as plate surface temperatures. The heated nozzle operated with a Mach 0.3 exit flow at a static temperature ratio of 2.7. Cooling air blowing ratios of 0, 1, and 2 were considered. Computational meshes for the flow domain were provided for participants. Workshop participants from eight separate organizations represented government, industry, and academia. A variety of flow solutions were obtained: Most of the flow solutions employed a Reynolds-averaged Navier–Stokes (RANS) approach, but scale-resolving simulations were used in some cases, including wall-modeled large-eddy simulation (LES) and hybrid RANS-LES approaches. One lattice Boltzmann solver was employed. For heat transfer to the test article, several analyses used a conjugate heat transfer approach. Effects of mesh sensitivity, flow solution approach, and wall heat transfer approach are considered. In general, a fully coupled three-dimensional conjugate heat transfer approach enabled significantly better prediction of surface temperatures than simpler wall temperature boundary treatments. Also, the scale-resolving approaches more accurately calculated the hot flow/film interaction and, hence, static temperatures immediately above the plate where the hot jet exhaust boundary layer interacted with the cooling film. Comparisons of plate surface friction drag and heat transfer obtained from the computations are also presented
Security Comparison of Powerline Communication and Wi-Fi Technologies for Internet of Things
No full textWith the rise of system automation, more devices require intelligence and communication capabilities with a network, which is commonly provided via Internet of Things (IoT) devices. Providing communication stack security for these networks becomes increasingly challenging as the expectations of the systems increase. This paper reviews the current status of physical security network technology and explores innovations made in the past five years to identify and solve cyber vulnerabilities in a variety of contexts and then relates them back to IoT applications. These networked devices are often produced as simply and cheaply as technically feasible, which results in them lacking computational capacity, robust networking hardware and inherent security measures. The review focuses on two main technologies, WiFi and Powerline communications (PLC), to compare research on guided and un-guided media. These technologies can both be considered shared, as it is typical for multiple users to be expected to utilize a shared channel. This means both mediums are vulnerable to wireless jamming, but the effectiveness of this varies greatly based on factors such as the environment, cable shielding and transceiver distance. Similarly, the propagation of signals from the two technologies jeopardizes the potential privacy of communications by leaving them vulnerable to various means of eavesdropping. This can be addressed by using methods such as encryption, which is usually implemented at higher layers of the communication stack to provide confidentiality but can also be applied at the physical layer. Encryption also has the challenge of ensuring key exchange occurs securely which requires unique solutions when the limits of IoT devices are considered. Eavesdropping can also be defeated by controlling the signal-to-noise ratio that is presented to unintended receivers. Additionally, methods of device fingerprinting are being developed to create more robust authentication regimes between devices. Several research opportunities have been proposed where new concepts in one medium are applied to the other
Computational Characterization of Ion Beam Uniformity of Planar Pinched-Beam Ion Diodes
No full textThis work presents a particle-in-cell (PIC) simulation-based analysis of ion-beam uniformity from planar diodes. Diode configurations with variations in anode geometry, thickness, and material were explored, including round, tapered, and monolithic polyethylene and Mylar designs. The ion fluence and current enclosed profiles were extracted to assess the quality of the beam across the anode surface. The results show that dual-anode planar geometries produce promising ion beam uniformities, with the most uniform case yielding a coefficient of variation of 10.62%. Round-edge designs mitigate magnetic field losses at anode tips, while thinner anodes enhance ion production through increased electron reflexing. These findings support the use of planar diodes in high-uniformity ion beam environments for materials testing applications
Indiscriminate disruption of conditional inference on multivariate Gaussians
No full textThe multivariate Gaussian distribution underpins myriad operations-research, decision-analytic, and machine-learning models (e.g., Bayesian optimization, Gaussian influence diagrams, and variational autoencoders). However, despite recent advances in adversarial machine learning (AML), inference for Gaussian models in the presence of an adversary is notably understudied. Therefore, we consider a self-interested attacker who wishes to disrupt a decisionmaker’s conditional inference and subsequent actions by corrupting a set of evidentiary variables. To avoid detection, the attacker also desires the attack to appear plausible wherein plausibility is determined by the density of the corrupted evidence. We consider white- and grey-box settings such that the attacker has complete and incomplete knowledge about the decisionmaker’s underlying multivariate Gaussian distribution, respectively. Select instances are shown to reduce to quadratic and stochastic quadratic programs, and structural properties are derived to inform solution methods. We assess the impact and efficacy of these attacks in three examples, including, a real estate evaluation application, an interest rate prediction task, and the use of linear Gaussian state space models. Each example leverages an alternative underlying model, thereby highlighting the attacks’ broad applicability. Through these applications, we also juxtapose the behavior of the white- and grey-box attacks to understand how uncertainty and structure affect attacker behavior
Structural Index Parameter for Capturing Aerothermal Effects in Conceptual-Level Vehicle Design
No full textThe three phases of vehicle conceptual design include parametric sizing, configuration layout, and configuration evaluation. During the parametric sizing phase, the ability to define and quantify the technology level of an aerospace system allows the assessment of candidate designs based on feasibility given current technology or indicates if one must advance a particular technology. To meet this need, the structural index (ISTR) parameter merits exploration to consider structural and aerothermal effects during the parametric sizing phase of conceptual design given materials, structural concepts, and manufacturing capability. This study showcases the utility of this structural/materials technology parameter for high-speed vehicles by modernizing and expanding upon Paul Czysz’s original structural index (ISTR) versus the surface temperature map. The modernized and expanded structural index (ISTR) map is constructed by selecting a temperature-through-thickness method for a given thermal protection system (TPS) that simplifies a given surface temperature and atmospheric pressure profile into a constant heat pulse. One can then size the TPS to keep the structural temperature within material limits. The newly generated structural index (ISTR) maps allow one to observe trends with variations in surface temperature, cruise time, average atmospheric pressure (PAVG), and TPS materials