We consider two-way amplify-and-forward relaying, where multiple full-duplex user pairs exchange information via a shared full-duplex massive multiple-input multiple-output (MIMO) relay. Most of the ...previous massive MIMO relaying works maximize the spectral efficiency (SE). By contrast, we maximize the non-convex energy efficiency (EE) metric by approximating it as a pseudo-concave problem, which is then solved using the classic Dinkelbach approach. We also maximize EE of the least energy-efficient user relying on the max-min approach. We also compare SE and EE of the proposed design with existing full-duplex systems and quantify the significant improvement achieved by the proposed algorithm. We also compare EE of the proposed full-duplex system to that of its half-duplex counterparts, and characterize the self-loop and inter-user interference regimes, for which the proposed full-duplex system outperforms the half-duplex ones.
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•Room temperature uniaxial tensile tests and uniaxial fatigue tests were conducted.•Laser shock peening improved the strength and fatigue life of ATI 718Plus.•Near surface ...microstructure changes due to laser shock peening were characterized.•Residual stress created by the laser shock peening hindered fatigue cracks.•Correlation of residual stress/microstructure and strength/fatigue life improvement.
The effects of Laser Shock Peening (LSP) on the microstructure, residual stress, hardness, strength, and fatigue life of ATI 718 Plus (718Plus) alloy was investigated and the results are reported. Microstructure before and after LSP and after mechanical testing was characterized using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD)/orientation imaging microscopy (OIM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). LSP led to severe surface plastic deformation, which, in turn, led to a high magnitude of surface compressive residual stresses (∼−725MPa) and changes in the near-surface microstructure. This change was in the form of high dislocation density forming dislocation entanglements and slip bands and formation of a few near-surface sub-grains, which, in turn, caused high surface hardening (∼8.3GPa). In addition, a 16% increase in the yield strength (∼175MPa) and around 15% increase in the endurance limit (∼110MPa) occurred in corresponding tests at room temperature. The improvement in fatigue life was due to the shielding and resistance to crack initiation that the LSP treatment provided to the material through the near-surface microstructure, hardening and high compressive residual stress. This shielding also hindered crack propagation, lowering its rate to a third compared with baseline. Results of studies of stress relaxation with cycles showed how the LSP shield was affected by cycling and, in turn, the improvement in fatigue life of 718Plus. The results have demonstrated that LSP is a powerful surface engineering technique that can improve mechanical properties and fatigue performance of many important metallic materials.
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•Uniaxial tensile tests and uniaxial fatigue tests were conducted at 650°C.•Laser shock peening improved the strength and fatigue life of ATI 718Plus.•Near surface microstructure ...changes due to laser shock peening were characterized.•Residual stress created by the laser shock peening hindered fatigue cracks.•Correlation of residual stress/microstructure and strength/fatigue life improvement.
Laser Shock Peening (LSP) is a mechanical surface treatment that induces large compressive residual stresses and microstructural changes in the material by using repetitive shocks from laser pulses. In this study, we investigate the use of LSP to improve the fatigue life of ATI 718 Plus (718Plus) at high temperature of 650°C. LSP led to severe surface plastic deformation, which, in turn, led to a high magnitude of surface compressive residual stresses and changes in the near-surface microstructure which caused high surface hardening. This change in the near-surface microstructure was in the form of high dislocation density forming dislocation entanglements and slip bands and formation of near-surface nanoscale sub-grains/crystallites that remained stable at elevated temperatures. In addition, LSP retained ∼−470MPa residual stress (68% of its initial residual stress) even after 140h exposure to 650°C. The retained residual stresses and the stable microstructure from the LSP increased the yield strength by ∼14% (∼140MPa) and endurance limit by ∼10% (∼90MPa) in corresponding tests at 650°C. This improvement in fatigue life was attributed to near-surface microstructure, hardening and high compressive residual stress. The estimated crack growth rates were 72% lower for LSP-treated 718Plus as compared with untreated material. The thermal-mechanical residual stress relaxation indicates the effectiveness of LSP in improving the fatigue life of 718Plus at 650°C.
In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on residual stresses, microstructure changes and mechanical properties of austenitic stainless steel 304 were ...investigated. The dynamic impacts induced by UNSM leads to surface nanocrystallization, martensite formation, and the generation of high magnitude of surface compressive residual stresses (−1400MPa) and hardening. Highly dense deformation twins were generated in material subsurface to a depth of 100µm. These deformation twins significantly improve material work-hardening capacity by acting both as dislocation blockers and dislocation emission sources. Furthermore, the gradually changing martensite volume fraction ensures strong interfacial strength between the ductile interior and the two nanocrystalline surface layers and thus prevents early necking. The microstructure with two strong surface layers and a compliant interior embedded with dense nanoscale deformation twins and dislocations leads to both high strength and high ductility. The work-hardened surface layers (3.5 times the original hardness) and high magnitude of compressive residual stresses lead to significant improvement in fatigue performance; the fatigue endurance limit was increased by 100MPa. The results have demonstrated that UNSM is a powerful surface engineering technique that can improve component mechanical properties and performance.
In this article, we characterize and review the unusual lack of threshold in fatigue crack growth (FCG) behavior for some alloys at low values of stress intensity factor ranges ΔK and its ...implications to damage-tolerant design approaches. This unusual behavior was first observed by Marci in 1996 in IMI 834 alloy. Conventional applications of linear elastic fracture mechanics to FCG analysis at constant R-ratio (or K
) assumes that (da/dN) decreases monotonically with decreasing ΔK and approaches the threshold value of ΔK
with (da/dN) ≤ 10
mm/cycle for a given R (or K
). However, instead of ΔK threshold behavior, some materials exhibit plateau or acceleration in da/dN rate with decreasing ΔK for long cracks tested in both constant R and K
conditions. This unusual (da/dN)-ΔK behavior is only observed experimentally but not understood and represents a challenge to scientists and engineers to model the safe fatigue life prediction of structures under low amplitude vibrating loads.
We derive post-processing signal-to-noise-plus-interference-ratio (PP-SINR) for minimum mean square error (MMSE) combining in multiple-input multiple-output (MIMO) filter bank multi-carrier (FBMC) ...systems based on offset quadrature amplitude modulation (OQAM). We show that the PP-SNR, derived by assuming imperfect receive channel state information, accurately characterizes the symbol error rate (SER) of MMSE combining in MIMO FBMC-OQAM systems. We numerically demonstrate the tightness of both PP-SINR and SER expressions.
A compact planar quad element wideband antenna for multiple-input-multiple-output (MIMO) system is proposed in this communication. A single element consists of a partially grounded printed monopole ...antenna loaded with a split ring resonator. The bandwidth of the antenna is from 2.2 to 6.28 GHz (96.2%), which covers LTE (2.2-3.8 GHz), Bluetooth (2.4 GHz), WLAN (2.4 and 5.1-5.8 GHz), WiMAX (2.3-5.7 GHz), and ISM bands (2.4/5.2/5.8 GHz). The fabricated antenna has an isolation greater than 14 dB between its elements, with a peak gain of 4 dBi and a peak efficiency of 91%. Polarization diversity is employed to accommodate four elements in an FR4 substrate, with an overall dimension of 0.33λ × 0.33λ × 0.01λ. The antenna has a simple planar design which is easy to fabricate with no intricate process involved.
Machine learning and artificial intelligence (ML/AI) are rapidly becoming an indispensable part of physics research, with domain applications ranging from theory and materials prediction to ...high-throughput data analysis. In parallel, the recent successes in applying ML/AI methods for autonomous systems from robotics to self-driving cars to organic and inorganic synthesis are generating enthusiasm for the potential of these techniques to enable automated and autonomous experiments (AE) in imaging. Here, we aim to analyze the major pathways toward AE in imaging methods with sequential image formation mechanisms, focusing on scanning probe microscopy (SPM) and (scanning) transmission electron microscopy ((S)TEM). We argue that automated experiments should necessarily be discussed in a broader context of the general domain knowledge that both informs the experiment and is increased as the result of the experiment. As such, this analysis should explore the human and ML/AI roles prior to and during the experiment and consider the latencies, biases, and prior knowledge of the decision-making process. Similarly, such discussion should include the limitations of the existing imaging systems, including intrinsic latencies, non-idealities, and drifts comprising both correctable and stochastic components. We further pose that the role of the AE in microscopy is not the exclusion of human operators (as is the case for autonomous driving), but rather automation of routine operations such as microscope tuning, etc., prior to the experiment, and conversion of low latency decision making processes on the time scale spanning from image acquisition to human-level high-order experiment planning. Overall, we argue that ML/AI can dramatically alter the (S)TEM and SPM fields; however, this process is likely to be highly nontrivial and initiated by combined human-ML workflows and will bring challenges both from the microscope and ML/AI sides. At the same time, these methods will enable opportunities and paradigms for scientific discovery and nanostructure fabrication.
Charge-transport and electrochemical processes are heavily influenced by the local microstructure. Kelvin probe force microscopy (KPFM) is a widely used technique to map electrochemical potentials at ...the nanometer scale; however, it offers little information on local charge dynamics. Here, we implement a hyperspectral KPFM approach for spatially mapping bias-dependent charge dynamics in timescales ranging from the sub-millisecond to the second regime. As a proof of principle, we investigate the role mobile surface charges play in a three-unit-cell LaAlO3/SrTiO3 oxide heterostructure. We explore machine learning approaches to assist with visualization, pattern recognition, and interpretation of the information-rich data sets. Linear unmixing methods reveal hidden bias-dependent interfacial processes, most likely water splitting, which are essentially unnoticed by functional fitting of the dynamic response alone. Hyperspectral KPFM will be beneficial for investigating nanoscale charge transport and local reactivity in systems involving a possible combination of electronic, ionic, and electrochemical phenomena.
•Ultrasonic nanocrystal surface modification (UNSM) were used to process 3D-printed metals.•Better surface finish, lower subsurface porosity were observed after UNSM treatment.•Tensile residual ...stresses were changed to compressive after UNSM treatment.•Significant improvement in fatigue performance was observed.
3D-printed metals have great potential for application in the biomedical and the aerospace industries. Unfortunately, they suffer from poor surface finish, high porosity, and high tensile residual stresses, leading to inferior mechanical properties as compared with traditional cast or wrought metals. In this study, we introduce an innovative method, ultrasonic nanocrystal surface modification (UNSM), for the processing of a 3D-printed Ti-6Al-4V alloy. The surface finish, microstructure, residual stresses and mechanical properties of the samples before and after UNSM treatment were characterized and compared. It was found that the UNSM treatment resulted in much better surface finish, lower subsurface porosity, and a high magnitude of compressive residual stresses, leading to significant improvement in rotation bending fatigue performance.
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