The Atomic Oxygen Erosion Resistance Effect and Mechanism of the Perhydropolysilazane-Derived SiOx Coating Used on Polymeric Materials in Space Environment

In this work, the atomic oxygen (AO) erosion-resistance effect and mechanism of the Perhydropolysilazane (PHPS) coating were investigated from the perspective of element distribution in the depth direction. The results revealed that the coating demonstrated good adhesion and intrinsic AO erosion-resistance, which was attributed to the composition gradient formed in the coating. Moreover, the oxygen ratio of the SiOx on top layer of the coating could be elevated during AO exposure, strengthening the Ar ion etching durability of the coating. According to these results, an AO erosion-resistance mechanism model of the PHPS-derived SiOx coating was finally obtained.

Loading–unloading contact analysis of a fractal rough surface with functional gradation

The present paper deals with a finite-element-based static loading–unloading analysis of a functionally graded rough surface contact with fractal characteristics. Two different gradation models, namely elastic and plastic gradations, are adopted. In these models, one out of yield strength and Young's modulus is varied spatially according to exponential functions, while the other is kept constant. In both these material models, separate inhomogeneity parameters control the variation of material properties. The gradation is such that throughout the top of the rough surface properties remain constant with variations in the depth direction being controlled by the above-mentioned parameters. Different fractal surfaces with different levels of roughness (governed by the values of fractal dimension and fractal roughness) have been analysed. The influence of the gradation parameters on the contact properties, viz. contact force, contact area, contact stress, etc., are investigated for both loading and unloading phases. It was found that for most of the loading phase, higher elastic, as well as plastic gradation parameter, causes higher contact force and contact area. However, in the case of the unloading of elastically graded surfaces, this trend is not maintained throughout. For the cases, where a substantial amount of yielding takes place during loading near the contact surface, the resulting contact area is found to be higher for the unloading phase in comparison with the same during the loading phase. The trend of plastic yielding at the vicinity of the contact surface is studied for varying gradation parameters. It is observed that the higher volume of yielded material is obtained for the higher value of elastic gradation parameter. On the other hand, the higher value of plastic gradation parameter causes more yielding to take place at the vicinity of the contact surface. Additionally, the effect of gradation on the energy dissipation due to plasticity after complete unloading is explored in detail.

Modeling and investigation of cutting force for SiCp/Al composites during ultrasonic vibration-assisted turning

The machining process of SiCp/Al composites is considerably difficult because of the addition of ceramic particles. As an effective machining method, ultrasonic vibration-assisted turning is used to process SiCp/Al composites, which can effectively reduce the cutting force, improve the surface quality, and reduce the tool wear. This study developed a cutting force prediction model for ultrasonic vibration-assisted turning of SiCp/Al composites, which comprehensively considers the instantaneous depth of cut and the instantaneous shear angle. This model divides the cutting force into the chip formation force considering the instantaneous depth of cut, the friction force considering the influence of SiC particles at tool-chip interface, the particle fracture force, and the ultrasonic impact force in the cutting depth direction. By comparing the predicted value of the main cutting force with the experimental values, the results present the same trend, which verifies the feasibility of the cutting force prediction model. In addition, the influence of vibration amplitude, depth of cut, and cutting speed on the main cutting force is analyzed. The systematic cutting experiments show that ultrasonic vibration-assisted turning can significantly reduce the cutting force and improve the machinability of SiCp/Al composites.

Comparative Study of High Temperature Anti-oxidation Property of Sputtering Deposited Stoichiometric and Si-rich SiC Films

Stoichiometric and silicon-rich (Si-rich) SiC films were deposited by Microwave Electron Cyclotron Resonance (MW-ECR) plasma enhanced RF magnetron sputtering method. As-deposited films were oxidized at 800, 900 and 1000 ℃ in air for 60 min. The chemical composition and structure of the films were analyzed by X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy and Fourier Transform Infrared spectroscopy (FT-IR). The surface morphology of the films before and after high temperature oxidation was measured by atomic force microscopy. The mechanical property of the films was measured by a Nano-indenter. The anti-oxidation temperature of the Si-rich SiC film is 100 ℃ higher than that of the stoichiometric SiC film. The oxidation layer thickness of the Si-rich SiC film is thinner than that of the stoichiometric SiC film in depth direction. The large amount of extra silicon in the Si-rich SiC film plays an important role in the improvement of its high temperature anti-oxidation property.

FE Analysis of Laser Shock Peening on STS304 and the Effect of Static Damping on the Solution

Laser shock peening creates compressive residual stress on the surface of the material, reducing stress corrosion cracking and increasing fatigue life. FE simulation of laser shock peening is an effective way to determine the mechanical effects on the material. In conventional FE simulations of laser shock peening, explicit analysis is used while pressure loads are applied and switched into implicit analysis to dissipate kinetic energy. In this study, static damping was adopted to dissipate kinetic energy without conversion into implicit analysis. Simulation of a single laser shock and multiple shocks was performed, and deformation and minimum principal stress were compared to evaluate the static damping effect. The history of the internal and kinetic energy were analyzed to compare the stabilization time depending on the damping value. Laser shock peening experiments were also performed on stainless steel 304 material. The residual stress of the specimen was measured by the hole drilling method and it was compared to the FE simulation result. The residual stress from the experiment and the simulation results showed similar distributions in the depth direction. Anisotropic residual stress distribution due to the laser path was observed in both results.

A method for well log data generation based on a spatio-temporal neural network

Well logging helps geologists find hidden oil, natural gas and other resources. However, well log data are systematically insufficient because they can only be obtained by drilling, which involves costly and time-consuming field trials. Additionally, missing or distorted well log data are common in old oilfields owing to shutdowns, poor borehole conditions, damaged instruments and so on. As a workaround, pseudo-data can be generated from actual field data. In this study, we propose a spatio-temporal neural network (STNN) algorithm, which is built by leveraging the combined strengths of a convolutional neural network (CNN) and a long short-term memory network (LSTM). The STNN exploits the ability of the CNN to effectively extract features related to pseudo-well log data and the ability of the LSTM to extract the key features from well log data along the depth direction. The STNN method allows full consideration of the well log data trend with depth, the correlation across different log series and the actual depth accumulation effect. The method proved successful in predicting acoustic sonic log data from gamma-ray, density, compensated neutron, formation resistivity and borehole diameter logs. Results show that the proposed method achieves higher prediction accuracy because it takes into account the spatio-temporal information of well logs.

Investigation of Factors Causing Nonuniformity in Luminescence Lifetime of Fast-Responding Pressure-Sensitive Paints

Factors that cause nonuniformity in the luminescence lifetime of pressure-sensitive paints (PSPs) were investigated. The lifetime imaging method of PSP does not theoretically require wind-off reference images. Therefore, it can improve measurement accuracy because it can eliminate errors caused by the deformation or movement of the model during the measurement. However, it is reported that the luminescence lifetime of PSP is not uniform on the model, even under uniform conditions of pressure and temperature. Therefore, reference images are used to compensate for the nonuniformity of the luminescence lifetime, which significantly diminishes the advantages of the lifetime imaging method. In particular, fast-responding PSPs show considerable variation in luminescence lifetime compared to conventional polymer-based PSPs. Therefore, this study investigated and discussed the factors causing the nonuniformity of the luminescence lifetime, such as the luminophore solvent, luminophore concentrations, binder thickness, and spraying conditions. The results obtained suggest that the nonuniformity of the luminophore distribution in the binder caused by the various factors mentioned above during the coating process is closely related to the nonuniformity of the luminescence lifetime. For example, when the thickness of the binder became thinner than 8 μm, the fast-responding PSPs showed a tendency to vary significantly in the luminescence lifetime. In addition, it was found that the luminescence lifetime of fast-responding PSP could be changed in the depth direction of the binder depending on the coating conditions. Therefore, it is important to distribute the luminophore uniformly in the binder layer to create PSPs with a more uniform luminescence lifetime distribution.

Seismic velocity structure along the North Anatolian Fault beneath the Central Marmara Sea and its implication for seismogenesis

Summary The offshore part of the North Anatolian Fault (NAF) beneath the Marmara Sea is a well-known seismic gap for future M > 7 earthquakes in the sense that more than 250 years have passed since the last major earthquake in the Central Marmara region. Although many studies discussed the seismic potential for the future large earthquake in this region on the basis of historical record, geodetic, and geological observations, it is difficult to evaluate the actual situation on the seismic activity and structure along the NAF beneath the Marmara Sea due to the lack of ocean bottom seismic observations. Using ocean bottom seismometer observations, an assessment of the location of possible asperities that could host an expected large earthquake is undertaken based on heterogeneities in the microseismicity distribution and seismic velocity structure. Specifically, seismic tomography and precise hypocenter estimations are conducted using offshore seismic data whose recording period is 11 months. About five times more microearthquakes are detected with respect to events recorded in a land-based catalog. A comparison with previously published results from offshore observation data suggests that the seismicity pattern had not changed from September 2014 to May 2017. The location accuracy of microearthquakes is greatly improved from only the land-based earthquake catalog, particularly for depth direction. There are several aseismic and inactive zones of microearthquake, and the largest one is detected using land-based seismic observation, whereas other zones are newly detected via offshore observations. The obtained velocity model shows a strong lateral contrast, with two changing points. The western changing point corresponds to a segmentation boundary, where the dip angle of the NAF segments changed. High-velocity zones from tomographic images are characterized by low seismicity eastward of the segment boundary. To the east of 28.50° E, the high-velocity zone becomes thicker in the depth direction and is characterized by low seismicity. Although the low seismic activity alone could be interpreted as both strong coupling and fully creeping, the high-velocity features at the same can be concluded that these zones are consist of brittle material and strong coupling. From comparison with other geodetic and seismic studies, we interpret these zones as locked zones that had been ruptured by the past large earthquakes and could be ruptured by future ones. These zones might accumulate strain since the mainshock rupture associated with the May 1766 Ms7.3 earthquake, the latest major earthquake in this region.

Three-dimensional and real-scale modeling of flow regimes in dense snow avalanches

Snow avalanches cause fatalities and economic loss worldwide and are one of the most dangerous gravitational hazards in mountainous regions. Various flow behaviors have been reported in snow avalanches, making them challenging to be thoroughly understood and mitigated. Existing popular numerical approaches for modeling snow avalanches predominantly adopt depth-averaged models, which are computationally efficient but fail to capture important features along the flow depth direction such as densification and granulation. This study applies a three-dimensional (3D) material point method (MPM) to explore snow avalanches in different regimes on a complex real terrain. Flow features of the snow avalanches from release to deposition are comprehensively characterized for identification of the different regimes. In particular, brittle and ductile fractures are identified in the different modeled avalanches shortly after their release. During the flow, the analysis of local snow density variation reveals that snow granulation requires an appropriate combination of snow fracture and compaction. In contrast, cohesionless granular flows and plug flows are mainly governed by expansion and compaction hardening, respectively. Distinct textures of avalanche deposits are characterized, including a smooth surface, rough surfaces with snow granules, as well as a surface showing compacting shear planes often reported in wet snow avalanche deposits. Finally, the MPM modeling is verified with a real snow avalanche that occurred at Vallée de la Sionne, Switzerland. The MPM framework has been proven as a promising numerical tool for exploring complex behavior of a wide range of snow avalanches in different regimes to better understand avalanche dynamics. In the future, this framework can be extended to study other types of gravitational mass movements such as rock/glacier avalanches and debris flows with implementation of modified constitutive laws.