Rocks extracted from deep mining operations often undergo various complex temperature and dynamic load disturbances. In order to investigate the influence of temperature on the mechanical ...characteristics and fracture mechanisms of sandstone under different loading conditions, thermal treatments were conducted on sandstone samples at temperatures of 25 °C, 200 °C, 400 °C, 600 °C, 800 °C, and 1000 °C. Subsequently, the quasi-static and dynamic mechanical properties of the treated samples were tested and recorded using an electronic universal testing machine and a Split Hopkinson Pressure Bar (SHPB). Simultaneously, detailed analyses and discussions of thermal damage and failure mechanisms of the specimens were carried out using Energy-Dispersive Spectroscopy (EDS), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Raman spectroscopy. The results indicate that with increasing temperature, the mass, density, and peak stress of the sandstone specimens decrease, while the porosity and peak strain increase. Transgranular (TG) fracture was identified as the primary failure mode during loading, accompanied by a small amount of intergranular (IG) fracture. Furthermore, high temperatures cause changes in the chemical properties of the sandstone, resulting in thermal decomposition of crystal grains and evaporation of mineral components, which are key factors that weaken the mechanical properties of sandstone. The significance of this study is to provide theoretical and technical support for site selection and safety assessment of underground engineering projects.
In this study, high-speed milling of marble was conducted with a coated carbide ball nose end mill. Single-factor cutting tests were designed to investigate the effect of milling parameters on ...cutting forces and marble chips. It was found that the cutting forces in three directions increased with the increasing feed rate and decreased with the increasing cutting speed. The morphology and particle size of marble chips were analyzed. The transgranular fracture was found to be the predominant failure mode in the milling process. In addition, it was found that the particle size of marble chips increased with the increasing feed rate and decreased with the increasing cutting speed. There was an obvious positive correlation between the cutting forces and chip formation. The experimental results were significant for a better understanding of the material removal mechanism and selection of cutting parameters in the machining of rock materials.
In this study, a Ti(C,N)-based cermet material was prepared through vacuum sintering. The research also investigates how holding time and maximum sintering temperature influence the material ...microstructure and mechanical properties. X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) were used to analyze the composition of the cermet. The microstructure of the cermet was analyzed and examined using a scanning electron microscope (SEM). A Vickers hardness tester was used to test the mechanical properties of the materials. As indicated by testing results, the hardness of the material decreases as the temperature of sintering increases, and its fracture toughness increases gradually as holding time increases. Ti(C,N)-based cermet manifested the optimal mechanical properties when sintering was conducted under 1400 °C with 80 min of holding time. Moreover, the material microstructure is significantly affected by the sintering process. The grain size of Ti(C,N) cermets increases as the sintering temperature increases. The microstructure tends to be uniform and the complete core-rim structures are established as the holding time increases.
Gust fronts (GFs) belong to the boundary layer convergence system. A strong GF can cause serious wind disasters, so its automatic monitoring and identification are very helpful but difficult in daily ...meteorological operations. By collecting convective weather processes in Hubei, Jiangsu, and other regions of China, 1422 GFs from 106 S-band new-generation weather radar (CINRAD/SA) volume scan data are labeled as positive samples by means of human–computer interaction, and the same number of negative samples are randomly tagged from no GF radar data. A deep learning dataset including 2844 labels with a positive and negative sample ratio of 1:1 is constructed, and 80%, 10%, and 10% of the dataset are separated as training, validation, and test sets, respectively. Then, the training dataset is expanded to 273,120 samples by data augmentation technology. Since the height of a GF is generally less than 1.5 km, three deep-learning-based models are trained for GF automatic recognition according to the distance from the radars. Three models (M1, M2, M3) are trained with the data at a 0.5° elevation angle from 65 to 180 km away from the radars, at 0.5° and 1.5° angles from 40 to 65 km, and at 0.5°, 1.5°, and 2.4° angles within 40 km, respectively. The precision, confusion matrix, and its derived indicators including receiver operating characteristic curve (ROC) and the area under ROC (AUC) are used to evaluate the three models by the test set. The results show that the identification precisions of the models are 97.66% (M1), 90% (M2), and 90.43% (M3), respectively. All the hit rates are over 89%, the false positive rates are less than 11%, and the critical success indexes (CSIs) surpass 82%. In addition, all the optimal critical points on the ROC curves are close to (0, 1), and the AUC values are above 0.93. These results suggest that the three models can effectively achieve the automatic discrimination of GFs. Finally, the models are demonstrated by three GF events detected with Qingpu, Nantong, and Cangzhou radars.
In the metal cutting process, a blocked chip needs to be avoided as it leads to low tool life, poor machined surface quality, and large cutting force, whereas an unbroken chip may scratch the ...machined surface and hinder efficient chip removal in metal cutting. For finishing machining of Fe-Cr-Ni stainless steel, the chips will not be broken easily without chip breakers. Therefore, in this paper, two-dimensional (2D) finite element models of orthogonal cutting were built with finite element software, AdvantEdge, to optimize the chip breaker parameters for finishing machining of Fe-Cr-Ni stainless steel. An optimized methodology was proposed and the chip breaker was optimized based on chip curl radius, tool stress, temperature, and cutting force. The smaller groove height of chip breaker is recommended with low cutting speed and large feed rate, while the larger groove height of chip breaker is recommended with high cutting speed and small feed rate. Other chip breaker parameters were also optimized.
The precise positioning of the laser focal spot on the substrate is an important issue for laser microfabrication. In this work, a diffraction pattern-based focal spot positioning method (DFSPM) is ...proposed to achieve the precise positioning of the laser focal spot on opaque substrates. A series of diffraction patterns of laser focus under-positioning, exact positioning and over-positioning were obtained to investigate the cross-section light distribution of the laser focal spot. According to the monotonic tendency of FWHM to exhibit light intensity at the focal spot cross-section away from the focal plane, the FWHM threshold of polynomial fitted curves was used to determine the exact positioning of laser focus. The ascending scanning method was used to obtain the diffraction patterns at various vertical positions and the FWHM threshold of light distribution at the exact position. The polynomial fitted curves verify the FWHM monotonic tendency of light intensity distribution at the focal spot cross-section along the optical axis. Precise positioning can be achieved with a 100 nm adjustment resolution. This work was expected to provide references for laser microfabrication on opaque materials.
The change in raindrop spectrum characteristics is an important factor affecting the accuracy of estimations of precipitation. The in-depth study of raindrop spectrum characteristics is of great ...interest for understanding precipitation process and improving quantitative radar precipitation estimation. In this paper, the raindrop size distributions at Longli (57913), Puding (57808) and Luodian (57916) stations in Guizhou were analyzed from the perspective of precipitation microphysical characteristics. The results showed that the raindrop size distribution was different among different regions. The correlation coefficients of the mass-weighted average diameter for the rain intensities at these three stations were 46.89%, 49.51%, and 47.03%, respectively, which were slightly lower than the normal correlation coefficients of the average volume diameter for the rain intensities: 67.80%, 71.28%, and 71.46%, respectively. Based on the data from the Guiyang weather radar, raindrop spectrometer, and automatic rain gauge, the dynamic Z-I relationship method and the LSTM neural network method were used to estimate precipitation. The correlation coefficients of the dynamic Z-I relationship method and the LSTM neural network method at the three stations studied were 0.8432, 0.7763, and 0.8658 and 0.8745, 0.9125, and 0.8676, respectively. Regarding the process of stratiform cloud precipitation, the correlation coefficients of the dynamic Z-I relationship method and LSTM neural network method at the three stations were 0.6933, 0.0902, and 0.1409 and 0.7114, 0.4984, and 0.4902, respectively. In the estimation of cumulative precipitation for 45 days from 1 July to 16 August 2020, the relative errors of the neural network estimation at the three stations were −4.25%, −11.35%, and −8.68% and the relative errors of the dynamic Z-I relationship estimation were −2.68%, −7.41%, and −21.23%, respectively. The final relative error of the neural network was slightly worse than that of the dynamic Z-I relationship in the cumulative precipitation estimations of Longli station and Puding station, but the overall correlation coefficients of the LSTM neural network method were better than those of the dynamic Z-I relationship method.
The failure mechanism and size effect during the quasi-static and dynamic shear tests of hat-shaped specimens were investigated in this study. Three types of specimens with different shear ring ...thicknesses (800, 400, and 50 μm) were designed. Quasi-static tests were carried out using an electronic universal testing machine, while dynamic impact tests were carried out using split Hopkinson pressure bar (SHPB) tests. The adiabatic temperature rises with different strain rates, and the shear ring thickness was calculated. We found that the adiabatic temperature rises of the specimens with shear ring thicknesses of 800 and 400 μm were much larger than those of the specimens with shear ring thicknesses of 50 μm. The failure surfaces after the SHPB test were investigated via scanning electron microscopy, and the failure surfaces after the SHPB test could be divided into three zones: tensile, shear, and impact zones. The effect of the shear ring thickness and impact speed on the failure surface morphology was discussed. The typical shear stress–strain curves could be divided into three sections: elastic, plastic rise, and plastic plateau sections. Subsequently, a modified Johnson–Cook constitutive model was employed to fit the shear stress–strain results, and the fitted curves showed good agreement with the tested curves.
The requirement of grooved surface and grooving forces is improved for the machining of precision and small parts in the modern industry. Reducing the grooving feed rate is an effective method to ...improve the surface quality and lower the cutting forces. But small feed rate results in the problems of chip breaking and cutting vibration. Improvement of tool geometry is the solution to the problems, however, The matching relationship between the tool geometry and the feed rate is not clear. In this paper, three grooving tools (N123G5-0300-0002-CM, GZD3002-MC, and N123G5-0300-0001-CF respectively) with different parameters have been selected and a series of grooving experiments have been performed. The experimental results show that the initial state and the steady state will produce two different kinds of chips during the grooving process. For CF tools, chip breaking occurs when the feed rate is higher than 0.04 mm/rev, and the feed rate should be higher than 0.06 mm/rev for the other two tools. An analytical model for calculating surface roughness considering the tool geometry parameters and feed rate has been proposed. The elastic recovery of the grooved surface has been analyzed to explain the error between analytical results and experimental results.
Features and transmission paths of electromagnetic interferences of coal mine underground were analyzed, which were in electrical transmission tunnel, traction network and electric locomotive tunnel ...and electromechanical chamber as well as areas around communication system. Electromagnetic interferences in above areas were tested. Through analyzing the tested data, electromagnetic environment in different areas was evaluated according to GB 9175-88 Hygienic standard for environmental electromagnetic waves. The evaluation results show that the tested environments all satisfy one level standard of environmental electromagnetic field.