Basic friction angle is an important input parameter in many peak shear strength criteria of rock joint. Reliable estimation of joint basic friction angle is essential for accurate determination of ...the corresponding peak shear strength. In this study, the basic friction angles of planar joint surface of three rocks (i.e., granite, marble, and sandstone) are studied using two commonly used methods, including tilt test and direct shear test. Although the basic friction angles determined from tilt test are about 4 to 5° smaller than those determined from direct shear test, the marble is found to have the largest basic friction angle in both tilt test and direct shear test. In direct shear test, there is about 2° difference of basic friction angles determined under low and high normal stress conditions, which is mainly associated with the shearing mechanism of joint surface. Friction generally occurs under low normal stress. On the other hand, shear-off is observed when the applied normal stress is high. To obtain a reliable basic friction angle using direct shear test, the test data under low normal stresses are suggested to be used. It is also seen from the results that the shear strength of planar joint surface is negligibly influenced by the cyclic shearing when the applied normal stress is low. The data in this study replenish the test data of basic friction angle of different rock types and are useful for establishing a database for the estimation of basic friction angle in future.
We studied the ultrasound velocity and attenuation coefficient in coal on the basis of acoustic emission analysis in the time and frequency domains. We also analyzed the mechanism of ultrasound ...attenuation in coal from a microscopic point of view. Previous research has indicated that the ultrasound velocity and the attenuation coefficient in coal correlate positively and negatively, respectively, with the coal density. In the present study, we found that the acoustic emission characteristics in the time and frequency domain are influenced by the quality of the coal and the fundamental frequency of the ultrasound. For example, the decay time was longer and the amplitude of the received signal was greater in anthracite than in fat coal. Moreover, with increasing fundamental frequency, the decay time decreased and the energy distribution in the transmitted wave became more concentrated around the fundamental frequency. In terms of energy dissipation, coal matrix deformation resulted in a pulling force and caused shear distortion owing to friction, the ultrasonic attenuation in coal is positively affected by the stress induced by ultrasonic waves, which means that a larger strain amplitude would lead to more energy dissipated in coal and an increase of the attenuation coefficient. This was the main cause of ultrasonic attenuation.
The purpose of this study is to experimentally investigate the stress effect on normally incident P-wave attenuation through coal specimens. Laboratory tests were carried out using a Split Hopkinson ...pressure bar (SHPB) system, and a modified method was proposed to determine the quality factor (Q) of P-waves through coal specimens. Larger quality factor denotes less energy attenuated during P-wave propagating through coal. Experimental results indicate that the quality factor and stress (σ) within coal specimens are positively correlated. The P-wave propagation through coal specimens causes crack closure at the beginning of the coal fracture process in SHPB tests, an innovative model was thus proposed to describe the relationship between the crack closure length and the dynamic stress induced by P-wave. Finally, the stress effect on P-wave attenuation through coal was quantitatively represented by a power function Q=a(c–bσ)−6, and the material constants a, b, and c were determined as 1.227, 1.314, and 0.005, respectively. The results obtained in this study would be helpful for engineers to estimate seismic energy attenuation and coal mass instability in coal mines.
•P-wave attenuation across coal was studied by a split Hopkinson pressure bar system.•A modified method was given to measure the energy attenuation of P-wave across coal.•A crack closure model was proposed to link the crack closure length and stress.•Stress effect on P-wave attenuation across coal was represented by a power function.
A novel bifunctional squaramide catalyzed vinylogous Michael–Michael cascade reaction of 3-alkylidene oxindoles with nitroolefin enoates was developed. This convenient, one-pot cascade reaction ...serves as a powerful tool for the enantioselective construction of potential bioactive chiral chromans, which have three continuous tertiary stereocenters, in moderate to good yields (up to 85%) with excellent stereoselectivities (up to >19:1 dr and >99% ee).
In this contribution, high ortho novolac resins modified with phenylboronic acid were synthesized. The thermal stability of novolac resins cured with hexamethylenetetramine (HMTA) and chemical states ...of B and N via a pyrolysis process were studied. For the cured
-novolac modified with phenylboronic acid, the temperature with maximum decomposition rate increased by 43.5 °C, and the char yield increased by 5.3% at 800 °C compared with cured
-novolac. Density functional theory (DFT) calculations show the existence of hydrogen bonding between N of HMTA and H of phenol in modified resin. Thus, N could still be found at high temperature and C=N structure could be formed via a pyrolysis process. B₂O₃ was obtained at 400 °C by the cleavage of B⁻O⁻C and B⁻C bonds and it reduces the oxygen loss which may take part in the formation of carbon oxides in the system. The melting B₂O₃ on the surface of the resin will prevent small molecules and carbon oxides from releasing. Moreover, introducing B into the system helps to decrease the interlayer distance and improve graphite structures via a pyrolysis process.
To study the near-field seismic impact of coal fractures in stress concentration zones, we established a source generation model based on finite dislocation source theory and dynamic fracture ...mechanics, derived an analytical expression for near-field seismic displacements caused by coal fractures in the zone and numerically computed the resultant near-field seismic displacements within the coal mass. The results show that (1) the larger difference between the vertical and horizontal normal stresses in the stress concentration zone leads to a greater fracture speed, which thereby causes a stronger seismic impact; (2) the P-wave component in the near-field seismic displacements mainly impacts on the middle of the roadway, while the SH- and SV wave components mainly affect the junctions between the roadway and both the roof and the floor, and the damage caused by the SH- and SV waves within the coal mass is more significant than that caused by the P-waves; and (3) the effective way to mitigate the seismic impact induced by coal fractures in stress concentration zones is to reduce the difference between the vertical and horizontal normal stresses as far as possible. It is hoped that this study will provide a better understanding of the seismic impacts induced by coal fractures in stress concentration zones and thus help engineers to discover ways to prevent roadway failure.
Abstract
Brittleness has a significant influence on rock failure under compression; however, the mechanism is rarely comprehensively discussed. This study numerically investigates the brittleness ...effect on microcracking behavior of crystalline rock using a grain-based model implemented into a two-dimensional particle flow code, with a focus on the discussion of how rock brittleness affects the failure mechanism. The simulated failure mode changes from tension to shear with decreasing rock brittleness, which is consistent with previous laboratory test results. As the brittleness gradually decreases in the model, the grain boundary (GB) tensile crack to shear crack ratio increases, and the corresponding fractures change from vertical or subvertical to an angle about 45° along the vertical direction. The propagation and coalescence of generated microcracks result in a transition of failure pattern from splitting to shear under uniaxial compression with a decreasing brittleness level in the rock. A transition from GB tensile crack to shear crack is also observed under direct tension when the brittleness index gradually decreases. The tension to shear transition mechanism is closely related to the relative strength of the mineral grain and mineral bonding. The relative strength of mineral and mineral bonding could be used as a parameter to characterize rock brittleness from a microscale viewpoint.
•A series of sulfate Gemini surfactants were synthesized and verified.•The molecular structure has influence on the surface and thermodynamic properties.•B-1–6 has the most excellent foaming ability ...and foam stability.
In this article, a series of sulfate Gemini surfactants with different lengths of hydrophobic tail chains and spacer groups were synthesized. Both 1H NMR spectra and mass spectrometry analysis verified that the synthesized products were sulfate Gemini surfactants. Krafft temperatures indicated that they showed better water solubility at low temperatures. The conductivity of the sulfate Gemini surfactant versus concentration under different temperature conditions was tested. The influence of the hydrophobic tail chain, spacer group, and temperature on the critical micelle concentration and micelle thermodynamic parameters were revealed. The structure–activity relationship between the molecular structure and the foaming performance was discussed. Finally, we studied the influence of temperature on the foaming performance, foam stabilization, and rheological properties of the foam system. The insights gained from this study may enrich theoretical research on sulfate Gemini surfactants and promote the applications of sulfate Gemini surfactants in various fields.
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