The low-temperature plasticity of aluminum nitride (AlN) is determined by the interaction between edge and screw dislocations. However, the motion of screw dislocations and their glide mechanisms have not been evaluated. In this study, the motion of a ⟨0001⟩{11̅00} screw dislocation in a single crystal of AlN is explored by molecular dynamics simulations using LAMMPS software with the Stillinger–Weber (SW) potential. Four modes of thermally activated motion are observed under different conditions of temperature and stress: double kinks, Shockley partials, self-pinning, and debris and dislocation loops. The mobilities of a ⟨0001⟩{11̅00} screw dislocation and a 1/3⟨112̅0⟩{11̅00} edge dislocation are compared under various conditions. Our results show that the mobilities of the ⟨0001⟩{11̅00} screw and 1/3⟨112̅0⟩{11̅00} edge dislocations are quite low at T < 600 K. The ⟨0001⟩{11̅00} screw dislocation moves faster at 900 < T < 1500 K and seems less dependent on the temperature than does the 1/3⟨112̅0⟩{11̅00} edge dislocation at 1200 < T < 2200 K. However, the opposite phenomenon is observed at higher temperatures. The mobility of the ⟨0001⟩{11̅00} screw dislocation is slightly lower than that of the 1/3⟨112̅0⟩{11̅00} edge dislocation at T > 1800 K, although the mobility difference can reach several orders of magnitude at 900 < T < 1200 K due to different Peierls barriers.
The average cyclic load of heavy-haul railway trains is generally larger than that of a conventional mixed passenger and freight railway. This load leads to more severe fatigue damage to structures, including the concrete in a tunnel invert. This study focuses on the fatigue damage of a tunnel invert under a cyclic load of 33 tonnes. The damage classifications for the tunnel inverts are given based on field investigations. With large-scale in-situ tests on the Zhang-Tang Heavy-Haul Railway Tunnel, the pressure–time distributions for the additional dynamic stresses on the surface of the track-bed for various classes of the surrounding rock are proposed. They were subsequently validated against numerical simulation using the ANSYS Workbench module. Fatigue damage of the tunnel invert is demonstrated using both numerical and monitoring methods. It has been observed that the damage to the tunnel invert becomes severe and extensive if the quality of the surrounding rock degrades. Damage zones develop first at the top of the invert and then expand to a deeper position, depending on the rock grade.
Classical molecular dynamics simulations are performed to investigate the motion of a-type edge dislocations in wurtzite aluminum nitride (AlN). The nucleation and propagation of kinks are observed via the dislocation extraction algorithm. Our simulation results show that the nucleation energy of the kink pair in AlN is 1.2 eV and that the migration energy is 2.8 eV. The Peierls stress of the 1/3⟨112̅0⟩{101̅0} edge dislocation at 0 K is 15.9 GPa. The viscous motion of dislocations occurs when τ > τ p , and the dislocation velocity is inversely proportional to the temperature and directly proportional to the applied stress. Below room temperature, the value of the critical resolved shear stress (CRSS) on the prismatic plane is the lowest, which suggests that the dislocation mobility on the prismatic plane is the easiest. The CRSS on the pyramidal plane is always the highest at all temperatures, which suggests that pyramidal slip is the hardest among these three slip systems.
Abstract The structure of a high-speed railway (HSR) bridge with a track system is complex, and the computational efficiency of seismic analysis for a fine HSR bridge with a track system model (BM) is low. This study considers an HSR bridge with CRTS II slab ballastless track system as a case study and develops a simplified model (SM) based on a method for simulating track constraints with equivalent springs. To obtain the optimal stiffness of SM equivalent springs, a finite element (FE) structural parameters updating framework utilizing an improved genetic algorithm (GA) is established, and the updated SM's dynamic characteristics and seismic response are compared to BM's. In addition, it uses SM to analyze the over-limit state and applicability of the bridge structure's critical components. The results indicated that the model simplification ratio of SM reaches 95%. The seismic response outcomes of SM and BM accurately agree with an average relative error of 3% and a maximum relative error of less than 5%. The over-limit state of the seismic response structure tends to be stable as the number of ground motion samples increases. The shear and bending capacity of the pier and the deformation of the fixed support have a high over-limit risk under longitudinal seismic, which is worth noting in the engineering design.
We extend a thermal-elastic stress model by the finite element method to evaluate anisotropic three-dimensional thermal stress in AlN bulk crystals grown on on-axis 2H-AlN and 6H-SiC seeds. The distribution of stresses in the growing AlN crystals at various crystal thicknesses is simulated based on the developed model. The simulation results show that a high von Mises stress layer with strong fluctuations at the 6H-SiC/2H-AlN interface is observed for the heteroepitaxial growth, and a critical crystal thickness of 1 mm for the stress relaxation is required to avoid cracking. On the contrary, a smooth evolution of the von Mises stress is observed for the homoepitaxial growth on the 2H-AlN seed. The maximum total resolved shear stress inside the crystal when using SiC seeds is slightly higher than that of using AlN seeds at the initial growth stage, while this phenomenon reverses after the crystal thickness exceeds approximately 3 mm. Whether using AlN or SiC seeds, the magnitude of the total resolved shear stress increases steadily, and the difference between the homoepitaxial and heteroepitaxial growth is quite small during the whole bulk AlN growth by the physical vapor transport process.
ABSTRACTThe longitudinal seismic response reduction in railway bridges is worthy of attention because it relates to the engineering cost. It is difficult to further explore the reduction effect because the complexity of traditional analysis model leads to the low efficiency in parametric analysis. In this paper, the track structure was simplified, and a simplified calculation model was established. The parametric analysis considering the span number and pier height on the simplified model seismic responses was carried out. The results showed that the reduction effect was extensive and there was a great internal force transferred to the subgrade-track structure.KEYWORDS: Track constrainthigh-speed railway simply-supported bridgereduction effectsimplified modelseismic responses Disclosure StatementNo potential conflict of interest was reported by the author(s).Additional informationFundingThis study was funded by the National Natural Science Foundations of China [U1934207, 52078487 and 51778630], the Innovation-driven Plan in Central South University [2020zzts159].
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