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Accidents frequently happen during tunnel and subway construction. This study defines the cave collapse types based on the investigation results of a cave collapse in Yanchang County, the precise scenario of the cave collapse, paired with Yan’an City. Flac3D software is used to simulate the slope before and after the excavation of the cave. The findings of comparing the stress and strain characteristics of the soil slope lead to the cave’s destruction. A comprehensive risk probability estimation method based on interval probability level of accident tree and Bayesian network, weight confidence index, and confidence interval is proposed to solve the problems of subjectivity and inaccurate results in tunnel risk assessment, and it is applied to the risk assessment of cave collapse. First, the Bayesian network was constructed by using accident tree, and the conditional probability (joint probability) of sensor nodes is obtained by using the dependence relationship between the case factors. Then, the piJ estimated value of expert J for the occurrence probability of the fundamental event XI was calculated using the interval probability grade division and weight confidence index approach. The sample space U and its statistics were constructed from the estimated values of all experts, and the probability range of the occurrence probability of the basic event Xi was obtained by introducing the confidence interval method. For risk inference, the probability range of all occurrences is derived and integrated with the conditional probability of a Bayesian network, ensuring the scientific correctness and precision of risk assessment. At the same time, the model can be used to determine the cause of an accident. According to our experimental results, cave collapse causes were investigated soon after an accident occurred, allowing for the development of risk management strategies to prevent future occurrences.

Under uniaxial shock compression, the steepness of the plastic shock front usually exhibits power law characteristics with the Hugoniot pressure, also known as the ``Swegle-Grady law.'' In this Letter, we show that the Swegle-Grady law can be described better by a third power law rather than the classical fourth power law at the strain rate between ${10}^{5}--{10}^{7}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. A simple dislocation-based continuum model is developed, which reproduced the third power law and revealed very good agreement with recent experiments of multiple types of metals quantitatively. New insights into this unusual macroscopic phenomenon are presented through quantifying the connection between the macroscopic mechanical response and the collective dynamics of dislocation assembles. It is found that the Swegle-Grady law results from the particular stress dependence of the plasticity behaviors, and that the difference between the third power scaling and the classical fourth power scaling results from different shock dissipative actions.

A Mylar flyer plate, being launched by an electric gun, has been applied to explore the ballistic performance of a 2024 alloy Whipple shield consisting of a 1.0-mm-thick bumper and a 3.2-mm-thick rear wall with a standoff of 70 mm between them. In comparison with a spherical projectile, experimental results show that the polymer plate, having about one-fourth mass of an aluminum sphere, can penetrate the same Whipple at impact velocities of 3 km/s and 5 km/s. The ballistic limit curves (BLCs) of the Whipple are obtained in the velocity range from 3 km/s to 12 km/s using 0.25-mm-thick and 0.5-mm-thick Mylar flyers at normal and/or oblique impacts. Different from spherical projectile, the BLCs obtained by a flyer plate oscillate abruptly with impact velocity. The development of the BLC depends on both the spreading feature of the fragmented bumper sheet and the kinetic energy it carries. In normal impact condition, the BLC turns to go upward at the flyer/bumper shock pressures of 41 GPa, 71 GPa and 121 GPa, exactly corresponding to the occurrence of melting in bumper materials under the interactions among rebounded stress pulses, under the first releasing wave propagating through shock-compressed state, and under direct shock compression, respectively. Among the three melting mechanisms, the shock-induced melting produces the most widely spread debris cloud with the finest fragments.

A dislocation-based constitutive model is applied to revisit the anomalous thermal hardening behavior of fcc metals under shock loading. Two scale-dependent dislocation motion mechanisms are found to dominate the thermal hardening behavior of fcc metals under shock loading. In particular, because of the shear stress decay with the propagation of the shock wave, the relativistic effect of dislocation motion is only significant near the impact surface, whereas the phonon drag mechanism dominates dislocation motion in a macroscopic fcc metal. Furthermore, we provide a detailed picture of the thermal hardening behavior on the continuum scale, in which the role of the newly generated stress wave from plastic deformation in the dynamic deformation process is highlighted. We show that the mechanical response at the elastic precursor is mainly controlled by the stress wave emanating from the plastic front.

Interpretation of the temperature dependence of the dynamic yield stress of shock-loaded metals has recently become a critical problem in shock wave physics. However, the temperature dependence of the dynamic yield stress of BCC metals is rarely investigated owing to the lack of an accurate description of the constitutive behavior of BCC metals subjected to high strain rate and high temperature. To unravel the underlying mechanism of the dynamic yield stress of BCC metals under such extreme conditions, we established a dislocation-based constitutive model in which the dislocation generation equation is proposed from the viewpoint of dissipation energy. When applied to shock-loaded BCC metals, this model reproduced the elastic–plastic wave characteristics observed in preheated plate-impact experiments quantitatively even at temperatures of >1000 K. It was found that forest hardening induced by thermally activated homogeneous nucleation (TA-HN) serves as the primary contributing factor to the thermal hardening behavior of vanadium at elevated temperature, while Peierls stress serves as the primary contributing factor to the thermal softening behaviors of other BCC metals. The novelty of this work lies in that the forest hardening mechanism, as a plastic hardening mechanism and usually regarded as temperature insensitive for BCC metals, has been proved to be temperature sensitive and influence dislocation motion significantly owing to TA-HN at high strain rate and high temperature. Based on this mechanism, we also predicted that the thermal hardening behavior would also occur in other BCC metals, e.g., molybdenum and tungsten, at temperature ranges beyond the limit of existing experiments.

A program MVFT3D of large-eddy simulation is developed and performed to solve the multi compressible Navier- Stokes equations. The SGS dissipation and molecular viscosity dissipation have been analyzed, and the former is much larger than the later. Our test shows that the SGS dissipation of Vreman model is smaller than the Smagorinsky model. We mainly simulate the experiment of fluid instability of shock-accelerated interface by Poggi in this paper. The decay of the turbulent kinetic energy before the first reflected shock wave–mixing zone interaction and its strong enhancement by re-shocks are presented in our numerical simulations. The computational mixing zone width under double re-shock agreement well with the experiment, and the decaying law of the turbulent kinetic energy is consistent with Mohamed and Larue’s investigation. Also, by using MVFT3D we give some simulation results of the inverse Chevron model from AWE. The numerical simulations presented in this paper allow us to characterize and better understand the Richtmyer-Meshkov instability induced turbulence, and the code MVFT3D is validated.

Dynamic tensile failure, e.g. spallation, of ductile metals is a complex phenomenon, due to its multiple tempo-spatial scales and structural levels involved on one hand, and due to its irreversible and nonlinear features as a non-equilibrium process on the other. Activated/Stimulated by tensile stress, non-uniform mesoscopic structures of materials evolve, leading to internal damage nucleation, growth, coalescence and final fracture, which consist the basic physical processes of dynamic tensile failure of ductile metals. Damage evolution on dynamic tensile fracture of ductile metals has been reported elsewhere, however, our ability to understand the mechanism of damage evolution in micro-scale and to accurately predict fracture in macro-scale is still limited. In this paper, we present investigations on macroscopic response, microscopic mechanism and physical models of dynamic tensile failure, based on our previous works. Current difficulties and challenges encountered when studying such a complex, multi-scale phenomenon are also discussed.

A novel explanation of the quasielastic release phenomenon in shock compressed aluminum is presented. A dislocation-based model, taking into account dislocation substructures and evolution, is applied to simulate the elastic plastic response of both single crystal and poly crystalline aluminum. The calculated results are in good agreement with experimental results from not only the velocity profiles but also the shear strength and dislocation density, which demonstrates the accuracy of our simulations. Simulated results indicate that dislocation immobilization during dynamic deformation results in a smooth increase of yield stress, which leads to the quasi-elastic release, while the generation of dislocations caused by the plastic release wave results in the appearance of a transition point between the quasielastic release and the plastic release in the profile., 13 pages and 12 figures

A novel system of macroscopic diversity with voting rule in a CDMA cellular system is suggested in order to raise the coverage, quality of service, and capacity of CDMA mobile communication system. The impact of macrodiversity on the performance of the CDMA cellular system is analyzed and investigated.