According to the positive definiteness of the quadratic type, and based on the mechanical stability criteria derived by Mouhat and Coudert, the mechanical stability criteria of monoclinic and triclinic crystal systems are supplemented. The mechanical stability criteria of the seven crystal systems are given. Based on the density functional theory of the first principle method, the elastic constants of nine kinds of SiO₂ belonging to different crystal systems are calculated. Combined with the mechanical stability criteria, the stabilities of SiO₂ in different space groups, including cubic P2₁3, hexagonal P6₃/mmc, trigonal P3₁21 and $R\overline 3 $, tetragonal P4₁2₁2 and $I\overline 4 $, orthorhombic Pbcn, monoclinic P2₁/c, and triclinic $P\bar 1$, are determined. The calculated results show that the nine types of SiO₂ are all mechanically stable at 0 GPa.

The lattice dynamics behavior of materials under high pressure can be studied by high-pressure Raman spectroscopy. However, Raman spectroscopic signal of metal samples at high pressure is difficult to obtain due to the fluorescence of the diamond in diamond anvil cell (DAC) and the strong reflection of the samples. In this work, we use DAC inclination scattering method to mitigate background noise. As a consequence, Raman spectroscopic signal of the hcp metal samples (Be, Re, Os) under high pressure have been achieved. In the case of Be, the pressure dependence of elastic constant C₄₄ is obtained by measuring the shear Raman mode E_2g at pressure up to 73 GPa. The proposed high-pressure Raman spectroscopy technique provides a new method to study bonding state, electronic structure, and phonon-electron coupling effects of metallic materials under high pressure.

In the high-pressure molding process of polymers, the pressure, pressurization rate and molecular weight of the polymers will affect the final crystalline structure of the products. Studying the relationship between different high-pressure processing methods and the crystalline structure of isotactic polypropylene (iPP) is helpful to deeply understand the influence of high-pressure molding process on the crystallization behavior of polymers. In this paper, wide-angle X-ray diffraction and differential scanning calorimetry were used to study the crystallization behavior and thermal properties of two kinds of iPP with different molecular weights at different pressurization rates and pressures. The results show that pressure is a key factor in determining the formation of mesophase iPP. The complete mesophase iPP can be prepared under sufficient pressure. Under enough pressure, the higher the pressurization rate, the easier to form mesophase iPP, ortherwise, to form γ-phase iPP. The higher the molecular weight, the higher pressure required for the preparation of mesophase iPP, and the greater the required critical pressurization rate. The thermal property analysis of the products shows that the thermal properties of the mesophase products obtained under different conditions are basically the same, but the crystal perfection of the γ-phase iPP is related to the pressurized condition.

Based on a two-stage multi anvil apparatus, with diamond anvil cell device, combined with the high-pressurein situX-ray diffraction (XRD) technology of Beijing Synchrotron Radiation Light Source, the phase transitions and grain size variation behaviors of germanium under high pressure were systematically investigated. It is found that the reciprocating phase transitions indeed have obvious effects on the grain refinement of germanium. At the same time, the amorphous regions are found in the bulk samples that have undergone five times reciprocating phase transitions, which provides a new way for the preparation of nanostructured and amorphous materials.

Interface friction is a common natural phenomenon. Based on the micro-contact fracture mechanism of friction, a two-dimensional interface friction model including a triangular micro bulge is established with linear elastic constitutive relationship and D-P failure criterion. The early dynamic behavior of the interface under transient loading is numerically calculated and analyzed by the finite element simulation method. The research shows that in the micro process of loading, there exist significant stress fluctuations and fine structure characteristics at frictional interfaces. The evolution of the wavefront in the near region of the interface has symmetrical diffusion. The interaction of the incoming stress disturbance and the micro bulge will induce the fracture of the bulge, resulting in a three-wave profile centered on the fracture surface: longitudinal wave, transverse wave, and interface wave. A new interesting phenomenon is that at the moment of loading, a micro stress disturbance is generated synchronously from the interface and propagates to the substrate in the form of longitudinal waves. More comparative examples and analysis show that the mechanism of this disturbance is related to the overall gravity micro-adjustment acting on the interface. This work reveals the early wave effect of interface friction and its micro fracture mechanism, which is expected to provide an effective way for earthquake prediction and to advance the earthquake prediction time.

Impulse is an important power parameter of explosive air shock wave, and the momentum-block test method is one of the ways to measure the impulse. This paper attempts to convert the momentum of the momentum-ball into the quantitative compression displacement of the spring by investigating the impact response characteristics of spring-mass system. Four different momentum-balls impact tests, such as polyformaldehyde, polytetrafluoron, aluminum and steel, were conducted to obtain the spring-mass compression response characteristics and momentum/energy conversion efficiency. The results show that the maximum compression displacement of the spring is linearly related to the loading velocity of the momentum-ball. The polyformaldehyde momentum-ball spring coupling system shows the most stable properties, which is suitable for the transfer carrier of shock wave impulse measurement. This work can provide a new method for shock wave measurement.

TB8 (Ti-15Mo-2.7Nb-3Al-0.2Si) is a metastable β titanium alloy, which plays an important role in the aerospace field. Microstructure, strain and strain rate are three important factors affecting mechanical properties of TB8 titanium alloy. Based on a universal material testing machine and a split Hopkinson pressure bar (SHPB) device, the effect of solution and aging heat treatment process on mechanical properties of TB8 titanium alloy was studied. Optical microscope (OM) and scanning electron microscope (SEM) were used to characterize the microstructure and section morphology of the specimens before and after deformation. The results show that short strip α phase precipitates inside the alloy after solution and aging treatment, and the size and quantity of secondary phase increase with aging temperature increasing. Under different loading conditions, the strain rate strengthening effect of TB8 titanium alloy before and after heat treatment is obvious, but the strain strengthening effect is not obvious under dynamic loading condition. With the increase of aging temperature, the yield strength of the alloy decreases and the plasticity increases. The failure mode of specimens under dynamic loading is typical shear failure. Adiabatic shear band is the precursor of crack formation and specimen failure.

Aero-engine is a high probability and high-risk component of bird strike events, which is of great significance to the study of bird strike resistance of fan blades. In this paper, based on the three-dimensional digital image correlation (DIC) in-situ strain measurement method, the static bird strike tests of TC4 titanium alloy hollow structure fan blades at different heights are carried out. In addition, a simulation model is established based on Johnson-Cook dynamic constitutive model and damage failure theory to better verify and describe the dynamic deformation response process and failure situation of aero-engine fan blades during bird strikes. It is found that the variation of bird strike velocity mainly affects the magnitude of blade deformation, but does not cause the change of blade characteristic mode. In the bird strike process, the middle root is a significant area of stress/strain localization, which is more prone to damage failure. It is found that with the increase of bird impact position, the critical bird impact velocity corresponding to the failure of the hollow fan blade at the blade root increases gradually, and the bird impact resistance of the whole structure is better. This experiment and the corresponding simulation study provide a certain reference for the anti-bird impact design of the TC4 titanium alloy hollow structure fan blade.

In this study, under the frame of equivalent static loads (ESL) method structural optimization and based on hard-kill bi-directional evolutionary structural optimization (hard-kill BESO), the topological optimization method for periodic porous sandwich structure under impact load was carried out. The commercial software ABAQUS was used to investigate the deformation patterns of the optimized periodic sandwich structure and the sandwich structures with trapezoidal, rectangular and random Voronoi cores under the impact load imposed by a rigid body with an initial velocity of 100 m/s. In the early stage of load, the upper half of the core layer of the optimized periodic sandwich structure is completely compressed and the energy absorption is higher than the other three structures. However, the total energy absorption of the optimal sandwich structure is slightly less than the other three due to the small plastic deformation at the end stage of load. To study the capabilities of the topologically optimized structure under different load conditions, the energy absorption performance of the four sandwich structures subjected to the rigid body impact loads at different velocities and three impulse loads were compared. After comprehensively considering the deflection at the centers of top and bottom panels, the specific energy absorption, the ratio of energy absorption of core layer, as well as the mean impact load, it shows that the optimized sandwich periodic structure performs higher energy absorption capability and resistance under the rigid body impact. The specific energy absorption of the optimal sandwich structure is less than the sandwich structure with rectangular core under rectangular impulse, losing advantages of the structural optimization. It indicates that the optimization design obtained under a single load condition cannot get the best performance for any load condition, and further research is required for different load conditions.

Based on equivalent single degree-of-freedom (SDOF) theory, a dynamic model of clamped elastoplastic circular plate under near air blast loading is established by considering the bending moment and the membrane force during the deformation process, and the whole process containing loading and unloading for circular plate is described. The finite element model is established using test parameters in the literature, and the dynamic response of clamped circular plate under air blast loading is analyzed by finite element simulation. After comparing the calculation results of the dynamic model and the finite element simulation results, the accuracy of the calculation results of the dynamic model is verified. The results show that the theoretical calculation results are in good agreement with the test results and the finite element simulation results. The proposed dynamic model can be applied to predict the large deformation of clamped circular plate under near air blast loading, and provides technical support for blast resistant structure.

Crack initiation and propagation is a long-standing difficult problem in solid mechanics, especially for elastic-brittle material. To explore the damage and crack propagation behavior of glass plates under impact loading, the element deletion, discontinuous Galerkin peridynamic (DG-PD), and meshless peridynamic (M-PD) methods are used to conduct numerical simulations, respectively. The JH-2 material model, and the maximum principal stress and maximum principal strain failure criteria are adopted in the element deletion method. The node separation operation and the critical energy release rate criterion are used in the DG-PD method. In the M-PD method, a self-programmed particle discretization method is utilized along with an appropriate computational domain, and a critical elongation criterion is imposed. The simulation results show that: (1) the element deletion method can roughly simulate the damage morphology of glass under impact loading, but it is insufficient in capturing crack bifurcation and penetration. (2) In the DG-PD method, circumferential cracks and radial cracks are observed, and the cracks are of high symmetry. In addition, there are a lot of glass fragments splashing at the impact point and the frame. (3) Radial cracks and circumferential cracks can be captured in the M-PD method, and the symmetry of the cracks is good. The size of horizon and the impact velocity show great influence on the dynamic responses of the glass plates. As far as the damage form is concerned, the M-PD method and the DG-PD method yield consistent results.

In order to investigate the dynamic mechanical characteristics of coal-rock composite engineering body under impact load, the basic mechanical parameters of pure coal and pure rock were obtained by laboratory tests for determining the parameters of HJC model. Based on the validity verification of the material model, LS-DYNA was employed to study the stress wave propagation features, failure process and failure characteristics of coal-rock combination bodies in dynamic splitting process considering the effects of impact loads, impact directions and loading angles. The results showed that: (1) the stress wave shapes of R-C and C-R samples are almost the same for different impact directions, but the stress amplitudes are slightly different. The comparison results showed that the incident wave amplitudes are similar, but for R-C samples, the amplitude of reflected wave is larger while that is smaller of transmitted wave. The difference gradually decreases with the increasing impact loads. (2) Under the action of different impact loads, the coal part is mainly damaged in the process of splitting, and the cracks generally appear in the coal part far from the interface, while the rock part commonly is damaged at the near side of the interface. (3) The failure modes of C-R and R-C samples are similar and mainly tensile and shear when the impact load is relatively low. The damage degree of the combination body is aggravated with increasing load, and the difference of failure modes becomes more obvious. (4) A method using the number of failure elements as an evaluation index is proposed to quantitatively analyze the breakage degree of the combination bodies. According to the numerical simulation results, the combination body damaged most seriously for the loading angle of 45°.

Numerical model of horsetail-bionic thin-walled structure (HBTS) under the lateral impact was constructed using ABAQUS. The effects of wall thickness, inner diameter and number of ribs on the crashworthiness performance and deformation modes were analyzed. The results indicate that the specific energy absorption and peak load of HBTS can be significantly enhanced with the increase of the number of ribs and the overall wall thickness. The changes in the wall thickness of each part significantly affects its deformation mode and crashworthiness performance. Based on the above results, the optimization software modeFRONTIER and the finite element analysis software ABAQUS were integrated to explore the influence of five design parameters, in terms of wall thickness, number of ribs, inner diameter and so on. Finite element models were uniformly distributed on the design space through parametric modeling method, hence the Kriging surrogate models for the specific energy absorption and peak load were established. Then, Pareto front was obtained using Kriging surrogate model-based multi-objective optimization method for the specific energy absorption maximization and peak load minimization simultaneously in one model. Finally, the distribution of each HBTS’s design parameters on Pareto front was analyzed and the optimization results were verified. The method is expected to provide new thoughts for the optimization design of the thin-walled structure.

Macroscopic deviation of shaped charge is one of the reasons for lateral deviation of induced jet. In order to study the influence of multiple coupled macro deviations on the lateral deviation of jet, theoretical model considering the effects of the coaxiality deviation, the liner thickness deviation and the position deviation on the lateral deflection of the jet was deduced according to the jet forming theory. The numerical simulations of the jet forming process for the shaped charge containing multiple macroscopic deviations were carried out. The results show that the coaxiality deviation and the position deviation both cause the jet to form a quadratic curve, and the liner thickness deviation makes the jet deflect, but still maintain a straight shape. For the cases of multiple macroscopic deviations coupling, the lateral displacement of the jet is approximately the vector sum of the displacements caused by each single factor. The results provide a reference for improving the stability of shaped charge.

In order to study the influence of flame characteristics on cook-off bomb in pool fire under the condition of fast cook-off, a fast cook-off model of pool fire was established, and the heat transfer characteristics of the cook-off bomb were obtained, then the influences of the placement height of the cook-off bomb and the size of the oil pool on the flame characteristics during the burning process were analyzed. The results show that with the increasing placement height of the bomb, the highest temperature region of the bomb surface shifts from the upper surface to the lower surface, and the peak radiation heat flux shifts from the top to the bottom of the bomb. With the increasing size of the oil pool, the surface temperature of the bomb becomes more uniform, the heat flux absorbed by the bomb increases, and the surface temperature of the bomb increases. Therefore, in the fast cook-off test, both the placement height of the bomb and the size of the oil pool affect the flame characteristics, and then the fast cook-off characteristics of the bomb.

In order to study the response characteristics of a new HMX-based cast explosive GOLA-1 (HMX-aluminum-ammonium perchlorate (AP)-binder (Kel-F)) under thermal stimulation, the cook-off tests with heating rates of 1.0 and 1.5 K/min were conducted, and the information such as the temperature rise of the explosive center point and the ignition time was obtained. On that basis, and combined with numerical simulation of cook-off, the ignition position and temperature of explosives were predicted. The numerical simulation is in good agreement with the experimental results. The ignition time deviations of GOLA-1 explosive at heating rates of 1.0 and 1.5 K/min are 1.3% and 1.7%, respectively, indicating that the established simulation model is reasonable. On above basis, numerical simulations at different heating rates were carried out. The results show that when the heating rate decreased to 0.4 K/min, the ignition position moved from the annular area at the bottom edge of the charge to the lower part of the central axis of the charge column. As the heating rate continued to decrease, the ignition position gradually moved to the upper part of the charge, while the heating rate had little effect on the ignition temperature.

The study of surface vibration attenuation law of urban underground engineering blasting construction is of great significance for the protection of adjacent buildings. This paper takes the Wuhan Metro Line 8 Phase Ⅱ liaison channel blasting excavation project as an example and uses a combination of field monitoring and ANSYS/LS-DYNA 3D finite element numerical simulation to analyze the characteristics of surface vibration hollow effect under the liaison channel blasting excavation and predict its attenuation law. The results indicate that where the surface vibration speed is significantly greater than the unexcavated area, there is a “cavity effect”; with the increase of the longitudinal distance between the mass point and the source of the explosion, the cavity effect amplification coefficient increases rapidly until the extreme value and then slowly decreases. Along both sides of the channel with the increase in distance, the amplification coefficient decreases, the effect of the cavity effect is weakened, at a distance of 8 m from the source (6 m behind the palm surface) to reach the maximum, 2−8 m from the source of the cavity effect should focus on vibration monitoring within the surface area. The excavation area with the blasting conditions is related to the coefficient of 58.52, and vibration attenuation coefficient of 1.43, while the unexcavated area of 152.09, and vibration attenuation coefficient of 1.74, the absorption coefficient of the layer media are 0.019, 0.023, respectively.

Water and mud inrush is one of the main engineering disasters in the construction stage of karst tunnel, so it is necessary to predict its potential risk level. Taking Yesanguan tunnel of Yiwan railway as the engineering background, the factors affecting the water inrush risk of the tunnel are summed up as engineering geological conditions, hydrogeological conditions and rock mass quality conditions by the literature survey method, and the corresponding evaluation index system is established. The interval judgment matrix of factors at each level is formed to determine the index weight, and the interval analytic hierarchy process (IAHP)-Fuzzy method is used to realize the classification of water inrush risk in karst tunnels. The analytic hierarchy process model and risk classification system of water and mud inrush in tunnels are formed by combining 3 first-level indexes and 11 second-level indexes. The index weights are determined by IAHP. The calculation results show that water pressure characteristics account for the highest proportion in all indexes, and high water pressure is the most direct cause of water inrush in Yesanguan tunnel. Through the verification calculation, the risk of water inrush in the tunnel is weak when the water pressure is 0.1 MPa, which effectively reduces the risk of water inrush. It is suggested to take active protective measures such as pre-grouting reinforcement to reduce the water pressure and guide the actual construction. The verification further proves the feasibility of the model.