Volume 13, Issue 1 -

A multi-thickness target plate impact experimental technique is proposed in this paper and adopted in the research on the so-called failure wave phenomenon in soda-lime glasses, in which, four sub-targets in different thicknesses embedded in the target ring are impacted simultaneously and the longitudinal stress temporal curves at the backing surface of each of the sub-targets are measured by four manganin piezo-resistive stress sensors. Hence, the failure wave trajectory under a certain dynamic loading can be obtained by only one test, which can reduce considerably the experimental expense as well as the experimental period, and, more importantly, the measurement uncertainty resulted from different loading conditions in repetitious impact experiments is avoided. It is found that the propagating velocity of failure wave is approximate to a constant and increases with the magnitude of the impact loading, and there always exists an initial delay time for the initiation of failure wave behind the precursory shock wave, which decreases with the magnitude of impact loads. Moreover, a numerical simulation for the failure wave propagation is carried out by using the LS-DYNA applied software, together with a statistical isotropic elastic microcrack model to describe the dynamic damage evolvement of soda-lime glasses. It is demonstrated that both the critical damage value distributions and the free surface particle velocity temporal curves can be used to determine the failure wave trajectory, and the numerical results are consistent substantially with the experimental data.

As orientation-dependence of shock-induced thermal responses and chemical reactions in energetic single crystals are related to anisotropic mechanical behavior, a crystal plasticity model for low-symmetric β -HMX single crystal with only limited operative slip systems has been developed. The model accounts for nonlinear elasticity, volumetric coupled with deviatoric behavior, and thermo-dynamical consistency. Simulations of planar impact on β -HMX single crystal show good agreement with existing particle velocity experimental data. An important feature of the model is its capability to capture the evolution of elastic-plastic wave profiles. The model provides new perspective to a range of complex, orientation-dependent mechanical and thermal behaviors than isotropic elastic-plastic constitutive descriptions. Pressure wave profiles show multiple fluctuations for single crystals with different orientations compared to that of isotropic medium. Simulated bulk temperature distribution and evolution with various characteristic features for different orientation single crystals are compared and discussed. Simulations for bicrystal samples show that the evolution and pattern of temperatures in different crystal pairs have great variations. Given that explosive grains are crystalline, the anisotropy of a crystal may be one of the sources of nonuniform temperature distribution.

A bio-shock tube with non-lethal pressure levels can be used to study blast-induced primary injuries. In this paper, a finite element shock tube model was developed to simulate the shock wave and gas flow in a specific bio-shock tube. The behaviors of the gases in the tube were described with an equation of state using the multi-material arbitrary Lagrangian–Eulerian (MMALE) formulation. The FE model-predicted temporal and spatial distributions of pressures were in reasonable agreement with those obtained experimentally. The characteristics of shock wave, unloading wave and rarefaction wave were well captured by this numerical model.

Magnetic materials subjected to shock wave compression will undergo shock wave demagnetization. To further investigate the demagnetization performance of Nd 2 Fe 14 B ferromagnets, an open magnetic circuit cylinder was used as the energy-carrying body. The discharge process and circuit characteristics during shock wave demagnetization were analyzed. A series of the shock wave demagnetization tests were performed using the shock wave from the detonation of high explosives acting along the axial direction of the cylindrical magnets. The induced electromotive force and current were measured. The results indicate that Nd 2 Fe 14 B ferromagnet begins to demagnetize at approximately 0.28 μs after the shock wave enters its body. The induced electromotive force and current in coils are caused by the destruction of the magnetism in the smallest units of magnetic domains rather than the mechanical damage of the magnet structure. The influence of the shock wave pressure on the effect of demagnetization was investigated by altering the pressure of the shock wave acting the magnets. The results show that Nd 2 Fe 14 B ferromagnets can be absolutely demagnetized and destroyed by a high shock wave pressure, and that demagnetization is insufficient under a low 13 GPa shock wave pressure. In order to reduce eddy current losses during the process of the shock wave demagnetization, a new design of a split magnet with insulation layers was proposed. This process is very effective and beneficial to improve the energy conversion during the shock wave demagnetization.

The vibration isolation and energy absorption capability of high porosity metallic foams with closed cells for large mass protection under low velocity impact is theoretically studied. To explore the underlying physical mechanisms of shock attenuation, a double degree of freedom (DDF) spring-damper-foam collision model is developed. The effects of key system parameters (such as spring stiffness, damping ratio, mass ratio, initial velocity and foam thickness) on optimal foam mass and minimum acceleration peak are discussed.

In modern warfare earth penetrating weapons are often used to defeat enemy’s hardened and deeply buried targets such as aircraft shelters and bunkers. A dual warhead system (DWS) is one of such weapons composed of a forward shaped charge (FSC) and a following through warhead (FTW). In this paper, an analytical model is first proposed to analyze the penetration of an FTW into concrete targets with pre-drilled holes and a DWS is then optimized in order to achieve its best penetration performance. The effects of various parameters on the performance of a dual warhead system penetrating a concrete target are delineated. It transpires that the present model predictions are in good agreement with available experimental data and that the results obtained may be useful for designing such weapon systems.

Static Mohr–Coulomb Failure Criterion for rocks has been used extensively in various rock engineering applications. In this model, the compressive strength, tensile strength, and shear strength are related. To investigate the applicability of the Mohr–Coulomb model to dynamic failures, we studied the correlation of the three dynamic rock properties: compressive strength, tensile strength and punch shear strength. The strengths are quantified using a split Hopkinson pressure bar (SHPB) system. The methods for acquiring these strengths were briefly discussed. A fine-grained sandstone, Longyou sandstone (LS) was studied using these methods. The results showed that the compressive strength calculated from the punch shear strength of LS matched well with the experimental UCS results, thus the punch shear strength could be effectively used to predict the UCS of rocks. The tensile strength calculated from the punch shear strength of LS also has exhibited a good trend with the experimental results of Brazilian tensile strength. We concluded that the famous Mohr–Coulomb criterion can be extended to the dynamic range. However, caution has to be taken in determining the loading rates for different strength results.

The damage and fragmentation of rock induced by impact loading is one of typical nonlinear problems in underground engineering. Rock specimen such as rock plate was found to become invalid under impact loading. This paper presents brittle fragmentation for dynamic damage of rock plate based on the edge-on impact (EOI) test with varying impact velocities (10 m s -1 to 40 m s -1 ) and impact angles. High-speed camera was used to study the relation among crack initiation and propagation, impact velocity and impact angle. The patterns of fracture of the crack under impact are observed and the influence of impact angle on the growth of the crack is analyzed. The results demonstrate that only when the impact velocity is less than a critical value does a damaged zone, whose size increases with the impact velocity, appear on the edge of the sample. When the velocity exceeds the critical value, some radial cracks occur around the damaged zone and extend outward from the boundaries of damaged zone. The radial crack length and damaged zone increase with impact velocity and are influenced by the impact angle. However, at a certain velocity, the damage zone decreases and the crack length increase. It shows that there was an obvious incubation period of crack forming. According to test results, the study found that the impact velocity of this jump is about 25 m s -1 –3 m s -1 and scope of the impact resistance optimized angle is about 15 ° –30 ° .

Quasistatic compressive strength of alumina cylinders was measured using specimen of two sizes. The strength of the material was dependent on the specimen size. High speed video photography showed that the failure of the cylindrical specimen started from axial splitting, and after failure the specimen fragmentized into small pieces. A fragmentation model was proposed for estimating the average sizes of the fragments. The calculated fragment sizes agreed reasonably well with the experimental data. By using Ls-Dyna software, the failure of cylindrical specimen containing pre-existing crack-like flaws had been simulated.

In this paper, the radiation characteristics of a reflector antenna with surface deformation were numerically simulated. The deformation of the reflector antenna under shock wave loading was simulated by LSDYNA software and the far-field patterns of these deformed reflector antennas were calculated by FEKO software. The structure-deformed reflector antennas obtained by LSDYNA software were used as the initial structure of the reflector antennas for calculating the radiation characteristics. The equations were derived to calculate the far field of the reflector antenna according to the physical optics method. As a consequence, according to the encounter modes of the shock wave and the antenna, the far-field patterns were investigated under three conditions of the shock wave frontal action, rear action and oblique action on the antenna.

By means of ping-pong balls, the dynamic buckling behaviours of thin-walled spherical shells under impact loading are studied both experimentally and numerically. First, the quasi-static tests were conducted on an MTS tester, in which the ball was compressed onto a PMMA plate. Apart from the force-displacement relationship, the evolution of the contact zone between the ball and the plate was obtained by a digital camera. In the impact tests, ping-pong balls were accelerated by an air-gun and then impinged onto a rigid plate with the velocity ranging 10–45 m / s. The local dynamic buckling processes of the ball were recorded by a high-speed digital camera, from which the impact duration, the maximum contact diameter, as well as the contact diameter at snap-through buckling under different impact velocities were obtained. It is found that with the same size of contact zone, the dynamic energy absorption of the ball is much larger than that in the quasi-static tests. To understand the dynamic effects in the impact process, numerical simulations were performed by using different material properties and different impact velocities. The comparison between the experimental and numerical results show that the kinetic energy absorption of the ball is induced by the strain-rate effect, local vibration of the ball and viscous-elastic effect, respectively.

In this paper, the dynamic buckling of an elastic cylindrical shell subjected to an axial impact load is analyzed in Hamiltonian system. By employing a symplectic method, the traditional governing equations are transformed into Hamiltonian canonical equations in dual variables. In this system, the critical load and buckling mode are reduced to solving symplectic eigenvalues and eigensolutions respectively. The result shows that the critical load relates with boundary conditions, thickness of the shell and radial inertia force. And the corresponding buckling modes present some local shapes. Besides, the process of dynamic buckling is related to the stress wave, the critical load and buckling mode depend upon the impacted time. This paper gives analytically and numerically some new rules of the buckling problem, which is useful for designing shell structures.

The micromechanical damage model proposed in Huang et al. (2002) [Huang C, Subhash G, Stanley JV. A dynamic damage growth model for uniaxial compressive response of rock aggregates. Mech Mater 2002;34:267–277] is modified to predict the dynamic damage evolution of concrete subjected to dynamic axial compression loads under different lateral confinement conditions. It shows that the stress–strain response, the compressive strength and the damage growth rate are strongly influenced by the confining pressure. It is also found that when the strain-rate exceeds 10 4 s -1 , the compressive strength tends to be greatly depended on strain-rates.