Modeling plastic shocks in periodic laminates with gradient plasticity theories

Steady plastic shocks generated by planar impact on metal-polymer laminate composites are analyzed in the framework of gradient plasticity theories. The laminate material has a periodic structure with a unit cell composed of two layers of different materials. First- and second-order gradient plasticity theories are used to model the structure of steady plastic shocks. In both theories, the effect of the internal structure is accounted for at the macroscopic level by two material parameters that are dependent upon the layer's thickness and the properties of constituents. These two structure parameters are shown to be uniquely determined from experimental data. Theoretical predictions are compared with experiments for different cell sizes and for various shock intensities. In particular, the following experimental features are well-reproduced by the modeling:

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the shock width is proportional to the cell size;

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the magnitude of strain rate is inversely proportional to cell size and increases with the amplitude of applied stress following a power law.

While these results are equally described by both the plasticity theories, the first gradient plasticity approach seems to be favored when comparing the structure of the shock front to the experimental data.     

Dynamic Contact Behavior of a Golf Ball during an Oblique Impact

The oblique impact between a golf ball and a rigid steel target was studied using a high-speed video camera. Video images recorded before and after the impact were used to determine the inbound velocity v _i, rebound velocity v _r, inbound angle θ_i, rebound angle θ_r, and the coefficient of restitution e. The results showed that θ_r and e decreased as v _i increased. The maximum compression ratio η_c, contact time t _c, average angular velocity , and tangential velocity , along the target were determined from images obtained during the impact. The images demonstrated that η_c increased with v _i while t _c decreased. In addition, and increased almost linearly as v _i increased. A rigid body model was used to estimate the final angular velocity ω* and tangential velocity ν_t* at the end of the impact; these results were then compared with experimental data.     

光滑粒子模拟方法在超高速碰撞现象中的应用

简要介绍了基于黎曼解的光滑粒子法,并将改进的SPH方法应用于超高速碰撞,对二维轴对称条件下的弹丸超高速碰撞薄板问题进行了数值模拟,研究了靶板厚度、弹丸速度、弹丸形状等因素对形成碎片云的影响。通过与实验数据比较,该算法模拟的碎片云的形状及特征与实验相吻合,验证了光滑粒子法对冲击动力学问题数值模拟的有效性。     

圆柱形弹体Taylor撞击方法研究

研究了圆柱形弹体垂直撞击刚性靶体的Taylor撞击问题,提出了弹体撞击过程中未发生变形部分的速度变化规律,即二次非线性变减速运动,并通过弹体的运动方程对Taylor撞击进行了理论分析;同时利用再生核质点法(Reproducing kernel particle method,RKPM)对Taylor撞击过程进行了数值分析。利用该理论对五种具体材料进行分析,结果表明,解析结果与试验结果及数值分析结果吻合较好。     

热声换热器的工作机制及换热特性研究

基于弱非线性热声理论,对热声换热器的换热特性进行了理论研究.获得了平行平板通道内二阶周期平均热流的解析解,并指出存在临界声导率比的模|Ya|_(cr)~I,使得二阶周期平均热流为零.当实际声导率比的模大于|Ya|_(cr)~I时,振荡流体从外热源吸热,为吸热器;当实际声导率比的模小于|Ya|_(cr)~I时,振荡流体向外放热,为放热器.获得了平行平板通道内二阶周期平均温度的解析解.计算分析了工作流体的物性参数、流动参数以及声导率比对二阶周期平均温度分布的影响,为进一步考察换热系数提供了依据。     

Improved CE/SE scheme with particle level set method for numerical simulation of spall fracture due to high-velocity impact

In the present paper, an Eulerian scheme combined with the hybrid particle level set method for numerical simulation of spall fracture due to high-velocity impact is proposed. An axisymmetric framework is established, based on an improved CE/SE scheme, to solve the high-velocity impact problems with large deformations, high strain rates and spall fractures. The hybrid particle level set method is adopted for tracking material interfaces and describing the formation and propagation of a crack. A novel representation of crack by level set is proposed. Numerical simulations are carried out and compared to the corresponding experimental results. The numerical results are in good agreement with the experimental data. The edge effects are reproduced and the decrease of scab thickness with increase in impact velocity is observed owing to the numerical analysis. It is proved that our computational technique is feasible and reliable for analyzing the spall fracture.     

Concerning eight mutually orthogonal latin squares

In this article, we provide a direct construction for 8 mutually orthogonal latin squares (MOLS)(48). Using this design together with one of Wilson's recursive constructions produces 8 new MOLS(v) for 88 other values of v. We also mention a few other new sets of 8 and 12 MOLS obtained recursively. © 2006 Wiley Periodicals, Inc. J Combin Designs 15: 255–261, 2007     

Toughness mechanism of polypropylene/elastomer/filler composites

The toughening mechanisms of polypropylene filled with elastomer and calcium carbonate (CaCO₃) particles were studied. Polypropylene/elastomer/CaCO₃ composites were prepared on a twin‐screw extruder with a particle concentration of 0–32 vol %. The experiments included tensile tests, notched Izod impact tests, scanning electron microscopy, and dynamic mechanical analysis. Scanning electron microscopy showed that the elastomer and CaCO₃ particles dispersed separately in the matrix. The modulus of the composites increased, whereas the yield stress decreased with the filler concentration. The impact resistance showed a large improvement with the CaCO₃ concentration. At the same composition (80/10/10 w/w/w), three types of CaCO₃ particles with average diameters of 0.05, 0.6, and 1.0 μm improved the impact fracture energies comparatively. The encapsulation structure of the filler by the grafting elastomer had a detrimental effect on the impact properties because of the strong adhesion between the elastomer and filler and the increasing ligament thickness. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1113–1123, 2005     

Double-hump resonance structure of the cross sections for electron impact ionization of Ar^5 ＋

Configuration-average distorted-wave calculations are carried out for electron-impact ionization of Ar^5 ＋ .Both direct ionization and the indirect excitation autoionization processes are included in our calculations. Our theoretical values are in quite reasonable agreement with the experimental data. The indirect processes contribute upto 50% to the total ionization cross sections. The possible origin of double-hump resonance structure of the crcessections is demonstrated and the contributions of metastable states are also taken into account.