Oblique impact testing of bicycle helmets

The performance of bicycle helmets was investigated in oblique impacts with a simulated road surface. The linear and rotational accelerations of a headform, fitted with a compliant scalp and a wig, were measured. The peak rotational accelerations, the order of 5 krad s⁻² when the tangential velocity component was 4 m s⁻¹, were only slightly greater than in comparable direct impact tests. Oblique impact tests were possible on the front lower edge of the helmet, a site commonly struck in crashes, without the headform striking the ‘road’. Data characterizing the frictional response at the road/shell and helmet/head interfaces, were generated for interpretation via FEA modelling.     

Physics of deformation and fracture at impact loading and penetration

The structure, dislocation substructure, and mechanical properties of the targets made of four aluminum alloys after a impact loading by kinetic energy projectile have been investigated. The formula for approximation of the ballistic limit velocity by indentation technique is proposed. It has been shown that the maximum nonequiaxiality of the grain shape, increase of dislocation density, and decrease of dislocation cell size correspond to the 40–70% of plastic deformation at static compression for the investigated aluminum alloys.     

颈椎椎间孔侧偏角度的研究

目的:测量颈椎椎间孔侧偏角度,研究颈椎斜位片在显示椎间孔中的作用,寻找清晰显示颈椎椎间孔的最佳投照角度。方法:选择无颈部病变者39例,做颈椎椎间孔螺旋CT扫描,层厚2-5mm。选取第二颈椎到第七颈椎之间的椎间孔,利用PPT文稿和Photoshop等工具进行测量。结果:颈椎椎间孔的侧偏角度悬殊较大,传统的45度斜位片并不适用于所有被检查者。结论:颈椎椎间孔侧偏角度因个体而异,对颈椎椎间孔的显示点片或直接查CT、MRI更好。     

The effect of temperature on the capability of industrial safety helmets to absorb impact energy

The fundamental document specifying the requirements and testing methods applicable to industrial safety helmets in European Union member states is the standard EN 397:2012. According to that standard, one of the most important parameters of a helmet is shock absorption, determined for an impact of a striker with a kinetic energy of 49 J. The shock-absorbing performance of a safety helmet involves absorbing the energy of a striking object associated with a deformation of the shell and cradle, as well as an increase in the force transferred to the user’s head. The paper presents a study conducted with the aim to estimate the actual amount of energy absorbable by various helmet types without exceeding the threshold value of the force acting on the user’s head. A method of testing helmet deformation and the force acting on the helmet during an impact exerted by a falling object is presented. The effect of the temperature used for conditioning various helmet types on their capability to absorb impact energy was determined. The causes of deterioration of that capability due to temperature changes are analyzed for various designs of helmets made of different materials, and possible solutions to that problem are offered.     

斜高线及其在三点透视作图中的应用研究

透视作图的关键在于如何确定3个主向灭点等作图参数的位置，然后合理地获取透视高度。传统的视线法需借助侧面投影求作建筑物高度棱线的透视。笔者基于量点法的透视作图，提出了斜高线的概念，证明了斜高线、真高线与透视高度的对应关系，并就其在三点透视作图中的应用进行了有效的研究。     

Temperature dependence of impact deformation behavior of single crystal Ni based superalloy

The impact test was carried out to investigate the intermediate temperature brittleness of single crystal Ni based superalloy. The samples were impacted at the velocity of ～5?m?s^??1. The results showed that the impact toughness also exhibited intermediate temperature brittleness, which is similar to the situation in tensile. The samples showed the highest impact toughness at 600°C but exhibited the lowest impact toughness at 760°C. Results showed that the variety of impact toughness was due to the deformation mechanism. At 600°C, a/2? dislocation slips on the {111} slip system were attributed to the high impact toughness; however, a/2? dislocation slips from octahedral {111} planes to cubic {100} planes resulted in significant work hardening, leading to the decrease of impact toughness.     

爆轰波斜入射金属材料的模拟计算

文章通过数值模拟方法,对B炸药-铝、B炸药-铜系统中爆轰波斜反射现象进行了研究,获得了介质分界面处压力随入射角度变化规律,并确定了发生膨胀反射时的临界入射角度。计算结果与前人的实验观测结果吻合。本模拟计算的金属中形成的冲击波压力比基于理想爆轰假设的理论结果更符合实际,计算精度可满足工程应用需求。     

含摩擦碰撞柔性机械臂动力学研究

本文研究了由柔性杆、柔性铰构成的机械臂的斜碰撞动力学问题。首先,采用高次刚柔耦合理论通过Lagrange方程得到系统的递推动力学模型。然后,引入碰撞力势能,运用连续法建模获得碰撞力所对应的广义力,得到系统的碰撞动力学方程。采用Hertz接触模型和Coulomb摩擦光滑修正模型分别建立法向和切向碰撞模型,提出碰撞/分离切换准则。通过光滑化处理摩擦接触,提高了大型复杂柔性多体系统含摩擦碰撞全局动力学的计算效率。最后对国际空间站机械臂Canadarm2的简化模型斜碰撞过程进行仿真,验证了所提模型和算法的有效性。     

Analysis of collapse behaviour of combined geometry metallic shells under axial impact

Collapse behaviour of aluminium thin conical frusta with shallow spherical caps (shells of combined geometry) is studied both experimentally and numerically. These shells were of four different thicknesses and were subjected to axial compression between two rigid platens under both quasi-static and impact loading. The R/t values of the spherical portion of the shells were varied between 27 and 218, and for the conical frusta portion, mean diameter-to-thickness (D_m/t) values were varied between 79 and 190. Quasi-static tests were performed on a UTM of 100 T capacity with digital recording facility. Impact experiments were carried out on a drop mass set-up. A three-dimensional non-linear finite element analysis was carried out using LS-DYNA. Numerical results thus obtained were validated with the experimental results. Typical time histories of the specimen deformation and load compression curves were obtained. The behaviour of these shells of combined geometry is compared with the response of the shells of spherical or conical geometries. A discussion on their deformation behaviour, mean buckling load and energy absorbed is presented, and influence thereon of various parameters is discussed.     

充气膜结构零应力态求解

充气薄膜管属于柔性结构,荷载作用下产生的变形,会引起其内压改变,进而导致外围薄膜刚度的变化,对其变形产生重要影响,表现出内充气体压力与外围薄膜变形相互耦合的特点。该文采用有限元方法分析气-膜耦合作用对充气薄膜管动力特性的影响及其随影响因素的变化规律。通过将内充气体看作小扰动线性势流以考虑气-膜耦合作用以及内充气体附加质量的影响;通过建立内充气体的三种等效模型,分别将内充气体作用等效为外围薄膜静力边界条件、考虑内充气体附加质量影响的静力边界条件以及小扰动线性势流体,并将其相应的有限元分析结果进行对比,研究气-膜耦合作用和内充气体附加质量对充气薄膜管自振特性的影响及其随初始内压、长细比、膜厚以及端部约束类型的变化规律。研究结果表明：气-膜耦合作用以及内充气体的附加质量对低阶自振模态没有明显影响;气-膜耦合作用对自振频率有较显著的影响作用,而内充气体附加质量的影响则较小;随初始内压和长细比的增加,气-膜耦合作用对频率的影响体现出因阶次不同而不同的变化规律;气-膜耦合作用对频率的影响随膜厚的增加而降低,随约束程度的减弱而增强。该文的研究成果揭示了气-膜耦合作用对充气薄膜管自振特性的影响规律,有助于深入认识充气薄膜管的动力行为,确保其设计计算的合理性和可靠性。     

Large-scale molecular dynamics simulations of hyperthermal cluster impact

Using multimillion-atom classical molecular dynamics simulations, we have studied the impact dynamics of solid and liquid spherical copper clusters (10–30 nm radius) with a solid surface, at velocities ranging from 100 m/s to 2 km/s. The resulting shock, jetting, and fragmentation processes are analyzed, demonstrating three distinct mechanisms for fragmentation. At early times, shock-induced ejection and hydrodynamic jetting produce fragments in the normal and tangential directions, respectively, while sublimation (evaporation) from the shock-heated solid (liquid) surface produces an isotropic fragment flux at both early and late times.     

吸波涂层斜入射特性研究

简要分析了吸波涂层（RAC）的斜入射特性，研究了几种吸波涂层在不同入射角（θi)下的吸波性能，指出在常规进气道等凹型腔体中使用RAC的主要要求是提高涂层的垂直极化性能。     

Ultra-elastic and inelastic impact of Cu nanoparticles

The degree of elasticity for the impact of a particle with a rigid wall is normally characterized with the restitution parameter, R. We examine such impact behavior of Cu nanoparticles with molecular dynamics simulations, for different particle sizes (1-15 nm in radius) and impact velocities (25-200 m s^− 1). The impact can be ultra-elastic (R > 1) or inelastic (R < 1). Ultra-elastic or inelastic impact may occur for the smallest nanoparticles soly due to fluctuations, and the impact is inelastic but can be highly elastic (R ∼ 0.9-1) for larger sizes. R decreases with increasing size and impact velocity in general. Impact-induced structure transitions (e.g., dislocations) can be reversible and induce irreversible heating regardless of their reversibility. Such heating along with remnant plasticity is the key mechanism for impact inelasticity. Inelastic impact may occur with little remnant plasticity.     

Molecular dynamics study of cluster impact on the (0 0 1) and (1 1 0) surfaces of fcc metals

Molecular dynamics simulations of the impact deposition of metal clusters on fcc metal surfaces are presented. Two-dimensional elongated islands are formed when the incident cluster travels parallel to the surface. For perpendicular incidence the results of the impact event are very sensitive to the relative cohesive properties of the cluster and substrate atoms.     

Toughening effects of Mo and Nb addition on impact toughness and crack resistance of titanium alloys

Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects of β sta-bilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb (～64J) were two times higher than that ofTi-6Al (～30J),indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb (15.4J,16.1J and 15.0J,respectively).However,the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb (46.7J and 48.3J,respectively) were about three times higher than that of Ti-6Al (14.4J),indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process,a mathematical model has been proposed to evaluate the effects of Al,Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory.Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy.Therefore,more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading.A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading.Dislocation pileup at α/β interfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb.Furthermore,deformation twinning facilitated the dislocation motion in α grains with hard orientations.The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.     

热敏CTP曝光参数中的斜排角度的设定及分析

使用适用于激光密排斜排热敏CTP的测控条,对国产热敏CTP的曝光参数中的激光密排斜排角度进行设定及研究,并分析了斜排角度对制版质量的影响。结果表明,激光密排斜排角度设置不正确,印版图文信息会发生扭曲、变形,影响制版质量。     

轴测体视图的轴测轴和轴向变形系数的图解法

在绘制物体的轴测图时，轴测轴应根据物体的形状特征来确定，绘制其体视图也是一样。轴测体视图是由一对斜轴测图构成，而绘制轴测体视图的关键是体视配对图中轴测轴和轴向变形系数的确定。介绍了轴测体视图轴测轴和轴向变形系数的确定方法，给出了一个应用实例，并对不同观察方法下的体视配对图的位置作了说明。     

Effect of superelastic shape memory alloy wires on the impact behavior of carbon fiber reinforced in situ polymerized poly(butylene terephthalate) composites

Low-velocity impact properties of shape memory alloy (SMA) wires and carbon fiber reinforced poly(butylene terephthalate) obtained by resin transfer molding were characterized. At the subcritical regime the dissipated energy is not affected by the presence of the wires. However SMA has a positive effect on the maximum absorbed energy, since the maximum allowable load is higher. The contribution of the SMA wires to the higher impact performance of the hybrid composite is suggested to be due to their energy absorbing capability, and also to the high reversible force that acts as a healing force.     

An experimental and numerical investigation on tyre impact

A key issue in the design of tyres is their capability to sustain intense impact loads. Hence, the development of a reliable experimental data basis is important, against which numerical models can be compared. Experimental data on tyre impact in the open literature is somewhat rare. In this article, a specially design rig was developed for tyre impact tests. It holds the test piece in a given position, allowing a drop mass with a round indenter to hit pressurised tyres with different impact energies. A high-speed camera and a laser velocimeter were used to track the impact event. From the laser measurement it was possible to obtain the impact force and the local indentation. A finite element study was then conducted using material properties from the open literature. By comparing the experimental measurements with the numerical results, it became evident that the model was capable of predicting the major features of the impact of a mass on a tyre. This model is therefore of value for the assessment of the performance of a tyre in extreme cases of mass impact.     

The impact response of traditional and BMX-style bicycle helmets at different impact severities

Bicycle helmets reduce the frequency and severity of severe to fatal head and brain injuries in bicycle crashes. Our goal here was to measure the impact attenuation performance of common bicycle helmets over a range of impact speeds. We performed 127 drop tests using 13 different bicycle helmet models (6 traditional style helmets and 7 BMX-style helmets) at impact speeds ranging from 1 to 10 m/s onto a flat anvil. Helmets were struck on their left front and/or right front areas, a common impact location that was at or just below the test line of most bicycle helmet standards. All but one of the 10 certified helmet models remained below the 300 g level at an impact speed of 6 m/s, whereas none of the 3 uncertified helmets met this criterion. We found that the helmets with expanded polystyrene liners performed similarly and universally well. The single certified helmet with a polyurethane liner performed below the level expected by the Consumer Product Safety Commission (CPSC) standard at our impact location and the helmet structure failed during one of two supplemental tests of this helmet above the test line. Overall, we found that increased liner thickness generally reduced peak headform acceleration, particularly at higher impact speeds.