考虑壁面滑移的磁流变胶泥缓冲器设计理论与落锤冲击试验

为提高汽车碰撞磁流变缓冲器力学模型的准确性,实现冲击作用下磁流变缓冲器动态特性的高精度预测,基于Herschel-Bulkley模型,同时考虑表观滑移和壁面滑移,建立了缓冲器理论力学模型。通过分析表观滑移和壁面滑移对缓冲器阻尼通道内部压力梯度的影响,结果表明,载液黏度较低时,受表观滑移影响,阻尼通道内部压力梯度有所降低,且在低速下影响更加显著;随着载液黏度的增加,在表观滑移作用下压力梯度有所增加,但对总体影响不大;壁面滑移使通道内部压力梯度明显降低,且随着滑移系数的增加,压力梯度变化更为显著;不同电流、冲击速度下的缓冲器落锤冲击试验表明,理论模型能够较好地预测、表征磁流变缓冲器的力学特性;磁流变胶泥在通道内流动主要受壁面滑移的影响,未出现明显的表观滑移。     

基于晶体塑性理论镁合金塑性变形行为研究概述

对镁及其合金的基本特点作了详细的介绍,总结概括了镁合金塑性变形过程中的滑移与孪生两种变形机制,阐述了镁合金的塑性变形是滑移与孪生两种变形机制相互竞争的结果,并对镁合金在塑性变形中孪生与退孪生现象做出了解释。简述了经典的晶体塑性模型及理论,如泰勒(Taylor)模型、萨克斯(Sachs)模型、粘塑性自洽模型(VPSC)以及晶体塑性有限元模型(CPFE)。分析国内外关于镁合金晶体塑性理论研究,主要包括塑性变形力学性能的研究、晶体塑性变形过程中各种滑移与孪生机制的开动及织构演化的研究,并利用VPSC和CPFE进行模拟,在宏观、微观、介观尺度上分析镁合金塑性变形过程中各变形机制的影响。     

基于ABAQUS CEL的高速水冲击载荷仿真与试验研究

针对水陆两栖飞机水箱汲水过程中形成的高速水流冲击问题，开展了高速水冲击载荷特性研究。提出了一种基于调节腔内气压的水冲击试验方法，通过调节储水容器内气压改变水流冲击速度，开展高速水流冲击试验研究；运用ABAQUS CEL数值仿真方法，预设水流初始速度，建立高速水流冲击的有限元模型，分析了水流速度和水管端部与平板间距对于冲击压力的影响。结果表明：数值仿真计算结果与试验结果相差不大，验证了该模型的准确性；水流速度越大，平板受到的冲击压力越大；水管端部与平板的间距越大，平板受到的水流冲击压力越小。     

高速铁路风障在横风与列车风耦合作用下的气动特性研究

针对单层、腔室型两种形式的开孔波纹板风障,采用滑移网格方法分别模拟横风条件下高速列车通过风障区域的过程,分析了在横风和列车风耦合作用下风障周围的绕流流场特性、风障面板气动荷载的时域特性及横风与列车风耦合脉动压力的频域特性。结果表明:在高速列车行经风障区域的过程中,无横风时头车产生的冲击作用要大于尾车的;存在横风作用时,列车头车产生的气动冲击作用与横风作用形成对冲,抵消了部分横风能量,而列车尾车则与横风作用相叠加,放大了横风对风障的气动作用;单层风障通过改变横风流向起到挡风减载作用,而腔室型风障同时可在腔室内部及尾流形成大量小漩涡来消耗横风能量,使用腔室风障能显著降低单个风障面板的气动荷载;该研究中,横风与列车风耦合作用于风障的脉动压力以及气动荷载的主频谱峰值集中在0.5~5 Hz内。     

工业纯钛TA2剪切带中微观组织的演变

剪切变形局域化是结构材料经受冲击时的一种重要失效机制,为研究密排六方晶体结构金属材料的绝热剪切带形成条件与扩展规律,采用HOPKINSON压杆装置对精加工后的工业纯钛帽形样品进行高速冲击,利用扫描电镜和高分辨透射电镜研究了剪切带形貌和剪切带微观组织的演化过程.结果表明,工业纯钛TA2经高速冲击后,在帽形样品的韧带部位形成了明显的剪切带,剪切带组织由细小的再结晶晶粒组成,剪切带内没有相变发生,剪切带内的动态再结晶过程通过渐进式亚晶位相差再结晶机制完成.     

基于OpenSees的钢筋混凝土梁粘结滑移数值分析

粘结滑移性能的数值分析是钢筋混凝土结构研究的热点,该文在总结现有粘结滑移本构关系的基础上,采用分离式思想在OpenSees中建立粘结滑移精细化分析模型,并通过拉拔试件与RC梁构件的数值分析研究梁中钢筋与混凝土的粘结滑移作用,同时考虑箍筋量对构件滑移分布的影响。经过该文分析可知,该数值分析模型具有一定的合理性,能较准确的反映出粘结应力、滑移等分布的规律;通过梁构件数值分析可知,其整体性能受到粘结性能、钢筋滑移量大小及箍筋配置等的影响,箍筋的配置对构件局部滑移会产生一定的影响,但对滑移的整体分布影响并不显著。     

碳纤维/环氧树脂复合材料高速冲击性能

采用树脂传递模塑(RTM)工艺制备碳纤维/环氧树脂复合材料,通过空气炮冲击实验研究树脂韧性和碳纤维类型对复合材料抗高速冲击性能的影响,并对高速冲击后的试样进行压缩性能测试,研究高速冲击损伤对复合材料剩余压缩性能的影响。结果表明:树脂的韧性可以降低复合材料遭受高速冲击时的内部损伤程度,大幅提高复合材料的抗高速冲击性能和冲击后剩余压缩性能;T700S碳纤维增强复合材料抗高速冲击性能优于T800H碳纤维增强复合材料;复合材料的破坏模式与冲击速率有关,冲击速率较低时,复合材料弹击面出现圆形凹坑,背弹面出现鼓包;冲击速率较高时,复合材料弹击面出现圆形通孔,背弹面出现沿纤维方向撕裂断口。     

铜单晶体循环应力应变曲线的平台行为的取向效应

系统讨论和总结了单滑移,双滑移和多滑移取向铜单昌体循环形变中呈现的不同平台行为。分析结果表明,晶体取强烈影响双滑移和多滑移铜晶体的循环变行为,平台区的出现与否及平台应力的高低,不仅与闰错反应模式和强度有关。     

铝锌镁锆合金超塑变形后位错结构与空洞长大的研究

本文对铝锌镁锆合金超塑变形后位错结构及空洞长大规律进行了研究。实验证明该合金在晶界滑移中存在位错蠕变的协调机制。在超塑变形中出现了品界位错并参与了品界滑移,它们是由晶界吸附晶格位错而形成的,或由晶界结构本身在品界滑移中形成的。各种应变速率下测定了空洞长大速率的参数η,探讨了η与 m 值的关系。     

高速列车耦合大系统动力学研究

根据高速铁路结构和技术特点,把高速列车以及与之相关并影响其动力学性能的线路、气流、供电和接触网等耦合系统作为一个统一的大系统,通过建立高速列车、线路、弓网及供电等子系统动力学模型,以及轮轨、弓网、流固和机电等耦合关系模型,形成高速列车耦合大系统动力学模型。针对高速列车运行模拟要求,给出基于循环变量方法的列车动力学建模及计算方法、基于滑移模型的车线耦合计算方法、基于松弛因子的流固耦合计算方法,实现高速列车耦合大系统动力学仿真。     

The role of numerical simulation in the study of high-velocity impact

The issue of the role of numerical simulation in solving the high-velocity impact problems is discussed. The applicability of some most often used models to describe the shock wave processes is analyzed. The model of deformation and fracture of materials under the action of dynamic loading, which combines the macro- and microstructural representations of the deformation and fracture mechanisms, is described. The solutions of some problems which demonstrate the posibilities of the model in studying physical regularities of the high-velocity impact process are presented.     

Crossed Micro-bands in the Martensite Variant Pairs of a Thermomechanically Trained Cu-Zn-Al Alloy

The crossed micro-bands in the martensite variant pairs of a thermomechanically trained Cu25.66Zn-4.02Al (wt-%) alloy have been studied. It was found that the micro-bands pass through the interface from one martensite variant to another and their orientation inside different variants is distinct, which would remain in the parent phase after the reverse transformation and play an important role in the formation of preferential martensite. In fact, the crossed micro-bands are micro-twins for accommodating the stress fields generated during thermomechanically training     

轴向碰撞弹性直杆动力后屈曲有限元分析

利用显式动力学有限元方法对弹性直杆的动力后屈曲进行了分析;模拟了弹性直杆轴向碰撞动力屈曲的变形及发展过程。分析中将碰撞杆视为无初始缺陷的理想直杆,将弹性直杆动力屈曲双特征参数的解答作为非线性动力后屈曲求解的初始条件,实现了对无缺陷理想直杆的动力后屈曲分析。计算结果与文献中的实验数据获得了很好的一致。计算结果同时也揭示了直杆动力屈曲变形发展的机理,以及轴向应力波和屈曲变形的相互作用规律。     

Initial wear characteristics of aging-treated ZG120Mn13 steel under steel shot at high-velocity impact

The initial wear of high manganese steel parts in practical use is related to their original hardness, external loading, surface machining accuracy, and the change of shape and dimension. The paper studies the size and coverage percentage of impact scars on the ZG120Mn13 high carbon high manganese steel surface, as well as the variation of weight loss under high-velocity steel shot by changing aging treatment process. Meanwhile, the relationship between the hardness of aging ZG120Mn13 steel and its initial wear characteristics is discussed. The results show that after 5 seconds, the size and coverage percentage decreased with the increment of hardness. However, in any different period, the wear mass losses of the steel increase with increment of hardness, and increase rapidly first, then slowly. Therefore, under high-velocity impact loading, the increment of hardness is beneficial to improving initial wear-resistance from the perspective of the deformation, but is conducive to improving the resistance from the mass loss. Consequently, we should not only emphasize the high hardness merely, but also consider the changing law of weight loss and initial deformation comprehensively, so that we can achieve the best initial wear resistance when the high manganese steel has the appropriate hardness.     

Shear band formation in lunar regolith by discrete element analyses

Few studies in shear band formation have considered the environmental conditions on the Moon, which however are significant for lunar regolith failure in future lunar exploration activities. This paper presents a numerical investigation into the mechanical behavior and strain localization of lunar regolith by means of the discrete element method (DEM). A micromechanical contact model for lunar regolith accounting for van der Waals forces and rolling resistance has been developed, then implemented into a DEM code, PFC2D, and finally applied to analyze the strain localization of lunar regolith through biaxial tests. Biaxial tests without considering van der Waals force effect were also performed as reference to compare with. The distributions inside the sample of grid deformation, void ratio, velocity, averaged pure rotation rate (APR), force chains and local stress during shear banding are analyzed. The simulations show that persistent bands are differently formed under Moon and Earth conditions. Van der Waals forces and rolling resistance play crucial roles in choosing persistent bands from various transient micro-bands before the peak state. Van der Waals forces lead to increased dilation and particle rotation, and enlarged “meso-voids” in force chain distributions within the persistent shear bands. The thickness (inclination to the horizontal) of shear band for the regolith under Moon condition is smaller (larger) than that for regolith under Earth condition.The fields of velocity and APR can reveal the finest heterogeneity in particle displacement (translation and rotation) in the form of transient micro-bands even at the very beginning of shear.     

Impact Simulation on Ductile Metal Pipe with Polymer Coating by a Coupled Finite Element and Meshfree Method

It is common knowledge that conventional finite element method (FEM) has intrinsic limitations in analyzing large deformation problems like high-velocity impact, explosion, etc. because of mesh distortion and tangling; while these problems can be easily avoided by the meshfree method (MM), the latter involves greater computation time. Therefore, in this article, in order to simultaneously utilize the respective advantages of the two methods, a coupled simulation method between both FEM and MM was employed to analyze the high-velocity impact on ductile metal pipe with polymer coating. The impacted area with large deformation was discretized by SPH (smoothed particle hydrodynamics) particles, a classic meshfree model, and the remaining section was modeled by FEM meshes. By this method, the interfacial shear stresses between the coating and the substrate and the residual stresses beneath the contact points were studied, which would have referenced values in analyzing failure modes of components with similar composite structure. Then, the results were compared with sole FEM and MM too.     

Warm spraying—a novel coating process based on high-velocity impact of solid particles

Abstract

In recent years, coating processes based on the impact of high-velocity solid particles such as cold spraying and aerosol deposition have been developed and attracting much industrial attention. A novel coating process called ‘warm spraying’ has been developed, in which coatings are formed by the high-velocity impact of solid powder particles heated to appropriate temperatures below the melting point of the powder material. The advantages of such process are as follows: (1) the critical velocity needed to form a coating can be significantly lowered by heating, (2) the degradation of feedstock powder such as oxidation can be significantly controlled compared with conventional thermal spraying where powder is molten, and (3) various coating structures can be realized from porous to dense ones by controlling the temperature and velocity of the particles. The principles and characteristics of this new process are discussed in light of other existing spray processes such as high-velocity oxy-fuel spraying and cold spraying. The gas dynamics of particle heating and acceleration by the spraying apparatus as well as the high-velocity impact phenomena of powder particles are discussed in detail. Several examples of depositing heat sensitive materials such as titanium, metallic glass, WC–Co cermet and polymers are described with potential industrial applications.     

Finite-element simulation for modeling composite plates subjected to soft-body,high-velocity impact for application to bird-strike problem of composite fan blades

We presented a numerical simulation to address the impact-induced deformation and damage of composite plates subjected to soft-body, high-velocity impacts for application to the bird-strike problem of composite fan blades. A new stabilized contact algorithm was developed based on the Lagrange multiplier method to predict appropriate impact forces applied to the plate, in order to solve soft-body impact at high velocity without causing severe numerical instabilities. The bird-strike impact on composite fan blade was simply modeled by discussing the damage characteristics of a unidirectional composite plate. Combining the model of a soft-body impactor with an appropriate contact algorithm, we could capture the transition from the global bending mode at low velocity to the local deformation mode at high velocity, enabling a discussion of the ballistic limit using the damage analysis of the laminate. As the impact velocity increased, the damage in the composite changed from bending-induced matrix-cracking to an intensive fiber-breakage mode causing local shear perforation. The damage mode transition allows us to detect the transition velocity as a ballistic limit, which is one of the critical factors for discussing the bird-strike resistance of composite fan blades.     

Microhardness and high-velocity impact resistance of HfB2/SiC and ZrB2/SiC composites

The results of Vickers microhardness and high-velocity impact tests on monolithic ZrB₂/SiC and HfB₂/SiC ultra-high temperature ceramic (UHTC) composites are presented. The UHTC materials exhibit fracture behavior typical of ceramics under indentation and impact loading. The materials are relatively hard with microhardness values of about 15 to 20 GPa. Cracks were observed to extend from the corners of indentations. Impacts of stainless steel and tungsten carbide spheres, with diameters in the 500 to 800 micron range and velocities of 200 to 300 m/s, produced minimal plastic deformation but significant radial and ring cracking at the impact sites. Impacts of micron-scale iron particles traveling at 1 to 3 km/s produced essentially no surface damage.     

A multiscale framework for high-velocity impact process with combined material point method and molecular dynamics

The equation of state (EOS) plays an important role in high-velocity impact process since phase transformation, melting, and even vaporization may happen under such extreme loading conditions. It is desired to adopt an accurate EOS covering a large range of points in the phase space. This paper proposes a combined molecular dynamics and material point method approach to simulate the high-velocity impact process. The EOS data are first obtained from a series of molecular dynamics computations, and the parameters are fitted. Then the EOS parameters are adopted in the material point method simulation to model the impact process. Simulation results show that the fitted EOS can be very accurate compared to experimental results. The shape of the debris cloud obtained by our multiscale method agrees well with that of the experiments. An empirical equation is also proposed to predict the fraction of melting material in the high-velocity impact process.