冲击荷载对具有轴向静应力红砂岩应力波传播速度的影响特性

岩体工程爆破开挖时，距爆源不同距离处的围岩体承受不同大小的冲击荷载和地应力。为研究冲击荷载和地应力对岩石应力波传播速度的影响，利用改进SHPB试验装置对红砂岩进行应力波传播试验，设置7个等级的轴向静应力和冲击速度，分别模拟工程中的地应力和冲击荷载大小。基于试验得到入射波和透射波起跳点的时间差，计算岩石的应力波传播速度，得到静应力和冲击速度对应力波传播速度的影响规律。构建岩石应力波传播速度与冲击速度的关系经验模型，探索拟合参数随轴向静应力的变化规律。通过测量冲击试验后岩石的声波波速，得到受载后岩石声波波速随冲击速度的变化规律，探究动载荷对岩石应力波传播速度的影响机理。研究结果表明，具有轴向静应力岩石的应力波传播速度随冲击速度的增加先增大后减小，二者呈高斯函数的变化关系。轴向静应力显著影响应力波传播速度与冲击速度的变化关系，各个拟合参数随着轴向静应力的增大呈现不同的变化趋势。在不同的轴向静应力工况下，受载后岩石的声波波速随着冲击速度的增大呈现“平缓减小—急剧减小”的变化趋势。研究结果有助于分析深部工程岩体爆破开挖应力波的传播规律以及邻近结构的稳定性分析。      

水平层状岩体边坡动力响应中的结构面效应研究

根据应力波传播原理分析了水平层状岩体边坡中应力波传播特征,建立了应力波在该类边坡中传播的模型.利用离散元软件UDEC分析了不同频率垂向压缩应力波作用下边坡动力响应规律中的结构面效应.结果表明:边坡中的水平层面对坡顶的动力响应有明显影响.低频应力波作用下,水平层状岩体边坡坡顶的垂向峰值速度较均质坡体相同部位的峰值速度的增加值随坡高增加而增大.较高频率应力波作用时,边坡顶部靠近坡面的垂向峰值速度高于无结构面边坡相同部位的峰值速度,远离坡面时情况相反;坡顶垂向峰值速度大小呈周期性变化,输入应力波频率越大该变化频次越高.研究结果将有助于进一步揭示各种不同岩体结构类型边坡在动力荷载作用下损伤机理及破坏模式.     

节理粗糙度对应力波传播及试样破坏影响的颗粒流模拟

为了研究节理粗糙度对应力波传播规律的影响以及粗糙节理试样受应力波作用发生破坏的微观机理,利用基于离散元方法的数值分析软件PFC^2D构建了SHPB系统的颗粒流数值模型。在已有SHPB物理试验的基础上对试验中采用的节理试样进行微观参数标定,研究了较低冲击荷载下节理粗糙度对应力波传播的影响规律以及较高冲击荷载下不同形貌节理试样的微观裂纹扩展和破坏机理。研究表明：节理面的存在会降低应力波的透射系数,且节理面粗糙度越大,透射系数越小;冲击载荷下裂纹在节理面处萌生并迅速扩展到试样完整部分尤其是试样端面,大部分裂纹形成于峰后阶段,且张拉裂纹占主导地位;节理面越粗糙,动态强度越低,试样越容易破坏,产生的裂纹数也越多。     

脉冲泵压环境膏体水分迁移转化与流变行为数值推演

通过微细观结构分析、低场核磁共振量化研究，科学、准确地描述膏体跨尺度颗粒群存在形态与水分赋存状态，并基于有限元和离散元耦合数值分析方法分析了泵压扰动下膏体颗粒流态演化规律.研究发现，膏体料浆中的吸附水、间隙水和弱自由水存在动态连通与转化行为，并主要以吸附水形式存在，低场核磁共振技术(Low-field nuclear magnetic resonance,LF-NMR)弛豫强度与吸附水峰面积非线性增强，与料浆的流动表现出显著的正相关性.液网结构与絮网结构反映了导水通道的活跃性与力链结构的强度，共同组成了膏体稳定性与流动性的双支撑骨架结构.通过Fluent-EDEM软件耦合模拟，分析了脉冲泵压环境颗粒运动行为，在速度差的影响下高、低流速颗粒冲击扰动加剧，力链接触作用增强，流态均匀性和整体颗粒运动稳定性可有效提高.     

围压对红砂岩应力波频谱和频带能量特性的影响

为研究围压对岩石应力波传播衰减特性影响, 对具有不同围压的红砂岩试件进行应力波传播试验.通过傅立叶变换得到反射波和透射波频谱数据, 分析频谱图形状和主频值随围压的变化规律.对反射波和透射波进行小波包分析, 研究不同围压工况下反射波和透射波频带能量分布的变化规律.结果表明, 随围压的增加, 反射波频谱图的幅值呈“减小—缓慢发展—增加”的趋势, 透射波频谱图的幅值逐渐增大后有略微减小, 且宽度也越来越窄.透射波主频值随围压的增加呈现“增大—稳定不变—减小”的趋势, 围压的两个临界值分别为单轴抗压强度的25 %和70 %, 反射波主频值变化与此相反.随围压的增加, 反射波主频带和透射波优势频带所占总能量百分比先增加后减小, 围压对低频带能量百分比的影响更大.      

钨铜复合粉末成形过程的颗粒尺度数值模拟

采用多粒子有限元方法对钨铜复合粉末的二维模压成形过程进行数值模拟。系统研究和分析了铜粉含量和压力对成形过程中压坯相对密度及分布、应力及分布、颗粒变形、孔隙填充行为以及力链结构的影响规律。结果表明：成形过程中应力主要集中在钨颗粒内部，并形成力链阻碍致密化的进程。孔隙的填充主要通过铜颗粒的变形来实现，相互接触的钨颗粒形成了难以填充的细小孔隙。钨铜颗粒边缘部位的应力都高于中心应力。随着压制压力的增加，铜颗粒比钨颗粒中心部位应力增长缓慢，沿压制方向上钨颗粒边缘应力值远大于颗粒中心部位。综合分析，钨颗粒的变形特性是阻碍成形致密化进程的关键因素，需要优化钨颗粒的排列结构，从而实现高性能压坯。     

起爆方式对间隔装药应力场分布及裂纹扩展的影响

通过模型实验研究了不同起爆方式对空气间隔装药炮孔两侧损伤分布的影响规律，借助数字图像相关实验系统，获取了全场应变场演化过程及空气段应变衰减规律，同时借助透射式焦散线实验系统，探究了起爆方式对预制裂纹动态断裂行为的影响。实验结果表明:柱状药包炮孔两侧产生的损伤范围具有显著的分形特征。采用外侧起爆时，空气段中心两侧均产生损伤，而采用其他起爆方式时，空气段均未出现损伤。不同起爆方式对空气段应变场径向压应变的影响主要体现在应变大小、衰减速度两个方面，对轴向拉应变的影响主要体现在时效性、衰减速度两个方面。不同起爆方式下预制裂纹端部断裂行为差别较大。采用内侧起爆、外侧起爆时，裂纹均为水平扩展，呈现典型Ⅰ型裂纹，裂纹起裂主要由拉伸破坏引起，异侧起爆时裂纹起裂为Ⅰ?Ⅱ混合型，具体表现为拉?剪破坏。基于数值模拟软件LS-DYNA，解释了预制裂纹端部起裂成因，得到了孔壁处应力场分布规律，不同起爆方式对炮孔轴向孔壁处压力分布影响显著，装药段主要体现在压力峰值位置和压力分布形态两个方面，空气段主要体现在压力峰值大小和压力分布形态两个方面。      

基于有限元仿真分析高速压实粉末时模壁摩擦因数对压制效果的影响

为研究粉末在高速压实下模壁摩擦因数的影响,分别选择Cu、Fe作为压制粉末,在MARC中建立微观的粉末模型,视冲头为刚体以计算得到的加载速度模拟压制粉末体。通过设定不同的边界摩擦因素,发现与传统静力压制相比,电磁脉冲高速压制时并非模壁的摩擦因数越小,压制件的密度越均匀,当速度超过60 m/s时,由于应力波叠加的作用,使得摩擦因数小于一定值以后,铜粉压坯在压制中期从下端先开始压实,最终导致粉末坯体的密度由下往上递减。分析发现当金属粉末颗粒密度大于4 g/cm3,同时应变敏感性指数C小于0.09时会出现最佳摩擦因数使得压制的毛坯密度均匀性最好,同时最佳摩擦因数在4~9 g/cm3密度范围内与密度成正比,在0.005~0.01的应变敏感性指数范围内与应变敏感性指数成反比。     

COREX熔化气化炉物料运动和气流分布

摘要：基于离散元数值计算方法(DEM),建立了熔化气化炉模型的离散元数学模型。应用此模型从颗粒尺度对气化炉3种软熔区域形状下的物料质量、运动速度、法向力、空隙度分布进行了研究。利用DEM计算空隙度数值,再结合计算流体力学软件对气化炉气流分布进行了计算。结果表明：软熔区域形状对气化炉料面形状和炉内下部法向力的分布影响很小,炉中心下部区域的物料所受的法向力较大;得到了3种软熔区域形状下,气化炉内空隙度分布平均数值;倒“V”型软熔区域结构可使气流分布均匀、稳定,利于气化炉生产实践。     

钢中非金属夹杂缺陷的红外热波无损检测

建立了钢中非金属夹杂缺陷热波散射的理论模型,基于非傅里叶热传导理论和虚拟镜像法,对含非金属夹杂缺陷的钢中热波散射场进行分析与计算,得到方程的一般解,通过数值计算,给出了不同类型非金属夹杂缺陷的几何参数和物性参数条件下钢的表面温度分布。研究结果表明:非金属夹杂缺陷改变了热波在钢中的传播特性,导致钢表面温度分布呈现双峰形式;同时,非金属夹杂缺陷的几何参数和物性参数也会影响钢的表面温度分布,从而影响热波的传播特性。     

基于离散元法的颗粒高速压制模拟及动态力学分析

采用三维离散单元法对高速压制条件下铝粉颗粒的动态响应进行了数值模拟,并与实验结果进行对比分析。结果表明,数值模拟与实验所测得的结果基本相同;在高速压制过程中,粉体的扰动呈不规则的弧形分布;在压制初期,会出现整体受力不均匀的现象,随着压制的进行,不均匀现象得到改善,粉体表现出较强的自组织性;在单次加载过程中,颗粒会发生多次碰撞;在致密化阶段,上层的部分颗粒会先发生变形并重排,中下层颗粒以重排为主;进入变形阶段后,所有颗粒的受力情况基本相同。     

Mathematical modeling of particle segregation during centrifugal casting of metal matrix composites

A one-dimensional transient heat-transfer model coupled with an equation for force balance on particles is developed to predict the particle segregation pattern in a centrifugally cast product, temperature distribution in the casting and the mold, and time for complete solidification. The force balance equation contains a repulsive force term for the particles that are in the vicinity of the solid/liquid interface. The solution of the model equations has been obtained by the pure implicit finite volume technique with modified variable time-step approach. It is seen that for a given set of operating conditions, the thickness of the particle-rich region in the composite decreases with an increase in rotational speed, particle size, relative density difference between particles and melt, initial pouring temperature, and initial mold temperature. With reduced heat-transfer coefficient at the casting/mold interface, the solidification time increases, which, in turn, results in more intense segregation of solid particulates. Again, with increased initial volume fraction of the solid particulates in the melt, both the solidification time and the final thickness of the particulate-rich region increase. It is noted that for Al-Al₂O₃ and Al-SiC systems, in castings produced using finer particles, lower rotational speeds, and an enhanced heat-transfer coefficient at the casting/mold interface, the volume fraction of particles in the outer layer of the casting remains more or less the same as in the initial melt. However, for castings produced with coarser particles at higher rotational speeds and reduced heat-transfer coefficients at the casting/mold interface, intense segregation is predicted even at the outer periphery of the casting. In the case of the Al-Gr system, however, intense segregation is predicted at the innermost layers.     

Simulation of the submerged energy nozzle-mold water model system using laser-optical and computational fluid dynamics methods

The present work describes quantitative digital particle image velocimetry measurements of a full-scale water model of a thin slab mold. Different casting speeds and two submerged entry nozzles with one and two outlet ports have been investigated. The flow pattern of the single-port nozzle shows a counterclockwise-rotating double vortex that is nearly steady-state but leads to high stationary surface waves. The flow jets out of the two-port nozzle oscillate and produce a transient flow pattern with low wave amplitudes. The amplitudes for the one-port nozzle show a linear variation with the volumetric flow rate. The experimental results lead to a good interpretation of the flow phenomena and are used to validate steady-state numerical simulations with the commercial program, CFX, on the basis of the Reynolds equations. To describe anisotropic turbulence effects, the Reynolds stress model (RSM) is used for the flat single-port nozzle and the standard k-ɛ model for the mold flow. The calculated mean velocities and wave amplitudes, predicted from pressure distribution at the water surface, are generally in the consensus of the experimental data. An erratum to this article is available at .     

综放开采顶煤与覆岩力链结构及演化光弹试验研究

综放开采过程中顶煤和覆岩由连续向非连续和散体介质状态转变,描述矿压在断裂覆岩和碎裂顶煤中的形成、传递方式和作用机理是综放开采矿压理论研究的难点.基于光弹试验原理,借助散体介质双轴加载双向流动光弹试验装置,对综放开采过程中散体顶煤与非连续覆岩关键层中力链网络结构及演化特征进行研究.研究发现:地层载荷在煤岩体中构成了错综复杂的弱-强力链网络.综放开采打破了初始力链网络结构的平衡,顶煤与覆岩中形成梁-拱复合力链拱结构,覆岩荷载以强力链形式传递到工作面前方煤体.随着工作面推进和顶煤放出,覆岩梁-拱力链网络不断扩展和演化,形成更大规模的力链拱结构.关键层弯曲、断裂和失稳运动,使工作面前方的强力链拱脚出现回缩现象,力链分布密度和强度增加,导致工作面来压现象.地层内水平力作用使顶煤与覆岩内的梁-拱力链结构效应较更加明显,整体拱结构形态愈加完整,强力链网络结构更加紧致,关键层断裂失稳时力链拱对工作面煤层的作用力更加显著.      

冲击加载条件下金属粉末的动态力学性能分析

为研究金属粉末在冲击加载过程中, 粉末预压力对压坯动态力学响应的影响, 设计了基于分离式霍普金森压杆装置(split Hopkinson pressure bar, SHPB)的金属粉末高应变率冲击加载实验, 并结合一维应力波理论对预压后粉末压坯的力学性能进行分析。结果表明:在冲击过程中, 金属压坯会表现出较为明显的应变率效应; 加载率越大, 材料的应变能越大, 预压力越大, 应变硬化率越大; 在相同的加载条件下, 预压力越大, 压坯临界位移越小。     

Study on Optimization of Parameters Affecting Simple Shear Extrusion of Pure Copper to Fabricate Fine Grain Structure

During last decades, severe plastic deformation (SPD) techniques have developed and increased their applications in variety of industries such as aerospace, nuclear, automotive, marine etc. One of emerging SPD process is simple shear extrusion (SSE). In the present study, an attempt was made to optimize parameters affecting SSE process of pure copper namely maximum distortion angle, channel length and ram speed on effective plastic strain and maximum punch force. Firstly, the process was numerically simulated by use of ABAQUS software. Then, the developed finite element model was verified by confirmatory experiment taking into account the punch force. 15 series of simulation runs were implemented incorporating three aforementioned factors within three levels. Then, response surface methodology was used here to correlate relationship between process parameters to forming force and effective strains. Sensitivity analysis was also carried out to find which factor had greatest impact on process quality characteristics. In order to find optimal parameter setting for maximum strain and strain rate as well as minimum punch force, desirability approach was utilized. Results revealed that distortion angle was the most significant factors affecting plastic strain and punch force, while the ram speed had greatest impact on strain rate. The specimens at optimum parameters (i.e. 45° distortion angle, 90 mm channel length and 0.3 mm/s ram speed that resulted in maximum plastic strain) were processed by SSE and mechanical properties and microstructure analysis were studied. It was found from the results that microstructure of the samples were significantly refined by performing SSE at optimum level that resulted in improvement of samples strength and hardness.     

Impact Response of Externally Strengthened Unreinforced Masonry Walls Using CFRP

Research reported herein investigates the out-of-plane impact resistance of unreinforced masonry (URM) walls strengthened with carbon fiber-reinforced polymer (CFRP) composites, externally applied in sheets to one face of the wall. Two analytical methods based on energy principle and wave propagation theory and a finite-element-based numerical model have been developed, assuming a perfect bond at composite–masonry interface with an equivalent stiffness of the system. Full-scale impact tests are conducted for verification purpose, where three 1.2?m tall URM concrete walls (one unstrengthened and two strengthened with continuous unidirectional and woven CFRP sheets) are vertically tested up to cracking using a pendulum drop-weight impact tester. The test results compare reasonably well with those obtained from the analyses and simulation. It is found that the energy and finite-element methods can provide reasonable estimates for peak impact force and wall deflection, whereas the wave propagation method is rather limited by its applicability. Parametric studies are conducted to examine the effect of impactor mass, velocity, amount of CFRP reinforcement, and property of masonry material using the developed models.     

Failure in thin sheets stretched over rigid punches

Criteria for diffuse and local necking in sheet metals stretched over a hemispherical punch have been developed. During stretching, a peak develops in the distribution of strain across the dome. This peak is a ring of thinned material around the pole which moves outward radially with continued deformation. The present approach essentially focuses on the loading rate of the region bounded by this ring of maximum thinning referred to here as thecrown. While diffuse instability is identified with a vanishing rate of crown loading, local necking is associated with the onset of a rapid rate of thinning under falling crown load. The principal surface strains were measured from incremental stretching of several different materials and time dependent constitutive relations were used to predict the instability strains. Such predictions agree with experimentally observed forming limits defined by the onset of visible necking and provide an understanding of the failure of biaxially stretched sheets deformed by a rigid punch. Since no satisfactory plasticity analyses have been developed to predict the strain distribution during punch stretching, the empirical input to the present instability criterion is required. Formerly with Res. Labs, General Motors Corporation