基于复变函数理论和D-P屈服准则的并行隧道合理间距

综合利用复变函数理论、解析延拓法和Schwarz交替法揭示相邻水平并行隧道的应力分布特征.在此基础上,结合考虑了中间主应力效应的D-P屈服准则建立相邻水平并行隧道力学模型.提出并行隧道塑性区贯穿半径的概念,建立求解方程,并通过数值模拟证明其正确性.采用隧道间塑性区临界贯穿状态下的间距作为隧道合理间距,与数值模拟软件FLAC3D计算得到的围岩位移量和沉降量随间距变化至基本不发生变化时所对应的隧道间距有较高的吻合性,从而表明其作为相邻水平并行隧道合理间距的可行性.      

基于应力三轴度的TA18钛合金损伤失效研究

对TA18钛合金材料设计不同缺口尺寸的拉伸试样,进行不同应力状态下的室温拉伸试验及断口形貌观察,通过试验和数值计算结合的方法研究TA18钛合金的韧性断裂特性,分析了应力状态对微观断裂机制的影响规律.利用Bridgman正向计算法修正颈缩失稳后的应力数据,建立了TA18钛合金的Johnson-Cook(J-C)本构模型,计算了拉伸试样的平均应力三轴度和断裂应变,回归确定了TA18钛合金损伤失效模型.结果 表明:不同应力状态下的TA18钛合金断裂应变各不相同,断裂应变随着应力三轴度的增大而减小,断口韧窝尺寸与应力三轴度呈正相关关系,所建立的损伤失效模型能够描述该材料的断裂特性.     

热轧双相钢微观应力-应变模型

利用扫描电子显微镜和透射电子显微镜对热轧后双相钢的微观组织进行分析,用Image-Pro Plus软件测定双相钢微观组织中各独立相的体积分数.根据多相材料中间混合法则和Swift方程,建立热轧双相钢微观应力-应变模型,并用DP590和DP780钢单向拉伸曲线进行验证.结果表明,该应力-应变关系微观模型基本阐明热轧双相钢微观组织参数与宏观力学性能的内在联系,能够准确地描述材料的变形行为,同时很好地预测热轧双相钢宏观的拉伸曲线.      

冷成型对低碳微合金钢拉伸行为及性能的影响

研究了冷成型对铁素体+珠光体组织类型低碳微合金热轧钢板拉伸行为及性能的影响。用弯曲和压平变形模拟高频直缝焊管制管及取样钢板所经历的冷成型过程。对冷成型前后材料拉伸行为和性能进行对比试验,结果表明,冷变形不同程度改变材料拉伸行为及性能,且这种变化是由冷变形方式和程度所决定;包申格效应试验分析结果表明,材料背应力随塑性应变增加而增大,且到达某一数值后当塑性应变继续增加,背应力增加幅度明显减小;有限元分析结果表明,冷成型导致的钢板性能变化是由包申格效应和加工硬化共同作用的结果,并给出了其形成机制。     

钨合金空心弹体垂直侵彻过程研究

进行了钨合金空心弹体垂直侵彻钢靶板和混凝土靶板的实验。通过对回收空心弹体的分析，指出钨合金空心弹体破坏的典型宏观特征和主要微观机制分别为：空心弹体危险区域的塑性大变形及断裂和危险截面处材料的绝热剪切失效。通过自行设计的弹载存储测试系统，成功地获得钨合金空心弹体危险截面处应变的时间历程。应变历程曲线真实地反映出侵彻目标过程中应力波效应致使弹体受到动态压缩和动态拉伸载荷的反复作用，压应力远高于拉应力，而且应力水平与目标靶板的材料性能密切相关。研究结果不仅有助于空心弹体侵彻模型的建立，而且为空心弹体的结构设计与选材提供理论依据。     

数值仿真技术在粉末冶金零件制造中的应用及研究进展

数值仿真技术可用于粉末零件制造的工艺设计与优化,为缺陷预测与预防、零部件性能改善提供有效的理论支持和技术指导。基于粉末零件压制和烧结机理的本构模型是进行准确可靠的数值计算的必要条件。本文主要介绍了粉末压制模拟中常用的本构建模方法,如粉末烧结体塑性力学方法、广义塑性力学方法及微观力学方法,和粉末烧结模拟中的微观结构模拟方法,如分子动力学法、相场法、蒙地卡罗法及元胞自动机法等,并对以上各种建模方法的计算原理、适用范围及近些年的应用进展做了简要介绍,最后对数值仿真技术在粉末零件制造中的发展方向进行了展望。     

几种金属材料织构参数与工艺参数的关系

一、前言 材料的宏观特性是由它的微观结构和性质所决定的。为了提高材料的性能,在进行工艺研究时必须对材料的微观结构、组织状态、晶粒大小及取向分布等进行观察,从而来指导工艺研究。文中介绍了三种不同金属材料的织构与加工工艺参数的关系。     

无底柱分段崩落采矿法回采方式对进路应力影响的模拟分析

以夏甸金矿无底柱分段崩落采矿法回采方式调整为工程背景,运用FLAC3D数值模拟分析软件,从应力变化规律、塑性破坏区变化特点和位移场分布特征等多个方面,对不同回采方式的采场地压变化规律进行数值模拟对比研究。研究发现无底柱分段崩落采矿法在回采过程中垂直应力集中区域出现在回采进路两帮的岩体中,水平应力集中区域出现在回采进路的顶底板中。当采用双翼阶梯式回采时,回采进路中应力分布较为均匀,总体应力较小,是最为合适的采矿方法。     

SiC陶瓷与Ni基高温合金连接件应力的有限元分析

陶瓷与金属连接具有重要的工程应用背景,然而却面临诸多技术难关,连接件的热应力缓解便是其中之一。本文作者采用弹性有限元方法,对采用不同材料作为中间层得到的实际连接尺寸的SiC陶瓷与Ni基高温合金连接件的应力进行计算,并结合各种材料的塑性对连接件的应力进行定性分析。计算结果表明,SiC陶瓷与Ni基高温合金直接连接产生的热应力很大。最大轴向拉应力位于陶瓷近缝区,导致连接件强度偏低或断裂。采用功能梯度中间层或软金属中间层能在一定程度上缓解热应力;硬金属中间层虽然不能缓解应力,但能改善应力分布状态,使最大轴向拉应力迁移出比较薄弱的陶瓷一侧,有利于连接强度的提高;采用软、硬金属复合中间层具有较好的缓解应力和改善应力分布的效果,但却较多地增加了连接件的界面,有可能导致负面效应,在实际工程应用中需要根据具体情况,权衡利弊,综合考虑。     

GH4169合金淬火残余应力的中子衍射及有限元分析

淬火工艺是工件制作过程中不可或缺的环节,而温度梯度又会导致残余应力的产生,因此准确表征GH4169工件内部残余应力及探索建立有效的有限元模拟分析对了解和改善其性能具有重要意义。利用中子衍射的手段研究了GH4169合金淬火残余应力分布,并对淬火过程的温度场和应力场进行有限元数值模拟分析。结果表明:淬火后工件中心的残余应力状态为3向拉应力,约为400MPa;工件近表层形成单向或双向压应力状态,约-300~-400MPa。将有限元计算得到的试样中心和表面处的残余应力和应变与中子衍射测试结果进行对比,结果表明二者应力分布规律较为一致。结合中子衍射和有限元模拟的结果,较为合理地阐述了快速冷却过程中残余应力的形成机制。     

Geomechanical Analysis of Unbound Pavements Based on Shakedown Theory

A procedure for analyzing the mechanical response of an unbound pavement to the repeated loading of traffic is presented. The pavement is modeled as a layered elastic/plastic structure, and its response is described by the concepts of shakedown theory. A critical shakedown load is identified as the key design parameter. Pavements operating at higher loads will eventually fail, and those operating at loads less than critical may initially exhibit some distress but will eventually shakedown to a steady state. Estimates of this critical load, for different types of pavement, are found by studying various types of failure mechanisms, such as rut formation and subsurface slip. Optimization procedures are then used to determine the most likely form of failure for a particular pavement. The effects of self-weight, dual loads, moisture content, relative strengths of the various layers, and nonassociated plastic flow are studied. Some preliminary implications for pavement design are discussed.     

Elasto-plastic load transfer in bulk metallic glass composites containing ductile particles

In-situ diffraction experiments were performed with high-energy synchrotron X-rays to measure strains in crystalline reinforcing particles (5 and 10 vol. pct W or 5 vol. pct Ta) of bulk metallic glass composites. As the composites were subjected to multiple uniaxial tensile load/unload cycles up to applied stresses of 1650 MPa, load transfer from the matrix to the stiffer particles was observed. At low applied loads, where the particles are elastic, agreement with Eshelby elastic predictions for stress partitioning between matrix and particles is found, indicating good bonding between the phases. At high applied loads, departure from the elastic stress partitioning is observed when the particles reach the von Mises yield criterion, as expected when plasticity occurs in the particles. Multiple mechanical excursions in the particle plastic region lead to strain hardening in the particles, as well as evolution in the residual strain state of the unloaded composite.     

Dislocation punching from spherical inclusions in a metal matrix composite

The dislocation punching (plastic relaxation) from spherical inclusions is examined. The plastic stran in the punched domain is determined by assuming that all geometrically necessary dislocations are punched out to the plastic-elastic boundary. By introducing impotent eigenstrains, the elastic state calculation is greatly simplified. The punching distance is determned by the balance between the punching force due to the elastic energy decrease and the retarding force due to the disspation energy needed for the plastic deformation. The punching distance is also examined in terms of stresses at the plastic domain boundary where the deformation carrying dislocations exist. The effect of the volume fraction of inclusions is discussed and is shown to sensitively depend on the elastic constants ofthe inclusions. An increase in flow stress due to statistically stored dislocations in the wake of punching is evaluated.     

Finite-element method simulation of effects of microstructure, stress state, and interface strength on flow localization and constraint development in Nb/Cr2Nb in situ composites

The effects of volume fraction of particles, stress state, and interface strength on the yield strength, flow localization, plastic constraint, and damage development in Nb/Cr₂Nb in situ composites were investigated by the finite-element method (FEM). The microstructure of the in situ composite was represented in terms of a unit rectangular or square cell containing Cr₂Nb particles embedded within a solid-solution-alloy matrix. The hard particles were considered to be elastic and isotropic, while the matrix was elastic-plastic, obeying the Ramberg-Osgood constitutive relation. The FEM model was utilized to compute the composite strength, local hydrostatic stress, and plastic strain distributions as functions of volume fraction of particles, stress state, and interface strength. The results were used to elucidate the influence of volume fracture of particles, stress state, and interface property on the development of plastic constraint and damage in Nb/Cr₂Nb composites.     

Cavity formation from inclusions in ductile fracture

The previously proposed conditions for cavity formation from equiaxed inclusions in ductile fracture have been examined. Critical local elastic energy conditions are found to be necessary but not sufficient for cavity formation. The interfacial strength must also be reached on part of the boundary. For inclusions larger than about 100? the energy condition is always satisfied when the interfacial strength is reached and cavities form by a critical interfacial stress condition. For smaller cavities the stored elastic energy is insufficient to open up interfacial cavities spontaneously. Approximate continuum analyses for extreme idealizations of matrix behavior furnish relatively close limits for the interfacial stress concentration for strain hardening matrices flowing around rigid non-yielding equiaxed inclusions. Such analyses give that in pure shear loading the maximum interfacial stress is very nearly equal to the equivalent flow stress in tension for the given state of plastic strain. Previously proposed models based on a local dissipation of deformation incompatibilities by the punching of dislocation loops lead to rather similar results for interfacial stress concentration when local plastic relaxation is allowed inside the loops. At very small volume fractions of second phase the inclusions do not interact for very substantial amounts of plastic strain. In this regime the interfacial stress is independent of inclusion size. At larger volume fractions of second phase, inclusions begin to interact after moderate amounts of plastic strain, and the interfacial stress concentration becomes dependent on second phase volume fraction. Some of the many reported instances of inclusion size effect in cavity formation can thus be satisfactorily explained by variations of volume fraction of second phase from point to point. This work has been presented in part orally at the Third International Conference on Fracture in Munich, Germany April 1973.     

A metallographic study of extruded reinforced composites based on iron and copper

Microprobe analysis has been applied to the boundary layers between fibers and matrix to examine the phase composition in relation to mode of extrusion for composite materials based on iron and copper as reinforced with molybdenum and steel fibers. The width of the interaction zone varies from 2 to 4 µm. The phases correspond to the phase diagrams for these systems. Fractography indicates the failure mechanism for reinforced composites under conditions of stress and strain. At the points of application of shock loads, there is planar transverse fracture in the fibers by the cleavage mechanism. The peripheral layers are subject to viscous failure in the fibers with the formation of necks. Extrusion produces plastic strain uniform throughout the fiber length, and there are no breaks in the fibers, which provides conditions for complete realization of the strength of the reinforcing phase.     

Homogenization of Masonry Using Numerical Simulations

Homogenization is one of the most important steps in the numerical analysis of masonry structures where the continuum method is used. In the present study, equivalent elastic properties, strength envelope, and different failure patterns of masonry material are homogenized by numerically simulating responses of a representative volume element (RVE) under different stress conditions. The RVE is modeled with distinctive consideration of the material properties of mortar and brick. In the numerical simulation, various displacement boundaries are applied on the RVE surfaces to derive the stress-strain relation under different conditions. The equivalent overall material properties of the RVE are averaged by integrating the stresses and strains over the entire area. Failure of masonry is defined by three different modes, namely, tensile failure of mortar (Mode I), shear failure of mortar or combined shear failure of brick and mortar (Mode II), and compressive failure of brick (Mode III). The homogenized elastic properties and failure model can be used to analyze large-scale masonry structures.     

Cavity formation from inclusions in ductile fracture

The previously proposed conditions for cavity formation from equiaxed inclusions in ductile fracture have been examined. Critical local elastic energy conditions are found to be necessary but not sufficient for cavity formation. The interfacial strength must also be reached on part of the boundary. For inclusions larger than about 100Å the energy condition is always satisfied when the interfacial strength is reached and cavities form by a critical interfacial stress condition. For smaller cavities the stored elastic energy is insufficient to open up interfacial cavities spontaneously. Approximate continuum analyses for extreme idealizations of matrix behavior furnish relatively close limits for the interfacial stress concentration for strain hardening matrices flowing around rigid non-yielding equiaxed inclusions. Such analyses give that in pure shear loading the maximum interfacial stress is very nearly equal to the equivalent flow stress in tension for the given state of plastic strain. Previously proposed models based on a local dissipation of deformation incompatibilities by the punching of dislocation loops lead to rather similar results for interfacial stress concentration when local plastic relaxation is allowed inside the loops. At very small volume fractions of second phase the inclusions do not interact for very substantial amounts of plastic strain. In this regime the interfacial stress is independent of inclusion size. At larger volume fractions of second phase, inclusions begin to interact after moderate amounts of plastic strain, and the interfacial stress concentration becomes dependent on second phase volume fraction. Some of the many reported instances of inclusion size effect in cavity formation can thus be satisfactorily explained by variations of volume fraction of second phase from point to point.