Upper and lower bounds for constitutive relation of crack-weakened rock masses under dynamic compressive loads

A micro-mechanics-based model is proposed to investigate the rate-dependent constitutive relation for crack-weakened rock masses subjected to dynamic compressive loads. The present micro-mechanical model reveals that the nucleation, growth and coalescence of sliding cracks dominate the failure and macroscopic properties of crack-weakened rock masses subjected to dynamic compressive loads. The interactions among multiple parallel sliding cracks in crack-weakened rock masses subjected to dynamic compressive loads are examined asymptotically in an explicit and quantitative manner in order to reveal fully their so-called shielding and magnification effects on the stress–strain relation. Based on the micro-mechanical framework and the asymptotic analysis, analytical upper and lower bounds are proposed for the rate-relation for rock masses containing multiple rows of echelon cracks subjected to dynamic compressive loads. The factors that affect the rate-dependent properties of crack-weakened rock masses have been analyzed. The strain energy density factor approach, which is related to crack growth velocity and dynamic fracture toughness of rock material, is employed in the analysis. The rate-dependent constitutive relation of crack-weakened rock masses is derived from micro-mechanical framework and the asymptotic analysis. The closed-form explicit expression for the rate-dependent constitutive relation of rock masses containing echelon cracks subjected to dynamic compressive loads is obtained. Finally, the present model is used to analyze the complete stress–strain relation and strength for jointed rock masses at shiplock slope of the Three Gorges Dam.     

冲击荷载下单层球面网壳的失效机理

为研究单层球面网壳在冲击荷载下的失效机理,在ANSYS/LS-DYNA中建立60m跨度K8型单层球面网壳与圆柱形冲击物的数值模型并进行数值分析,总结了网壳结构的4种失效模式。通过对失效全过程的分析,从能量的角度将失效过程分为能量施加、能量传递与损失、能量消耗3个阶段。之后分别从能量传递与杆件破坏形式2方面揭示了网壳的失效机理。能量分析表明:剩余能量(Elf)对结构最终动力响应及失效模式起决定作用,而Elf只是初始冲击能量中除去冲击物穿透损失与网壳局部破坏损失后的剩余部分。通过对杆件破坏形式的分析发现:杆件的破坏可能滞后于冲击荷载的作用,且杆件的破坏形式决定其传递能量的能力,当杆件发生拉伸破坏时,其强度被充分利用,传递的能量最多,Elf值较大,网壳整体破坏严重。杆件的破坏形式与Elf及网壳整体的失效模式间有很好的对应关系。     

Constitutive relationship of brittle rock subjected to dynamic uniaxial tensile loads with microcrack interaction effects

A micromechanical model is proposed to describe both stable and unstable damage evolution in microcrack-weakened brittle rock material subjected to dynamic uniaxial tensile loads. The basic idea of the present model is to classify the constitution relationship of rock material subjected to dynamic uniaxial tensile loads into four stages including some of the stages of linear elasticity, pre-peak nonlinear hardening, rapid stress drop, and strain softening, and to investigate their corresponding micromechanical damage mechanisms individually. Special attention is paid to the transition from structure rearrangements on microscale to the macroscopic inelastic strain, to the transition from distribution damage to localization of damage and the transition from homogeneous deformation to localization of deformation. The influence of all microcracks with different sizes and orientations are introduced into the constitutive relation by using the statistical average method. Effects of microcrack interaction on the complete stress-strain relation as well as the localization of damage for microcrack-weakened brittle rock material are analyzed by using effective medium method. Each microcrack is assumed to be embedded in an approximate effective medium that is weakened by uniformly distributed microcracks of the statistically-averaged length depending on the actual damage state. The elastic moduli of the approximate effective medium can be determined by using the dilute distribution method. Micromechanical kinetic equations for stable and unstable growth characterizing the ‘process domains’ of active microcracks are taken into account. These ‘process domains’ together with ‘open microcrack domains’ completely determine the integration domains of ensemble averaged constitutive equations relating macro-strain and macro-stress. Theoretical predictions have shown to consistent with the experimental results.     

Analysis of the localization of damage and the complete stress-strain relation for mesoscopic heterogeneous brittle rock subjected to compressive loads

A micromechanics-based model is established. The model takes the interaction among sliding cracks into account, and it is able to quantify the effect of various parameters on the localization condition of damage and deformation for brittle rock subjected to compressive loads. The closed-form explicit expression for the complete stress-strain relation of rock containing microcracks subjected to compressive loads was obtained. It is showed that the complete stress-strain relation includes linear elasticity, nonlinear hardening, rapid stress drop and strain softening. The behavior of rapid stress drop and strain softening is due to localization of deformation and damage. Theoretical predictions have shown to be consistent with the exoerimental results.     

含缝拱坝强震失效破坏振动台试验研究

为探究强震时含缝拱坝结构的承载能力及失效路径,在振动台上开展了一系列拱坝模型强震失效破坏试验.基于弹性力-重力准则设计试验,分别进行了不分缝、只有横缝、诱导缝与横缝联合作用这三种情况的振动台模型试验,试验中考虑了材料率相关性的影响,忽略了库水-拱坝相互作用.结果表明,强震作用时由于设缝处的抗拉能力较低,导致缝位置首先发...     

Study on catastrophic mechanism for rotor drop transient vibration following magnetic bearing failure

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Response of an infinite beam resting on a tensionless elastic foundation subjected to arbitrarily complex transverse loads

This paper presents an investigation into the static response of an infinite beam supported on a unilateral (tensionless) elastic foundation and subjected to arbitrary complex loading, including self-weight. A new numerical method is developed to determine the initially unknown lengths that remain in contact. Based on the continuity conditions at the junctions of contact and non-contact segments, the response of the whole beam may be expressed through the displacement constants of the initial segment, reducing the contact problem to two nonlinear algebraic equations with two unknowns. The technique has been named the transfer displacement function method (TDFM). Comparison with the exact results of a particular limiting case shows the expected complete agreement. Finally, an example of a beam with several contact segments is presented and verified by the application of equilibrium conditions.     

爆炸载荷作用下加筋板的失效模式分析及结构优化设计

通过对爆炸载荷下具有1根加强筋的固支矩形加筋板的有限元模拟,探讨了抗爆加筋板结构优化设计方法,分析了加筋板的失效模式以及加强筋相对刚度和冲击载荷强度对加筋板失效模式的影响,指出了失效模式Ⅰ下的3种变形模态以及失效模式Ⅱ下的2种子失效模式,得到了失效模式Ⅰ下加强筋和加筋板最大挠度的近似计算公式,提出了单根加筋板的两种失效模式的判别条件,并对具有1根加强筋的固支矩形加筋板抗爆结构进行了优化设计。结果表明,通过数值模拟或模型实验可以求得任意加筋板结构由发生塑性大变形到发生破损的临界条件,从而确定抗爆性能最强时加筋板的质量与加强筋横截面尺寸及间距间的关系,实现对抗爆加筋板结构的优化设计。     

火炮制退机节制环失效微观机理的实验研究

为了研究火炮制退机节制环的故障原因和失效机理,基于金属腐蚀磨损理论和微观分析技术,综合运用光学金相显微镜分析、扫描电镜及能谱分析、显微硬度测定、电感耦合等离子体质谱法和火焰原子吸收光谱法等分析测试手段,结合报废节制环和从大修工厂待修火炮上采集的驻退液样本,从微观层面分析了节制环失效机理。研究表明,节制环作为制退机产生后坐阻力的关键部件之一,主要受到冲击磨损、化学腐蚀和气蚀破坏的复合作用而失效。     

弹体对混凝土材料先侵彻后爆炸损伤破坏效应的数值模拟研究

基于Kong-Fang混凝土材料模型和LS-DYNA的流固耦合和重启动算法,开展了某新型钻地武器先侵彻后爆炸对混凝土靶体的毁伤破坏效应研究。通过模拟大口径缩比弹侵彻实验和预制孔爆炸实验,验证了材料模型及其参数的可靠性。在此基础上,进一步对预制孔装药爆炸建模、不考虑弹壳的重启动建模和考虑弹壳的重启动建模3种方法进行了比较。数值计算结果表明,由于爆轰产物的外泄,不考虑侵彻预损伤的预制孔装药爆炸方法得到的爆坑直径仅为3倍弹径,且损伤破坏模式与其他2种方法得到的损伤破坏模式区别较大。重启动建模方法继承了弹体侵彻过程中累积的损伤,爆坑直径在原有侵彻损伤破坏的基础上明显增大;且由于弹壳变形破碎消耗部分能量,考虑弹壳时模拟得到的爆坑直径（约14.5倍弹径）略小于不考虑弹壳时模拟得到的爆坑直径（约16倍弹径）;但由于破碎弹头的二次侵彻作用,考虑弹壳时模拟得到的爆坑深度比不考虑弹壳时模拟得到的爆坑深度增加约5%。上述研究结果可为进一步开展钻地武器先侵彻后爆炸毁伤破坏效应的实验研究提供参考。     

平面应力条件下闭合断续节理岩体力学特性试验研究

通过平面应力条件下闭合断续节理岩体的破坏试验,采用自行研制的高压柔性加载装置施加垂向和侧向荷载,用千分表和激光散斑照相技术量测试体位移场,用应变花量测试体平面应力场． 结果表明,激光散斑法用于测定闭合断续节理岩体的位移场效果良好,其测值比对应点千分表测值偏小１４％ 左右．据所测得的位移场和应力场,研究了裂纹尖端应力场的演变及裂纹扩展的路径,分析了试体的变形和破坏特征,测定了翼裂纹初裂角,探讨了试体的破坏机理．     

应用三维有限元法探讨RC梁柱节点的破坏机理

为了探究节点加强后RC梁柱节点的抗剪承载力、延性提高以及节点破坏模式由剪切破坏转变为梁弯曲破坏的实质,本文在RC节点的三维有限元分析的基础上探讨RC梁柱节点加强后的破坏机理。通过详细考察加固前后节点内部不同位置混凝土的应力-应变发展规律,探讨加强前后节点混凝土的宏观损伤发展过程;同时,通过考察核心混凝土的应力-应变关系及发展过程定量探讨加强钢板、梁柱主筋及箍筋对核心混凝土的约束作用。基于上述混凝土损伤过程的宏观分析,可以得出由于梁主筋的粘结加强、加强钢板以及箍筋对混凝土约束作用,使节点核心混凝土的实际强度增大、损伤延迟,RC节点由加强前的节点剪切破坏模式转变成梁端的弯曲破坏模式,从而提高RC梁柱节点的抗剪承载力和延性。     

Determination of the first ply failure load for a cross ply laminate subjected to uniaxial tension through computational micromechanics

A method for determining the in situ strength of fiber-reinforced laminas for three types of transverse loading including compression, tension and shear is presented. In the framework of this method, an analysis of local stresses that are responsible for the coalescence of matrix cracks is carried out by using a multi-fiber unit cell model and finite element method. The random distribution of fibers, fiber–matrix decohesion and matrix plastic deformations are taken into account in the micromechanical simulations. The present study also shows that the nonlinear hardening behavior of matrix reflects more realistically the influence of plastic deformations on the in situ transverse strength of lamina than the perfectly plastic behavior of matrix. The prediction of the in situ transverse strength is verified against the experimental data for a cross ply laminate subjected to uniaxial tension.     

舱内爆炸载荷及舱室板架结构的失效模式分析

通过对典型半穿甲导弹打靶实验中舰艇结构破坏模式的观察,结合数值模拟,分析了舱内爆炸载荷的特征以及舱内爆炸下舱室板架结构的失效模式。结果表明,舱内爆炸下,舱室板架结构承受的冲击载荷及失效模式与敞开环境爆炸下加筋板结构承受的冲击载荷及失效模式有较大区别,其动态响应难以用敞开环境爆炸下加筋板结构的动态响应描述;舱内爆炸载荷主要有壁面反射冲击波、角隅汇聚冲击波以及准静态气体压力,其中两壁面和三壁面角隅汇聚冲击波的强度分别为相同部位壁面反射冲击波强度的5倍和12倍以上;舱室板架结构主要有4种失效模式,其中模式Ⅲ、Ⅳ较常发生;舱室板架结构加强筋布置在迎爆面将使板架中部的局部破坏程度增加,但有利于削弱角隅汇聚冲击波强度,减小板架沿角隅部位的撕裂破坏。     

大型舰船在水下接触爆炸下的毁伤与防护研究综述

大型舰船受到水中兵器的巨大威胁,尤其是在水下接触爆炸情况下,船体结构将产生严重的局部毁伤,给舰船的战斗力乃至生命力带来严峻挑战。本文以大型舰船水下防护结构为研究对象,简要概述了各国海军大型舰船水下防护结构形式的发展历程,分析了水下接触爆炸下的毁伤载荷以及对舷侧多舱防护结构的毁伤机理,总结了基于具体结构和不同毁伤元的防护措施;并针对目前的研究现状,提出了有待进一步研究的问题。以期为舰船的水下防护设计提供参考,从而提高我国大型舰船的结构抗毁伤能力

     

应用混合变量余能原理求解不同载荷作用下四边简支矩形板的弯曲

由功的互等定理导出了弯曲矩形板混合变量的总余能,应用混合变量余能原理求解四边简支矩形板弯曲问题,给出了四边简支矩形板的混合变量余能表达式,求解了不同载荷作用下四边简支矩形板的弯曲问题,并将计算的不同载荷作用下的挠度和弯矩值与ANSYS有限元的计算值比较,给出了一种求解四边简支矩形板的新方法。     

单轴压缩条件下溶蚀礁灰岩细观变形破坏特征研究

礁灰岩的溶蚀对海洋工程和岛礁工程的稳定性有明显的影响.孔隙型溶蚀是礁灰岩常见的溶蚀方式.本文通过构建随机溶蚀孔洞的离散元模型,模拟孔洞型溶蚀礁灰岩的细观变形破坏特征,分析其变形破坏规律与裂隙演化特征.结果 表明,基于随机聚类算法构建的溶蚀礁灰岩离散元模型能够很好地模拟孔洞型溶蚀作用,且其变形破坏特征具有明显的规律;随着...     

饱水对煤层顶板砂岩单轴压缩破坏能量影响的分析

为研究饱水对砂岩力学参数和能量特征的影响,利用RMT-150B岩石力学系统对煤层顶板砂岩自然和饱水试样进行单轴压缩试验。试验结果表明:饱水对砂岩试样的强度和变形参数均有不同程度的影响,软化系数为0.79,弹性模量降幅为5.25%,变形模量降幅为5.92%;饱水后砂岩试样峰值前吸收能量、可释放弹性能和耗散能均有不同程度降低,吸收能量降幅36.8%,可释放弹性能降幅为34.4%,耗散能降幅为57.7%;饱水后砂岩储蓄能量的能力有较大减弱,脆性减弱,塑性明显增强;饱水砂岩试样压缩过程中积蓄可释放能难以使试样滑移破坏,仍需要吸收部分能量使试样逐步失稳破坏;饱水对砂岩试样压缩过程吸收能量、可释放能量比例关系的影响较小,而对耗散能比例关系的影响较大;自然状态下砂岩试样峰值前相同应变条件下吸收能量、可释放能均明显高于饱水试样对应能量值;深部巷道位置确定和支护设计时应充分考虑水对巷道围岩弱化的影响,对于完整坚硬围岩采用高压注水软化可有效防止冲击地压发生和减缓灾害程度。     

风荷载作用下上海地区外墙外保温系统性能研究

高层建筑随着高度的增加受到风荷载值也增加,外墙外保温系统受到风荷载影响亦增加.外墙外保温系统在交付后,开裂、脱落、渗水现象时有发生,严重时会造成脱落,存在安全质量隐患.本文根据上海地区外墙外保温的实际情况,对外墙外保温系统的风荷载设计值、空腔风压力损失、粘结力等进行计算,综合分析了风荷载对外墙外保温系统的影响及破坏机理,并提出了针对性的措施.     

双波护栏-汽车碰撞动力响应与吸能机理研究

我国西部山岭重丘区山高谷深,高速公路跨线天桥、弯道及坡道较多,这些特殊路段的防撞设施多采用立柱加密的双波护栏.通过ANSYS/LS-DYNA有限元数值仿真,对加密立柱双波护栏在汽车碰撞作用下的动力响应进行了研究,分析了双波护栏的碰撞能量转换与吸能机理,结果表明加密立柱的双波护栏对车辆碰撞具有良好的吸能、导向作用.