基于Voronoi建模和Cohesive单元的PBX细观力学模型

针对PBX的细观力学模型存在模型结构与真实PBX细观形貌不符、破坏行为描述不全面(仅考虑界面脱粘)、研究对象单一(大多针对PBX-9501)等问题,建立了基于Voronoi建模和Cohesive单元的PBX细观力学模型。首先,基于光学显微镜下的细观结构图,采用Voronoi方法构建了更贴近实际形貌的PBX炸药细观模型;然后,采用内聚力有限元方法在炸药晶体内部、黏结剂内部及炸药颗粒/黏结剂界面处都引入了Cohesive单元,以在二维尺度上实现任意路径破坏行为的模拟;最后,针对某HMX基-F2311黏结剂炸药和某HMX基-F2313黏结剂炸药标定了模型参数,模拟了这两种炸药的准静态拉伸行为。结果表明,两种炸药数值模拟的拉伸变形曲线、破坏应力、破坏应变都与试验数据吻合良好,表明该模型不仅适用于不同PBX炸药材料(线弹性PBX、非线性PBX)的变形行为描述,还可以分析PBX炸药在拉伸破坏过程中的细观机理,刻画不同PBX炸药的裂纹萌生、扩展和贯穿的过程。

Finite Element Simulations of the Localized Failure and Fracture Propagation in Cohesive Materials with Friction

Strain localization frequently occurs in cohesive materials with friction(e.g.,composites,soils,rocks)and is widely recognized as a fundamental cause of progressive structural failure.Nonetheless,achieving high-fidelity simulation for this issue,particularly concerning strong discontinuities and tension-compression-shear behaviors within localized zones,remains significantly constrained.In response,this study introduces an integrated algorithmwithin the finite element framework,merging a coupled cohesive zone model(CZM)with the nonlinear augmented finite elementmethod(N-AFEM).The coupledCZMcomprehensively describes tension-compression and compressionshear failure behaviors in cohesive,frictional materials,while the N-AFEM allows nonlinear coupled intraelement discontinuities without necessitating extra nodes or nodal DoFs.Following CZM validation using existing experimental data,this integrated algorithm was utilized to analyze soil slope failure mechanisms involving a specific tensile strength and to assess the impact of mechanical parameters(e.g.,tensile strength,weighting factor,modulus)in soils.

Investigation of FRP and SFRC Technologies for Efficient Tunnel Reinforcement Using the Cohesive Zone Model

Amid urbanization and the continuous expansion of transportation networks,the necessity for tunnel construction and maintenance has become paramount.Addressing this need requires the investigation of efficient,economical,and robust tunnel reinforcement techniques.This paper explores fiber reinforced polymer(FRP)and steel fiber reinforced concrete(SFRC)technologies,which have emerged as viable solutions for enhancing tunnel structures.FRP is celebrated for its lightweight and high-strength attributes,effectively augmenting load-bearing capacity and seismic resistance,while SFRC’s notable crack resistance and longevity potentially enhance the performance of tunnel segments.Nonetheless,current research predominantly focuses on experimental analysis,lacking comprehensive theoretical models.To bridge this gap,the cohesive zone model(CZM),which utilizes cohesive elements to characterize the potential fracture surfaces of concrete/SFRC,the rebar-concrete interface,and the FRP-concrete interface,was employed.A modeling approach was subsequently proposed to construct a tunnel segment model reinforced with either SFRC or FRP.Moreover,the corresponding mixed-mode constitutive models,considering interfacial friction,were integrated into the proposed model.Experimental validation and numerical simulations corroborated the accuracy of the proposed model.Additionally,this study examined the reinforcement design of tunnel segments.Through a numerical evaluation,the effectiveness of innovative reinforcement schemes,such as substituting concrete with SFRC and externally bonding FRP sheets,was assessed utilizing a case study from the Fuzhou Metro Shield Tunnel Construction Project.

Extended finite element-based cohesive zone method for modeling simultaneous hydraulic fracture height growth in layered reservoirs

In this study,a fully coupled hydromechanical model within the extended finite element method(XFEM)-based cohesive zone method(CZM)is employed to investigate the simultaneous height growth behavior of multi-cluster hydraulic fractures in layered porous reservoirs with modulus contrast.The coupled hydromechanical model is first verified against an analytical solution and a laboratory experiment.Then,the fracture geometry(e.g.height,aperture,and area)and fluid pressure evolutions of multiple hydraulic fractures placed in a porous reservoir interbedded with alternating stiff and soft layers are investigated using the model.The stress and pore pressure distributions within the layered reservoir during fluid injection are also presented.The simulation results reveal that stress umbrellas are easily to form among multiple hydraulic fractures’tips when propagating in soft layers,which impedes the simultaneous height growth.It is also observed that the impediment effect of soft layer is much more significant in the fractures suppressed by the preferential growth of adjoining fractures.After that,the combined effect of in situ stress ratio and fracturing spacing on the multi-fracture height growth is presented,and the results elucidate the influence of in situ stress ratio on the height growth behavior depending on the fracture spacing.Finally,it is found that the inclusion of soft layers changes the aperture distribution of outmost and interior hydraulic fractures.The results obtained from this study may provide some insights on the understanding of hydraulic fracture height containment observed in filed.

基于XFEM与Cohesive模型分析PBX裂纹产生与扩展

利用扩展有限元法(XFEM)分析PBX-9502带孔板状试件在整体压缩下由局部裂纹萌生到裂纹扩展全过程的开裂破坏机理。采用应力状态相关的强度面、非关联流动法则及Cohesive模型,描述了材料在复杂应力状态下的非线性本构行为以及材料的破坏行为。进行了数值模拟结果与美国洛斯阿拉莫斯国家实验室(LANL)试验结果的对比。结果表明,含孔洞的平板在整体压应力环境下孔洞周围产生局部拉伸应力,这种拉伸条件导致局部裂纹萌生。数值模拟的裂纹发展趋势与试验结果相吻合,包括裂纹时程的整体走势和拐点、启裂时刻、裂纹初期扩展速度等。基于扩展有限元方法和内聚模型法,可模拟高聚物粘结炸药(PBX)含能材料的裂纹萌生、扩展。

基于Cohesive单元法的弃置井水泥塞-套管界面胶结失效数值模拟

水泥塞-套管界面胶结失效对弃置井井筒完整性提出了严峻挑战。考虑地下流体与水泥塞的流固耦合作用,基于cohesive单元方法,建立水泥塞-套管-水泥环-地层系统三维有限元模型,模拟垂直井水泥塞-套管界面裂缝剥离过程,研究地应力对界面裂缝损伤演化的影响,并分析水平地应力、水泥塞力学参数及界面性质对裂缝剥离高度的影响。结果显示:水平地应力均匀时剥离裂缝沿着整个界面圆周延伸且高度相等,水平地应力非均匀时剥离裂缝倾向于沿着界面某一圆周角扩展且在最大水平地应力方向具有较大的高度;水泥塞弹性模量从30 GPa减小到1 GPa,裂缝剥离高度降低9.3 m,临界法向强度从0.25 MPa增大到2.0 MPa,裂缝剥离高度降低6.5 m,表明较低的弹性模量及较大的临界法向强度有利于减小水泥塞-套管界面胶结失效的风险;水泥塞泊松比从0.35减小到0.10,裂缝剥离高度仅降低2.0 m,临界剪切强度从0.5 MPa增加到4.0 MPa,裂缝剥离高度仅降低3.3 m,表明泊松比和临界剪切强度对界面胶结失效影响较小。建立的模型能够为水泥浆配方优选和井筒弃置工艺优化提供指导。

Numerical study of fatigue damage of asphalt concrete using cohesive zone model

In order to investigate the fatigue behavior of asphalt concrete, a new numerical approach based on a bi-linear cohesive zone model （CZM） is developed. Integrated with the CZM, a fatigue damage evolution model is established to indicate the gradual degradation of cohesive properties of asphalt concrete under cyclic loading. Then the model is implemented in the finite element software ABAQUS through a user-defined subroutine. Based on the proposed model, an indirect tensile fatigue test is finally simulated. The fatigue lives obtained through numerical analysis show good agreement with laboratory results. Fatigue damage accumulates in a nonlinear manner during the cyclic loading process and damage initiation phase is the major part of fatigue failure. As the stress ratio increases, the time of the steady damage growth stage decreases significantly. It is found that the proposed fatigue damage evolution model can serve as an accurate and efficient tool for the prediction of fatigue damage of asphalt concrete.

基于Cohesive单元的石拱桥主拱圈二相数值模拟方法

为解决石砌体材料非均质的描述问题,提出一种基于Cohesive单元的石拱桥主拱圈两相数值模拟方法。视石砌体为两相材料(砌块和砌缝),采用实体单元模拟砌块并引入非线性本构描述其破坏行为,在相邻砌块间插入Cohesive单元考虑砌缝砂浆的剪切和拉伸破坏。通过室内试验与数值模拟对比验证方法的有效性及适用性,分析了砌缝抗剪摩擦系数μ、加载位置等敏感参数对拱桥承载力的影响。结果表明:基于Cohesive单元的石砌体两相数值模型,可以有效描述石砌体材料的非均匀性及石拱桥的破坏过程(尤其是砌缝剪切滑移破坏行为),可为石拱桥极限承载力评估提供重要信息,如荷载-位移曲线、破坏模式等。此外,研究结果还发现主拱圈破坏机制由拱的受弯、受剪特性决定,并与砌缝抗剪摩擦系数μ强相关。

Survey Research into Cohesive Device Misuses in Business English Writing

Cohesive devices in students’business English writing are regarded as the object of the research.Based on Haliday and Hasan’s cohesion theory,this paper introduces commonly-used cohesive devices in English writing.With the method of quantitative data,use and misuse frequency of cohesive devices in students’writings can be known.The paper will also analyze why misuses happen through qualitative data analysis and explore right ways of using cohesive devices.

Analysis on the Cohesive and Coherent Features of John Kennedy's First Inaugural Address

John Kennedy's first inaugural address is one of the widely appreciated speeches worldwide. It is famous not only for calling up the American people to well serve the country, but also for its extraordinary linguistic power to arouse the listeners' emotions, which lies to a great extent in the marvelous employment of the cohesive and coherent devices in the process of its delivery. Cohesion and coherence are two elementary and significant concepts in the theoretical system of discourse analysis. There-fore, they play an important role in the structuring, arrangement, interpretation and analysis of a discourse. In this sense, it is significant to analyze the cohesive and coherent features of John Kennedy's first inaugural address in order to obtain a penetrating comprehension of the speech in many aspects. A detailed analysis on the cohesive and coherent features of the speech has been conducted in this paper. In the aspect of cohesion in the address, the devices employed fall into two categories: structural cohesion and non-structural cohesion. Structural cohesive devices used in the discourse are mainly grammatical cohesion and lexical cohesion like repetition, ellipsis, conjunction, etc. Non-structural methods adopted in the speech are transitivity, mood and modality, thematic progression, parallel structure and so on. In the aspect of coherence, five levels of coherent methods have been employed, namely, lexical level, syntax level, semantic level, phonological level and social semiotic level. The neat intermingling of the cohesive and coherent methods function cooperatively and lead to the smooth going of the text.

界面应力传递重新分析及Cohesive模型参数的确定

在经典剪滞理论中引入双线性cohesive模型表征纤维/基体之间的非理想界面,重新分析了纤维增强复合材料中的应力传递机理,得到了考虑界面因素的应力分布。用上述结果解释了单丝段裂实验过程中的现象,讨论了界面参数和材料性能对应力分布的影响。基于上述理论,建立了用cohesive单元表征界面的模拟单丝段裂实验的三维有限元模型,结合单丝段裂实验结果,提出了一种估测cohesive界面刚度参数的新方法。数值和理论分析结果与实验结果对比,吻合良好,可以为材料的界面性能分析和材料设计提供参考依据。

基于cohesive单元海域天然气水合物储层水力压裂模拟

天然气水合物作为一种高效清洁能源,广泛分布于我国南海海域的沉积地层中。我国先后于2017年和2020年成功开展了2次试开采,但由于海域天然气水合物特殊的赋存条件,单井水合物试采仍然面临着开采范围小、高产稳产时间短等问题。为了提高水合物的开采范围,基于cohesive单元进行了水合物储层二维水力压裂数值模型研究,比较了100 m×100 m和20 m×20 m两种模型的裂缝半长和宽度,得出了当注入压力为25 MPa时,压裂裂缝半长均为6 m,最大宽度分别为5.8、5.5 mm,构建尺寸较大的模型得出的实验结果更加准确。并且研究了裂缝宽度随注入时间的变化规律,随着注入压力和注入量的不断增加,初期裂缝宽度急速变大,后续在地应力和注入流体压力的共同作用下裂缝出现“阶梯式”的扩展规律。该研究在页岩气和煤层气等非常规能源储层水力压裂模型分析中得到了成功运用,为海域天然气水合物储层水力压裂提供一定的理论指导。

COHESIVE ZONE FINITE ELEMENT-BASED MODELING OF HYDRAULIC FRACTURES

Hydraulic fracturing is a powerful technology used to stimulate fluid production from reservoirs. The fully 3-D numerical simulation of the hydraulic fracturing process is of great importance to the efficient application of this technology, but is also a great challenge because of the strong nonlinear coupling between the viscous flow of fluid and fracture propagation. By taking advantage of a cohesive zone method to simulate the fracture process, a finite element model based on the existing pore pressure cohesive finite elements has been established to investigate the propagation of a penny-shaped hydraulic fracture in an infinite elastic medium. The effect of cohesive material parameters and fluid viscosity on the hydraulic fracture behaviour has been investigated. Excellent agreement between the finite element results and analytical solutions for the limiting case where the fracture process is dominated by rock fracture toughness demonstrates the ability of the cohesive zone finite element model in simulating the hydraulic fracture growth for this case.

Rheological Properties and Incipient Motion of Cohesive Sediment in the Haine Estuary of China

The Haihe cohesive sediment, which is typical in China, is studied systematically for its basic physical and incipient motion properties. Following the requirements of dredging works in the Haihe Estuary, cohesive sediment samples were taken from three locations. Laboratory experiments were conducted to determine the rheological properties of these samples and to examine the incipient motion of the cohesive sediment. It is found that the cohesive sediment has an obvious yield stress tau(b), which increases with the mud density in a manner of an exponential function, and so does the viscosity parameter eta. The cohesive sediment behaves like a Bingham fluid when its density is below 1.38 similar to 1.40 g/cm(3), and when denser than these values, it may become a power-law fluid. The incipient motion experiment also revealed that the incipient velocity of the cohesive sediment increases with die density in an exponential manner. Therefore, the incipient motion is primarily related to the density, which is different from the case for non-cohesive sediment in which the incipient motion is con-elated with the diameter of sand particles instead. The incipient motion occurs in two different ways depending on the concentration of mud in the bottom. For sufficiently fine particles and a concentration lower than 1.20 g/cm(3), the cohesive sediment appears as fluidized mud, and the incipient motion is in the form of instability of an internal wave. For a higher concentration, the cohesive sediment appears as general quasi-solid-mud, and the incipient motion can be described by a series of extended Shields curves each with a different porosity for newly deposited alluvial mud.

Wave-induced pore water pressure in marine cohesive soils

Cyclic triaxial tests and numerical analyses were undertaken, in order to evaluate the wave-induced pore water pressure in seabed sediments in the Hangzhou Bay. The cyclic triaxial tests indicate that the rate of pore water pressure generation in cohesive soils decreases with time, and the development of the pore water pressure can be represented by a hyperbolic curve. Numerical analyses, taking into account the generation and dissipation of pore water pressure simultaneously, suggest that the pore water pressure buildup in cohesive soils may increase with time continuously until the pore water pressure ratio approaches to 1, or it may decrease after a certain time, which is controlled by drain conditions. These phenomena are different from those in sands. For waves with a retum period of 100 a in the Hangzhou Bay, if the wave duration is more than 60 h, then the pore water pressure ratio will be close to 1 and soil fabric failure will take place.

Maximum Force of Inclined Pullout of A Torpedo Anchor in Cohesive Beds

Torpedo anchors have been used in mooring systems for deep-water oil and gas projects owing to their prominent advantages, such as low cost and easy installation. The maximum force of torpedo anchors is crucial not only to the safety and stability of vessels and other marine facilities, but also for an economical design. It is necessary to develop reliable formula for fast predicting their maximum inclined force of a torpedo anchor in cohesive beds. In this study, the maximum inclined force of a torpedo anchor vertically embedded in cohesive beds was extensively investigated. 316 sets of inclined pullout laboratory tests were carried out for 9 differently shaped torpedo anchors which were vertically embedded in different cohesive beds. The loading curves were automatically acquisitioned and their characteristics were analyzed. The load angle relative to the horizontal varied from 20° to 90°. A new formula for fast calculating the maximum inclined force of the torpedo anchor vertically embedded in cohesive beds was obtained based on force analysis and a nonlinear regression on the data from the present and other studies. Effect aspects on the tests are discussed and further studies are highlighted.

Utilization of soil nailing technique to increase shear strength of cohesive soil and reduce settlement

This article deals with the assessment of the soil nailing technique with a vertical inclusion to improve the geotechnical parameters of cohesive soil. A series of unconfined compression tests and direct shear tests were carried out to establish the stressestrain relationship and strength characteristics of the reinforced clay sample by vertical steel nails. The shear strength performance of the new composite material was tested by varying the number of vertical inclusions, the embedment depth and the alignment radius. The results confirmed that the vertical bars/inclusions shared the vertical applied load with clay. Increase in the number of vertical inclusions significantly increases the shear strength and the stiffness with a remarkable reduction in settlement. When the clay samples were reinforced with six inclusions along the perimeter, the shear strength was increased to 231% for the embedment depth ratio equal to 0.85. To obtain the optimum effect in eliminating shear failure, the vertical inclusions should be extended to a deeper zone with sufficient numbers. It has been found that the vertical inclusions significantly influence the shear strength, and the brittle or general shear failure of the unreinforced sample can be diverted to partial/plastic shear failure.

A Study of Cohesive Devices in English IM

The main purpose of the present research is to reveal features of the use of cohesive devices in English IM(Instant Message).In order to be accurate and explicit,Oxford WordSmith Tools(Version 4.0) is applied to investigate the frequency of each type of cohesive devices used by online-communicators.Cohesion in English IM is characterized by a number of distinguishing features:1) this genre of discourse makes abundant use of cohesive ties such as personal reference,lexical reiteration and ellipsis;2) substitution and comparative reference are rarely applied;3) The variety of conjunctions used is limited,and the most frequently used conjunctions are and,so,but and or.In analyzing the cohesive effect of each cohesive device,it is held that in order to overcome the limitations of the messaging systems and to achieve smooth communication,participants mainly adopt explicit cohesive ties which could achieve stronger force of cohesion in IM communication.

Multiscale modeling of heterogeneous propellants from particle packing tograin failure using a surface-based cohesive approach

In the present work, a computational frame- work is established for multiscale modeling and analysis of solid propellants. A packing algorithm, considering the am- monium perchlorate （AP） and aluminum （A1） particles as spheres or discs is developed to match the size distribution and volume fraction of solid propellants. A homogenization theory is employed to compute the mean stress and strain of a representative volume element （RVE）. Using the mean results, a suitable size of RVE is decided. Without consider- ing the interfaces between particles and matrix, several nu- merical simulations of the relaxation of propellants are per- formed. The relaxation effect and the nonlinear mechanical behavior of propellants which are dependent on the applied loads are discussed. A new technology named surface-based cohesive behavior is proposed to describe the phenomenon of particle dewetting consisting of two ingredients： a damage initiation criterion and a damage evolution law. Several ex- amples considering contact damage behavior are computed and also nonlinear behavior caused by damaged interfaces is discussed in this paper. Furthermore the effects of the criti- cal contact stress, initial contact stiffness and contact failure distance on the damaged interface model have been studied.

Modeling shale with consideration of bedding planes by cohesive finite element method

Shale contains distributed directional bedding planes,which make the shale transverse isotropic.To model shale with consideration of the bedding planes,a cohesive finite element method(CFEM)is developed based on the randomized triangular mesh.The interface orientation generated from such mesh tends to be uniformly distributed with the element number increasing.To represent the bedding plane,the interfaces aligned with the bedding plane are assigned the cohesive law that characterizes the bedding plane while the other interfaces are assigned the cohesive law that characterizes the matrix.By this means,the anisotropy characteristics of the stiffness and the strength of shale are well represented.The simulation examples demonstrate that the bedding plane has a significant influence on the fracture trajectory,which is consistent with the observation in the experiment.It is suggested that this modeling method of shale is feasible.It provides an alternative approach to fracture simulation in shale.