页岩气高效开采中钻井完井和水力压裂的关键力学问题

本文研究了在页岩气高效开采中钻井完井和水力压裂缝网改造的关键力学问题. 提出了页岩多孔弹性介质的本构、强度和断裂韧性的各向异性模型, 指出了微观均匀假设与微观各向同性假设在页岩多孔弹性本构中的简化应用, 给出了横观各向同性多孔弹性岩石材料常数的简化测量方案, 讨论了基于修正的能量最大释放率下裂纹扩展的弱面模型, 提出了裂纹扩展禁止区现象. 阐述了钻井完井过程中的多孔弹性介质井壁稳定性和剪切破坏的时间效应, 提供了多种破坏模式下井壁许可压力范围的显式表达式, 并与传统广义胡克定律得到结果对比. 给出了水平井水力压裂缝网改造技术, 包含水力压裂的大物模实验技术、水力压裂过程中基于扩展有限元与有限体积法的耦合流体/固体/裂缝扩展的数值模拟方法, 并与黏性主导的水力压裂解析解结果对比, 针对性分析了川渝地区的水力压裂施工现场实践应用问题. 建立了基于数据驱动的页岩气采收率预测方法, 指出了机器学习中极限梯度爬升法在小数据集情况下的有效性.      

FRP-混凝土界面粘结行为的参数影响研究

FRP-混凝土界面的粘结性能对FRP加固混凝土结构力学行为和破坏模式有着重要影响。本文对表征FRP-混凝土界面粘结性能的三个重要参数(界面初始刚度、最大剪应力、界面破坏能)开展研究,通过13个单剪试件的试验考察了混凝土强度、胶层厚度和粘结长度等因素对界面粘结行为的影响,根据试验结果拟合了界面破坏能、最大剪切应力与胶层剪切刚度、混凝土强度之间的函数关系。在试验研究基础上,构建了外贴FRP-混凝土界面粘结的有限元模型。通过有限元分析考察了界面破坏能等三个参数不变的前提下,不同的局部粘结滑移本构关系对界面粘结行为的影响;进而研究了其中一个参数变化时引起的界面粘结性能改变。研究结果表明:界面粘结承载力随着胶层厚度增加而逐渐提高;胶层厚度与界面破坏能成正比,与峰值剪应力成反比;当界面破坏能等三个参数保持不变时,局部粘结滑移本构关系对FRP-混凝土界面粘结性能的影响较小;三个参数中的一个增大时将延缓界面破坏的过程。     

加载角度对层理页岩裂纹扩展影响的实验研究

采用分离式霍普金森压杆（SHPB）系统对页岩进行冲击实验,研究层理角度对页岩动态断裂过程的影响,在裂尖设置裂纹扩展计,借助高速摄影和数字图像相关（DIC）技术对页岩中心切槽半圆盘弯曲（NSCB）试件断裂的全过程进行研究,得到了不同加载角度下页岩的动态起裂韧度、裂纹扩展速度、断裂过程中应变场和水平位移场的变化规律。实验发现:不同加载角度下,页岩的动态起裂韧度具有显著的各向异性,加载角度与动态起裂韧度呈正相关;加载角度对试样的裂纹扩展速度具有显著影响,与裂纹扩展速度呈负相关;当冲击速度较低时,切槽方向是裂纹扩展的优势方向,而当冲击速度较高时,试样会产生沿层理弱面的次生裂纹,次生裂纹对试样的断裂具有显著影响。     

Progressive failure constitutive model of fracture plane in geomaterial based on strain strength distribution

Progressive failure constitutive model of fracture plane in geomaterial based on strain strength distribution is proposed. The basic assumption is that strain strength of geomaterial comply with a certain distribution law in space. Failure of tensile fracture plane and shear fracture plane in representative volume element (RVE) with iso-strain are discussed, and generalized failure constitutive model of fracture plane in RVE is established considering combined effect of tension and shear. Fracture plane consists of elastic microplanes and fractured microplanes. Elastic microplanes are intact parts of the fracture plane, and fractured microplanes are the rest parts of the fracture plane whose strain have ever exceeded their strain strength. Interaction mode on elastic microplanes maintains linear elasticity, while on fractured microplanes it turns into contact and complies with Coulomb’s friction law. Intact factor and fracture factor are defined to describe damage state of the fracture plane which can be easily expressed with cumulative integration of distribution density function of strain strength. Strong nonlinear macroscopic behavior such as yielding and strain softening can be naturally obtained through statistical microstructural damage of fracture plane due to distribution of strain strength. Elastic–brittle fracture model and ideal elastoplastic model are special cases of this model when upper and lower limit of distribution interval are equal.     

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.     

单轴压缩下页岩类脆性材料细观损伤演化的离散元分析

了解广泛存在的类似页岩的脆性材料各向异性对工程安全具有重要意义。本研究将页岩视为粘结颗粒材料,基于离散单元方法研究了横观各向同性脆性页岩的损伤演化。再现了不同层理角的页岩试样的破坏模式,并对比了实验和数值模拟的抗压强度和弹性模量。引入微裂纹的概念,通过定义裂纹密度函数,系统地研究了单轴压缩条件下,页岩层理角对细观结构的影响。此外基于平均配位数建立了配位数变化与细观损伤的联系,并根据配位数的变化与裂纹数量将加载过程分为三个阶段,分析了不同阶段配位数与裂纹数量的对应变化关系。研究表明,页岩的裂纹密度随着层理角的增加而增加,而试样的平均配位数在加载过程中先上升后剧烈下降,颗粒集合体在单轴压缩条件下的应力应变及裂纹数量曲线与平均配位数曲线有良好的一致性。该研究揭示了横观各向同性脆性岩石的破坏过程和内在机理,将为页岩类脆性材料的工程应用提供理论指导。     

Analysis of Damage and Failure in Anisotropic Ductile Metals Based on Biaxial Experiments with the H-Specimen

Background

Dependence of strength and failure behavior of anisotropic ductile metals on loading direction and on stress state has been indicated by many experiments. To realistically predict safety and lifetime of structures these effects must be taken into account in material models and numerical analysis.

Objective

The influence of stress state and loading direction on damage and failure behavior of the anisotropic aluminum alloy EN AW-2017A is investigated.

Methods

New biaxial experiments and numerical simulations have been performed with the H-specimen under different load ratios. Digital image correlation shows evolution of strain fields and scanning electron microscopy is used to visualize failure modes on fracture surfaces. Corresponding numerical studies predict stress states to explain damage and fracture processes on the micro-scale.

Results

The stress state, the load ratio and the loading direction with respect to the principal axes of anisotropy affect the width and orientation of localized strain fields and the formation of damage mechanisms and fracture modes at the micro-level.

Conclusions

The enhanced experimental program with biaxial tests considering different loading directions and load ratios is suggested for characterization of anisotropic metals.

     

页岩气高效开采的可压裂度和射孔簇间距预测

页岩储层的可压裂性是影响页岩气产量的关键因素. 本文基于断裂力学理论, 以高围压下岩石层理弱面的剪切破坏为主要研究对象, 依据岩石抗拉强度和层理弱面抗剪强度的比值关系, 首先提出了可压裂度的概念, 给出了无量纲的定性曲线图, 涵盖了岩石脆性矿物质含量, 粘性主导和韧性主导裂缝尖端流体压强、射孔簇分布间距的综合地质与工程因素. 接着提出了一种新的表征高围压下页岩可压裂度的无量纲参数, 在保证达到充分解吸附的最小压裂间距前提下, 依据该参数可计算水平井压裂中的射孔簇间距, 可作为工程参考指标. 本文将断裂力学理论结合水力压裂高效开采页岩气工程, 具有力学理论意义和工程应用前景.      

Mesoscale modeling of nonlinear elasticity and fracture in ceramic polycrystals under dynamic shear and compression

Dynamic deformation and failure mechanisms in polycrystalline ceramics are investigated through constitutive modeling and numerical simulation. Two ceramics are studied: silicon carbide (SiC, hexagonal crystal structure) and aluminum oxynitride (AlON, cubic crystal structure). Three dimensional finite element simulations incorporate nonlinear anisotropic elasticity for behavior of single crystals within polycrystalline aggregates, cohesive zone models for intergranular fracture, and contact interactions among fractured interfaces. Boundary conditions considered include uniaxial strain compression, uniaxial stress compression, and shear with varying confinement, all at high loading rates. Results for both materials demonstrate shear-induced dilatation and increasing shear strength with increasing confining pressure. Failure statistics for unconfined loading exhibit a smaller Weibull modulus (corresponding to greater scatter in peak failure strength) in AlON than in SiC, likely a result of lower prescribed cohesive fracture strength and greater elastic anisotropy in the former. In both materials, the predicted Weibull modulus tends to decrease with an increasing number of grains contained in the simulated microstructure.     

EXPERIMENTAL INVESTIGATIONS OF STRENGTH,TOUGHNESS AND FAILURE MECHANISM OF THE METAL/EPOXY/METAL ADHESIVE SYSTEM

本文系统地开展了金属/环氧/金属胶结体系的强韧机理及失效行为实验研究,针对铝合金圆棒与铝合金圆棒通过环氧树脂胶层的各种斜截面方向粘结,实验观测了该体系的拉伸变形和失效行为,测量了界面失效载荷对胶层厚度和粘结界面倾斜角的依赖关系;通过引入胶结界面平均正应力、平均剪应力、平均正应变、平均剪应变等概念,可对界面失效强度进行测量,获得界面强度与界面粘结角度以及胶层厚度的关系,进而获得了铝合金/环氧胶层/铝合金体系的强度失效面以及胶结界面的断裂能和胶结体系的能量释放率.上述研究结果为深入认识金属胶结体系的强韧性能和失效机制提供了科学依据,对金属胶结体系的优化设计和性能评判具有重要指导意义.研究结果表明,铝合金/环氧胶层/铝合金体系的拉伸失效总体呈弹脆性破坏特征,失效表现为胶层粘结界面的断裂,失效强度和界面断裂能在胶层厚度为百微米量级时表现出强烈的尺度效应：界面粘结强度随着胶层厚度的减小而显著增大,临界状态的平均正应力和平均剪应力在强度破坏面上近似位于同一圆上,界面断裂能随着胶层厚度的减小而显著减小;与此同时,界面失效强度和界面断裂能也密切依赖于界面粘结角度.     

SHPB冲击作用下层状千枚岩多尺度破坏机理研究

通过分离式霍普金森杆对层状千枚岩施加动态载荷,得到不同层理倾角下层状千枚岩的动态抗压强度与宏观破坏模式。采用三维激光仪获得断裂面细观形貌,引入分形几何定量计算断口面粗糙度;结合SEM观察到的微观尺度下不同层理倾角断口破坏机理,分析了不同层理倾角下层状岩石的动态破坏机制。研究结果表明:动态压缩下层理弱面对岩石的抗压强度影响较大;不同层理倾角千枚岩的断口形貌分形维数随层理倾角增大呈U型变化;从强度与裂纹扩展两方面考虑层理弱面对层状岩石破坏特征的影响,对于层理倾角为0°的试样,强度由岩石基质控制,但层理弱面仍对岩石破坏的裂纹分布与走向产生较大影响;对于层理倾角为22.5°的试样,强度与裂纹走向受岩石基质与层理弱面共同控制;对于层理倾角为45°~67.5°的试样,强度与裂纹走向受层理弱面控制;而对于层理倾角为90°的试样,动态抗压强度受岩石基质的影响较大,在层理弱面较早形成纵向宏观裂纹,导致该层理弱面角度下裂纹受层理弱面的影响较大。     

Prediction of progressive failure in multidirectional composite laminated panels

A mechanism-based progressive failure analyses (PFA) approach is developed for fiber reinforced composite laminates. Each ply of the laminate is modeled as a nonlinear elastic degrading lamina in a state of plane stress according to Schapery theory (ST). In this theory, each lamina degrades as characterized through laboratory scale experiments. In the fiber direction, elastic behavior prevails, however, in the present work, the phenomenon of fiber microbuckling, which is responsible for the sudden degradation of the axial lamina properties under compression, is explicitly accounted for by allowing the fiber rotation at a material point to be a variable in the problem. The latter is motivated by experimental and numerical simulations that show that local fiber rotations in conjunction with a continuously degrading matrix are responsible for the onset of fiber microbuckling leading to kink banding. These features are built into a user defined material subroutine that is implemented through the commercial finite element (FE) software ABAQUS in conjunction with classical lamination theory (CLT) that considers a laminate as a collection of perfectly bonded lamina (Herakovich, C.T., 1998. Mechanics of Fibrous Composites. Wiley, New York). The present model, thus, disbands the notion of a fixed compressive strength, and instead uses the mechanics of the failure process to provide the in situ compression strength of a material point in a lamina, the latter being dictated strongly by the current local stress state, the current state of the lamina transverse material properties and the local fiber rotation. The inputs to the present work are laboratory scale, coupon level test data that provide information on the lamina transverse property degradation (i.e. appropriate, measured, strain–stress relations of the lamina transverse properties), the elastic lamina orthotropic properties, the ultimate tensile strength of the lamina in the fiber direction, the stacking sequence of the laminate and the geometry of the structural panel. The validity of the approach advocated is demonstrated through numerical simulations of the response of two composite structural panels that are loaded to complete failure. A flat, 24-ply unstiffened panel with a cutout subjected to in-plane shear loading, and a double notched 70-ply unstiffened stitched panel subjected to axial compression are selected for study. The predictions of the simulations are compared against experimental data. Good agreement between the present PFA and the experimental data are reported.     

Numerical failure analysis of a stretch-bending test on dual-phase steel sheets using a phenomenological fracture model

Advanced High Strength Steels (AHSS) are increasingly used in automotive industry due to their superior strength and substantial weight advantage. However, their compromised ductility gives rise to numerous manufacturing issues. One of them is the so-called ‘shear fracture’ often observed on tight radii during stamping processes. Since traditional approaches, such as the Forming Limit Diagram (FLD), are unable to predict this type of fractures, great efforts have been made to develop failure criteria that could predict shear fractures. In this paper, a recently developed Modified Mohr–Coulomb (MMC) ductile fracture criterion (Bai and Wierzbicki, 2010) is adopted to analyze the failure behavior of a Dual Phase (DP) steel sheet during stretch-bending operations. The plasticity and ductile fracture of the present sheet are fully characterized by a Hill’48 orthotropic model and a MMC fracture model, respectively. Finite element models with three different element types (3D, shell and plane strain) were built for a Stretch Forming Simulator (SFS) test (Shih and Shi, 2008), numerical simulations with four different R/t values (die radius normalized by sheet thickness) were performed. It has been shown that the 3D and shell element simulations can predict failure location/mode, the upper die load–displacement responses as well as wall stress and wrap angle at the onset of fracture for all R/t values with good accuracy. Furthermore, a series of parametric studies were conducted on the 3D element model, and the effect of tension level (clamping distance), tooling friction, mesh size and fracture locus on failure modes and load–displacement responses were investigated.     

A study of the in situ transverse tensile lamina strength and the classical solution of the opening mode crack in a 0/90/0 laminate

In this paper, the classical solution of the opening mode crack in the 90° layer of 0/90/0 laminates has been determined by means of Fourier transformations and the procedure of Copson for a pair of dual integral equations. The fracture behavior and the in situ transverse strength of the 90° layer have been quantitatively studied in graphite/epoxy laminates, based on the solution obtained above. The results show that the stress intensity factor of this kind of laminates, which is different from that of a single unidirectional 90° layer, decreases with the increase in thicknessb, or modulusE _L orG _LT of the 0° layer and also decreases with the decrease in the thickness of the 90° layer. So the lamination effect manifests itself and thein situ transverse strength of the 90° layer is thereby enchanced. The theoretical calculations agree with the experimental data presented by D.L. Flaggs. Supported by National Natural Science Fundation of China.     

A micromechanics method to study the effect of domain switching on fracture behavior of polycrystalline ferroelectric ceramics

ＩｎｔｒｏｄｕｃｔｉｏｎＲｅｃｅｎｔｌｙ ,ｔｈｅｆｅｒｒｏｅｌｅｃｔｒｉｃｃｅｒａｍｉｃｓｈａｓｓｕｃｈｅｘｃｅｌｌｅｎｔｃｈａｒａｃｔｅｒｉｓｔｉｃｓｏｆｐｉｅｚｏｅｌｅｃｔｒｉｃｉｔｙａｎｄｐｙｒｏｅｌｅｃｔｒｉｃｉｔｙｅｔｃ .ｔｈａｔｉｔｂｅｃｏｍｅｓｏｎｅｏｆｔｈｅｍｏｓｔｉｍｐｏｒｔａｎｔｆｕｎｃｔｉｏｎａｌｍａｔｅｒｉａｌｓ.Ｆｏｒｉｎｓｔａｎｃｅ ,ｔｈｅｗｉｄｅｌｙａｐｐｌｉｅｄｓｅｎｓｏｒｓ,ｔｒａｎｓｄｕｃｅｒｓａｎｄａｃｔｕａｔｏｒｓｅｔｃ .ａｒｅｍａｄｅｏｆｔｈｅｆｅｒｒｏｅ…     

A combined experimental–numerical investigation of ductile fracture in bending of a class of ferritic–martensitic steel

We present a combined experimental–numerical study on fracture initiation at the convex surface and its propagation during bending of a class of ferritic–martensitic steel. On the experimental side, so-called free bending experiments are conducted on DP1000 steel sheets until fracture, realizing optical and scanning electron microscopy analyses on the post mortem specimens for fracture characterization. A blended Mode I – Mode II fracture pattern, which is driven by cavitation at non-metallic inclusions as well as martensitic islands and resultant softening-based intense strain localization, is observed. Phenomena like crack zig-zagging and crack alternation at the bend apex along the bending axis are introduced and discussed. On the numerical side, based on this physical motivation, the process is simulated in 2D plane strain and 3D, using Gurson’s dilatant plasticity model with a recent shear modification, strain-based void nucleation, and coalescence effects. The effect of certain material parameters (initial porosity, damage at coalescence and failure, shear modification term, etc.), plane strain constraint and mesh size on the localization and the fracture behavior are investigated in detail.     

Metallic sandwich panels subjected to multiple intense shocks

The mechanical response and fracture of metal sandwich panels subjected to multiple impulsive pressure loads (shocks) were investigated for panels with honeycomb and folded plate core constructions. The structural performance of panels with specific core configurations under multiple impulsive pressure loads is quantified by the maximum transverse deflection of the face sheets and the core crushing strain at mid-span of the panels. A limited set of simulations was carried out to find the optimum core density of a square honeycomb core sandwich panels under two shocks. The panels with a relative core density of 4%–5% are shown to have minimum face sheet deflection for the loading conditions considered here. This was consistent with the findings related to the sandwich panel response subjected to a single intense shock. Comparison of these results showed that optimized sandwich panels outperform solid plates under shock loading. An empirical method for prediction of the deflection and fracture of sandwich panels under two consecutive shocks – based on finding an effective peak over-pressure – was provided. Moreover, a limited number of simulations related to response and fracture of sandwich panels under multiple shocks with different material properties were performed to highlight the role of metal strength and ductility. In this set of simulations, square honeycomb sandwich panels made of four steels representing a relatively wide range of strength, strain hardening and ductility values were studied. For panels clamped at their edge, the observed failure mechanisms are core failure, top face failure and tearing at or close to the clamped edge. Failure diagrams for sandwich panels were constructed which reveal the fracture and failure mechanisms under various shock intensities for panels subjected to up to three consecutive shocks. The results complement previous studies on the behavior and fracture of these panels under high intensity dynamic loading and further highlights the potential of these panels for development of threat-resistant structural systems.     

Bedding plane-emb e dde d augmente d virtual internal bonds for fracture propagation simulation in shale

To effectively simulate the fracture propagation in shale, the bedding plane(BP) effect is incorporated into the augmented virtual internal bond(AVIB) constitutive relation through BP tensor. Comparing the BP-embedded AVIB with the theory of transverse isotropy, it is found the approach can represent the anisotropic properties induced by parallel BPs. Through the simulation example, it is found that this method can simulate the stiffness anisotropy of shale and can represent the effect of BPs on hydraulic fracture propagation direction. Compared with the BP-embedded virtual internal bond(VIB), this method can account for the various Poisson's ratio. It provides a feasible approach to simulate the fracture propagation in shale.     

SHEAR BEAM MODEL FOR INTERFACE FAILURE UNDER ANTIPLANE SHEAR (Ⅰ)—FUNDAMENTAL BEHAVIOR

The propagation of interlayer cracks and the resulting failure of the interface is a typical mode occurring in rock engineering and masonry structure. On the basis of the theory of elasto~plasticity and fracture mechanics, the shear beam model for the solution of interface failure was presented. The concept of `cohesive crack’ was adopted to describe the constitutive behavior of the cohesive interfacial layer. Related fundamental equations such as equilibrium equation, constitutive equations were presented. The behavior of a double shear beam bonded through cohesive layer was analytically calculated. The stable propagation of interface crack and process zone was investigated.