纤维和颗粒增强复合材料中的裂纹扩展

颗粒和纤维增强复合材料中的裂纹扩展,是用双扭试件(Double—Torsion)在扫描电子显微镜下,直接观察裂纹扩展,并同时进行力学测量的方法来进行研究的。这些结果加深了对裂纹尖端应变场、裂纹扩展机理和影响复合材料韧性因素等的理解。      

碳纤维编织复合材料弯曲损伤破坏声发射监测

为研究碳纤维编织复合材料的弯曲损伤与破坏行为,结合声发射(AE)与数字图像相关(DIC)方法互补实验技术,对复合材料在四点弯曲载荷作用下的失效过程进行监测,动态获取AE特征信号和对应的散斑图像,并根据DIC算法得到复合材料在加载过程中的全场变形和应变。结果表明:AE信号在碳纤维编织复合材料中近似呈指数衰减,与谐振式AE传感器相比,宽频带式AE传感器表现出更好的测量性能;复合材料弯曲加载前期,无明显损伤出现,AE信号较少,对应的位移场和应变场变化平稳;复合材料弯曲破坏对应较高持续时间、幅度和相对能量的AE信号。复合材料损伤演化过程的AE响应行为及对应位移场、应变场的变化反映复合材料的变形与损伤破坏过程,为碳纤维编织复合材料的无损评价、健康监测与失效分析提供借鉴。     

斜纹编织碳纤维/环氧树脂复合材料II型分层性能及损伤演化表征

为研究编织复合材料在静载及疲劳载荷下的分层特性及损伤演化模式,对斜纹编织CF3052/3238A碳纤维/环氧树脂复合材料II型静开裂及疲劳开裂性能进行了测试。结果表明:斜纹编织CF3052/3238A碳纤维/环氧树脂复合材料裂纹扩展行为受纬向纤维影响存在周期性局部受阻现象,分层破坏模式除层间开裂外还存在纬向纤维脱粘;斜纹编织CF3052/3238A碳纤维/环氧树脂复合材料裂纹扩展速率符合Paris公式,不同加载控制模式下编织复合材料疲劳驱动力增长规律存在本质区别:恒幅疲劳载荷下斜纹编织复合材料疲劳驱动力呈抛物线型单调增长;而恒幅疲劳位移下复合材料疲劳驱动力随分层长度呈波峰型分布;采用基于载荷控制模式和位移控制模式下的疲劳驱动力模型,可对斜纹编织CF3052/3238A碳纤维/环氧树脂复合材料进行损伤演化表征,其表征效果良好,具有工程参考价值。      

正交复合材料Ⅰ型裂纹动态响应焦散线实验研究

利用高速摄影技术与光学焦散线技术相结合，对冲击下正交复合材料Ⅰ型裂纹尖端的动态应力集中问题进行了光测力学实验研究。记录了两种层状碳纤维／环氧树脂复合材料在冲击作用下Ⅰ型裂纹尖端的动态焦散斑图，提取反映材料局部应力集中问题的力学参数：焦散斑特征尺寸，进而比较两种复合材料的抗冲击性能。这些研究工作对于复合材料冲击性能评估具有工程应用价值和科学研究意义。     

正交复合材料I型裂纹动态响应焦散线实验研究

利用高速摄影技术与光学焦散线技术相结合,对冲击下正交复合材料I型裂纹尖端的动态应力集中问题进行了光测力学实验研究。记录了两种层状碳纤维/环氧树脂复合材料在冲击作用下I型裂纹尖端的动态焦散斑图,提取反映材料局部应力集中问题的力学参数:焦散斑特征尺寸,进而比较两种复合材料的抗冲击性能。这些研究工作对于复合材料冲击性能评估具有工程应用价值和科学研究意义。     

TiB2颗粒增强锌基合金中短,长疲劳裂纹扩展行为

研究了ＴｉＢ２颗料增强锌基合金及其基体材料中由缺口起始的疲劳短、长裂纹的扩展行为。通过精抛光试样表面跟踪裂纹路径，在同一试样上采用降低施加负荷的方法，测定了短、长疲劳裂纹的扩展速率，并沿裂纹路径观察了增强粒子与裂纹尖端的作用。研究结果表明：在该复合材料及基体材料中均存在小裂纹扩展延迟现象，此现象与缺口塑性有关，复合材料中还与粒子与裂纹尖端的作用有关，两种材料在相同的ΔＫ水平下，短、长疲劳裂纹具有不     

三维编织复合材料研究及应用现状

正三维编织复合材料是利用编织技术,把经向、纬向及法向的纤维束(或纱线)编织成一个整体,即为预成型结构件(简称"预制体"),然后以预制体作为增强材料进行树脂浸渍固化而形成的复合材料结构。由于增强纤维在三维空间多向分布,阻止或减缓了冲击载荷作用下复合材料层间裂纹的扩展,使得复合材料层间性能大大提升。因此,三维编织复合材料较普通层合复合材料具有更高的冲击损伤容限和断裂韧性。三维编织技术可按实际     

金属泡沫材料平面应变裂纹缓慢扩展的奇异摄动分析

利用奇异摄动分析法研究了金属泡沫材料中平面应变Ⅰ-型裂纹的纹缓慢扩展问题，对裂纹尖端附近的应力场和速度场进行了分析，并得到了扩展裂纹尖端速度场和角应力函数的的最低阶摄动解。     

不同应力比下Cu/WCp复合材料疲劳裂纹扩展行为及影响机制分析

通过粉末冶金工艺制备了一种高压电触头用Cu/WCp颗粒增强复合材料。研究了不同应力比下Cu/WCp颗粒增强复合材料的疲劳裂纹扩展行为,并结合裂纹闭合模型和两参数驱动力模型分析了应力比对Cu/WCp颗粒增强复合材料疲劳裂纹扩展速率的影响机制。研究结果表明:随着应力比R的增大裂纹扩展速率增大,尤其在近门槛值附近裂纹扩展速率差别最明显。裂纹闭合模型和两参数驱动力模型均可以较好地将不同应力比R下(da/d N-ΔK)关系曲线关联起来,且两参数驱动力模型的相关性更好。这说明导致不同应力比R下Cu/WCp颗粒增强复合材料疲劳裂纹扩展速率差异的原因主要是Kmax引起裂纹尖端单调损伤,其次是裂纹闭合效应。根据SEM断口分析发现高应力比的断面较低应力比的粗糙,低应力比时断口以基体撕裂为主而高应力比时以颗粒基体脱粘为主。     

基于相场法的复合材料失效分析研究进展

预测复合材料的失效行为，对复合材料结构设计具有重要意义。由于其失效模式和失效机制较复杂，传统的计算断裂力学方法和基于损伤力学的数值方法在对其进行失效分析存在一定困难。相场法结合了断裂力学和损伤力学的优点，无需额外的判据便可精确捕捉裂纹的萌生、扩展和扭结行为，近年来被广泛地应用于复合材料的失效分析。本文首先简要介绍相场法的基本理论，给出了基本的断裂能模型和控制方程。然后着重介绍了基于相场法的复合材料失效分析的研究进展，梳理了相场法在复合材料领域的应用范围。最后，对相场法模拟复合材料在疲劳、疲劳湿热环境下和冲击下的损伤进行了展望。     

考虑界面过渡层的编织CMC裂尖混合度的研究

对于界面断裂的三维编织CMC,其断裂混合度Ψ对材料的断裂韧性影响显著。尽管由于一个不确定的参量——断裂特征长度,界面断裂混合度几乎难以确定,但是与界面断裂混合度相关的裂纹尖端断裂混合度Ψ_tip可以唯一确定。这样,就可以用裂纹尖端断裂混合度Ψ_tip研究三维编织CMC的断裂韧性。本文通过数值方法分析了倾斜角度、界面层厚度与混合度的关系,进一步研究了三维编织CMC的裂纹扩展路径对于裂尖混合度的影响,为材料断裂韧性的优化设计提供了依据。     

Dynamic fields generated by rapid crack growth

Stresses and strains near a rapidly propagating crack tip are affected by the mass density of the material. This paper starts with a brief summary of analytical results for near-tip dynamic fields as predicted by linear elastic fracture mechanics. Next, exact expressions are derived for dynamic crack-line strains, for mode-III crack propagation in a nonlinear elastic material and in an elastic perfectly-plastic material. These expressions are valid on the crack line from the moving crack tip to the moving boundary with the region of linearly elastic deformation. For steady-state crack growth, a critical strain criterion is used to compute the relation between external load and crack tip speed. The required external load increases with crack-tip speed.     

Fracture parameters for interfacial cracks: an experimental-finite element study of crack tip fields and crack initiation toughness

Crack tip measurements and analysis of interfacial parameters for PMMA-aluminum bimaterial system are presented. A variety of crack tip mode-mixities are obtained by subjecting asymmetric four-point-bend specimens to different boundary loads. The crack tip fields are mapped using the optical method of Coherent Gradient Sensing (CGS). The complex stress intensity factors and the associated crack tip mixities () are measured from CGS fringe patterns. The asymptotic expansion field for interface cracks is used for extracting fracture parameters by accounting for higher order contributions to the experimental data. The measurements are compared with complementary finite element computations. A linear relationship between crack tip mixity and the applied load mixity is experimentally demonstrated in this large elastic mismatch system. The fracture load and hence the energy release rate G _cr () at crack initiation is measured as applied load mixities are varied. Limited discussion on the influence of surface roughness prior to bonding on the fracture toughness is included. Positive and negative shear on the crack plane produce different failure responses in this bimaterial system and the observed asymmetry is akin to the one predicted by the T&H model that includes crack tip nonlinearty.     

Measurements of crack tip strain field in wood at the scale of growth rings

The fracture mechanisms of wood have often been interpreted on the scale of cell walls. Although this scale is important, the scale of growth rings needs to be considered in the same context. In the present study, the crack tip strain field of radial TR cracks at the scale of growth rings is measured by electronic speckle photography. The methodology is discussed in detail as well as the data reduction scheme. The tip is in the earlywood layer and the crack plane of the TR crack is perpendicular to the stiffer latewood layer. Increasing opening mode load is applied in-situ as the crack is observed by reflected light optical microscopy. Strains are measured on direct images of the microstucture. In contrast to some other methodologies, this allows direct correlation between strain field and microstructure. In the softer earlywood, tangential strains extend considerable distances in the tangential direction. Due to the stiff latewood, the strain is heavily constrained in the radial direction. This nature of the local strain field has been largely neglected, despite its obuius significance to TR crack growth mechanisms.     

Size effect on the fracture toughness of metallic foil

The size effects on fracture behavior of Cu foil are investigated by a new optical technique, the digital speckle correlation method (DSCM). Displacement and strain fields around a crack tip are analyzed for different thicknesses of Cu foil. Then, the J integral and fracture toughness J _C are evaluated directly from the strain fields around the crack tip. The fracture toughness J _C is obtained as a function of foil thickness. The results indicate that J _C indeed depends on foil thickness within a certain range of thickness (the thickness varies from 20 micron to 1 millimeter in this work).     

Stable crack growth,instability, fatigue and fracture of a 50/50 blend of poly(2,6-dimethyl-1,4-phenylene oxide) and polystyrene

The fracture behaviour of a 50/50 blend of poly(2,6-dimethyl-1,4-phenylene oxide)/poly-styrene has been studied. The crack propagation behaviour is strongly influenced by the temperature, crack driving force and the nature of the crack tip craze zone. A fracture map outlining the regions of stable crack growth as a function of temperature, crack velocity and crack driving force has been determined. At high temperatures and low crack growth velocities, stable crack propagation proceeds through a single-craze crack tip damage zone, while at lower temperatures and high crack velocities, a multiple-craze crack tip zone is observed. Corresponding behaviour can be observed under fatigue loading conditions. An instability leading to very high-speed fracture occurs at a critical crack velocity, thus limiting the stable crack propagation regime to lower velocities. The various reported measures of fracture toughness, such as those based on crack initiation, peak load and the onset of crack instability, are discussed.     

Geometrical relationships between side necking and plastic zone size near a crack tip in ductile metals. Part I: Modified boundary layer solutions

The plastic zone size is regarded as the measure of a material's resistance, and it also determines the fracture behaviour. Recently, stereo digital speckle photography (SDSP) has been found to be useful for measuring in-situ the side necking developed on the lateral surfaces of a specimen during a standard fracture test procedure for J_IC . Because plastic deformation occurs without any volume change, the in-plane plastic zone developed around a crack tip should be accompanied by out-of-plane deformation, that is, side necking. With the aid of the new measurement technique, side necking is expected to act as a gauge for indicating the plastic zone size. As a preliminary study, the geometrical relationships between side necking and the plastic zone size near a crack tip in ductile metals are explored by using a finite element model with modified boundary conditions. As parameters representing the geometrical similarity between side necking and the plastic zone, the shapes of each region, the distances from the crack tip to the boundaries of each region, r_p and r_s and the areas of each region, A_p and A_s are examined for their sensitivities to variables such as mode mixity, hardening exponent and so on. Among them, the areas, A_p and A_s seem to be the best for application because an excellent linearity between them is maintained in a wide range of mode mixity and load level regardless of the hardening exponent, specimen thickness and yield stress.     

基于两种破裂判据的裂隙岩体单轴压缩起裂分析

该文引入Rankine最大拉应力准则和Mohr-coulomb剪切破坏准则分别作为岩石基质的拉伸和压剪破裂判据,分析了单轴压缩下裂隙岩体的起裂机制。根据含单个椭圆裂隙的无限域岩体在单轴压缩下的应力理论解,编制了Matlab程序,计算分析了不同短轴与长轴比k和倾角α(加载轴与裂隙长轴间的夹角)下的岩石基质应力集中系数、两种不同起裂机制的破裂函数值、开裂位置和开裂临界荷载。对多裂隙岩体,采用ABAQUS有限元软件进行了应力计算和起裂机制分析。计算结果表明:1)与单裂隙岩体相比,多裂隙岩体的岩石基质应力集中系数略大、起裂临界荷载略小,但起裂位置相同;2)随着裂隙倾角α的增大,岩石基质的主拉应力集中区由裂隙端部附近很小的区域逐渐变为裂隙中部的大面积区域,而主压应力集中区则反之;3)存在临界裂隙倾角α0,其值在45°附近。当裂隙倾角0<α≤α0时,在裂隙端部同时有拉应力和压剪应力集中,拉破裂临界荷载小于压剪破裂临界荷载,但随着裂隙轴比的增大二者逐渐相等,表明岩体受拉破裂和压剪破裂共同影响越来越明显;当α0<α≤90°时,尽管拉破裂临界荷载大于压剪破裂临界荷载,但首先发生在裂隙端部的压剪破裂区范围很小,而随后将在裂隙中部或端部发生大量的拉伸破裂。上述分析结果与实验现象较为吻合。     

Influence of local fracture energy distribution on maximum fracture load of three-point-bending notched concrete beams

The maximum fracture load of a notched concrete beam has been related to the local fracture energy at the cohesive crack tip region analytically in this paper, and then the correlation between the size effects on the maximum fracture loads and the RILEM specific fracture energy is established. Two extreme conditions have been established, namely zero crack-tip bridging with zero local fracture energy and maximum crack-tip bridging with the maximum size-independent fracture energy. It is concluded that the local fracture energy at the crack tip region indeed varies with the initial crack length and the size of specimen. The tri-linear model for the local fracture energy distribution is confirmed by using the proposed simple analytical solution.     

Finite strain fracture of plates and shells with configurational forces and edge rotations

We propose a simple and efficient algorithm for FEM‐based computational fracture of plates and shells with both brittle and ductile materials on the basis of edge rotation and load control. Rotation axes are the crack front nodes, and each crack front edge in surface discretizations affects the position of only one or two nodes. Modified positions of the entities maximize the modified mesh quality complying with the predicted crack path (which depends on the specific propagation theory in use). Compared with extended FEM or with classical tip remeshing, the proposed solution has algorithmic and generality advantages. The propagation algorithm is simpler than the aforementioned alternatives, and the approach is independent of the underlying element used for discretization. For history‐dependent materials, there are still some transfer of relevant quantities between elements. However, diffusion of results is more limited than with tip or full remeshing. To illustrate the advantages of our approach, three prototype models are used: tip energy dissipation linear elastic fracture mechanics (LEFM), cohesive‐zone approaches, and ductile fracture. Both the Sutton crack path criterion and the path estimated by the Eshelby tensor are employed. Traditional fracture benchmarks, including one with plastic hinges, and newly proposed verification tests are solved. These were found to be very good in terms of crack path and load ∕ deflection accuracy. Copyright © 2013 John Wiley & Sons, Ltd.