纤维增强复合材料层合板分层破坏的研究

本文对纤维增强复合材料层合板的分层破坏进行了大量的试验,同时用三维有限元进行应力分析。试验和分析结果表明此类层合板的分层总是发生在θ/90界面上,该界面上不仅层间剪应力大而且层间正应力也大。通过对不同θ/90界面的临界能量释放率的测定表明,对层合板不同的θ/90分层界面的G_ⅠC和G_ⅡC是随θ角的变化而变化。文中对一个Ⅰ型,Ⅱ型耦合型能量释放率分层判据的应用作了改进,试验结果表明此改进是有效的。     

碳纤维/环氧树脂复合材料动态拉伸试验研究与损伤分析

利用MTS试验机和分离式Hopkinson拉杆对2种碳纤维/环氧树脂(T300/epoxy)层合板试件[(45/-45)₄]_S和[(0/45/90/-45)₂]_S进行了准静态(应变率10-5~10-4 s-1)、中速(应变率10-1~101 s-1)和高速(应变率102~104 s-1)冲击拉伸试验。在热力学框架内建立了基于损伤能释放率的弹塑性动力损伤本构模型,用该损伤模型来分析试件的动态拉伸失效过程。模型中提出了3种基本损伤机制(纤维断裂、基体开裂及面内剪切)的演化规律,通过对损伤阈值黏性归一化的方法考虑了应变率对损伤演化的影响。编写了该模型有限元用户材料子程序,并模拟了拉伸试验过程,计算结果表明该模型能够较好地模拟碳纤维/环氧树脂层合板动态拉伸失效过程。     

复合材料裂纹敏感性的评定──(Ⅱ)HTA/6376和T300/M10的Ⅱ型分层扩展行为

应用ENF试验研究了HTA/6376和T300/M10两种碳/环氧复合材料的静态与疲劳层间断裂行为。在静态载荷下,两种材料均呈现脆性不稳定和稳定的裂纹扩展特性。在R=0.1且△G_I大幅度变化的疲劳加载过程中,两种材料呈现稳定的裂纹扩展。采用位移控制方法,确定了裂纹扩展速率与循环应变能释放率的关系和应变能释放率门槛值。与T300/914C相比,HTA/6376和T300/M10具有较高的抗裂纹扩展能力。     

复合材料的Ⅰ型层间断裂韧性

本文采用铰链式双悬臂梁试件对碳/双马来酰亚胺复合材料的Ⅰ型层间断裂韧性进行了研究,分析比较了层间断裂韧性G_IC的表达方法,用三次多项式和幂函数拟合实验柔度的方法得到的结果比较满意,实验结果表明纤维桥连对单向层合板的G_IC的影响是显着的,用刀片切割桥连纤维后G_IC值下降百分之二十,分散性也有显着下降。另外发现G_IC值随试件厚度增加而增大。     

正交异性纯弯梁应力强度因子的研究

本文运用复合材料断裂理论推导出带单边切口的纯弯梁不同纤维增强方向时v位移分量与K_Ⅰ、K_Ⅱ的关系式。用贴片云纹干涉法与有限元法确定纯弯梁裂纹尖端v位移,从而得出应力强度因子值。文中给出了复合材料纯弯梁不同裂纹长度,不同纤维增强方向时的云纹图,最后将测试结果与有限元计算值进行了误差对比。     

Z-pin增强复合材料层合板断裂韧性试验研究

针对Z-pin增强复合材料层合板, 开展了断裂韧性的试验研究。研究选取了3种Z-pin直径(0.28、 0.52、 0.80mm)且每种直径下分别以3种分布形式(5×5、 8×8、 10×10)排布Z---pin的增强方式, 为了确定比较基准, 试验中同时测试了不含Z-pin的复合材料层合板试样。通过Z-pin拔出试验测试了3种直径Z-pin从基体拔出过程中的载荷位移关系。利用双悬臂梁试验和端部开口弯曲试验分别测试了不含Z-pin和含Z-pin试样的Ⅰ型断裂应变能释放率G_ⅠC、 Ⅱ型断裂应变能释放率G_ⅡC。试验结果表明:? 与不含Z-pin的结构相比, Z-pin增强试样的Ⅰ型断裂应变能释放率G_ⅠC增大了83%~1110%, Ⅱ型断裂应变能释放率G_ⅡC增大了23%~438%; 在相同Z-pin体积含量下, 与增大Z-pin直径相比, 增大Z-pin分布密度能更有效地提高复合材料层合板的断裂韧性。      

“离位”增韧技术在碳纤维/RTM聚酰亚胺复合材料中的应用

研究了“离位”增韧对RTM聚酰亚胺（PI-9731）树脂基复合材料力学性能的影响。结果表明: 当增韧剂的质量分数为15%时, 经粉末法和薄膜法“离位”增韧G827／PI-9731复合材料的室温层间剪切强度从增韧前的97.9MPa分别提高到108MPa和110MPa, 高温（288℃）层间剪切强度变化不大。G827／PI-9731复合材料经粉末法“离位”增韧后, Ⅰ型断裂能释放率从增韧前的310J/m2提高到410J/m2, Ⅱ型断裂能释放率从增韧前的590J/m2提高到939J/m2。而经过薄膜法“离位”增韧后, 其复合材料的Ⅰ型断裂能释放率提高到459J/m2, Ⅱ型断裂能释放率提高到1100J/m2。经电镜分析表明, 由于热塑性聚酰亚胺的引入, 在复合材料层间区域形成热固/热塑相反转结构, 在裂纹扩展的过程中, 包覆热塑性聚酰亚胺的PI-9731粒子发生明显取向和变形, 从而提高韧性。      

克服纤维桥连对GIC影响的隔离层法

本文分析了纤维桥连产生的原因和它对Ⅰ型层间断裂韧性G_IC的影响,提出了一种克服纤维桥连的“隔离层”法,从试件铺层设计入手,力图将纤维嵌套的可能性减到最低.有限元计算结果表明,隔离层的引入并没有改变层间裂纹尖端的应力分布和应力集中,试件的弯曲刚度计算表明,当隔离层的铺层角取±10°～±20°时,可以使±θ隔离层带来的试件弯扭耦合效应降到最低.实验结果证明了这种方法的可行性.     

Ti/Al_3Ti层状复合材料层间断裂性能研究

金属间化合物基层状复合材料Ti/Al_3Ti是采用Ti-6Al-4V箔和Al箔按照一定顺序叠加后,在真空环境下热压烧结成由韧性金属Ti和金属间化合物Al_3Ti组成的叠层结构。利用ENF(End-Notch-Flexure)和MMF(Mixed-Mode-Flexure)测试方法,对Ti/Al_3Ti层状复合材料的Ⅱ型及Ⅰ+Ⅱ型层间断裂能、层间断裂行为以及能量释放率等方面进行了研究。结果表明,Ti/Al_3Ti层间断裂时,裂纹在复合材料界面处发生起裂,而在传播过程中发生偏转,最终导致Al_3Ti层开裂。由此可见,在实验过程中裂纹的表现形式是在Ti/Al_3Ti界面及Al_3Ti层中共同扩展;Ⅰ+Ⅱ型层间起裂能量释放率G_(Ⅰ+Ⅱ)为46 J/m~2,Ⅱ型层间起裂能量释放率G_Ⅱ为1453 J/m~2,表明Ⅱ型层间断裂比Ⅰ+Ⅱ型层间断裂更困难。     

复合材料层板+θ/-θ层间断裂韧性研究

本文讨论了纤维增强复合材料层板沿+θ/-θ层间断裂韧性的研究方法;提出在满足一定力学条件时,可沿用机械载荷实验方法测得G_C中“纯机械部分”G_m;结合数值方法计算得出“纯温度部分”G_T,从而得到G_C.给出一个针对DCB试验的充分条件,据此设计铺层并对T300/648和T300/QY8911进行试验和分析(θ=0～30°)、讨论了大变形的影响并提出一个新的修正系数.结合θ对G_C的影响及断口微观形貌作了进一步讨论.     

基于小波内聚力模型的界面裂纹扩展

利用区间B样条小波良好的局部化性能,将内聚力模型（CZM）引入小波有限元法（WFEM）数值分析中,以区间B样条小波尺度函数作为插值函数,构造小波内聚力界面单元,推导了小波内聚力界面单元刚度矩阵,基于虚拟裂纹闭合技术（VCCT）计算界面裂纹应变能释放率（SERR）,采用β-Κ断裂准则,实现界面裂纹扩展准静态分析。将WFEM和传统有限元法（CFEM） 的SERR数值分析结果与理论解进行比较,结果表明:采用WFEM和CFEM计算的SERR分别为96.60 J/m2 和 101.43 J/m2,2种方法的SERR数值解与理论解相对误差分别为1.85%和3.06%,这明确表明WFEM在计算界面裂纹扩展方面能用较少单元和节点数获得较高的计算精度和效率。在此基础上,探讨了界面裂纹初始长度和双材料弹性模量比对界面裂纹扩展的影响,分析结果表明:界面裂纹尖端等效应力随界面裂纹初始长度的增加而增加;双材料弹性模量比相差越大,界面裂纹越易于扩展,且裂纹扩展长度也越大,因此可通过调节双材料弹性模量比来延缓界面裂纹扩展。      

T应力对Ⅰ-Ⅱ复合型裂纹扩展的影响

利用最大周向正应力判据MTS重新分析研究了脆性破坏的Ⅰ-Ⅱ复合型裂纹扩展,其中考虑了平行于裂纹方向的非奇异项T应力。以平板中的斜裂纹处于双向受力为研究对象,通过两个方向力的不同组合以及裂纹与受力方向的夹角变换得到包括纯I型和纯II型在内的Ⅰ-Ⅱ复合型裂纹,分析了T应力对裂纹扩展方向以及断裂时的应力强度因子的影响,并将预测结果与现有的实验数据进行了比较。在此基础上,给出了不同T应力条件下通用的Ⅰ-Ⅱ复合型裂纹扩展条件,可用于给定几何试件的脆性断裂判定。分析结果表明:裂纹尖端非奇异项T应力对裂纹扩展的影响是不可忽略的,尤其是对II型断裂的影响更为明显。     

Analysis of unidirectional (0°) fiber-reinforced laminated composite double cantilever beam specimen using higher order beam theories

Mathematical models, for the stress analysis of unidirectional (0°) fiber-reinforced laminated composite double cantilever beam (DCB) specimen using classical beam theory, first and higher order shear deformation beam theories, have been developed to determine the mode I strain energy release rate (SERR) for unidirectional composites. In the present study, appropriate matching conditions at the crack tip of the DCB specimen have been derived by using variational principles. SERR has been calculated using compliance method. In general, the performance of shear deformation beam models of DCB specimen with variationally derived matching conditions at the crack tip is good in determining the SERR for medium to long crack lengths. Performance of higher order shear deformation beam model (having quadratically varying transverse displacement over the thickness) of DCB specimen, with non-variationally derived matching conditions at the crack tip, is good in determining the SERR for all the crack lengths in comparison with the available theoretical and finite element solutions in the literature. Higher order shear deformation beam theories having varying transverse displacement over the thickness are more appropriate to analyze DCB specimen as they predict the appropriate nature of the interlaminar normal stress at the crack tip and its distribution ahead of the crack tip.     

Fracture analyses and experimental results of crack growth under general mixed mode loading conditions

In this paper detailed results of 3D finite element (FE) and mixed mode analyses of different fracture specimens are presented and discussed. Special interest is taken in 3D and mode coupling effects to be found in strain energy release rate (SERR) results along crack fronts, in particular adjacent to corners, where a crack front intersects a free surface of a specimen. It will be shown that these effects stay small if they are related to Poisson’s ratio but that they can also be considerably pronounced if they are related to the global deformation behaviour of the specimen. The computational fracture analysis is based on the calculation of separated energy release rates (SERRs) by the aid of the modified virtual crack closure integral (MVCCI)-method in order to calculate the local SERR-distributions along the crack front. Furthermore some qualitative experimental results will show the influence of these variable mixed mode I, II and III loading conditions along the crack front on crack initiation and on the further development of 3D crack growth in the specimens.     

奇异薄层单元及其在双材料界面断裂分析中的应用

构造了一种平面应力奇异薄层单元并证明了其具有-1/2阶奇异性。用此单元研究了双材料界面层的刚度对界面裂纹尖端场的影响。研究发现:对于I 型界面断裂, 减小界面法向刚度对K_Ⅰ、K_Ⅱ的影响远大于减小界面切向刚度, 且法向和切向刚度的减小对K_Ⅱ的影响均大于对K_Ⅰ的影响, 降低法向刚度会显著改变裂尖正应力和剪应力的分布, 而降低切向刚度只明显改变剪应力的分布, 对正应力的分布影响不大;对于界面II 型断裂, 则减小切向刚度对K_Ⅰ、K_Ⅱ的影响远大于减小法向刚度, 且切向、法向刚度的减小对K_Ⅰ影响均大于对K_Ⅱ影响, 降低切向刚度会显著改变裂尖正应力和剪应力的分布, 而降低法向刚度只明显改变裂尖正应力的分布, 对剪应力的分布影响不大。随着界面刚度增大, 应力强度因子和裂尖应力分布均趋近无厚度理想界面情况。     

HT有限元在Ⅰ、Ⅱ与Ⅲ型复合弹性断裂问题中的应用

探讨了HT有限元应用于Ⅰ、Ⅱ和Ⅲ型复合裂纹的弹性断裂问题。分析了Ⅲ型弹性断裂问题的HT有限元方法及高阶奇异性应力强度因子KΙΙΙ,同时,对Ⅰ和Ⅱ型断裂问题的HT有限元原理及断裂强度因子KΙ和KΙΙ的计算也进行了阐述。特别地,在计算三个强度因子时,引入了一种新的方法——附加试函数法,它主要用于满足裂尖特殊的边界条件,提高了三个奇异应力强度因子的精确性与可靠性。最后,根据HT有限元计算结果,讨论了奇异应力强度因子无量纲化系数K/Kc随裂纹单元特殊T函数项数、细划单元数、单元高斯点数及裂尖不同附加试函数的变化规律;获得了应力强度因子精确度和可靠度,并与其它有限元结果进行了比较,阐述了此方法的优越性。     

Analytical Model for Prediction of Reduced Strain Energy Release Rate of Single-Side-Patched Plates

A study was undertaken to develop an analytical model that can predict how much reduction in Strain Energy Release Rate (SERR) can be achieved by repairing a cracked plate using a single-side bonded patch. The plate may be subjected to inplane or out-of-plane bending loading. Furthermore, the plate may be flat or curved in a cylindrical shape. The model helps to select patch material (i.e., elastic modulus of the material) and the appropriate patch size in order to reduce the SERR at the crack tip of the patched base plate. In other words, the analytical model can be utilized to select the patch material and patch dimensions required to achieve the desired SERR for a cracked base plate with known modulus, thickness, and crack size. The model is based on axial and bending stresses of the single-side strap joint configuration, which are related to the SERR at the crack tip of a plate with a single-side patch repair. In order to verify the analytical model, finite element analyses were conducted to determine stresses as well as SERR in many different patched plates. The numerical study confirmed the validity of the analytical model in predicting the reduction ratio of SERR resulting from the single-side patch repair.     

Ⅰ型和Ⅱ型Dugdale模型解析元列式及其半解析有限元法

利用弹性平面扇形域哈密顿体系的方程,通过分离变量法及共轭辛本征函数向量展开法,推导了两个圆形奇异超级解析单元列式,这两个超级单元能够分别准确地描述Ⅰ型和Ⅱ型Dugdale模型平面裂纹尖端场。将该解析元与有限元相结合,构成半解析的有限元法,可求解任意几何形状和载荷的Ⅰ型或Ⅱ型裂纹基于Dugdale模型的裂纹尖端塑性区尺寸和裂纹尖端张开位移(CTOD)或裂纹尖端滑开位移(CTSD)的计算问题。对典型算例的计算结果表明方法简单有效,具有令人满意的精度。     

Mixed-mode delamination growth of laminar composites by using three-dimensional finite element modeling

ABSTRACT Delamination is one of the most frequent failure modes in laminated composites. Its importance is crucial, because a delamination can occur in the interior of a panel without any noticeable damage on the surface, drastically reducing its strength and stiffness. A study has to be made on critical dimensions of delaminations and their shape, through the calculation of the strain energy release rate (SERR), G. This study was performed numerically, for a given geometry, with varying loads and shapes of delamination, in pure and mixed‐mode propagation. All numerical values were obtained with three‐dimensional finite element (FE) analyses from a commercial package. The use of three‐dimensional analyses in simple geometries helps establish the basis for the more complex ones, and the correspondence with the usual analytical or numerical bi‐dimensional plane‐strain analysis. The conclusions were (a) G is not constant along the crack tip, even for mode I propagation and straight crack tip; (b) the mean value of G obtained from a three‐dimensional analysis equals the value obtained in bi‐dimensional plane‐strain analysis; (c) in mixed‐mode propagation, the method exhibits a good correlation with experimental results and (d) the shape and mode partitioning of the SERR depend not only on the loading, but also on the shape of the crack front.