ENVIRONMENTAL FATIGUE CRACK PROPAGATION IN METAL/COMPOSITE LAMINATES

Abstract— Flight simulation fatigue crack propagation tests with gust spectrum loading were carried out on laminates of (i) aluminium alloy 2024-T3/carbon epoxy, (ii) aluminium alloy 7475-T761/carbon-epoxy and (iii) titanium alloy Ti–6Al–4V/carbon-epoxy, each with a nominal weight saving of 30% as compared to equivalent 2024-T3 panels. The performance of the 2024-T3/carbon-epoxy laminates was much superior to those of the other material combinations. Overall, there was a fairly strong effect of changing from an environment of normal air to an air plus water spray environment. Probable reasons for this strong effect are discussed.     

金属基复合材料内部损伤仿生愈合

本文根据生物体损伤愈合的原理,对金属基复合材料内部纤维开裂、分离和折断损伤的愈合进行了尝试,提出增强纤维可以模仿生物体内纤维结构,设计成为空心结构,内部填充愈合剂。模拟试验结果表明,增强纤维开裂、分离和折断内部损伤可以通过管内愈合剂的输送、浸润和填充达到愈合。     

复合材料损伤阻抗和损伤容限的性能表征

复合材料结构的设计要求和使用经验提出了复合材料体系损伤阻抗和损伤容限性能表征的需求。在试验研究的基础上,本文作者指出长期以来一直使用的CAI (冲击后压缩强度) 的物理意义比较含混,有时可能误导材料研究和设计选材,同时提出应分别用典型层压板静压痕力-凹坑深度曲线的最大压痕力F_max来表征损伤阻抗性能,用凹坑深度-压缩破坏应变曲线门槛值CAIT(Compression failure strain After Impact Threshold)来表征损伤容限性能,同时给出了测试方法的建议。      

光滑与含孔复合材料压-压疲劳实验研究

本文新设计了光滑与含孔两种复合材料试样的抗失稳装置, 简述了这两种试样的压-压疲劳实验概况, 分析和讨论了其疲劳破坏机理, 提出复合材料疲劳损伤的“转捩点”概念。     

复合材料层板损伤过程的刚度分析

本文对典型铺设的[0₂/±45₂/90₂]_s碳/环氧复合材料层板中的典型损伤状态进行了实验观察,测定出损伤对层板刚度引起的下降率.建立了横向裂纹扩展、分层伴以横向裂纹扩展的三维有限元分析模型,计算出对层板刚度引起的下降率,并与实验值进行了比较.结果表明,横向裂纹和分层是层板的主要损伤型式,分层损伤扩展是一个主导性的稳定的损伤扩展过程,是导致刚度下降的主要因素.     

带圆孔玻璃/聚脂层板的疲劳损伤扩展

本文用云纹法研究了交变载荷作用下带圆孔玻璃纤维/聚脂层板的损伤扩展。云纹法能显示全埸位移应变分布,试件出现损伤时,能显示损伤区分布及裂纹扩展;由此得到柔度变化进而可以预测疲劳寿命。避免了在损伤分布不均匀时,因标距选取而影响灵敏度的问题。      

复合材料单钉接头三维逐渐损伤破坏分析

针对纤维增强复合材料螺栓双盖板接头, 发展了面内静拉伸三维逐渐损伤模型。并对损伤累积过程中出现的四种基本损伤机理类型(纤维断裂、纤维-基体剪切、基体开裂、分层) 及其之间的相互关联性进行了分析模拟, 并能成功预测其接头层合板静拉伸强度、破坏模式及损伤与扩展的整个过程。同时用参数化设计方法, 对任意铺层、任意尺寸单钉接头进行建模, 使分析工作更加方便。用本模型计算的结果与文献[ 4 ]提供的9 种不同铺层尺寸接头实验结果进行了比较分析, 对比结果非常令人满意。      

TIME-DEPENDENT FATIGUE CRACK GROWTH IN TITANIUM METAL MATRIX COMPOSITES

Abstract The interaction of fatigue and creep in a titanium metal matrix composite was studied by employing loading frequencies of 10 Hz (in both air and vacuum environment) and 0.1 Hz with and without hold times (in air) at 500°C. It was shown that, for the same loading frequency, the crack growth rate is lower in vacuum than in air. In an air environment, however, where the influence of load-related creep and environmental effects exist, it was shown that a decrease in the loading frequency leads to a decrease in the crack growth rate. This behavior is interpreted in terms of the redistribution of fiber and matrix stresses occurring in response to the creep-related relaxation of matrix stresses. The result of this stress redistribution is the generation of a compressive axial residual stress in the matrix phase in the region of the composite ahead of the crack tip. As the crack bridges the fibers in this region, the release of the matrix residual compressive stress leads to the closure of the matrix fractured surfaces at the crack tip, thus leading to a decrease in the crack tip driving force. To support this concept, experimental measurements of the crack opening displacement at different loading frequencies are presented. In addition, a simple model is proposed to describe the nature of the residual stresses developed in the matrix phase during cyclic loading. Results of this model have been examined using finite element analysis. The influence of time-dependent effects during a fatigue cycle was, furthermore, investigated by carrying out high frequency fatigue tests on specimens which have been previously subjected to creep deformation. Results of these tests in terms of the crack growth rate and associated crack closure, support the conclusion that a predeformed matrix produces a decrease in the crack growth rate of the corresponding composite.     

含有基体开裂和内部分层的复合材料层板的刚度特性

本文分析了复合材料层板中两种类型的损伤,一种是层内损伤卽基体开裂,另一种是层间损伤卽内部分层。文中取损伤向量作为唯象理论的内部状态变量,确定了这两种损伤的强度与全部刚度系数之间的关系。分析表明,层内开裂损伤不僅使得材料弹性模量下降而且还改变了层板的初始正交異性对称性;层间分层损伤不改变层板初始正交異性对称性但使得弹性模量降低。应用本文分析方法对几种层板的模量变化进行了预算,并同实验结果进行了比较,发现两者之间吻合得很好。     

检测复合材料层板损伤区的揭层法

本文介绍了由S.M.Freeman提出的检测复合材料层板内部损伤的新方法——揭层法(Deply technique),根据作者的实际经验详细描述了该方法的实现过程及操作步骤,并给出了部分检测结果。      

A THREE-DIMENSIONAL MODELLING OF LOW VELOCITY IMPACT DAMAGE IN COMPOSITE LAMINATES

The modelling of the behaviour of laminates during low velocity impact requires, first, an analysis of the efficiency of the numerical tool used. A logical development in relation to the numerical structure of the code and the experimental observations is proposed. Four aspects of the code implementation are discussed. The first one is the representation of the impact loading by a macro-contact model using the Lagrange multiplier technique. The second is the development of this technique on the meso-scale to represent the assembly of plies of the laminate. The third step is the modelling of the evolution of the non homogeneous cracked state in the composite with an averaging technique, based on the Laws–Dvorak–Hejazi model, developed on the finite element scale. The fourth step is the modelling of the delamination between plies as a lack of contact set off by a mixed criterion matrix-cracking/interfacial forces. Numerical results on a glass-epoxy [02/906/02] laminate impacted at 27 J are in a good agreement with experimental observations.     

复合材料层合板准静态横压损伤及其压缩破坏研究

为了考察一种小试件CAI试验方法的有效性,本文作者对复合材料层合板的准静态横向压缩特性和损伤,以及损伤后的压缩破坏进行了试验研究,采用C扫描、热揭层等技术对层合板内的损伤进行测量,并将含准静态横压损伤层合板的剩余压缩强度与低速冲击后板的压缩强度进行了比较。结果表明:在横压过程中存在分层损伤起始门槛压缩载荷值或压入深度值,以及横压载荷极限值;小板试件各界面的分层面积沿厚度方向的分布和继后的压缩破坏形式等与SACMA的CAI试验标准的情况相异。     

MODELLING THE CONDITIONS FOR FATIGUE FAILURE IN METAL MATRIX COMPOSITES

Abstract— An investigation on the fatigue crack growth (FCG) and fatigue failure in metal matrix composites (MMCs) has been conducted using a model based on micromechanical elasto-plastic fracture mechanics (EPFM) principles. To evaluate the model, comparisons between experimental and predicted fatigue life have been made for two silicon carbide strengthened (SCS)-6/Ti-based MMCs. Conditions for crack arrest and crack instability have also been considered in order to define the fatigue damage limits. Crack arrest occurs from the added effects of fibre bridging and the constraint provided by the fibre on matrix microplasticity, while crack instability is achieved when the fibre constraint effect is minimum and the fatigue resistance of the material is reduced due to the accumulation of fatigue damage. Comparisons of the predicted fatigue damage limits with experimental results show good agreement which underlines the usefulness of a microstructural fracture mechanics model.     

粘弹性基体复合材料层合板的固化残余应力和曲率变化

粘弹性基体复合材料层合板因铺层取向差异而产生的固化残余应力会随层合板在室温下放置时间的推移而变化, 这一过程可通过非对称层合板的曲率变化得到反映。本研究通过非对称板的曲率变化实验来观察板内残余应力的变化过程, 并根据细观复合材料力学理论和适宜的变形分析方法建立一种与残余应力最终稳定分布相对应的铺层修正折减模量矩阵, 用于进行层合板固化残余应力和曲率变化的渐近值预测, 并依此确定固化后残余应力的变化范围。     

复合材料层合板多尺度交互渐进损伤分析

纤维增强复合材料强度的准确表征是复合材料力学性能研究的核心问题之一。该文以碳纤维增强树脂基复合材料层合板为研究对象,基于宏观-细观多尺度分析方法,根据复合材料的物理失效模式分别给出了基体和纤维的细观失效准则,同时考虑基体失效对复合材料层合板纤维轴向力学性能的影响。提出了新的刚度退化方式,可准确表征复合材料层合板的损伤演化过程,开展了复合材料层合板四点弯模型的多尺度交互渐进损伤分析和试验验证。结果表明：基于多尺度方法的复合材料层合板宏-细观交互渐进损伤分析结果与试验结果吻合较好,新的刚度退化方式可以准确模拟层合板的失效过程。     

A THEORETICAL MODEL FOR PREDICTING FATIGUE CRACK GROWTH RATES IN FIBRE-REINFORCED METAL LAMINATES

This paper is concerned with the development and application of an analytical model for predicting fatigue crack growth in fibre-reinforced metal laminates (FRMLs). An analytical model for the distribution of bridging traction is first introduced. Based upon observations of the delamination shapes in FRMLs under fatigue loading and a model for characterizing delamination growth in FRMLs, a model for predicting crack growth rates in CCT specimens of FRMLs is developed. The model is applied to two GLARE laminates (2/1, 3/2 lay-ups) under various cyclic stress levels and stress ratios. The predicted crack growth rates are compared with experimental data. The predicted crack growth rates agree well with the experimental results.     

界面相性态对纤维增强复合材料内应力传递的影响

本文用有限元法研究了具有基体裂纹的纤维增强复合材料内的应力传递问题.假设纤维与基体的界面为非理想的,文中运用“弹簧层”模型首先分析了在不同的组分弹性模量比、纤维体积含量与边界约束条件下,界面相性态对复合材料内的应力传递的影响,然后进一步考察了在几种典型的损伤模式下界面附近的应力分布情况.     

复合材料层板中基体性能对层间应力分布的影响

本文分析了复合材料层板中基体性能的变化对层间应力的影响,差重讨论了层板自由边附近及层间界面上的应力分布.分析结果表明,基体性能的变化会对层间立力的分布产生显著的影响.因此,对于实际复合材料结构,有可能通过选用合适的基体来改变复合材料层板自由边附近的层间应力分布,从而减轻率合材料层板的分层破坏.      

复合材料层板界面缺陷的有限元分析

基于对多种界面缺陷的一致表达,建立了能够反映复合材料层合结构界面缺陷的有限元模型.通过引入横向剪切变形函数来反映界面粘贴状况;对于界面局部缺陷问题,通过缺陷边缘处相邻单元的几何矩阵的简单匹配来满足缺陷边缘的连续条件.只需简单选择界面柔度系数,可方便地处理界面理想粘贴、弱粘贴和脱层三种界面粘贴状况.有限元形式简单,仅涉及个节点自由度.算例验证了有限元模型的精度,讨论了各种参数时界面缺陷对结构的影响.     

LIQUID PHASE SINTERED METAL MATRIX COMPOSITE MATERIALS

Iron-base and aluminum-base composite materials reinforced with various ceramic particulates have been fabricated via powder metallurgy and liquid phase sintering. The advantage of this manufacturing route is that conventional powder metallurgy processing equipment can be used to fabricate metal matrix/ceramic composites. Furthermore, this approach makes it possible to manufacture these composites to near-net-shape. A number of matrix/ceramic combinations have been examined: Fe-C-Si and Fe-Cu with ZrO₂ additions and a Al-Cu-Si-Mg alloy with SiC or Al₂O₃ additions. The interfacial structures were characterized and found to play a significant role in controlling the properties of the composites. Reinforcement was observed in several systems. However, a glassy interfacial layer forms when Si additions and oxide reinforcements are present; the resultant particle/matrix bond strength is weak and reinforcement.