界面脱粘对陶瓷基复合材料疲劳迟滞回线的影响

采用细观力学方法对脆性纤维增强的陶瓷基复合材料拉-拉疲劳载荷下应力-应变迟滞回线进行了研究,将拉梅公式与库仑摩擦法则相结合分析了界面脱粘区以及粘结区复合材料细观应力场.根据卸载与重新加载时纤维相对基体滑移机制,分析了加卸载纤维轴向应力分布,结合断裂力学界面脱粘准则确定了初始加载界面脱粘长度ls、卸载界面反向滑移长度y以及重新加载界面滑移长度z',讨论了界面脱粘能和界面摩擦系数对初始界面脱粘、卸载界面反向滑移、重新加载界面滑移以及加卸载迟滞回线的影响.并与Pryce-Smith模型和试验数据进行对比表明:该文模型与试验曲线吻合的较好.     

纤维增强陶瓷基复合材料疲劳迟滞回线模型研究

纤维增强陶瓷基复合材料初始加载到疲劳峰值应力时, 基体出现裂纹, 纤维/基体界面发生脱粘. 在疲劳载荷作用下, 纤维相对基体在界面脱粘区往复滑移使得陶瓷基复合材料出现疲劳迟滞现象. 建立了纤维陶瓷基复合材料疲劳迟滞回线细观力学模型, 采用断裂力学方法确定了初始加载纤维/基体界面脱粘长度、卸载界面反向滑移长度与重新加载新界面滑移长度, 分析了4种不同界面滑移情况的疲劳迟滞回线. 假设正交铺设与编织陶瓷基复合材料疲劳迟滞回线主要受0°铺层、轴向纱线内纤维/基体界面滑移的影响, 预测了单向、正交铺设与编织陶瓷基复合材料在不同峰值应力与不同循环的疲劳迟滞回线, 与试验结果吻合.      

INVESTIGATION ON FATIGUE HYSTERESIS LOOPS MODELS OF FIBRE-REINFORCED CERAMIC-MATRIX COMPOSITES

纤维增强陶瓷基复合材料初始加载到疲劳峰值应力时, 基体出现裂纹, 纤维/基体界面发生脱粘. 在疲劳载荷作用下, 纤维相对基体在界面脱粘区往复滑移使得陶瓷基复合材料出现疲劳迟滞现象. 建立了纤维陶瓷基复合材料疲劳迟滞回线细观力学模型, 采用断裂力学方法确定了初始加载纤维/基体界面脱粘长度、卸载界面反向滑移长度与重新加载新界面滑移长度, 分析了4种不同界面滑移情况的疲劳迟滞回线. 假设正交铺设与编织陶瓷基复合材料疲劳迟滞回线主要受0°铺层、轴向纱线内纤维/基体界面滑移的影响, 预测了单向、正交铺设与编织陶瓷基复合材料在不同峰值应力与不同循环的疲劳迟滞回线, 与试验结果吻合.     

钢纤维增强砂浆的轴拉损伤力学模型

基于Li Fa Ming的平行棒模型,对短钢纤维增强砂浆的平板试件,在直接拉伸条件下的破坏行为进行了分析,假设试件由N根相互平行的复合棒组成,每根复合棒又由一根纤维棒和S根砂浆棒组成,考虑纤维在基体中分布的方向因子和长度因子.砂浆的损伤可按连续损伤力学进行处理,将Loland模型和Mazars模型加以改进来描述.依据多根纤维的拉拨模型,假定纤维与基体间界面的损伤由纤维脱粘长度与纤维插入长度的比值来描述,复合材料的损伤包括基体的损伤和纤维的损伤,借助已有的试验数据和文献资料来确定本构模型中的各种参数,成功建立了短钢纤维增强砂浆直接拉伸应力一应变全曲线模型.所建模型与试件在直接拉伸试验下的应力-应变全曲线进行了对比,结果较为吻合.     

碳／环氧层板螺接接头拉脱疲劳的实验研究

拉脱破坏是复合材料结构机械连接接头破坏的一种模式。本文通过实验研究了拉脱疲劳累积损伤的扩展过程，并应用累积损伤理论定义和测定了损伤参数，提出了损伤极限的概念，简单阐述了损伤破坏机理。     

一种新的界面脱粘和纤维拔出模型

对纤维增强复合材料中界面的脱粘和纤维的拔出行为进行了研究,通过纤维间距d来考虑纤维之间的相互影响,改变脱粘段的剪切强度和粘结段的临界能量释放率,推导出了纤维拉拔荷载和纤维脱粘长度之间的变化关系,与StangH的模型进行了对比,当纤维间距较大时,纤维之间的相互影响相对较小,此时与StangH的单根纤维拉拔情况较为相符,但当纤维间距较小时,由于临近纤维的影响,使得在相同脱粘长度的情况下,纤维拉拔荷载和纤维拔出端位移有减小的趋势,改变复合材料板层的厚度,由于影响了基体的变形,界面的脱粘和纤维的拔出行为也受到了相应的影响。     

弱界面端初始失效位置的试验初探

通过双材料受压试验研究了弱奇异性界面端的界面初始脱粘点的位置问题。实验材料采用钢和有机玻璃,根据弱奇异性要求和界面端应力分布特征,设计、制作了试件,并进行了初始脱粘实验。由实验发现楔形角为30°、45°和55°试件的界面初始脱粘点位置均在界面内部,脱粘是由有限应力引起的。63°是个临界角楔形角,大于63°试件,脱粘多会从界面端开始,即脱粘由奇异的界面应力引起的。此外界面正应力若为拉应力时,它对界面脱粘有很大的贡献。     

纤维／基体界面裂纹扩展断裂能

本文通过研究纤维／基体界面裂纹断裂能，分析了几种界面脱粘断裂功的理论表达式；着重研究了单纤维拔出试验中界面残余压应力及界面摩擦对界面裂纹能量释放率的重要性．     

纤维复合材料界面横向拉伸分析

以单纤维十字型横向拉伸试验为研究对象,对纤维/基体界面采用弹性-软化双线性内聚力模型,建立了纤维复合材料在横向拉伸作用下界面法向失效过程的解析模型。得到了沿纤维/基体圆周界面的法向应力分布,纤维/基体界面的状态与界面承载力和单纤维复合材料承载力的关系,以及内聚力参数和试件几何尺寸对它们的影响。结果表明:纤维/基体圆周界面在脱粘前经历全部弹性及弹性+软化两种状态;当界面为弹性状态时,界面法向应力随界面强度线性增加;当界面为弹性+软化状态时,界面软化范围随界面裂纹萌生位移的增加而增大;界面初始脱粘位置与拉伸荷载方向重合;界面初始脱粘时的界面承载力随界面强度及界面裂纹萌生位移的增加而增加,随界面裂纹生成位移的增加而降低;单纤维复合材料的脱粘荷载受基体截面尺寸的影响,当纤维体积含量相同时,沿荷载方向截面尺寸的增大对提高脱粘荷载更显著。     

含空心球涂层粒子复合材料界面的脱粘应力分析

在球对称拉伸载荷作用下针对空心球涂层复合材料分析了空心球涂层粒子增强复合材料的局部应力场，得到了界面临界脱粘应力的解析表达式．讨论了各相几何参数对非均匀涂层空心球粒子临界脱粘应力的影响，比较了均匀涂层和非均匀涂层的脱粘应力．结果表明：在球对称拉伸下界面脱粘更容易发生在涂层相与基体相界面间，空心球的壁厚和涂层厚度是影响界面临界脱粘应力的重要因素，因而选择适当的空心球、涂层厚度和提高界面粘结能将有利于提高界面的临界脱粘应力．     

FRP加固RC梁界面疲劳损伤红外检测分析

FRP-混凝土界面剥离损伤的探测是界面力学分析的一个难点。基于三个标准试件探讨了红外检测方法对FRP-混凝土界面剥离探测的精度、可行性以及剥离判断的标准,并对常幅疲劳荷载下FRP加固钢筋混凝土(RC)梁界面的疲劳行为进行了跟踪记录,分析了界面的疲劳破坏过程。试验结果表明,FRP加固RC梁界面存在初始的未粘结区,在疲劳加载的初期界面剥离快速增加,随后在大部分疲劳寿命期内保持稳定,在最后数千次加载循环内界面损伤失稳发展导致整个加固构件的破坏。文中基于红外数据给出了每个阶段的疲劳加载次数和界面剥离损伤的面积。     

Micro-mechanical modelling of fatigue damage in titanium metal matrix composites

In this work, the benefits from the blending between micro-structural fracture mechanics and elasto-plastic fracture mechanics in the analysis of fatigue damage in titanium metal matrix composites (TMCs) is presented. The efficiency of interfacial debonding and fibre bridging (FB) are shown not only to control crack growth but also to be responsible for severe crack growth changes taking place throughout the material's response under fatigue. A possible way to extract answers about the fatigue threshold, the operational life of the material and finally the fracture toughness is given in detail.     

Numerical analysis of interface fatigue of fiber reinforced composites

A numerical analysis of the interface debonding process is carried out for fiber–matrix composites under cyclic loading. The results show that the interface friction plays an important role in resisting debonding under fatigue. The degradation coefficient of interface friction influences the debonding velocity. An approximate formula of interface friction is found for fatigue debonding.     

Bond decay at bar–concrete interface under variable fatigue loads

The bond decay at the bar–concrete interface under variable fatigue loads is investigated in this paper. Two kinds of loading paths are considered: low- and high-amplitude cycles. Based on the shear-lag model, the governing equations of the problem are established and solved with reference to three different interfacial degradation models. By the aid of Paris formula, the interfacial debonding rate, the debonded length, and the pull-out force are studied. The effect of the amplitude of the middle applied load and of several material parameters on bond decay is discussed. It is found that the power-degradation model is suitable for short embedment lengths, while both the linear-degradation model and the modified power-degradation model are advantageous in study of bond in long embedment length. It is also found that interfacial friction plays an important role in resisting interfacial debonding, in spite of the effects of Poisson's contraction.     

SGBEM modeling of fatigue crack growth in particulate composites

The symmetric-Galerkin boundary element method (SGBEM) has previously been employed to model 2-D crack growth in particulate composites under quasi-static loading conditions. In this paper, an initial attempt is made in extending the simulation technique to analyze the interaction between a growing crack and clusters of perfectly bonded particles in a brittle matrix under cyclic loading conditions. To this end, linear elastic fracture mechanics and no hysteresis are assumed. Of particular interest is the role clusters of inclusions play on the fatigue life of particulate composites. The simulations employ a fatigue crack growth prediction tool based upon the SGBEM for multiregions, a modified quarter-point crack-tip element, the displacement correlation technique for evaluating stress intensity factors, a Paris law for fatigue crack growth rates, and the maximum principal stress criterion for crack-growth direction. The numerical results suggest that this fatigue crack growth prediction tool is as robust as the quasi-static crack growth prediction tool previously developed. The simulations also show a complex interplay between a propagating crack and an inclusion cluster of different densities when it comes to predicting the fatigue life of particulate composites with various volume fractions.     

Dynamic interfacial debonding initiation induced by an incident crack

When a crack propagates towards a weak interface, interface debonding may occur before the incident crack reaches the interface. This phenomenon refers to the “Cook–Gordon mechanism”. In this investigation, an equivalent dynamic Cook–Gordon mechanism is studied both experimentally and analytically. Two strength-based criteria incorporating dynamic fracture mechanics analysis are proposed to predict the initiation location of interface debonding ahead of a dynamic incident crack. As validation, a comparison is made between the analytical predictions and experimental measurements. Results show that the strength-based criteria can effectively predict the initiation of interface debonding. Meanwhile, effects of the stress intensity factor and the T stress of the incident crack, on the interfacial debonding initiation are investigated. It is concluded that high-stress intensity factors of the incident cracks will easily induce interfacial debonding initiation, and changing the T stress is an effective way to control interfacial debonding initiation. Furthermore, high-interfacial tensile strengths rather than shear strengths, tend to suppress interfacial debonding initiation induced by a mode-I incident crack.     

Experimental Investigation and Finite Element Analysis of Fatigue Crack Growth in Pipes Containing a Circumferential Semi-elliptical Crack Subjected to Bending

In this paper, a circumferential external surface flaw in a metallic round pipe under cyclic bending loading is considered. Because of very rapid changes in the geometrical parameters around the crack front region, the mesh generation of this region must be done with great care. This may lead to an increase in the run time which makes it difficult to reach valid results and conclusions. Because of the advantages of the sub-modeling technique in problems which need very high mesh density, this method is used. Stress intensity factors in mode I condition are determined using three-dimensional finite element modeling with 20 node iso-parametric brick elements in the ANSYS 9.0 standard code and the singular form of these finite elements at the crack front. In order to estimate the analysis error, the structural parameter error in energy norm criterion was used. Because of the advantages of non-dimensional analysis, this method is employed, and the stress intensity factors are normalized. For the analysis of the fatigue crack growth, the Paris law is used. The propagation path of the surface flaw is obtained from the diagram of aspect ratio versus relative crack depth. The fatigue crack growth analysis (the relative crack depth against loading cycles diagram) of different initial crack aspect ratio under cyclic loading is also considered. Fatigue shape development of initially semi-elliptical external surface defects is illustrated. The effect of the Paris exponent (material constant) on fatigue crack propagation is shown as well. Moreover, the fatigue crack growth of several specimens is assessed experimentally using a manually-constructed experimental set up. Finally, the experimental results obtained by cyclic bending loading tests are compared with the numerical results. The experimental results show good conformity with the finite element results.     

Cohesive zone laws for fatigue crack growth: Numerical field projection of the micromechanical damage process in an elasto-plastic medium

Cohesive zone failure models are widely used to simulate fatigue crack propagation under cyclic loading, but the model parameters are phenomenological and are not closely tied to the underlying micromechanics of the problem. In this paper, we will inversely extract the cohesive zone laws for fatigue crack growth in an elasto-plastic ductile solid using a field projection method (FPM), which projects the equivalent tractions and separations at the cohesive crack-tip from field information outside the process zone. In our small-scale yielding model, a single row of discrete voids is deployed directly ahead of a crack in an elasto-plastic medium subjected to cyclic mode I K-field loading. Damage accumulation under cyclic loading is captured by the growth of voids within the micro-voiding zone ahead of the crack, while the evolution of the cohesive zone law representing the micro-voiding zone is inversely extracted via the FPM. We show that the field-projected cohesive zone law captures the essential micromechanisms of fatigue crack growth in the ductile medium: from loading and unloading hysteresis caused by void growth and plastic hardening, to the softening damage locus associated with crack propagation via a void by void growth mechanism. The results demonstrate the effectiveness of the FPM in obtaining a micromechanics-based cohesive zone law in-place of phenomenological models, which opens the way for a unified treatment of fatigue crack problems.