基于频率变化预测玻璃纤维增强树脂复合材料层合板的剩余疲劳寿命

复合材料结构在疲劳过程中的累积损伤将导致结构刚度下降,并进一步引起结构的动态参数如频率发生衰减。因此,可以将结构疲劳状态与结构频率联系起来,基于频率预测结构的剩余疲劳寿命。本文首先基于复合材料在纵向、横向和面内剪切三个方向的疲劳特性,结合ABAQUS与Umat子程序开发了三维有限元模型模拟复合材料层合板中的疲劳损伤演变,并构建了不同疲劳状态下对应的模态分析模型,由此获得了疲劳过程中的频率衰减曲线。之后,基于疲劳过程的频率变化量训练了人工神经网络,用于预测玻璃纤维增强复合材料层合板的剩余疲劳寿命。特别地,在当前的数值模型中为每个单元分配了符合高斯正态分布的材料属性,以模拟实际情况下复合材料性能的离散性。结果表明,疲劳模型数值模拟结果与已有文献的疲劳实验数据吻合,基于频率变化量训练的人工神经网络可以成功预测玻璃纤维增强复合材料试件的剩余疲劳寿命。      

层板复合材料疲劳性能的实验研究

对层板复合材料在拉伸 -拉伸疲劳载荷作用下的初始静刚度、初始静强度、剩余刚度、剩余强度、疲劳寿命进行了实验研究 ,取得了大量的有意义的实验数据 ,分析了层板复合材料的初始静刚度、初始静强度和疲劳寿命的概率分布 ,讨论了层板复合材料在不同应力水平下剩余刚度随疲劳循环周次的衰减变化及损伤破坏的形式 ,得到了一些有意义的结论     

层板复合材料疲劳性能的实验研究

对层板复合材料在拉伸－拉伸疲劳载荷作用下的初始静刚度、初始静强度、剩余刚度、剩余强度、疲劳寿命进行了实验研究,取得了大量的有意义的实验数据,分析了层板复合材料的初始静刚度、初始静强度和疲劳寿命的概率分布,讨论了层板复合材料在不同应力水平下剩余刚度随疲劳循环周次的衰减变化及损伤破坏的形式,得到了一些有意义的结论。     

电阻法预报CFRP层板疲劳损伤演变过程

通过类比方式,以电阻变化作为复合材料的损伤参数,建立了用电阻损伤参数预报CFRP层板疲劳损伤的数学模型,通过拉-拉疲劳试验,对CFRP层板的疲劳损伤与疲劳寿命预报进行研究,建立了累积损伤中电阻与疲劳损伤状态之间的关系.研究表明,在疲劳损伤过程中,电阻损伤参数的变化同材料的疲劳损伤过程密切相关,电阻损伤规律可分为下降阶段、平稳阶段和破坏阶段;碳纤维材料的电阻变化能够反映纤维的损伤程度和破坏形式,可以作为表征复合材料纤维损伤的有效参数.     

基于超声导波的复合材料结构疲劳损伤监测实验研究

为快速评估复合材料结构的疲劳损伤状况,采用超声导波和时-频分析相结合的方法,对疲劳状态下的复合材料结构进行在线连续监测。对玻璃纤维增强复合材料试件进行拉伸疲劳试验,使用激光引伸计获取试件纵向刚度的变化,探究试件内部疲劳损伤的累积情况。利用小波变换对由压电传感元件激励和接收的超声导波信号在时频域进行分析,提取与疲劳损伤有关的信号特征;最后,通过多元偏值分析引入马氏平方距离,融合多个信号特征,确定复合材料试件中疲劳损伤的存在性以及表征其演变过程。实验结果表明所提出方法在复合材料结构疲劳损伤连续监测方面的有效性。     

C/C复合材料的疲劳性能及损伤研究现状

碳/碳复合材料作为理想的高温结构材料,在服役过程中不可避免地涉及疲劳加载的情况,其疲劳行为的研究具有十分重要的意义.通过对近年来C/C复合材料疲劳行为的研究情况进行的综述,总结出了疲劳特点以及疲劳过程中材料微观结构的变化特点.并在此基础上,对今后的研究工作发表了一些看法.     

基于声发射技术的陶瓷基复合材料声速特性

基于Christoffel方程,运用复合材料刚度矩阵与弹性常数间的关系,将正交各向异性模型运用于2D-C/SiC复合材料的声学特性中,得到材料声速的表达式。通过循环加卸载试验测量了2D-C/SiC复合材料整个拉伸过程中不同应力水平处的声速变化,研究了声速对2D-C/SiC复合材料的损伤表征。研究发现,随着应力水平的不断增加,声速逐渐下降,2D-C/SiC复合材料损伤程度对声波在材料中的传播速度有较大影响;引入卸载模量和再加载模量,代替声速理论计算切线模量,理论结果与试验结果吻合良好,误差随载荷增加而增大;声波速度随2D-C/SiC复合材料损伤而发生衰减的关系,根据此衰减关系建立了基于声速的损伤表征量。      

孔隙演化与C/C复合材料的疲劳行为

孔隙是C/C复合材料结构中重要的组成部分,它直接影响着材料的疲劳行为,因此,在研究C/C复合材料的疲劳行为时,考察基体以及界面中孔隙的变化具有重要的意义.综述了C/C复合材料原始孔隙结构的特点,分析了在疲劳加载过程中碳/碳复合材料孔隙结构的演化规律,强调了孔隙结构变化对碳/碳复合材料疲劳强化所作出的积极贡献,进而为其疲劳机理的研究提供依据.     

光纤传感器监测复合材料固化过程

设计了两种新型光纤传感器,一种是通过测量光纤末端近场模斑谱反映光纤埋置 脂折射率的变化,另一种行微弯衰减方法测量复合材料固化中铺层被压缩的进程。利用这两种传感器进行了复合材料固化监测实验,结果表明,发现光纤模斑传感器的信号可以反映树脂基体的粘度的变化历程,光纤微弯传感器的信号可以反映铺层被压缩的过程,监测这两个关键的参数变量不但有利于操作工艺的优化,而且为建立复合材料因化过程的智能化在线监控系统奠     

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。     

Monitoring quasi-static and cyclic fatigue damage in fibre-reinforced plastics by Poisson’s ratio evolution

Even if the extent of fatigue damage in fibre-reinforced plastics is limited, it can already affect the elastic properties. Therefore, the damage initiation and propagation in composite structures is monitored very carefully. Beside the use of nondestructive testing methods (ultrasonic inspection, optical fibre sensing), the follow-up of the degradation of engineering properties such as the stiffness is a common approach.In this paper, it is proved that the Poisson’s ratio can be used as a sensitive indicator of fatigue damage in fibre-reinforced plastics. Static tests, quasi-static cyclic tests and fatigue tests were performed on [0°/90°]_2s glass/epoxy laminates, and longitudinal and transverse strain were measured continuously. The evolution of the Poisson’s ratio ν_xy versus time and longitudinal strain ε_xx is studied. As the transverse strain measurement is crucial to monitor the degradation of the Poisson’s ratio, three techniques were applied to measure the transverse strain (strain gauges, mechanical extensometer and external optical fibre sensor).Finally, the technique has been applied to a totally different material: a carbon fabric thermoplastic composite. The results show a very similar degradation of the Poisson’s ratio, although no stiffness degradation can be observed during fatigue loading of this material.It is concluded that the degradation of the Poisson’s ratio can be a valuable indicator of fatigue damage, in combination with the stiffness degradation.     

Monitoring of non-homogeneous strains in composites with embedded wavelength multiplexed fiber Bragg gratings: A methodological study

For the detection of problems like fastening failures and delamination in composites, the employment of embedded fiber Bragg grating (FBG) sensors is considered an effective tool to measure non-homogeneous internal strains near damage or interfaces of dissimilar materials which in turn can help identify the severity of fracture. In this work wavelength multiplexed FBGs are used to measure strains during gradual adhesive fastening failure in a single lap joint and delamination in a uniaxial composite, during monotonic and fatigue loads. It is shown that such measurements can be effectively used to determine the stresses near the joint and bridging tractions in delamination. The particular method can provide a significant tool in experimental mechanics and fatigue of composites as well as composite assemblies. Experimental measurements are in very good agreement with the numerical analysis concurrently used for the verification of the experimental procedure’s feedback.     

Poisson's ratio as a sensitive indicator of (fatigue) damage in fibre-reinforced plastics

Even if the extent of damage in fibre‐reinforced plastics is limited, it already affects the elastic properties. Therefore, the damage initiation and propagation in composite structures is monitored very carefully. Beside the use of nondestructive testing methods (ultrasonic inspection, optical fibre sensing), the follow‐up of the degradation of engineering properties such as the stiffness is a common approach. In this paper, it is investigated if the Poisson's ratio can be used as a sensitive indicator of (fatigue) damage in fibre‐reinforced plastics. Static, cyclic and fatigue tests have been performed on [0°/90°]_2s glass/epoxy laminates, and axial and transverse strain were measured continuously. The evolution of the Poisson's ratio ν_xy versus time and axial strain ɛ_xx is studied. It is concluded that the degradation of the Poisson's ratio can be a valuable indicator of damage, in combination with the stiffness degradation.     

Optical fibre sensors for health monitoring of bonded repair systems

In the aircraft industry the use of externally bonded composite repairs has become an accepted way of repairing fatigue, or corrosion, damaged metallic structural components. However, current NDI and damage assessment techniques for composite repairs are passive and generally performed on ground. The challenge is to develop new techniques utilising recent analytical and experimental tools. This report examines the use of optical fibre sensors. Optical fibres offer a means of monitoring the load transfer process in these repairs, and can therefore be used to provide an indication of the integrity of the repair. This paper describes the use of an array of fibre Bragg grating strain sensors (FBGs) for the in situ monitoring of bonded repairs to aircraft structures and, in particular, the monitoring of crack propagation beneath a repair. In this work the FBGs have been multiplexed using a combination of wavelength and spatial techniques employing a tunable Fabry–Pérot (FP) filter to track individual gratings. The multiplexed FBGs were then surface-mounted on a boron–epoxy unidirectional composite patch bonded to an aluminium component. The sensors were located so as to monitor the changing stress field associated with the propagation of a crack beneath the patch. The ability of relating experimental results to sensor readings is then confirmed using both a thermo-elastic scan of the patch and 3D finite element analysis.     

Flexural behaviour of UHTCC‐layered concrete composite beam subjected to static and fatigue loads

As an engineered material, ultra‐high toughness cementitious composite (UHTCC) exhibits the characteristics of pseudo strain hardening and multiple cracking under uniaxial tension. It can be applied as the reinforcing and protective material of concrete structures. In this paper, static and fatigue flexural tests were carried out on UHTCC‐layered concrete composite beams, for which UHTCC layer was used on the tension side. Under both static and fatigue loads, plane section assumption was suitable for such composite beams, and a good bond strength was achieved between the two layers. For static specimens, the UHTCC layer enhanced the ductility of the concrete layer. While under cyclic loads, because of the reinforcing effect of UHTCC, more than one crack were formed in the concrete layer, which led to a ductile deformation. Furthermore, the fatigue damage process of the composite beam was analysed.     

Effect of fatigue loading on the modal properties of composite structures and its utilization for prediction of residual life

The design of composite structures for fatigue loading utilizes S-N curves to predict the fatigue life based on coupon testing. Due to the complex manufacturing process, the initial fabrication inconsistencies may cause the predicted number of cycles to failure to be off by one or two orders of magnitude. An experimental method is developed to predict the residual fatigue life of composite structures and continuously refine the prediction during the service life of these structures. Global effects on modal parameters, natural frequencies and damping ratios are investigated during the fatigue process. The Random Decrement signal processing technique is used. Changes of modal parameters are correlated with the prediction of fatigue failure life for selected graphite/epoxy composite specimens. The nature of the proposed method lends itself to real-time applications, especially, in service structures, as a nondestructive indicator.     

Critical review on the assessment of fatigue and fracture in composite materials and structures

This paper provides a critical review of common approaches to assess fatigue and fracture in composite materials and structures. It explains how fatigue in composite materials and structures should be understood based on the observed fatigue phenomena and mechanisms. In relation to these phenomena, the selection of proper similitude conditions for predictions is discussed and it is explained how mechanistic models can be developed that describe fatigue damage growth in its sequence from first initiation to final failure. An explanation is given for the fact that these mechanistic damage growth approaches have not yet led to suitable prediction models. In addition, this paper illustrates that in-depth understanding of individual damage mechanics may pave the road towards further material optimization with respect to fatigue and durability.     

Fatigue and post-fatigue behaviour of carbon/epoxy non-crimp fabric composites

This paper focuses on the static, fatigue and post-fatigue tensile properties of a biaxial carbon/epoxy non-crimp fabric composite. In a series of quasi-static tensile tests, the stress–strain level where damage initiates was determined. This stress level was then used as the maximum stress level in tensile–tensile fatigue tests in the fibre direction. It was found that in fibre direction, this load level can be considered safe for fatigue up to very high cycle numbers. The damage evolution during the tests was monitored at certain cycle times with X-ray radiography. The post-fatigue residual static tensile properties were determined after different numbers of cycles. A series of tensile–tensile fatigue tests at various higher stress levels allowed for the fatigue life curves to be constructed in each of the four testing directions. This revealed that the damage initiation load level is well below the practical fatigue limit of the material.     

轮胎疲劳损伤过程的数值监测

利用系统分析的方法和信号处理技术,建立了轮胎在疲劳损伤过程的数学模型,结合大量的试验结果,定义了轮胎的疲劳损伤参数,并在此基础上以在线的方式,对轮胎疲劳损伤过程进行监测和数值跟踪描述、经轮胎耐久性疲劳试验结果表明：基于系统分析的方法所定义的轮胎疲劳损伤参数,能够有效跟踪在轮胎的复合层、橡胶层和复合层与橡胶层之间的结构内部的疲劳损伤的发展时间、疲劳裂纹扩展过程以及各层间结合的疲劳强度、裂纹扩展阻力等数值结果．