Residual Strains using Integrated Continuous Fiber Optic Sensing in Thermoplastic Composites and Structural Health Monitoring

The evolution of spatially resolved internal strain/stress during the manufacturing of thermoplastic composites and subsequent relaxation from water intake are evaluated using an in-situ fiber optic sensor corresponding to a coated optical glass fiber with a nominal diameter of 160 μm. Unidirectional carbon fiber-polyamide 6 composites are produced using compression molding with an embedded fiber optic for strain measurement. The distributed fiber optic based strain sensor is placed in an arrangement to capture 0, 45, and 90° strains in the composite to resolve in-plane strain tensor. Strains are monitored in the direction of fiber optic sensor along its length at high resolution during the various stages of compression molding process. Results indicate considerable internal strains leading to residual stress at the end of processing step along the off-axis (45°) and transverse (90°) directions, and small strains in the carbon fiber pre-preg (0°) direction. At the end of compression molding process, an average of 7000 and 10,000 compressive micro-strains are obtained for residual state of strain in the off-axis and transverse direction. Since water/moisture infusion affects the mechanical properties of polyamide-6 matrix resin, these composite panels with embedded sensors targeted for marine applications are monitored in a water bath at 40 °C simulating accelerated testing conditions. Using the same fiber optic sensor based technique, the strain relaxation was observed during water uptake demonstrating in-situ strain monitoring during both manufacturing and subsequent composite implementation/application environment. The technique presented in this paper shows the potential of optimizing time-temperature-pressure protocols typically utilized in thermoplastic manufacturing, and continuous life-cycle monitoring of composite materials using a small diameter and inexpensive distributed fiber optic sensing.     

Measurement of inhomogeneous strain fields by fiber optic sensors embedded in a polymer composite material

Experimental results of strain field measurement in polymer composite specimens by Bragg grating fiber optic strain sensors embedded in the material are considered. A rectangular plate and a rectangular plate with “butterfly” shaped cuts are used as specimens. The results of uniaxial strain experiments with rectangular plates show that fiber optic strain sensors can be used to measure the strains, and these results can be used to calculate the calibration coefficients for fiber optic strain sensors. A gradient strain field is attained in a plate with cuts, and the possibility of measuring this field by fiber optic strain sensors is the main goal of this paper. The results of measurements of gradient strain fields in the plate with cuts are compared with the results obtained by using the three-dimensional digital optic system Vix-3D and with the results of numerical computations based on finite element methods. It is shown that the difference between the strain values obtained by these three methods does not exceed 5%.     

树脂基三维编织复合材料粘弹性性能的有限元分析

根据材料的细观结构,采用APDL语言分别建立了纤维束和三维编织复合材料两级单胞的参数化几何模型;推导了Prony级数表示的树脂粘弹性本构方程,对模型进行了组分材料参数设置;对纤维束单胞模型进行扫掠式网格划分,对三维编织复合材料单胞模型进行线-面-体式网格划分;对两级单胞模型均施加合理的边界条件,使单胞边界上的位移满足周期性和连续性。以有限元模型为基础,计算了三维编织复合材料的粘弹性能,并给出了材料粘弹性效应随工艺参数变化的规律。计算结果表明:三维编织复合材料编织方向的粘弹性效应随编织角的增大而增强,随纤维体积比的增大而减弱。该结果与已有实验结论一致。     

Experimental and analytical characterization of multidimensionally braided graphite/epoxy composites

This paper presents results of an investigation of a novel, through-the-thickness fiber-reinforced composite material. The generic name for this composite technology is multidimensional (X-D) braiding. X-D braided composites consist of a net-shaped, densely braided fiber skeleton which is rigidized with a structural epoxy-resin system. This material is an alternative to the conventional laminated composite and has the potential for being more resistant to delamination and matrix cracking. This paper describes results of the mechanical characterization of one graphite fiber system which was braided into panels in which two braid parameters could be investigated. The variables investigated included the effect of edge condition and braid pattern on the tensile, compressive and flexural properties of the braided panels. These properties were obtained in the braid direction only. The cutting of the specimen edges substantially reduced both tensile and flexural strengths and moduli. Of the three braid patterns investigated, 1×1, 3×1, and 1×1×1/2 F, the 3×1 braid pattern showed superior tensile performance, while the 1×1×1/2 F braid pattern exhibited superior flexural properties. The development of an analytical method for modeling the tensile performance of the multidimensionally (X-D) braided composite is also presented. The fiber geometry in X-D braids was modeled based on the braid parameters used in the construction of these composites. By the nature of the symmetry of the resulting braided structure, an analytical model based on classical lamination theory was used to determine the extensional stiffness in the three principal geometric directions of a braided composite. These analytical results are shown to compare favorably with those obtained experimentally. Finally, to further validate the ability of this material to contain damage, multidimensionally braided and conventionally laminated panels were impacted and the resulting damage was nondestructively determined. The multidimensionally braided material was shown to reduce the area of damage caused by impact by a factor of three for the energy levels tested.     

埋光纤光栅传感器智能土木结构应变监测

采取与土木工程施工特点相适应的操作工艺与保护方法，将光纤Bragg光栅传感器埋入钢筋混凝土结构中，分别在实验室和实际工程施工现场对混凝土内部的应变进行了测量。实验中光纤Bragg光栅传感性能良好，能够监测混凝土内部应变状态，显示出成活率高、绝对值测量、寿命长等优势，为建立基于光纤传感器的结构健康监测系统奠定了基础。     

基于准分布式FBG传感器的均质材料内部应变测试的试验研究

采用环氧树脂材料作为试验载体,使用没有任何保护性封装的、运用波分复用技术在同一光纤上刻入三个布拉格光栅的基础FBG传感器,来监测液体树脂流动及固化成型的全过程,并使用固化成型的环氧树脂板进行三点弯曲试验。将所得数据与有限元模拟数值解进行比较,来验证在去除封装、将传感器自身应变影响降到最低的情况下,FBG传感网络是否依然能够有效地反映出试验对象在各种情况下的内部应变。本文给出了FBG光纤光栅在逐级静力荷载加载下材料内部三点弯曲的应变图,并通过数值计算分析了未封装光纤在复合材料内部受力过程中的力学性能。     

不同温度下2种钨结构增强Zr基非晶合金的动态压缩性能

利用SHPB装置对钨丝增强Zr基非晶复合材料和钨骨架增强Zr基非晶复合材料进行了3种环境温度下多种应变率的动态压缩性能测试。比较了2种材料的动态力学性能,发现二者均具有应变率敏感性和较强的塑性变形能力。但二者承载机制存在较大差异。钨丝增强结构变形主要表现在钨丝的失稳,由数值模拟初步分析了这种局部结构失稳控制的变形以及热失稳现象;钨骨架增强结构变形前期钨骨架起主要承载作用,而不是各成分的共同作用,这导致材料的屈服强度比纯非晶和纯钨的低。     

Homogenization of long fiber reinforced composites including fiber bending effects

This paper presents a homogenization method, which accounts for intrinsic size effects related to the fiber diameter in long fiber reinforced composite materials with two independent constitutive models for the matrix and fiber materials. A new choice of internal kinematic variables allows to maintain the kinematics of the two material phases independent from the assumed constitutive models, so that stress–deformation relationships, can be expressed in the framework of hyper-elasticity and hyper-elastoplasticity for the fiber and the matrix materials respectively. The bending stiffness of the reinforcing fibers is captured by higher order strain terms, resulting in an accurate representation of the micro-mechanical behavior of the composite. Numerical examples show that the accuracy of the proposed model is very close to a non-homogenized finite-element model with an explicit discretization of the matrix and the fibers.     

稀土处理炭纤维填充聚四氟乙烯复合材料在干摩擦条件下的摩擦磨损性能研究

研究了稀土元素(RE)处理炭纤维表面的最佳添加量和不同炭纤维表面处理对聚四氟乙烯(PTFE)复合材料在干摩擦条件下摩擦磨损性能的影响,并利用扫描电子显微镜对其磨损表面进行观察和分析.结果表明:当稀土元素在表面改性剂中的含量为0.3%时,炭纤维填充聚四氟乙烯复合材料的摩擦磨损性能最佳;在干摩擦条件下,表面处理炭纤维填充聚四氟乙烯复合材料的摩擦系数比未经处理炭纤维填充聚四氟乙烯复合材料的低,且其耐磨性较好;稀土处理使得复合材料的界面强韧性得到明显改善,从而提高了其摩擦磨损性能.     

粘弹性树脂基三维编织复合材料的热膨胀性能研究

首先利用均匀化理论并结合有限元法研究了三维编织复合材料的粘弹性效应,根据松弛模量的计算结果研究了材料热膨胀系数随时间的变化关系,在此基础上,给出了编织结构和工艺参数(编织角、纤维体积比)对材料初始热膨胀系数的影响规律,计算结果与实验值吻合较好。本文工作为深入研究三维编织复合材料的热粘弹性能提供了基础。     

倾斜内锁型三维机织陶瓷基复合材料的细观力学模型

利用平均化方法提出了倾斜内锁型三维机织陶瓷基复合材料弹性性能分析的三维细观力学模型,对材料的弹性性能进行了预测。这个力学模型考虑了倾斜内锁型三维机织陶瓷基复合材料经向纤维束的弯曲和纬向纤维束的平直,纤维束的横截面形状尺寸和相邻纤维束之间的孔洞以及材料制造过程中碳纤维性能下降对弹性性能的影响。基于层合板理论,提出两种单胞应变状态假设分别对材料的九个弹性常数进行了推导计算,结果表明两种方法理论的预测值非常接近。计算结果与实验值比较吻合,表明所提出的细观力学模型是合理的,可以为纺织陶瓷基复合材料的优化设计提供有价值的参考。     

Optimization of Digital Image Correlation for High-Resolution Strain Mapping of Ceramic Composites

Digital image correlation (DIC) is assessed as a tool for measuring strains with high spatial resolution in woven-fiber ceramic matrix composites. Using results of mechanical tests on aluminum alloy specimens in various geometric configurations, guidelines are provided for selecting DIC test parameters to maximize the extent of correlation and to minimize errors in displacements and strains. The latter error is shown to be exacerbated by the presence of strain gradients. In a case study, the resulting guidelines are applied to the measurement of strain fields in a SiC/SiC composite comprising 2-D woven fiber. Sub-fiber tow resolution of strain and low strain error are achieved. The fiber weave architecture is seen to exert a significant influence over strain heterogeneity within the composite. Moreover, strain concentrations at tow crossovers lead to the formation of macroscopic cracks in adjacent longitudinal tows. Such cracks initially grow stably, subject to increasing app lied stress, but ultimately lead to composite rupture. Once cracking is evident, the composite response is couched in terms of displacements, since the computed strains lack physical meaning in the vicinity of cracks. DIC is used to identify the locations of these cracks (via displacement discontinuities) and to measure the crack opening displacement profiles as a function of applied stress.     

Micromechanics-based constitutive modeling for unidirectional laminated composites

A micromechanics-based constitutive model is developed to predict the effective mechanical behavior of unidirectional laminated composites. A newly developed Eshelby’s tensor for an infinite circular cylindrical inclusion [Cheng, Z.Q., Batra, R.C., 1999. Exact Eshelby tensor for a dynamic circular cylindrical inclusion. J. Appl. Mech. 66, 563–565] is adopted to model the unidirectional fibers and is incorporated into the micromechanical framework. The progressive loss of strength resulting from the partial fiber debonding and the nucleation of microcracks is incorporated into the constitutive model. To validate the proposed model, the predicted effective stiffness of transversely isotropic composites under far field loading conditions is compared with analytical solutions. The constitutive model incorporating the damage models is then implemented into a finite element code to numerically characterize the elastic behavior of laminated composites. Finally, the present predictions on the stress–strain behavior of laminated composite plate containing an open hole is compared with experimental data to verify the predictive capability of the model.     

On the effect of fiber creep-compliance in the high-temperature deformation of continuous fiber-reinforced ceramic matrix composites

Creep models for unidirectional ceramic matrix composites reinforced by long creeping fibers with weak interfaces are presented. These models extend the work of Du and McMeeking (1995) [Du, Z., McMeeking, R. 1995. Creep models for metal matrix composites with long brittle fibers. J. Mech. Phys. Solids 43, 701–726] to include the effect of fiber primary creep present in the required operational temperatures for ceramic matrix composites (CMCs). The effects of fiber breaks and the consequential stress relaxation around the breaks are incorporated in the models under the assumption of global load sharing and time-independent stochastics for fiber failure. From the set of problems analyzed, it is found that the high-temperature deformation of CMCs is sensitive to the creep-compliance of the fibers. High fiber creep-compliance drives the composite to creep faster, leading however to greater lifetimes and greater overall strains at rupture. This behavior is attributed to the fact that the greater the creep-compliance of the fibers, the higher the creep rate but the slower the matrix stress relaxation – since the matrix must deform with a rate compatible with the more creep-resistant fibers – and therefore the less the load carried by the main load-bearing phase, the fibers. As a result, fewer fibers fail and less damage is accumulated in the system. Moreover, the greater the creep-compliance of the fibers, the slower the matrix shear stress relaxation – and thus the lower the levels of applied stress for which this effect becomes important. The slower the shear stress relaxes, the slower the “slip” length increases. Due to the Weibull nature of the fibers, the fiber strengths at the smaller gauge length of the slip length are stronger; therefore fewer fibers undergo damage. Hence, high fiber creep-compliance is desirable (in the absence of any explicit creep-damage mechanism) in terms of composite lifetime but not in terms of overall strain. These results are considered of importance for composite design and optimization.     

Evaluation of Failure Behavior of Transversely Loaded Unidirectional Model Composites

A leading reason for the limited use of laminated composite materials in primary structural applications is that the failure initiates in the ply oriented transverse to the direction of the applied load at a much lower strain level than that which would cause the ultimate failure of the laminate. Previous studies indicate that transverse failure is manifested as either cavitation-induced failure of the matrix system or fiber-matrix debonding. The mechanism causing the failure initiation event is not decidedly known and depends on the local stress field of the constrained matrix that is a function of fiber spacing. In the present study a model composite system using a transparent matrix is employed in a cruciform-shaped specimen to evaluate the viability of several transverse failure theories. The cruciform-shaped specimen utilizes a low strain-to-failure 828/D230 RT cured epoxy and stainless steel wires arranged such that a fiber is placed at the intersection of face diagonals of four remaining fibers located at corners of a square. The transverse failure mechanism is observed in-situ via the reflected light method and recorded utilizing high resolution, high magnification microscope cameras. A parametric study is conducted using three dimensional finite element models to analyze the stress state in the cruciform specimen as a function of fiber spacing. The result of the 3-D FE models in conjunction with experimental observations are used to evaluate the transverse failure theories suggested in the literature. In addition this data will be used to develop a comprehensive failure criterion for transversely loaded multi-fiber composites that encompasses the dependence on fiber spacing.     

绞合式光纤应变传感器

研究了一种可用于工程结构状态监测的本征型强度调制光纤应变传感器,由两根以上多模光纤相互绞合形成,分析了该传感器的应变传感原理,得到其既能测量拉应变,又有测定压应变的结论,传感器对应变的响应具有良好的线性和重复性、灵敏度高、无迟滞现象,对钢筋拉应变、混凝土压就矿业的实验结果与理论分析一致,表明该传感器是适合土木工程测量、结构状态监测等方面的较为理想的光纤传感器。     

Modeling of fiber-reinforced cement composites: Discrete representation of fiber pullout

The discrete modeling of individual fibers in cement-based materials provides several advantages, including the ability to simulate the effects of fiber dispersion on pre- and post-cracking composite performance. Recent efforts in this direction have sought a balance between accurate representation of fiber behavior and computational expense. This paper describes a computationally efficient approach to representing individual fibers, and their composite behavior, within lattice models of cement-based materials. Distinguishing features of this semi-discrete approach include: (1) fibers can be positioned freely in the computational domain, irrespective of the background lattice representing the matrix phase; (2) the pre- and post-cracking actions of the fibers are simulated with little computational expense, since the number of system degrees of freedom is independent of fiber count. Simulated pullouts of single fibers are compared with theory and test results for the cases of perfectly-plastic and slip-hardening behavior of the fiber–matrix interface. To achieve objective results with respect to discretization of the matrix, pullout forces are distributed along the embedded lengths of fibers that bridge a developing crack. This is in contrast to models that lump the pullout force at the crack surfaces, which can lead to spurious break-off of matrix particles as the discretization of the matrix is refined. With respect to fracture in multi-fiber composites, the proposed model matches theoretical predictions of post-cracking strength and pullout displacement corresponding to the load-free condition. The work presented herein is a significant step toward the modeling of strain-hardening composites that exhibit multiple cracking.     

Microstructure and mechanical properties of three-dimensional five-directional braided composites

Three-dimensional (3D) five-directional braided composites are significant structural materials in the fields of astronauts and aeronautics. On the basis of the 3D five-directional braiding process, three types of microstructural unit cell models are established with respect to the interior, surface and corner regions. The mathematical relationships among the structural parameters, such as fiber orientation, fiber volume fraction, the yarn packing factor, are derived. By using these three unit cell models, a micromechanical prediction procedure is described to simulate the stiffness and strength properties of 3D five-directional braided composites. Only the in situ constituent fiber and matrix properties of the composites and the fiber volume proportion are required in the simulation. The stress states generated in the constituent fiber and matrix materials are explicitly correlated with the overall applied load on the composites. The predictive stiffness and strength are in good agreement with available experimental data, which demonstrates the applicability of the present analytical model.     

空心光纤网络埋入复合材料中性能影响的研究

空心光纤网络可用来进行复合材料力学性能的监测,同时又可对材料的损伤进行自修复。空心光纤网络与复合材料性能的影响主要有三方面：1）复合材料对空心光纤的影响;2）空心光纤对复合材料的影响;3）空心光纤与复合材料力学性能的匹配。本文测试了三种规格的空心光纤埋入复合材料中受到的影响,依据国家有关的复合材料测试标准,对树脂基复合材料埋与不埋大直径空心光纤进行了对比实验,并论证了光纤与材料力学性能匹配点存在的必然性。从而为空心光纤网络用于复合材料的自诊断与自修复提供了研究的依据。     

润滑条件下短纤维增强镁基复合材料的摩擦磨损行为研究

采用挤压浸渍预制件工艺制备了氧化铝短纤维增强镁基复合材料,并探讨了纤维取向对润滑条件下复合材料摩擦磨损行为的影响.结果表明:挤压浸渍工艺制备的镁基复合材料具有纤维二维择向分布,不同纤维分布对复合材料在润滑条件下的耐磨性能和磨损机制有较大影响.滑动方向垂直于纤维排列方向时,复合材料的磨痕深度小于平行方向,但相对应的钢球的磨损量则高于平行方向.滑动方向垂直于纤维排列方向时复合材料的磨损机制主要包括纤维破碎和基体的磨粒磨损;滑动方向平行于纤维排列方向时复合材料的磨损机制主要表现为纤维剥落和磨粒磨损.