Exploiting carbon nanotube networks for damage assessment of fiber reinforced composites

An approach for damage inspection of composite structures utilizing carbon nanotubes (CNT) networks is investigated. CNT are dispersed in an epoxy using a processing technique compatible with commonly employed composite manufacturing techniques and subsequently used as matrix for a structural glass fiber reinforced composite. The developed electrical conductivity of the composite system is verified experimentally. The electrically conductive CNT network within the GFRP is exploited through distributed electrical voltage measurements to sense and, ultimately, locate damage in the plane of the composite plate. Damage in the form of cracks or delamination interrupts the continuity of the CNT network separating and isolating regions of the conductive network. Employing electric potential fields these changes can become measurable and can provide information for inversely locating the damage. Electrical Resistance Tomography (ERT) is formulated and experimentally applied to measure changes in the potential fields and deliver electrical conductivity change maps which are used to identify and locate changes in the CNT networks. These changes are correlated to capture the damage in the composite. Different damage modes are studied to assess the capabilities of the technique. The technique shows sensitivity to very small damages; less than 0.1% of the inspected area. The solution of the inverse ERT problem delivers a conductivity change maps which offers an effective localization with nearly 10% error and an inspection area suppression of around 75%. The proposed methodology to create CNT networks enables the application of ERT for Non-Destructive Evaluation of composite materials, previously not possible due to lack of conductivity, thus offering damage sensing and location capabilities even in-situ.     

多层结构的玻璃纤维织物/环氧树脂复合材料的降噪性能评价

根据Kirchhoff-Mindlin理论要求,多层结构面板的隔声性能要考虑到复合结构中流体层和固体层之间的整个交互作用:复杂的声波传递过程中在固体和流体层之间多次反射和系统共振,通过对隔声性能与填充在两层面板之间物质密度的关系,以及对硬质物体的组合和空气层厚度的研究,来探讨评价隔声可变因素及对空腔共振约束的隔声低谷的影响.以玻璃纤维为增强材料,环氧树脂为基体,制备复合树脂板,并设计了多层复合结构,利用混响室-静音箱法对材料隔声性能进行了测试分析.研究表明:多层玻璃纤维织物增强环氧树脂复合材料呈一定的刚性,在各频率的隔声量随着复合材料厚度的增加而增加;声音传播过程中所产生的弯曲波对柔性材料隔声性能的影响远大于对刚性材料的影响.不同颗粒填充多层介质复合材料平均隔声量随材料面密度的增加而增加,与材料面密度的指数成良好的线性相关性.     

Damage detection of glass fiber reinforced composites using embedded PVA–carbon nanotube (CNT) fibers

Polyvinyl alcohol–carbon nanotube (PVA–CNT) fibers were embedded in glass fiber reinforced plastic composites and used as strain sensors for damage monitoring of the composite. Sensing of the structural integrity of the composite was made by the in situ measurement of the electrical resistance of the embedded PVA–CNT fiber during the mechanical tests. The multi-functional materials were tested in tensile progressive damage accumulation (PDA) tests. These tests aimed to seek the electrical response of untreated and pre-stretched PVA–CNT fibers with known level of progressively induced damage to the composite. The advantages and disadvantages of each PVA–CNT fiber used as a sensor are analyzed; the electrical resistance readings of the PVA–CNT fibers were correlated with known parameters that express the induced damage of the composite.     

Structural health monitoring of glass fiber reinforced composites using embedded carbon nanotube (CNT) fibers

In the present work, carbon nanotube (CNT) fibers had been embedded to glass fiber reinforced polymers (GFRP) for the structural health monitoring of the composite material. The addition of the conductive CNT fiber to the non-conductive GFRP material aims to enhance its multi-function ability; the test specimen’s response to mechanical load and the insitu CNT fiber’s electrical resistance measurements were correlated for sensing and damage monitoring purposes. It is the first time this fiber is used in composite materials for sensing purposes; CNT fiber is easy to be embedded and does not downgrade the material’s mechanical properties. Various incremental loading–unloading steps had been applied to the manufactured specimens in tension as well as in three-point bending tests. The CNT fiber worked as a sensor in both, tensile and compression loadings. A direct correlation between the mechanical loading and the electrical resistance change had been established for the investigated specimens. For high stress (or strain) level loadings, residual resistance measurements of the CNT fiber were observed after unloading. Accumulating damage to the composite material had been calculated and was correlated to the electrical resistance readings. The established correlation between these parameters changed according to the material’s loading history.     

Preparation of continuous carbon nanotube networks in carbon fiber/epoxy composite

In order to optimize carbon nanotube (CNT) dispersion state in fiber/epoxy composite, a novel kind of CNT organization form of continuous networks was designed. The present work mainly discussed the feasibility of preparing continuous CNT networks in composite: Fiber fabric was immersed into CNT aqueous solution (containing dispersant) followed by freeze drying and pyrolysis process, prior to epoxy infusion. The morphologies of fabric with CNTs were observed by Scanning Electron Microscope. The relationship between CNT networks and flowing epoxy resin was studied. Properties of composite, including out-of-plane electrical conductivity and interlaminar shear strength (ILSS), were measured. The results demonstrated that continuous and porous CNT networks formed by entangled CNTs could be assembled in fiber fabric. Most part of them were preserved in composite due to the robustness of network structures. The preserved CNT networks significantly improved out-of-plane electrical conductivity, and also have an effect on ILSS value.     

Preparation and mechanical properties of carbon nanotube grafted glass fabric/epoxy multi-scale composites

In the present paper, carbon nanotubes (CNTs) were chemically grafted onto surfaces of the amino silane treated glass fabric by a novel chemical route for the first time to create 3D network on the glass fibers. The chemical bonding process was confirmed by Fourier transform infrared spectroscopy and scanning electron microscopy. The glass fabric/CNT/epoxy multi-scale composite laminates were fabricated with the CNT grafted fabrics using vacuum assisted resin infusion molding. Tensile tests were conducted on fabricated multi-scale composites, indicating the grafting CNTs on glass fabric resulted a decrease (11%) in ultimate tensile strength while toughness of the multi-scale composite laminates were increased up to 57%. Flexural tests revealed that the multi-scale composite laminates prepared with CNT grafted glass fabric represent recovering after first load fall. The interfacial reinforcing mechanisms were discussed based on fracture morphologies of the multi-scale composites.     

Mechanical Properties and Interlaminar Fracture Toughness of Glass‐Fiber‐Reinforced Epoxy Composites Embedded with Shape Memory Alloy Wires

The effects of the content and position of shape memory alloy (SMA) wires on the mechanical properties and interlaminar fracture toughness of glass‐fiber‐reinforced epoxy (GF/epoxy) composite laminates are investigated. For this purpose, varying numbers of SMA wires are embedded in GF/epoxy composite laminates in different stacking sequences. The specimens are prepared by vacuum‐assisted resin infusion (VARI) processing and are subjected to static tensile and three‐point‐bending tests. The results show that specimens with two SMA wires in the stacking sequence of [GF₂/SMA/GF₁/SMA/GF₂] and four SMA wires in the stacking sequence of [GF₄/SMA/GF₂/SMA/GF₄] exhibit optimal performance. The flexural strength of the optimal four‐SMA‐wire composite is lower than that of the pure GF/epoxy composite by 5.76% on average, and the flexural modulus is improved by 5.19%. Mode‐I and II interlaminar fracture toughness tests using the SMA/GF/epoxy composite laminates in the stacking sequence of [GF₄/SMA/GF₂/SMA/GF₄] are conducted to evaluate the mechanism responsible for decreasing the mechanical properties. Scanning electron microscopy (SEM) observations reveal that the main damage modes are matrix delamination, interfacial debonding, and fiber pullout.

     

Evaluation of the mechanical behavior of epoxy composite reinforced with Kevlar plain fabric and glass/Kevlar hybrid fabric

In this study, composite plates were manufactured by hand lay-up process with epoxy matrix (DGEBA) reinforced with Kevlar fiber plain fabric and Kevlar/glass hybrid fabric, using to an innovative architecture. Results of the mechanical properties of composites were obtained by tensile, bending and impact tests. These tests were performed in the parallel direction or fill directions of the warp and in a 90° direction. FTIR was used in order to verify the minimum curing time of the resin to perform the mechanical tests, and scanning electron microscopy was used to observe reinforcement and matrix fractures. Composites with Kevlar/glass hybrid structure in the reinforcing fabric showed the better results with respect to specific mechanical strength, as well as bending and impact energy.     

Determination of stiffness properties of braided composites for the design of a hip prosthesis

This paper describes an experimental procedure used to determine the stiffness properties of two different composites made of braided glass and hybrid carbon–glass fibre reinforced epoxy resin. Tubular specimens manufactured by reinforcing an epoxy resin system with commercial braided preforms were used to determine the elastic constants. All specimens were manufactured using compression moulding technique assisted with internal pressure. The stiffness properties were determined from axial and circumferential strains recorded from strain gauges using internal water pressure tests. Identical types of composite laminates were used to manufacture two prototypes of a composite femoral prosthesis with controlled stiffness.     

Perspective Measurement of the Out‐of‐plane Displacement and Normal Strain Field Distributions Inside Glass Fibre‐reinforced Resin Matrix Composite

A depth‐resolved wavenumber‐scanning interferometer (DRWSI) was built up to measure the out‐of‐plane displacement and normal strain field distributions on the front surface, rear surface and internal glass fibres of a glass fibre‐reinforced resin matrix composite before and after loading. Series of the fringe patterns were recorded, while the wavenumber of the laser, monitored online by an optical wedge, was scanned by tuning the temperature. Random sampling Fourier transform is used to overcome the non‐linearity of the wavenumber series. In the end, the distributions of the out‐of‐plane displacements and normal strain field are presented as the applied loads were 10 µm, 20 µm and 30 µm, respectively. In conclusion, DRWSI is a suitable method to measure the mechanical properties inside resin composite non‐destructively.     

Strain-sensitive Raman spectroscopy and electrical resistance of carbon nanotube-coated glass fibre sensors

This paper presents the development of glass fibres coated with nanocomposites consisting of carbon nanotubes (CNTs) and epoxy. Single glass fibres with different CNT content coating are embedded in a polymer matrix as a strain sensor for composite structures. Raman spectroscopy and electrical response of glass fibres under mechanical load are coupled for in situ sensing of deformation in composites. The results show that the fibres with nanocomposite coating exhibit efficient stress transfer across the fibre/matrix interface, and these with a higher CNT content are more prone to fibre fragmentation at the same matrix strain. A relationship between the fibre stress and the change in electrical resistance against the fibre strain is established. The major finding of this study has a practical implication in that the fibres with nanocomposite coating can serve as a sensor to monitor the deformation and damage process in composites.     

The Wear Behaviour of Composite Materials with Epoxy Matrix Filled with Hard Powder

The wear behaviour of composite materials, sliding under dry conditions against smooth steel counterface, has been investigated. The composite materials consisted of glass woven fabric reinforcing three different systems of matrix: epoxy resin, epoxy resin filled with powders of silica and epoxy resin filled with powders of tungsten carbide. The powders were mixed in a volumetric fraction of 6% with the epoxy resin. Three laminates were manufactured by hand lay up technology. The sliding tests have been conducted on the specimens, cut from the three laminates, with a pin on disk apparatus. The results put in evidence different wear behaviours of the composite materials observed at different values of sliding speed and pressure. The presence of different wear mechanisms has been appreciated by SEM-micrographic examinations.     

Deformation and energy absorption of composite egg-box panels

Static compressive tests of composite egg-box panels, whose stacking sequences and number of plies were controlled, were carried out to investigate their deformation behaviour and energy absorption capacity. Silicon rubber moulds were first moulded from an aluminium egg-box panel template. These moulds were in turn used to fabricate composite specimens. Two fabric prepregs, carbon/epoxy plain weave fabric and glass/epoxy 4-harness satin weave were draped over the rubber mould with various stacking angles. The specimens were cured in an autoclave using vacuum bag degassing moulding and an appropriate cure cycle. The nominal stress–strain relations of the specimens were compared and multiply-interrupted compressive tests were used to identify fracture initiation and development. The energy absorption per unit mass of composite egg-box panels were compared with that of an aluminium egg-box panel. From the test results it was concluded that the compressive behaviour of the composite structure is affected by the local stacking sequence of the fabrics and by shear deformation during initial lay-up and draping. By considering the stress–stain behaviour, energy absorption and material cost, the optimal material and draping condition were proposed for a composite egg-box panel.     

Improved adhesion between nickel–titanium SMA and polymer matrix via acid treatment and nano-silica particles coating

Shape memory alloy (SMA) composites are the desirable candidate for smart materials that used in intelligent structures. However, the overall mechanical performance of SMA composites depends immensely on the quality of the interaction between SMA and polymer matrix. Therefore, it is necessary to find out an approach to enhance the interfacial property of this composite. In this paper, we modified nickel–titanium SMA wire with nano-silica particles before and after acid treatment. The modification effect on the interfacial strength between SMA and epoxy resin was evaluated. Contact angle analysis, scanning electron microscopy (SEM) observation, and single fiber pull-out test were carried out. The bonding characteristics between modified wire and liquid/cured resin were investigated. We then embedded SMA wire into woven glass fabric/epoxy composite laminates, and manufactured this hybrid composites via vacuum assisted resin transfer molding processing. Three-point-bending test of the hybrid composites was performed to validate the modification effect. Fiber pull-out experiment demonstrates that the interfacial shear strength increases by 6.48% by nano-silica particles coating, while it increases by 52.21% after 8 h acid treatment and nano-silica particles coating simultaneously. For hybrid composites, flexural strength of the two specimens increases by 19.8 and 48.2%, respectively. In SEM observation, we observed large debonding region in unmodified composites, while interfacial adhesion between modified wire and epoxy keeps strong after flexural damage.     

碳纤维增强羟基磷灰石/环氧树脂复合材料的制备与力学性能

采用树脂传递模塑(RTM)工艺制备了碳纤维增强环氧树脂以及碳纤维增强羟基磷灰石(HA)/环氧树脂两种复合材料，并测试了其力学性能。结果表明，RTM工艺可以基本保证环氧基体均匀浸入碳纤维织物内部。碳纤维增强HA，环氧复合材料的冲击韧性高于碳纤维增强环氧复合材料，而弯曲强度和弯曲模量低于碳纤维增强环氧复合材料。两种复合材料的弯曲强度远高于人体皮质骨，弯曲模量与皮质骨非常接近。动态力学分析(DMA)表明加入HA后，复合材料的贮存模量和内耗降低，玻璃化转变温度升高。     

整体中空复合材料隔声性能的实验研究

为了探讨整体中空复合材料结构与隔声性能之间的关系, 设计并制备了不同高度、不同面板厚度以及不同芯材的玻璃纤维整体中空织物/环氧树脂复合材料。采用混响室-消声室法对其隔声性能进行了测试分析。研究表明: 整体中空复合材料的结构对其隔声性能有明显的影响。复合材料的隔声性能随着结构高度的增加逐渐提高, 面板厚度对材料的隔声效果影响较大, 芯材排列形式对其隔声性能影响相对较小; 8 形整体中空复合材料的隔声性能略高于 88 形和 X 形。      

Composite robot end effector for manipulating large LCD glass panels

Recently, the design and the manufacture of light robot end effectors with high stiffness have become important in order to reduce the deflection due to the self-weight and weight of glass panel, a part of LCD, as the size of glass panels as well as robot end effectors increases. The best way to reduce the deflection and vibration of end effectors without sacrificing the stiffness of end effectors is to employ fiber reinforced composite materials for main structural materials because composite materials have high specific stiffness and high damping. In this work, the end effector for loading and unloading large glass panels were designed and manufactured using carbon fiber epoxy composite honeycomb sandwich structures. Finite element analysis was used along with an optimization routine to design the composite end effector. A box type sandwich structure was employed to reduce the shear effect arising from the low modulus of honeycomb structure. The carbon fiber epoxy prepreg was hand-laid up on the honeycomb structure and cured in an autoclave. A special process was used to reinforce the two sidewalls of the box type sandwich structure. The weight reduction of the composite end effector was more than 50% compared to the weight of a comparable aluminum end effector. From the experiments, it was also found that the static and dynamic characteristics of the composite end effector were much improved compared to those of the aluminum end effector.     

Auto-accelerative water damage in an epoxy composite reinforced with plain-weave flax fabric

Water diffusivity was measured through 12 wet–dry cycles, for epoxy resin reinforced with plain-weave flax fabric. Electron microscopy revealed micro-cracks that provided routes for water uptake. Water damage was characterised by the volume of pores, expressed as percent of the total volume of the composite. Water diffusivity doubled for every increase of 2.3% in the volume of pores, until a plateau was reached after several cycles, when water diffusivity was 10 times as great as on first immersion. The amount of water absorbed by the flax–epoxy composite was an order of magnitude larger than that reported for unsaturated polyester resin reinforced by plain-woven E-glass fabric, yet the extent of water damage and associated changes in diffusivity were similar. Results were consistent with a damage mechanism in which both flax fibres and matrix become swollen when wet, but the fibres shrink faster than the matrix when dried.     

跨尺度预测非屈曲织物增强复合材料的刚度和强度

为了预测非屈曲织物增强复合材料的力学性能, 建立了纤维束的正六边形单胞和非屈曲织物复合材料的长方形单胞, 并重点推导了正六边单胞的方程边界条件。通过跨尺度逐级计算这两个单胞的有效弹性常数, 得到了非屈曲碳纤维织物增强环氧树脂基复合材料的宏观有效弹性性能和强度。对该非屈曲织物复合材料在拉伸载荷下的累计失效进行了有限元损伤分析。结果表明: 初始损伤发生在富树脂区或横向纤维束, 损伤在富树脂区与横向纤维束内逐步扩展, 最后向纵向纤维束扩展并迅速导致整体失效; 非屈曲织物增强复合材料的面内拉伸模量的计算预测值非常接近实验值, 面内拉伸强度计算值略小于实验值。     

Ultra‐Light and Scalable Composite Lattice Materials

Architected lattice materials are some of the stiffest and strongest materials at ultra‐light density (<10 mg cm^?3), but scalable manufacturing with high‐performance constituent materials remains a challenge that limits their widespread adoption in load‐bearing applications. We show mesoscale, ultra‐light (5.8 mg cm^?3) fiber‐reinforced polymer composite lattice structures that are reversibly assembled from building blocks manufactured with a best‐practice high‐precision, high‐repeatability, and high‐throughput process: injection molding. Chopped glass fiber‐reinforced polymer (polyetherimide) lattice materials produced with this method display absolute stiffness (8.41 MPa) and strength (19 kPa) typically associated with metallic hollow strut microlattices at similar mass density. Additional benefits such as strain recovery, discrete damage repair with recovery of original stiffness and strength, and ease of modeling are demonstrated.