Evaluation of water degradation of vinylester and epoxy matrix composites by single fiber and composite tests

Degradation of the mechanical properties of vinylester and epoxy matrix composites exposed to water has been approached by monitoring the strengths of glass and carbon fibers and resins. In addition, the fiber/matrix (F/M) interface strengths and debond lengths of single-fiber composites were determined and test results were compared to test results of macroscopic composite specimens. The single-fiber tensile test results indicate a substantial loss of the tensile strength of glass fibers and the fragmentation tests reveal loss of F/M shear strength and substantial debonding for both glass and carbon fiber composites after water exposure. The transverse strengths of the composites are also degraded to large extents. The tests results identify water degradation of the F/M interface as a major strength limiting mechanism.     

Transverse Tensile Properties of 3 Dimension-4 Directional Braided C<Subscript>f</Subscript>/SiC Composite Based on Double-Scale Model

In this thesis, a double-scale model for 3 Dimension-4 directional(3D-4d) braided C/SiC composites(CMCs) has been proposed to investigate mechanical properties of it. The double-scale model involves micro-scale which takes fiber/matrix/porosity in fibers tows into consideration and the unit cell scale which considers the 3D-4d braiding structure. Basing on the Micro-optical photographs of composite, we can build a parameterized finite element model that reflects structure of 3D-4d braided composites. The mechanical properties of fiber tows in transverse direction are studied by combining the crack band theory for matrix cracking and cohesive zone model for interface debonding. Transverse tensile process of 3D-4d CMCs can be simulated by introducing mechanical properties of fiber tows into finite element of 3D-4d braided CMCs. Quasi-static tensile tests of 3D-4d braided CMCs have been performed with PWS-100 test system. The predicted tensile stress-strain curve by the double scale model finds good agreement with the experimental results.     

Modeling elastic properties of short flax fiber-reinforced composites by orientation averaging

Natural fibers of plant origin, used as reinforcement in polymer matrix composite materials, exhibit highly anisotropic elastic properties due to their complex internal structure. Mechanical properties can be evaluated not only by tests but also by mechanical models reflecting the principal morphological features of fibers. Such a FEM model is applied to estimate the elastic properties of a unit cell of a short-fiber-reinforced composite, an elementary flax fiber embedded in a polymer matrix. Orientation averaging approach is used for prediction of the stiffness of short flax fiber reinforced polymer matrix composite. The numerical estimates of Young’s modulus are compared to the test results of extruded flax/polypropylene composite.     

Thermomechanical properties of bio-based composites made from a lactic acid thermoset resin and flax and flax/basalt fibre reinforcements

Low viscosity thermoset bio-based resin was synthesised from lactic acid, allyl alcohol and pentaerythritol. The resin was impregnated into cellulosic fibre reinforcement from flax and basalt and then compression moulded at elevated temperature to produce thermoset composites. The mechanical properties of composites were characterised by flexural, tensile and Charpy impact testing whereas the thermal properties were analysed by dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). The results showed a decrease in mechanical properties with increase in fibre load after 40 wt.% for the neat flax composite due to insufficient fibre wetting and an increase in mechanical properties with increase fibre load up to 60 wt.% for the flax/basalt composite. The results of the ageing test showed that the mechanical properties of the composites deteriorate with ageing; however, the flax/basalt composite had better mechanical properties after ageing than the flax composite before ageing.     

Modeling the effect of reinforcement discontinuity on the tensile strength of UD flax fiber composites

To exploit the potential of natural fibers as reinforcement of polymer matrix composites, aligned bast fiber composite materials are being produced and studied. Bast fiber reinforcement is discontinuous due to the limited length of natural fibers, which needs to be reflected in predictive models of mechanical properties of composites. The strength in tension in the fiber direction of an aligned flax fiber-reinforced composite is modeled assuming that a cluster of adjacent fiber discontinuities is the origin of fracture. A probabilistic model of tensile strength, developed for UD composites containing a microdefect, is applied. It follows from the theoretical analysis that the experimental tensile strength as a function the fiber volume fraction can be described with acceptable accuracy assuming the presence of a cluster of ca. 4 × 4 elementary fiber discontinuities.     

Mechanical behavior of carbon fiber reinforced polyamide composites

The purpose of this work is to compare tensile, compressive and interlaminar shear properties of different carbon reinforcement/polyamide composites obtained by interfacial polymerization and hot compression molding techniques. Two types of composite matrices were studied: polyamide 6 and polyamide 6/6, both reinforced by fabric and unidirectional carbon fibers. The effects of the fiber volume fraction and the matrix on mechanical properties were analyzed through tensile, interlaminar shear and compressive tests. In general, the results have shown a slight increase of the composite elastic modulus, tensile and compressive strength with the increase of carbon fiber content. The microscopic damage development within selected composites during the loading has been observed through optical and scanning electron microscope techniques and has shown that shear failure at the fiber/matrix interface has been mostly responsible for damage development, initiated at relatively low stress.     

High strain rate visco-damageable behavior of Advanced Sheet Molding Compound (A-SMC) under tension

Advanced Sheet Molding Compound (A-SMC) is a serious composite material candidate for structural automotive parts. It has a thermoset matrix and consists of high weight content of glass fibers (50% in mass) compared to standard SMC with less than 30% weight fiber content. During crash events, structural parts are heavily exposed to high rates of loading and straining. This work is concerned with the development of an advanced experimental approach devoted to the micro and macroscopic characterization of A-SMC mechanical behavior under high-speed tension. High speed tensile tests are achieved using servo-hydraulic test equipment in order to get required high strain rates up to 100 s⁻¹. Local deformation is measured through a contactless technique using a high speed camera. Numerical computations have led to an optimal design of the specimen geometry and the experimental damping systems have been optimized in terms of thickness and material properties. These simulations were achieved using ABAQUS explicit finite element code. The developed experimental methodology is applied for two types of A-SMC: Randomly Oriented (RO) and Highly Oriented (HO) plates. In the case of HO samples, two tensile directions were chosen: HO-0° (parallel to the Mold Flow Direction (MFD)) and HO-90° (perpendicular to the MFD). High speed tensile tests results show that A-SMC behavior is strongly strain-rate dependent although the Young's modulus remains constant with increasing strain rate. In the case of HO-0°, the stress damage threshold is shown an increase of 63%, when the strain rate varies from quasi-static (0.001 s⁻¹) to 100 s⁻¹. The experimental methodology was coupled to microscopic observations using SEM. Damage mechanisms investigation of HO and RO specimens showed a competition between two mechanisms: fiber-matrix interface debonding and pseudo-delamination between neighboring bundles of fibers. It is shown that pseudo-delamination cannot be neglected. In fact, this mechanism can greatly participate to energy absorption during crash. Moreover, the influence of fiber orientation and imposed velocity is studied. It is shown that high strain rate and oriented fiber in the tensile direction favor the pseudo-delamination.     

基于内聚力模型的形状记忆合金短纤维增强树脂基复合材料的模拟分析

通过单纤维拔出实验和单轴拉伸实验, 测定了形状记忆合金(SMA)增强树脂基复合材料的界面脱粘剪切强度和单向随机分布SMA短纤维增强复合材料的拉伸强度。根据蒙特卡罗法和边界条件控制方程, 编写了适于软件调用的单向随机分布短纤维增强复合材料的APDL语言生成程序, 建立数值模拟模型。基于指数型内聚力模型, 对SMA纤维与环氧树脂基体界面分离(即界面脱粘)过程进行了有限元模拟。结果表明: 相同纤维体积分数下, 随着纤维长细比的减小, 复合材料整体弹性模量逐渐降低; 温度驱使SMA纤维弹性模量发生变化, 可以有效提高复合材料整体弹性模量。     

Mechanical Properties of Thermoplastic and Thermoset Composites Reinforced with 3D Biaxial Warp-knitted Fabrics

In this study, two types of thermoplastic matrices (low melting point polyethylene terephthalate (LPET) fiber and polypropylene (PP) fiber) and glass fiber/epoxy resin/multi-walled carbon nanotubes (MWCNTs) were used to fabricate the thermoplastic and thermoset composite materials with 3D biaxial warp-knitted fabrics. Thermoplastic and thermoset composites were fabricated using hot-press and resin transfer molding (RTM) methods. The fabricated samples were tested with tensile and three-point flexural tests. In thermoplastic composites, samples in the 90° direction and LPET matrix showed the best tensile and flexural properties with an improvement of 39 and 21% tensile modulus and strength, 16 and 8% flexural modulus and strength compared to the PP samples in the same direction. In thermoset composites, samples in the 90° direction and MWCNTs showed the best improvement of the flexural modulus and strength with 97 and 58% compared to the samples without MWCNTs. This improvement can most likely be attributed to an increase in interfacial adhesion due to the presence of the carbon nanotubes.     

Development of a carbonization-in-nitrogen method for measuring the fiber content of carbon fiber reinforced thermoset composites

Carbon fiber reinforced thermoset composites such as carbon fiber epoxy composites are widely used in aircraft and aerospace, and are being increasingly used in automotive applications because of their lightweight characteristics, high specific strength, and stiffness. The carbon fiber content in the composite plays a critical role in enhancing structural performance. The carbon fibers contribute to the strength and stiffness; therefore, the mechanical properties of the composite are greatly influenced by the carbon fiber content. Measurement of carbon fiber content is essential for product quality control and process optimization. In this work, a novel carbonization-in-nitrogen (CIN) method is developed to characterize the fiber content in carbon fiber thermoset composites. A carbon fiber composite sample is carbonized in a nitrogen environment at elevated temperatures, alongside a neat resin sample. The carbon fibers are protected from oxidization while the resin (the neat resin and the resin matrix in the composite sample) is carbonized under nitrogen environment. The neat resin sample is used to calibrate the resin carbonization rate and calculate the amount of the resin matrix in the composite sample. The new method has been validated on several thermoset resin systems, and found to yield accurate estimation of fiber content in carbon fiber thermoset composites.     

低成本PVA纤维对超高韧性水泥基复合材料力学性能的影响

超高韧性纤维增强水泥基复合材料（ECC）因其出色的高韧性及多缝开裂特性备受关注,然而一直以来因配比中进口PVA纤维的使用导致高昂的价格限制了其在工程中的大规模应用。为了进一步降低成本及实现原材料的本土化,研究低成本国产PVA纤维对ECC力学性能的影响十分必要。通过单轴拉伸、压缩、三点抗弯及单裂缝拉伸等宏观、细观试验研究两种国产低成本PVA-ECC的力学性能,并借助纤维分散性试验及SEM,探讨纤维的分散等微观特征。结果表明,低成本国产纤维在基体中具有良好的分散性,尽管其纤维桥接余能、最大桥接应力及PSH指数低于进口纤维,但均能满足能量与强度准则,即便相对较差的纤维A试件的3 d、7 d及28 d的极限拉伸应变也可达到2.52%、3.34%及3.08%,可实现良好的应力硬化行为及饱和多缝开裂特性,满足ECC的使用要求。     

Seawater effects on transverse tensile strength of carbon/vinylester as determined from single-fiber and macroscopic specimens

The main objective of this work is to characterize the effect of seawater immersion on the transverse tensile strength of a carbon/vinylester composite. A recently developed single-fiber test was used to determine the tensile strength of the fiber/matrix interface at dry and seawater saturated conditions. Measured loads at onset of fiber/matrix debonding were combined with finite element stress analysis to determine the tensile strength of the fiber/matrix interface. It was found that seawater exposure had an insignificant influence on the interface strength. The transverse tensile modulus and strength of macroscopic composite test specimens were slightly reduced by exposure to seawater. The modulus only slightly exceeds the lower bound estimate for both the dry and seawater saturated composite. This may be a consequence of insufficient load transfer between fiber and matrix due to interface voids. The fiber/matrix interface strengths measured at dry and seawater saturated conditions were used as input in the Cooper–Kelly micromechanical model to predict the transverse tensile strength of the composite. Predictions of strength at dry and seawater saturated conditions, based on measured fiber/matrix interface strengths, were unconservative, which again may be the results of voids at the fiber/matrix interface.     

Flax fibre and its composites – A review

In recent years, the use of flax fibres as reinforcement in composites has gained popularity due to an increasing requirement for developing sustainable materials. Flax fibres are cost-effective and offer specific mechanical properties comparable to those of glass fibres. Composites made of flax fibres with thermoplastic, thermoset, and biodegradable matrices have exhibited good mechanical properties. This review presents a summary of recent developments of flax fibre and its composites. Firstly, the fibre structure, mechanical properties, cost, the effect of various parameters (i.e. relative humidity, various physical/chemical treatments, gauge length, fibre diameter, fibre location in a stem, oleaginous, mechanical defects such as kink bands) on tensile properties of flax fibre have been reviewed. Secondly, the effect of fibre configuration (i.e. in forms of fabric, mat, yarn, roving and monofilament), manufacturing processes, fibre volume, and fibre/matrix interface parameters on the mechanical properties of flax fibre reinforced composites have been reviewed. Next, the studies of life cycle assessment and durability investigation of flax fibre reinforced composites have been reviewed.     

Behavior of cement based matrices reinforced by randomly dispersed microfibers

A study on behavior of cement based matrices reinforced by randomly distributed microfibers is summarized. Effects of volume fraction, length and type of fiber, and type of cement based matrix were experimentally examined using uniaxial tensile specimens and three-point bend beams. The matrix fracture properties were measured by a RILEM recommended test procedure. A confocal microscopy technique was used to measure fracture surface roughness, a parameter that was shown to correlate with fracture properties. By incorporating the obtained matrix fracture properties, fiber aspect ratio, and fiber-matrix interface bond into a fracture mechanical R-curve approach, mechanical responses of cement based matrices reinforced by fibers can be predicted.     

Developing rigid gliadin based biocomposites with high mechanical performance

We aim to produce unidirectional fiber composites with high mechanical performance based on flax fibers and a rigid gliadin matrix. As a fraction from wheat gluten, gliadin is soluble in alcohol containing media. The fabrication process did not involve any further solvents or plasticizers. Finally, samples were cooled at different rates. Overall, the cooling rate does not strongly affect the mechanical properties although slowly cooled materials contain a higher amount of non-disulfide cross-links, next to disulfide bonds within the gliadin matrix. At 40% fiber volume fraction, flax/gliadin composites with a flexural modulus and strength of respectively 21.5 GPa and 240 MPa were obtained when loaded in the longitudinal direction. These high values demonstrate that in this composite fabrication process, a good impregnation of the polymer matrix in between the fiber bundles has been achieved. However, the fiber–matrix adhesion, as measured by transverse flexural and tensile tests, was still relatively modest.     

Comparison of chemical vapor deposition and chemical grafting for improving the mechanical properties of carbon fiber/epoxy composites with multi-wall carbon nanotubes

By engineering the fiber/matrix interface, the properties of the composite can be changed significantly. In this work, we increased the effective surface area of the fiber/matrix interface, to facilitate additional stress transfer between fibers and matrix, by grafting carbon nanotubes on to carbon fibers (in the form of carbon fabric) by two different methods: (1) chemical vapor deposition (CVD) method and (2) a purely chemical method. With the CVD process, carbon nanotubes (CNT) were directly grown on carbon fiber substrate using chemical vapors. For the chemical method, CNT with carboxyl groups were grafted on functionalized carbon fiber via a chemical reaction. The morphology of CNT/carbon fibers was examined by scanning electron microscope (SEM) which revealed uniform coverage of carbon fibers with CNT in both of CVD method and chemical grafting method. CNT-grafted woven carbon fibers were used to make carbon/epoxy composites, and their mechanical properties were measured using three-point bending and tension tests which showed that those with CNT-grafted carbon fiber reinforcements using the CVD process has 11 % higher tensile strength compared to those containing carbon fibers modified with the chemical method. Also, composites with CNT-grafted carbon fibers with chemical method showed 20 % higher tensile strength compared to composites with unmodified carbon fibers. The results of tensile test revealed that both CVD and chemical grafting could significantly improve the mechanical properties of the carbon fiber composites.     

Transverse stiffness properties of unidirectional fiber composites containing debonded fibers

First-principles micromechanics modeling for the determination of transverse stiffness properties of a unidirectional fiber composite with fiber–matrix interfacial debonding is presented. The composite has a packing arrangement of a periodic square array of fibers, but contains randomly distributed debonded fibers. The finite element method is employed for the exact treatment of local microscopic stress and strain fields in a representative volume element of the composite material, and of the nonlinear problem of separation and contact of fiber and matrix at debonded interface. The randomness of the distribution of debonded fibers is dealt with by means of the Monte Carlo method, and the composite stiffness properties are found as ensemble average properties over a large number of representative volume elements. Bimodular behavior of the composite under transverse loading, i.e., different stiffnesses in tension and compression, is accurately captured.     

纤维涂层法制备SiCf/Cu复合材料及其性能分析

为研究纤维涂层法制备SiCf/Cu复合材料的性能特点,通过磁控溅射法先后将Ti6Al4V界面改性层和基体Cu涂层涂覆到SiC纤维表面,并通过真空热压法将被涂覆的纤维制备成SiCf/Cu复合材料.对Ti6Al4V涂层、Cu涂层以及复合材料进行了微观分析,并测试了复合材料的拉伸强度.研究表明,复合材料的Cu基体由致密而细小的晶粒组成;Ti6Al4V提高了纤维/基体界面结合强度,复合材料轴向抗拉强度高达500 MPa,界面脱粘主要发生在纤维表面的碳涂层与纤维之间.     

Effect of Reactive Graphitic Nanofibers (r-GNFs) on Tensile Behavior of UHMWPE Fiber/Nano-Epoxy Bundle Composites

Ultra-high molecular weight polyethylene (UHMWPE) fibers have good mechanical and physical properties and effective radiation shielding functions, which are significant for aerospace structures. In our previous work, nano-epoxy matrices were developed based on addition of reactive graphitic nanofibers (r-GNFs) in a diluent to form a blend. It is found that improved wettability and enhanced adhesion of the matrices to UHMWPE fibers can be obtained. In this study, a series of nano-epoxy matrices with different concentrations of r-GNFs (up to 0.8 wt%) and different weight ratios of r-GNFs to a reactive diluent (1:4, 1:6, 1:7, and 1:9) were prepared. Composite bundle specimens of UHMWPE fiber/nano-epoxy were fabricated and their tensile behavior was investigated. All load-displacement curves of the UHMWPE/nano-matrix bundle composites under tensile loading showed three regions corresponding to the three deformation and failure stages of the materials: 1) elastic deformation stage, 2) plateau stage, and 3) UHMWPE fiber failure stage. The nano-epoxy with 0.3 wt% of r-GNFs and with 1:6 ratio of r-GNFs to the diluent proved to be the best matrix for UHMWPE fiber composites with enhanced tensile properties. For the resulting composite, the load level and consumed energy in the plateau stage were increased by 8% and 30% over the UHMWPE fiber/pure-epoxy specimens, respectively. This UHMWPE fiber composite with the optimized nano-epoxy matrix also possesses the highest initial stiffness and ultimate tensile strength among all the resulting UHMWPE fiber composites. These results laid a foundation for us to fabricate UHMWPE fiber reinforced composite laminates in the near future.     

Fabrication and mechanical properties of aramid/nylon plain knitted composites

The presence of fibre/matrix interfaces strongly influences the overall mechanical properties of composites. In order to produce fully recyclable fiber reinforced composites with improved adhesion properties, polyethylene and polypropylene materials were previously used as single-polymer composite materials. In this paper, another breed of single-polymer composite material has been defined as the ‘one-unity’ composite. Polyamide materials were chosen and combined with aramid fibre in an attempt to achieve better interfacial bonding. Weft-knitting technique was used to produce textile reinforcements for aramid/nylon composite processing. Aramid/epoxy knitted composites were also fabricated to compare them with aramid/nylon thermoplastic composites. Mechanical properties of aramid/nylon and aramid/epoxy composites and their relationships to the fibre/matrix interfacial adhesion and interactions have been investigated. With the increase in processing time, tensile modulus and strength of aramid/nylon composites have increased and decreased, respectively. Furthermore, scanning electron microscopic observations clearly indicated that longer molding time has resulted in stronger adhesion property between fiber and matrix. Aramid/nylon knitted composites have revealed comparable strength property in the course direction, albeit they have inferior tensile strength in the wale direction when compared to that in aramid/epoxy composites. In aramid/nylon knitted composites, while tensile modulus exhibited an increasing trend, there were clear drops in tensile strengths with longer molding time. This indicates that there could be an optimum molding condition at which maximum tensile properties can be obtained. Aramid/nylon knitted composites exhibited relatively better interfacial bonding properties than Aramid/epoxy composites, which suffered fibre/matrix debonding.