Self-healing of damage in fibre-reinforced polymer-matrix composites

Self-healing resin systems have been discussed for over a decade and four different technologies had been proposed. However, little work on their application as composite matrices has been published although this was one of the stated aims of the earliest work in the field. This paper reports on the optimization of a solid-state self-healing resin system and its subsequent use as a matrix for high volume fraction glass fibre-reinforced composites. The resin system was optimized using Charpy impact testing and repeated healing, while the efficiency of healing in composites was determined by analysing the growth of delaminations following repeated impacts with or without a healing cycle. To act as a reference, a non-healing resin system was subjected to the same treatments and the results are compared with the healable system. The optimized resin system displays a healing efficiency of 65% after the first healing cycle, dropping to 35 and 30% after the second and third healing cycles, respectively. Correction for any healability due to further curing showed that approximately 50% healing efficiency could be achieved with the bisphenol A-based epoxy resin containing 7.5% of polybisphenol-A-co-epichlorohydrin. The composite, on the other hand, displays a healing efficiency of approximately 30%. It is therefore clear that the solid-state self-healing system is capable of healing transverse cracks and delaminations in a composite, but that more work is needed to optimize matrix healing within a composite and to develop a methodology for assessing recovery in performance.     

Influence of hydrostatic pressure on the mechanical behavior and properties of unidirectional, laminated, graphite-fiber/epoxy-matrix thick composites

This paper reviews and gives new insight into earlier work by the author and his co-workers on the experimental investigation of the influence of superimposed hydrostatic pressure on the mechanical behavior and properties of the epoxy used for the matrix and unidirectionally laminated, graphite-fiber/ epoxy-matrix thick composites. The direction of the fibers was, respectively, 0°, 45° and 90° for the compressive test samples and 0°, 45° -45° and 90° for the shear samples.

Hydrostatic pressure induces very significant, often dramatic changes in the compressive and shear stress/ strain behavior of composites, and consequently in the elastic, yielding, deformation and fracture properties. The range of pressures covered for the compressive experiments was 1 bar to 4 kbar, and for the shear tests 1 bar to 6 kbar. The shear modulus (G) of the epoxy increased bilinearly with pressure, with the break, or the discontinuity point, occurring at 2 kbar. The compressive elastic modulus (E) and the shear modulus (G) of the composites increase in the same manner as for the epoxy. The break, which is located at 2 kbar, represents a pressure at which physical changes in the molecular motion of the matrix epoxy occur. That is, segmental motion of molecules between the cross-links is frozen in by 2 kbar pressure. This pressure is known as the secondary glass transition pressure of the epoxy at room temperature. Alternatively, the sub-zero secondary glass transition temperature of the epoxy is shifted to ambient temperature by 2 kbar pressure. The increase in the moduli may also be given a mechanical interpretation. The elastic or shear modulus of an isotropic, elastic material due to small compressive or shear deformations, respectively, superimposed on a finite volume deformation, which is caused by hydrostatic pressure, increases with pressure. Such an increase in E or G has been predicted using finite deformation theory of elasticity.

The normally brittle epoxy develops yielding when the superimposed hydrostatic pressure exceeds 2 kbar. The shear yield stress (1% off-set) of the epoxy increases linearly with pressure above 2 kbar. This kind of yielding behavior can be predicted by a pressure-dependent yield criterion. The compressive yield strength of the 45° and 90° composites increases bilinearly with pressure, and the shear yield strength of the 0°, 45° and 90° composites also increases bilinearly with pressure. This bilinear behavior is also due to the secondary glass transition pressure of the matrix epoxy, being located at 2 kbar. The fracture strength of the composites also increases with pressure linearly and the greatest increase occurs in the 45° composite in compression and in the −45° composite in shear. The fracture modes of the composites undergo changes with increasing hydrostatic pressure. For instance, the 0° composite undergoes a brittle-ductile transition under shear stress, while no such transition appears to set in under compressive stress. The fracture mode of the 45° composite changes from matrix failure at lower pressures to fiber failure at high pressures under shear stress.     

Lignin/epoxy composites

This paper presents some possibilities for the use of lignin/epoxy resins in blends and composites with epoxy resins. A compatibility study was carried out by optical and electron microscopy, viscosimetric determinations and thermo-optical analysis in order to establish optimum synthesis conditions of molding mass (cast resins). Lignin/epoxy composites including various fillers (lead soap, alum earth, talc, chalk, sand, trihydrate aluminium oxide, glass fibers), plasticizer (dibutylphthalate and polyester C₆) and pigments (iron-oxide and titanium dioxide) have been obtained. Lignin/epoxy composites are characterized by good dielectric, mechanical and adhesive properties. These composite materials can be used in the electronics industry.     

炭纤维增强C/SiC双基体复合材料的制备及性能(英文)

以针刺炭纤维整体毡为预制体,联用化学气相沉积法与熔融渗硅法制得炭纤维增强C/SiC双基体(C/C-SiC)复合材料;研究了C/C-Si材料的显微结构、力学性能和不同制动速度下的摩擦磨损性能及机理。结果表明:C/C-SiC材料具有适中的纤维/基体界面结合强度,弯曲强度和压缩强度分别达240MPa和210MPa,具有摩擦系数高(0.41～0.54),磨损小(0.02cm3/MJ),摩擦性能稳定等特点.随着制动速度提高,C/C-Si材料的摩擦磨损机制也随之变化:在低速制动条件下主要表现为磨粒磨损;中速时以黏着磨损为主;高速时以疲劳磨损和氧化磨损为主。     

热固性复合材料固化过程三维有限元模拟和变形预测

分析了复合材料热固化过程中各种复杂的物理化学变化之间的相互影响,在此基础上建立了复合材料固化过程数值模拟和固化变形预测的三维有限元分析模型。采用整体-子模块方法将固化过程分为热-化学、流动-压实和应力-变形三个相对独立的子模块。热-化学模块的控制方程基于Fourier 热传导方程和树脂固化动力学方程建立,解决了温度和固化度之间的强耦合问题。流动-压实模块的控制方程基于Darcy定律和有效应力原理建立,反映了树脂流动和纤维网络紧密压实之间的流固耦合关系。应力-变形模块建立了考虑热载荷和固化收缩载荷时复合材料层合板的有限元方程。各模块之间的相互作用通过它们之间的数据交换来实现,以树脂在固化过程中的凝胶点和玻璃化转化点为判断依据确定是否运行各模块及其子程序。典型结构的计算结果与实验对比验证了本文三维有限元模型的有效性。     

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.     

Technology development and applications of composites at HAL

Composite materials for aerospace applications through in-house R & D and through collaboration with overseas aerospace organizations and National Laboratories covering a wide spectrum including glass/carbon/kevlar fibre reinforced plastics, metal and Nomex honeycomb sandwich structures, laminated composites, metal matrix composites and metallo-ceramic composites.     

石墨表面镀Si对石墨/Al复合材料热物理性能的影响

研究了石墨粒径及表面镀Si处理对石墨/Al复合材料热物理性能的影响。结果表明:在盐浴过程中石墨表面形成了SiC层,这不仅增强了石墨-Si/Al复合材料的界面结合力,而且抑制了Al4C3相的产生。随着石墨鳞片体积分数从50%增加到70%,复合材料X-Y方向的热导率从492 W/(m·K)增加到654 W/(m·K),而且体积分数为50%的镀Si石墨/Al复合材料抗弯强度达到了81 MPa,相比未镀覆的提高了53%,是理想的定向导热电子封装材料。随着石墨粒径从500μm减小到150μm,石墨-Si/Al复合材料X-Y面方向的热导率由654 W/(m·K)降低到445 W/(m·K),但Z方向的热导率和复合材料抗弯强度变化不明显。     

Evaluation of creep compliances of unidirectional fibre-reinforced composites

In this paper, a method is proposed for the determination of the viscoelastic behaviour of unidirectional fibre-reinforced composites. The method is based on the model of Laws and McLaughlin. The interface problem is taken into account by using anisotropic elastic coefficients for the fibre. The non-linearity of the matrix is also considered and integrated in the model. Comparisons with experimental data are performed on glass/epoxy materials.     

玻璃纤维增强环氧复合材料上超疏水表面层的制备

先采用真空袋压法制备含CaCO3/环氧树脂表面功能层的玻璃纤维增强环氧树脂复合材料,再通过化学刻蚀与表面修饰,在玻璃纤维增强环氧树脂复合材料上制备出超疏水表面。采用扫描电镜和动/静态接触角分析仪,表征表面的形貌和疏水性,结果表明,在复合材料表面构建了具有微-纳米尺度二元粗糙结构;采用1%(质量分数)的硬脂酸修饰后,其表面与水的接触角最高达160.03°;制备的超疏水表面结构在室温环境下具有长期的稳定性。     

Machining of aluminium based metal matrix composites

Although metal matrix composites (MMCs) are generally regarded as extremely difficult to machine, it is also acknowledged that their machining behaviour is not fully understood. The work reviewed here confirms this widely held view but also suggests that the machinability of these materials can be improved by appropriate selection of the reinforcing phase, its volume fraction, size, and morphology as well as the composition and hardness of the matrix material. Cemented carbide tools can be used to machine some of the less abrasive materials at slow speeds but if higher production rates are required or the more abrasive materials are to be machined, polycrystalline diamond tooling is required.     

锂离子电池用硅/石墨/碳复合负极材料的电化学性能

采用湿法混料及高温热解法制备了锂离子电池用硅/石墨/碳复合负极材料,并研究了不同配方的复合材料结构及电化学性能。研究发现,硅含量为20%(质量分数)时,复合材料首次可逆容量为865mAh/g,循环30次后仍为757mAh/g,容量保持率可达88%,大大改善了硅基材料作为锂离子电池负极材料的电化学性能。     

Finite element micromechanical modelling of unidirectional fibre-reinforced metal-matrix composites

Typical finite element formulations and models for unidirectional composite materials are reviewed. The application of micromechanical finite element analysis to the modelling of unidirectional fibre-reinforced metal-matrix composites is demonstrated by presenting some studies from recent publications. It is shown that while analytical models offer a simple tool for obtaining the overall response of composites, finite element analysis provides more accurate and detailed characterisation of composite properties for complicated geometries and constituent property variations. Various effects that influence the stress/strain response and fibre/matrix deformation of composites are studied through modelling. These effects include the fibre coating and reaction layer, fibre shape and distribution, metallurgical and environmental factors, stress distributions and damage. It is demonstrated that the properties and constituent phase interaction of metal-matrix composites are best modelled by finite element analysis. It is emphasized that in order to obtain good predictions, the models must be coupled with first-hand characterisations of the constituent phases and their interactions, including the thermal history of the specimens.     

BN表面改性对BN/环氧树脂复合材料导热性能的影响

采用十八烷基三甲基溴化铵（OTAB）阳离子表面活性剂对BN微米片进行有机化改性,研究了BN表面改性对BN/环氧树脂复合材料导热性能的影响。当OTAB浓度为0.6 g · L^-1时,BN表面的OTAB吸附量接近饱和。BN表面改性提高了环氧树脂对BN的浸润性,降低了BN的导热系数。SEM观察及黏度测试结果表明：BN表面改性改善了BN/环氧树脂复合材料的界面性能及体系相容性。由于界面热阻的降低,改性BN/环氧树脂复合材料的导热系数高于未改性BN/环氧树脂复合材料,当BN填充量为30%（填料与树脂基体的质量比）时,改性BN/环氧树脂复合材料的导热系数为1.03 W （m · K）^-1,是未改性BN/环氧树脂导热系数（0.48 W （m · K）^-1）的2.15倍。     

Tensile and compressive damage coupling for fully-reversed bending fatigue of fibre-reinforced composites

ABSTRACT Due to their high specific stiffness and strength, fibre-reinforced composite materials are winning through in a wide range of applications in automotive, naval and aerospace industry. Their design for fatigue is a complicated problem and a large research effort is being spent on it today. However there is still a need for extensive experimental testing or large safety factors to be adopted, because numerical simulations of the fatigue damage behaviour of fibre-reinforced composites are often found to be unreliable. This is due to the limited applicability of the theoretical models developed so far, compared to the complex multi-axial fatigue loadings that composite components often have to sustain in in-service loading conditions.
In this paper a new phenomenological fatigue model is presented. It is basically a residual stiffness model, but through an appropriate choice of the stress measure, the residual strength and thus final failure can be predicted as well. Two coupled growth rate equations for tensile and compressive damage describe the damage growth under tension–compression loading conditions and provide a much more general approach than the use of the stress ratio R . The model has been applied to fully-reversed bending of plain woven glass/epoxy specimens. Stress redistributions and the three stages of stiffness degradation (sharp initial decline – gradual deterioration – final failure) could be simulated satisfactorily.     

碳纤维蜂窝复合材料加工表面质量影响研究

为研究碳纤维蜂窝复合材料切削过程中切削宽度和切削方向对表面质量的影响,本文开展了高速切削碳纤维蜂窝实验,分析了切削过程中切削力的变化特征,获得了切削宽度和切削方向对切削力的影响规律。同时,观测了碳蜂窝典型表面缺陷形貌,分析了不同切削宽度和切削方向下表面缺陷的分布规律。进一步以蜂窝壁损伤面积占比作为表面质量的评价方法,定量研究了切削宽度和切削方向对于碳蜂窝表面质量的影响。实验结果表明：在碳纤维蜂窝复合材料加工过程中,减小切削宽度和改变切削方向可有效降低加工损伤。切削力随着切削宽度的增大呈现出增大的趋势;毛刺、撕裂、孔壁破损以及孔壁开胶等损伤现象也随着切削宽度的增大而增加。此外,碳纤维蜂窝复合材料的表面质量还与切削方向有关,相较于沿双层孔壁方向切削,当切削宽度分别为5、10及15 mm时,沿垂直于双层孔壁方向切削的损伤分别减小了22.5%、13.4%和8.7%。     

Role of alumina in flexure of glass/epoxy composites

The influence of particulate additions of alumina on the flexural properties of glass-fabric/epoxy composites was studied. The additions improved translaminar flexural strength, while decreasing interlaminar strength. The translaminar bending modulus showed an increasing trend whereas its interlaminar value showed a decrease, up to additions of 3 vol%. The mechanisms of deformation and the fracture features have been discussed with the aid of scanning electron microscopy.     

Constitutive modelling of fibre-reinforced composites with unidirectional plies using a plasticity-based approach

This paper presents the development of a constitutive model able to accurately represent the full non-linear mechanical response of polymer-matrix fibre-reinforced composites with unidirectional (UD) plies under quasi-static loading. This is achieved by utilising an elasto-plastic modelling framework. The model captures key features that are often neglected in constitutive modelling of UD composites, such as the effect of hydrostatic pressure on both the elastic and non-elastic material response, the effect of multiaxial loading and dependence of the yield stress on the applied pressure.The constitutive model includes a novel yield function which accurately represents the yielding of the matrix within a unidirectional fibre-reinforced composite by removing the dependence on the stress in the fibre direction. A non-associative flow rule is used to capture the pressure sensitivity of the material. The experimentally observed translation of subsequent yield surfaces is modelled using a non-linear kinematic hardening rule. Furthermore, evolution laws are proposed for the non-linear hardening that relate to the applied hydrostatic pressure.Multiaxial test data is used to show that the model is able to predict the non-linear response under complex loading combinations, given only the experimental response from two uniaxial tests.     

Natural fibre-reinforced composites for bioengineering and environmental engineering applications

Recently, the mankind has realized that unless environment is protected, he himself will be threatened by the over consumption of natural resource as well as substantial reduction of fresh air produced in the world. Conservation of forests and optimal utilization of agricultural and other renewable resources like solar and wind energies, and recently, tidal energy have become important topics worldwide. In such concern, the use of renewable resources such as plant and animal based fibre-reinforce polymeric composites, has been becoming an important design criterion for designing and manufacturing components for all industrial products. Research on biodegradable polymeric composites, can contribute for green and safe environment to some extent. In the biomedical and bioengineered field, the use of natural fibre mixed with biodegradable and bioresorbable polymers can produce joints and bone fixtures to alleviate pain for patients. In this paper, a comprehensive review on different kinds of natural fibre composites will be given. Their potential in future development of different kinds of engineering and domestic products will also be discussed in detail.     

Effect of UV and water spraying on the mechanical properties of flax fabric reinforced polymer composites used for civil engineering applications

The lack of data related to durability is one major challenge that needed to be addressed prior to the widespread acceptance of natural fibre reinforced polymer composites for engineering applications. In this work, the combined effect of ultraviolet (UV) radiation and water spraying on the mechanical properties of flax fabric reinforced epoxy composite was investigated to assess the durability performance of this composite used for civil engineering applications. Specimens fabricated by hand lay-up process were exposed in an accelerated weathering chamber for 1500 h. Tensile and three-point bending tests were performed to evaluate the mechanical properties. Scanning electron microscope (SEM) was used to analyse the microstructures of the composites. In addition, the durability performance of flax/epoxy composite was compared with synthetic (glass and carbon) and hybrid fibre reinforced composites. The test results show that the tensile strength/modulus of the weathered composites decreased 29.9% and 34.9%, respectively. The flexural strength/modulus reduced 10.0% and 10.2%, respectively. SEM study confirmed the degradation in fibre/matrix interfacial bonding after exposure. Comparisons with other composites implies that flax fabric/epoxy composite has potential to be used for civil engineering applications when taking its structural and durability performance into account. Proper treatments to enhance its durability performance will make it more comparable to synthetic fibre reinforced composites when considering as construction building materials.