Single fibre fragmentation test for assessing adhesion in fibre reinforced composites

The single fibre fragmentation test for measuring the properties of the fibre–matrix interface in fibre-reinforced composites is reviewed. Special emphasis has been paid to the recent stress transfer models in single fibre composites and its application to the development of a suitable data reduction technique for the fragmentation test. The complexities of the correlation of the micromechanical results to the properties of the macrocomposites have been highlighted.     

Compression creep behaviour of steel fibre reinforced cement composites

The paper presents the results of an experimental investigation to determine the influence of steel fibre reinforcement on the creep of cement matrices under compression. Creep tests were carried out at a number of applied stress-strength ratios ranging between 0.3 to 0.9. Melt extract and hooked steel fibres were used at volume fractions ranging between 0 and 3% by volume of a mix. Three types of cement matrices were used namely cement paste, mortar and two mix proportions of concrete. The results indicate that steel fibres restrain the creep of cement matrices at all stress-strength ratios, the restraint being greater at lower stresses and at higher fibre contents. Steel fibres are effective in restraining only the flow component of creep of cement matrices, the delayed elastic component being unaffected. The reduction in creep of cement pastes, due to fibre reinforcement, is much greater than that for mortar or concrete matrices. Mathematical expressions are given for the creep of steel fibre reinforced cement matrices.

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Résumé Cet article présente les résultats d’une étude expérimentale en vue de déterminer l’influence du renforcement de fibres d’acier sur le fluage de matrices de ciment en compression. On a exécuté les essais de fluage à un certain nombre d’intervalles correspondant à des rapports contrainte/déformation allant de 0,3 à 0,9. On a utilisé des fibres d’acier dans des proportions se situant entre 0 et 3% par volume de mélange, et on s’est servi de trois types de matrices de ciment: pate de ciment, mortier et béton selon deux dosages. Les résultats montrent que les fibres d’acier empêchent le fluage des matrices de ciment dans tous les rapports de contrainte/déformation, l’effet étant plus important pour les contraintes faibles et les teneurs en fibres plus élevées. Les fibres d’acier n’agissent qu’en s’opposant à la déformation plastique des matrices de ciment sans que l’élasticité différée en soit affectée. La diminution du fluage des pates de ciment due au renforcement des fibres est beaucoup plus importante que pour le mortier ou les matrices de béton. On donne des formules mathématiques pour le fluage des matrices de ciment renforcé de fibres d’acier.

     

Rheology of low carbon fibre content reinforced cement mortar

It is recognised that the addition of carbon fibres to a brittle cement matrix results in a less dense composite with enhanced ductility, improved impact resistance and increased toughness. In addition, the reinforcing effect of fibres in the cement often produces superior flexural strength and marked improvements in post-cracking behaviour. Further, carbon fibres influence the electrical properties of the composite which could, potentially, make it a smart material, with a range of applications. Despite attention directed towards the mechanical and electrical properties of carbon fibre reinforced cement (CFRC), there is a dearth of information of the influence of fibre additions on the rheological properties of the resulting composite. To this end, this paper describes an investigation using the Viskomat NT into the influence of carbon fibre additions (fibre length in the range 3–12 mm and volume in the range 0–0.5%) on the rheological properties of CFRC. Within the limitations of the instrument and testing procedure it is shown that CFRC’s conform to the Bingham model: increasing fibre volume and fibre length increase both the yield stress and plastic viscosity.     

A criterion for optimum adhesion applied to fibre reinforced composites

The effects of physical adhesion on the mechanical properties of a composite structure are examined in this work. A criterion for optimum adhesion between matrix and reinforcing fibres is proposed based on maximizing the wetting tension. It is shown that the maximum wetting tension criterion best fulfils two important requirements for a strong interface:(i) the physical interactions at the molecular level between the resin and the fibres must be maximized, and (ii) the liquid resin must spontaneously wet the fibre surface in order to minimize the flow density at the interface. The conditions on the surface energy of the various phases leading to maximum wetting tension are analysed considering three mixing rules: two based on dispersive–polar interactions, and a third one based on acid–base interactions. The optimum adherend for a given adhesive, and the optimum adhesive for a given adherend, are examined. The analysis shows that maximum wetting tension is obtained when the substrate and adhesive surface energies are very high and equal, so that their polar and dispersive components are equal when the polar–dispersive mixing rule is used, and e.g. their Lifshitz–van der Waals’ components are equal and the acid component of one phase is equal to the basic component of the other phase when the acid–base approach is considered. It is shown using data from the literature that interfacial strength correlates with the wetting tension for fibre reinforced composites. Additional observations show that under poor wetting conditions the voids tend to concentrate at the fibre–resin interface, whereas under favourable wetting conditions they tend to coalesce in regions away from the fibre surface. This revised version was published online in November 2006 with corrections to the Cover Date.     

The oxidative stability of carbon fibre reinforced glass-matrix composites

The environmental stability of carbon fibre reinforced glass-matrix composites is assessed. Loss of composite strength due to oxidative exposure at elevated temperatures under no load, static load and cyclic fatigue as well as due to thermal cycling are all examined. It is determined that strength loss is gradual and predictable based on the oxidation of carbon fibres. The glass matrix was not found to prevent this degradation but simply to limit it to a gradual process progressing from the composite surfaces inward.     

Nickel- and copper-coated carbon fibre reinforced tin-lead alloy composites

Nickel and copper were deposited over brominated, surface treated, and pristine P-100 carbon fibres using cementation and electroplating techniques. The fibres were brominated by bromine vapour for 48 h and then desorbed at 200 °C in air for 12 h. The anodic oxidation treatment of the fibres involved electrochemical etching in a dilute sodium hydroxide electrolyte for 3 min. Electroplated coated fibres showed better tensile properties than cementation coated fibres. In addition, nickel coating exhibited better bonding with the carbon fibres compared to copper coating. The effect of bromination and surface treatment was improved adhesion between coating and fibres. Nickel- and copper-coated fibres, which were brominated, anodically oxidized, and pristine, reinforced tin-lead alloy composites were fabricated by squeeze casting. The composites containing coated treated carbon fibres had higher tensile and shear strength than the ones containing coated pristine carbon fibres. Moreover, the composite with coated brominated carbon fibres had better tensile strength and shear strength than the surface treatment. The results also showed the composites containing nickel-coated fibres had higher tensile and shear strength than the ones containing copper-coated fibres.     

Role of fibre surface—matrix combination in carbon fibre reinforced epoxy composites

The effect of surface treatment of carbon fibres with concentrated as well as dilute nitric acid on the mechanical properties of carbon fibres has been reported. The role of the fibre—matrix interface in carbon fibre reinforced epoxy resin composites has been studied. Composites have been made both with untreated and surface treated carbon fibres and epoxy resin Araldite LY556 with different hardeners. Mechanical properties as well as fracture behaviour of these composites suggest that it is the physical interlocking between the fibres and the matrix, along with some chemical bonding between the two, and not the pure chemical bonding which yield better composites.     

碳纤维水泥基材料电阻的非线性研究

研究了碳纤维水泥基材料(CFRC) 非线性电阻的伏安特性,并着重讨论了不同的碳纤维掺量和温度对CFRC材料电压-电阻关系的影响.结果表明:在较小外加电压下(＜2V)CFRC材料的电压-电阻曲线会出现明显的平台区,随着电压的进一步增大,其电阻逐渐降低呈现非线性特性.相同纤维掺量的水泥基材料随温度的升高其电压-电阻曲线下降斜率基本保持不变,但初始电阻值下降;而在温度保持不变时,随碳纤维掺量的增加,电阻随电压下降的趋势逐渐减缓.     

Recycling of carbon fibre reinforced composites using water in subcritical conditions

In this paper, a method of chemical recycling of thermosetting epoxy composite was discussed. Water was used to be reaction medium and the decomposition of carbon fibre reinforced epoxy composites was studied. Experiments were devised in order to identify the significant process parameters that affect fibre reinforced composite recovery potential including temperature, time, catalyst, feedstock, and pressure. Experiments were performed in a batch-type reactor without stirring. Under the condition that the temperature was 260 °C and the ratio of resin and water was 1:5 g/mL, the decomposition rate could reach 100 wt.% and the carbon fibres were obtained. The results from the Scanning Electron Microscopy (SEM) and Atomic Force Microscope (AFM) measurements showed that the fibres were clean and no cracks or defects were found. The average tensile strength of the reclaimed fibres was about 98.2% than that of the virgin fibres.     

碳纤维增强油井水泥石的力学性能

油井水泥石在井下易脆裂,造成油井层间封隔失效,进而影响油井开采。为了解决这一问题,需要对水泥石进行降脆增韧。首先,考察了甲基纤维素和羧甲基纤维素对碳纤维的分散效果;然后,研究了碳纤维对油井水泥石抗压强度、抗折强度和劈裂抗拉强度的影响,并模拟井下环境测试了水泥石单轴和三轴应力-应变曲线;最后,使用扫描电子显微镜对碳纤维增强水泥石的微观形貌进行观察,探讨碳纤维对水泥石的增韧机制。结果表明:0.2wt%的羧甲基纤维素溶液可有效分散碳纤维;养护28d后,0.3wt%碳纤维增强水泥石的抗压强度、抗折强度和劈裂抗拉强度较纯水泥石的分别提高8.6%、31.5%和52.4%,三轴直接加载条件下,其弹性模量较纯水泥石的低49.5%;经过分散的碳纤维在水泥石中乱向分布,形成三维网络结构,通过桥联、剥离及拔出耗能作用增强水泥石。研究结果为解决油井水泥石易脆裂的问题提供了理论参考。     

Pressure-sensitive properties and microstructure of carbon nanotube reinforced cement composites

Carbon nanotubes (CNTs) treated by using a mixed solution of H₂SO₄ and HNO₃ were uniformly dispersed into cement paste by means of ultrasonic energy. Electrical resistivity and pressure-sensitive properties under cyclic compressive loading of this composite were analyzed and compared to that of untreated-CNT reinforced cement paste. Results show that the addition of treated or untreated CNTs to cement paste leads to a notable decrease in volume electrical resistivity and a distinct enhancement in compressive sensitivity. The microstructures of these cement composites were analyzed by using scanning electron microscope. The microscopic observation reveals that both treated and untreated CNTs were dispersed homogenously in the cement matrix. For untreated CNT-reinforced cement composites, the CNTs with glossy surface were zigzag and cling to cement matrix; the bridging of cracks and a well three-dimensional meshwork were also observed. For treated-CNT reinforced cement composites, the surface of CNTs was covered by C–S–H, which leads to a higher mechanical strength. The contact points of the treated-CNTs in composites were much fewer than that of the untreated-CNTs in cement matrix composites, which leads to a higher compressive sensitive properties and a lower electrical conductivity.     

Carbon fibre reinforced carbon produced by polymer impregnation and pyrolysis

Carbon fibre reinforced carbon (C/C) is an attractive material for intermediate and high temperature applications due to its specific properties like low density, high strength and chemical stability. Unfortunately the material oxidizes, so that in an oxidative environment a protective coating has to be applied. Polymer impregnation and pyrolysis is a cost effective production technique to produce C/C materials. In the present work, an abstract of a research program funded by the German Science Foundation (DFG), the mechanical properties of C/C as a function of processing temperature and test temperature have been described. In the program the behaviour of two-dimensionally reinforced (2D) material and unidirectional reinforced (1D) materials has been investigated. All materials experience a strength reduction as a result of carbonization of the polymer matrix at temperatures up to 1000°C. An additional heat treatment above 1000°C causes a partial recovery of the strength. The 1D C/C material shows up to testing temperatures of 1800°C a 10 % loss of strength, whereas for the 2D C/C the strength increases by 10 % at 1500°C in comparison with the room temperature results.     

Mechanical behaviour of carbon and glass hybrid fibre reinforced polyester composites

Mechanical behaviour of carbon fibre/glass mat/polyester resin hybrid composites of sandwich construction is studied through tension, flexure, impact and post-impact tension tests. Tensile and flexural strength, modulus and failure strain values are compared to the calculated values. Total impact fracture energy and residual (after impact) tensile strength values of hybrid composites are analysed with regard to corresponding values of carbon/polyester composites. Failure of tested coupons was analysed by visual inspection and observation by scanning electron microscopy.     

Study on UV laser machining quality of carbon fibre reinforced composites

The large differences in material properties of the carbon fibre and the epoxy resin in carbon fibre reinforced plastic (CFRP) composite make laser processing very challenging. The heat affected zone (HAZ) has been the major obstacle for wide industry applications of laser machining of CFRP composites. This paper investigates the quality of CFRP machined by a diode pumped solid state (DPSS) UV laser. The results show that minimum HAZ (about 50 μm) is achievable in machining of CFRP composite by using short pulsed UV laser. The study found that heat is easily accumulated in the material during laser processing, especially when the carbon fibres are sliced into small pieces. The paper discusses how to make use of the heat accumulation and how to avoid potential damage by the heat accumulation. Bearing strengths test and fracturing mechanisms study were conducted. Method of characterization of thermal damage in polymer matrix is developed.