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.     

Interface characteristics and mechanical properties of carbon fibre reinforced copper composites

Interface characteristics of carbon fibre reinforced copper matrix composites materials with various interface states and their effect on the flexural strength of composites have been studied. Interfacial states are mechanical bonding, dissolution bonding and reaction bonding. To a certain extent, raising the interfacial strength enables an increase in the flexural strength due to prevention of carbon fibre being pulled out under low stress during fracture process of composites. Raising the interfacial bondage strength, causes the brittleness of composites to increase; the fracture surface of composites is converted from a fibre pull-out model to a fibre even model. While strengthening the interface bondage, the extent of chemical reaction and dissolution at the interface must be controlled to avoid degrading the carbon fibre.     

Plasma surface modification of advanced organic fibres

The interlaminar shear strength, interlaminar fracture energy, flexural strength and modulus of extended-chain polyethylene/epoxy composites are improved substantially when the fibres are pretreated in an ammonia plasma to introduce amine groups on to the fibre surface. These property changes are examined in terms of the microscopic properties of the fibre/matrix interface. Fracture surface micrographs show clean interfacial tensile and shear fracture in composites made from untreated fibres, indicative of a weak interfacial bond. In contrast, fracture surfaces of composites made from ammonia plasma-treated fibres exhibit fibre fibrillation and internal shear failure as well as matrix cracking, suggesting stronger fibre/matrix bonding, in accord with the observed increase in interlaminar fracture energy and shear strength. Failure of flexural test specimens occurs exclusively in compression, and the enhanced flexural strength and modulus of composites containing plasma-treated fibres result mainly from reduced compressive fibre buckling and debonding due to stronger interfacial bonding. Fibre treatment by ammonia plasma also causes an appreciable loss in the transverse ballistic impact properties of the composite, in accord with a higher fibre/matrix interfacial bond strength.     

The crystallization and interfacial bond strength of nylon 6 at carbon and glass fibre surfaces

The nucleation and crystallization of nylon at the interface in glass-fibre and carbon-fibre reinforced nylon 6 composites has been investigated by electron microscope studies of sectioned and etched bulk specimens and solution cast and melt crystallized thin films. The fracture energies of the composites were obtained from tensile strength tests and the interfacial bond strengths were calculated from fibre pullout measurements. The fibres are shown to nucleate a columnar structure at the interface with marked differences between the structures nucleated by glass fibres and by carbon fibres and also between that nucleated by type I and type II carbon fibres. The structure around glass fibres was non-uniform and influenced to some extent by the presence of the size coating on the fibre surface. In the carbon-fibre composites the columnar structure was due primarily to physical matching of the graphite crystallites. Surface treatment of the carbon fibres to improve chemical bonding is shown to have a significant effect on bond strength which cannot be explained in terms of the columnar structure at the fibre surface. The treated fibres gave rise to only small amounts of fibre pull-out and low fracture energies whereas the untreated fibres showed extensive pull-out which was reflected in high fracture energies.     

Fracture behaviour of long fibre reinforced thermoplastics

This paper deals with the fracture performance of injection moulded long glass fibre composites based on polybutylene terephthalate (PBT) and polypropylene (PP) matrices. The tensile behaviour of these composites is analysed using the shear lag theory taking into consideration the interfacial shear strength, fibre length distribution and fibre orientation in the mouldings. The fracture performance is investigated using the post yield fracture mechanics approach. The crack growth resistance of the PP and PBT long fibre composite was found to increase with increasing fibre volume content up to 35%. Above 35% a plateau in the fracture performance was observed. A combination of high fibre degradation and a change in the fibre orientation pattern of the moulded pieces is found to be responsible for the plateau region in the performance of the high concentration system. In fact, the dependence of the maximum crack growth resistance of the composites on fibre length and fibre orientation is also controlled by testing temperature. The competition between fibre-induced matrix deformation and the fibre pull-out determines the ability of the composites to resist crack propagation.     

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized. This revised version was published online in November 2006 with corrections to the Cover Date.     

Deformation micromechanics in model carbon fibre-reinforced composites

Raman spectroscopy has been used to study the deformation micromechanics of the single-fibre pull-out test for a carbon fibre/epoxy resin system using surface-treated and untreated versions of the same type of PAN-based fibre. It has been possible to determine the detailed strain distribution along embedded fibres and it has been found that it varies with the level of strain in the fibre outside the resin block. The variation of interfacial shear stress along the fibre/matrix interface has been determined using the balance of forces equilibrium and this has been compared with the single values of interfacial shear strength determined from conventional pull-out analyses. It has been demonstrated that it is possible to identify situations where the interface is well-bonded, partially debonded or fully debonded and also to follow the failure mechanisms in detail. It has been found that the level of interfacial adhesion is better for the surface-treated fibre and that, for the untreated fibre, interfacial failure takes place by the cohesive failure of a weakly-bonded surface skin that appears to be removed by the surface pretreatment process.     

过氧化物的引发作用对玻璃纤维增强聚丙烯界面结合的影响

采用过氧化物溶液涂覆玻璃纤维,将处理过的玻璃纤维丙烯复合,采用单丝临界长度法测定了复合体系的界面剪切强度,研究了过氧化物的引发作用对玻璃纤维／聚丙烯复合体系界面结合的影响,探索了复合工艺条件对体系界面结合的影响,并考察了所形成界面的耐水性能,结果表明,涂覆于玻纤表面的过氧化物在复合过程中能引发聚丙烯与玻璃纤维表面含双键的偶联剂反应,在纤维与基体之间形成较界面结合,过氧化物的这种引发作用对界面的耐水     

Plasma surface modification of advanced organic fibres

A marked improvement in the interlaminar shear strength and flexural strength of aramid/ epoxy composites is observed when the fibres are pretreated in an ammonia or ammonia/ nitrogen gaseous discharge (plasma) to introduce amine groups on to the fibre surface. Scanning electron and optical microscopic observations are used to examine the microscopic basis for these results. Scanning electron micrographs of shear fracture surfaces show clean fibre/matrix separation in composites made from untreated fibres, indicative of weak interfacial bonding. In contrast, shear fracture surfaces of composites containing plasma-treated fibres exhibit clear evidence of fibre fibrillation and matrix cracking, suggesting stronger interfacial bonding. Optical microscopic examination of flexure specimens shows that enhanced strength results mainly from reduced compressive fibre buckling and debonding, due to an increase in fibre/matrix interfacial bond strength. This increase is not accompanied by any significant change in the interlaminar fracture energy or flexural modulus of the composites, but there is an appreciable loss in transverse ballistic impact properties. These results are also examined in terms of the observed increase in fibre/matrix interfacial strength.     

Flexural failure mechanisms and global stress plane for unidirectional composites subjected to four-point bending tests

Glass fibre-reinforced epoxy and polyester composites of different fibre/matrix interface strengths exhibited tensile, compressive and shear failure modes in four-point bending tests. The flexural tensile mechanism comprised fibre ridging, transverse matrix cracking and longitudinal matrix cracking; the flexural compressive mode was caused by microbuckling of fibres. The interface strength appeared to affect each of these failure mechanisms, with the flexural tensile mode associated with the strongest and the shear failure mode corresponding to the poorest interface condition. The apparent flexural strength also decreased rapidly as the interface degraded. These phenomena are rationalized by a newly developed ‘global stress plane’, the theoretical basis of which is that the dependency of the interlaminar shear strength on the interfacial shear strength is larger than that of the longitudinal compressive strength, which in turn is larger than that of the longitudinal tensile strength.     

Matrix Modification for Improved Reinforcement Effectiveness in Polypropylene/Glass Fibre Composites

The influence of matrix modification on the interfacial shear strength (IFSS) and the mechanical performance of polypropylene/glass fibre composites is investigated. Two different modifiers were used: a highly reactive hyperbranched polymer grafted polypropylene (HBPgPP) and a maleic anhydride grafted polypropylene (MAHgPP). The interfacial shear strength increased with the addition of the modifiers, with HBPgPP giving the highest values. To evaluate the effects of the matrix modification on the composite strength, a method to normalise the composite strength with respect to fibre orientation and fibre concentration is presented. The normalised strength values followed the same trend as the measured IFSS values, namely that the HBPgPP modified composite displayed the highest strength and the unmodified material the lowest.     

Effects of fibre coating (size) on properties of glass fibre/vinyl ester composites

Effects of fibre coating (size) on transverse cracking has been investigated. Two glass fibre/vinyl ester model composites were studied, denoted CA and NoCA and based on different size compositions. Various single fibre tests were not able to quantify the interfacial failure of CA as the interface never failed. The CA size consisted of a film former and a methacrylsilane-coupling agent whereas the NoCA size did not contain any coupling agent. The study reveals limitations with single fibre composite tests for fibre/matrix combinations with high interfacial toughness. Cross-ply laminates based on NoCA demonstrated significant inferior transverse cracking toughness as compared with CA laminates. Composites based on commercially sized glass fibre were also investigated and they performed almost as poorly as the NoCA material, demonstrating large potential for improvement in commercial composites. Results further indicate that the remarkable transverse cracking toughness of the CA material stems partly from strong fibre/matrix adhesion but also from high ductility of the matrix region close to the fibre surface.     

Transcrystallisation in PP/flax composites and its effect on interfacial and mechanical properties

The effect of flax fibre reinforcement on the crystallisation behaviour of polypropylene (PP) was investigated using a hot-stage polarising optical microscope. To follow the crystallisation kinetics, crystallisation temperature and time were varied. At crystallisation temperatures between 130 and 138 °C the most uniform and thickest transcrystalline layers were formed. The effect of transcrystallisation on the interfacial shear strength (IFSS) in micro-composites was studied by the fibre pull-out test method. It was found that the IFSS of the PP/flax system is slightly decreased with the presence of a transcrystalline interphase. Finally, the influence of the formation of a transcrystalline zone on the macromechanical properties of compression moulded PP/flax composites was studied.     

Effects of interfacial coating and temperature on the fracture behaviours of unidirectional Kevlar and carbon fibre reinforced epoxy resin composites

The enhancement of transverse fracture toughness of unidirectional Kevlar and carbon fibre reinforced epoxy resin composites (KFRP and CFRP) has been studied using polymer coatings on the fibres. The results obtained show a substantial improvement in the impact fracture toughness of both KFRP and CFRP with polyvinyl alcohol (PVAL) coating without any loss of flexural strength; but there is only a moderate increase in impact toughness with other types of coating (i.e. carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer and polyvinyl acetate (PVA)) with some reduction in flexural strength. The dependence of impact fracture toughness of these composites (with and without PVAL coating) on temperature was analysed on the basis of existing theories of toughening mechanisms from measurements of fibre-matrix interfacial properties, debond and fibre pull-out lengths and microscopic observations. The beneficial effect of fibre coating with PVAL on transverse fracture toughness is shown to sacrifice little damage tolerance of the composites against delamination fracture.     

Tailoring of dual-interface in high tenacity PP composites – Toughening with positive hybrid effect

The study proves the feasibility of manufacturing injection moulded polypropylene composites reinforced with short rayon cellulose fibres of two selectively tailored fibre–matrix interfaces. The originally developed method relies on selective chemical grafting of two different polymer waxes onto the surface of cellulose fibres in order to obtain two different strengths of fibre–matrix interfaces in one composite. This selective tailoring of a dual-interface is meant to improve the notched impact strength without deteriorating of its flexural strength. Compatibilised fibres have a strong interphase, which conditions the transfer of strain from the matrix to fibres during deformation. Fibres tailored for a weak interface more efficiently hinder the crack propagation at crash. A 32% improvement of composite notched impact strength was achieved with merely a 5% deterioration of its flexural strength. Its specific properties are on the level or better than those of polypropylene counterpart reinforced with the same content of glass fibres.     

Shear strength and fracture toughness of carbon fibre/epoxy interface: effect of surface treatment

Textile-reinforced composites have become increasingly attractive as protection materials for various applications, including sports. In such applications it is crucial to maintain both strong adhesion at fibre–matrix interface and high interfacial fracture toughness, which influence mechanical performance of composites as well as their energy-absorption capacity. Surface treatment of reinforcing fibres has been widely used to achieve satisfactory fibre–matrix adhesion. However, most studies till date focused on the overall composite performance rather than on the interface properties of a single fibre/epoxy system. In this study, carbon fibres were treated by mixed acids for different durations, and resulting adhesion strength at the interface between them and epoxy resin as well as their tensile strength were measured in a microbond and microtensile tests, respectively. The interfacial fracture toughness was also analysed. The results show that after an optimum 15–30 min surface treatment, both interfacial shear strength and fracture toughness of the interface were improved alongside with an increased tensile strength of single fibre. However, a prolonged surface treatment resulted in a reduction of both fibre tensile strength and fracture toughness of the interface due to induced surface damage.     

Mechanical performance optimization through interface strength gradation in PP/glass fibre reinforced composites

A new design for thermoplastic composites based on the gradation of the interlaminar interface strength (IGIS) has been developed with the aim of coupling high impact resistance with high static properties. IGIS laminates have been prepared by properly alternating layers of woven fabric with layers of compatibilized or not compatibilized polymeric films. To prove the new concept, polypropylene (PP) and glass fibres woven fabrics have been used to prepare composites by using the film stacking technique. Maleated PP, able to compatibilize polypropylene with glass fibres, has been used to manage the interface strength layer by layer.The flexural and low-velocity impact characterizations have shown that the presence of the coupling agent in conventional composite structures (prepared with fully compatibilized polymeric layers) improves the static flexural properties through the strengthening of the matrix/fibre interface but considerably lowers the low velocity impact resistance of the composite, in terms of maximum load before fibre breakage and recovered energy after impact. The use of the IGIS design, that grade the interface strength through the laminate thickness, allows to prepare composites with both high flexural properties and high impact resistance, without affecting the balance and type of the reinforcement configuration.     

Matrix morphology and fibre pull-out strength of T700/PPS and T700/PET thermoplastic composites

The main objective of this fundamental study was to investigate effects of processing conditions and resulting matrix morphology on interfacial bond strength of fibre reinforced thermoplastic composites. Using a hot stage microscope, single fibre pull-out samples were produced with T700S high strength carbon fibre and two semicrystalline thermoplastic matrices, polyphenylene sulphide (PPS) and polyethylene terephthalate (PET), respectively. Processing temperatures and cooling histories were the major variables in sample preparation. The T700S fibre had no clear effect on the surrounding PPS and PET matrix morphology, as long as direct cooling at constant rates was selected. A transcrystalline phase around the fibres could be induced in the T700S/PPS system, if isothermal crystallization was carried out at 227°C. Fibre pull-out tests were conducted at room temperature and two basic failure paths were observed, i.e. debonding at the fibre-matrix interface and cohesive failure of the matrix close to the fibre surface. The results indicate that slow cooling rate and a resulting coarse spherulitic morphology around the fibres correlate with high interfacial shear strength. In fact somewhat higher strength values were obtained for samples with transcrystalline layers around the fibres.     

Fibre fragment distribution in a single-fibre composite tension test

Single fibre fragmentation tests are performed for brittle fibres with Weibull strength distribution and different surface treatments. The fragmentation process is modelled and closed-form expressions for break spacing distribution are obtained. The model accounts for the effect of finite fibre length on the initial fragmentation as well as for break interaction on the advanced fragmentation stage. It is assumed that the exclusion zone due to fibre–matrix interface failure and stress recovery in the fibre is linearly dependent on the applied load. This assumption is validated experimentally. The derived theoretical average fragment length dependence on applied load is used to determine the fibre strength distribution parameters and the effective interfacial shear stress for carbon/epoxy single fibre composites with different fibre surface treatment and for glass/vinylester single fibre composite. Fragment length distribution is predicted for several load levels. Predictions are in good agreement with experimental data.     

玻璃纤维毡增强聚丙烯复合材料的冷热循环疲劳特性

通过测定玻璃纤维毡增强聚丙烯复合材料在不同温度区间内经冷热循环后的弯曲性能和动态力学性能以及玻璃纤维增强聚丙烯复合体系经冷热循环后界面剪切强度的变化, 研究了该材料的冷热循环疲劳特性。结果表明,在一定温度区间内的冷热循环会对玻璃纤维毡增强聚丙烯的界面造成损伤, 使材料的力学性能下降; 随着冷热循环温度区间温差的增大、冷端温度的降低、循环次数的增多, 形成的热应力对材料的界面损伤越严重; 不同的复合体系由于其界面松弛热应力的能力不同, 在同样条件下的冷热循环过程中, 界面所受到的损伤程度有差异。