Fatigue behaviour characterization of the fibre-matrix interface

The fracture behaviour of FRP composite materials is significantly influenced by the behaviour of the fibre-matrix interfacial bond. Thus far interfacial bond mechanical characterization has been based upon the critical strength and critical fracture energy of debonding. Characterization of the fatigue behaviour of the interfacial debonding process, however, may be more valuable for composite design and fibre-matrix selection. A fracture mechanics model of interfacial bond fatigue based on the mode II strain energy release rate (G _II) is presented. An expression forG _II is derived for a single fibre in matrix cylinder model. By fitting the model to single fibre pull-out fatigue test data, fatigue crack propagation plots for specific fibre-matrix combinations can be drawn. These should prove useful for the development of fatigue resistant FRP composite materials.     

Fracture toughness of the fibre-matrix interface in glass-epoxy composites

The failure process arising at a broken fibre end in polymer matrix composite materials has been studied experimentally and analytically using the finite element method. A series of experiments were carried out using S-glass and E-glass single filaments, with different sizings and/or coupling agents, embedded in epoxy matrices with different moduli. A finite element analysis was used to simulate the experiments and calculate the change in strain energy accompanying the observed fracture mode. The strain energy release rate upon arrest of the crack, G _arrest, was then calculated. The measured interface debonding energies varied from G _arrest=57–342 J m^–2, depending primarily on the nature of the fibre sizing and the ratio of moduli of the fibre and matrix. Transverse and shear matrix cracks were characterized by G _arrest values of 58–103 J m^–2. Subtle changes in the constituent properties or fibre surface treatment resulted in a change in the fracture mode. This measurement and analysis technique may suggest reasons for the variability of previous measurements of interfacial adhesion, and provide a standard method for characterizing fracture modes at broken fibre ends.     

XPS study of carbon nitride films deposited by hot filament chemical vapor deposition using carbon filament

Amorphous carbon nitride, a-CN_x, thin films were deposited by hot filament CVD using a carbon filament with dc negative bias voltage on the substrate. The effects of the negative bias and the filament components on the binding structure of the films are investigated by XPS. The composition ratio of graphite to amorphous carbon in the filaments affects the bonding structure of carbon and nitrogen in the films, although the nitrogen content in the films is almost same as 0.1. The nitrogen content in the films changes from 0.1 to 0.3 as the negative bias changes from 0 to − 300 V.     

AFM,SEM and XPS characterization of PAN-based carbon fibres etched in oxygen plasmas

Atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) have been used to investigate changes in topography and surface chemical functionality on PAN-based carbon fibres exposed to low-temperature, lowpower, oxygen plasmas. Unsized, type II, Cellion 6000 carbon fibres were treated in oxygen plasmas for 2–60 min at a power of 25 W. Increasing treatment time caused an increase in oxidation from surface alcohol(ether) to carbonyl and carboxyl species, but the total amount of oxidized carbon near the surface remained constant. SEM confirmed that treatments longer than 15 min resulted in pitting on the fibre surface, but even treatments of 60 min did not significantly reduce the overall fibre diameter. AFM showed surface morphology changes after oxygen plasma treatments for 2 and 15 min. 1 m×1 m AFM scans of untreated fibres showed processing grooves with a distribution of depths. Enlarged images along these grooves revealed that their walls were smooth. Oxygen plasma treatments of 2 min roughened fibre surfaces and created holes of the order of 50 nm evenly distributed with a spacing of 150 nm along the bottoms of the grooves. Treatment for 15 min smoothed the overall topography and resulted in smaller holes, of the order of 5–10 nm, with a spacing of < 50 nm. Calculated RMS roughnesses from the AFM data showed an initial increase in roughness with treatment, followed by a decrease to final values lower than those for untreated fibres.     

Numerical characterisation of the fibre-matrix interface crack growth in composites under transverse compression

Inter-fibre failure under compression transverse to the fibres is studied at micromechanical level. Interfacial fracture mechanics concepts, associated to both the open model and the contact model, are applied. A numerical study is performed using the boundary element method aimed at explaining the origin and evolution of the damage at micromechanical level, considered as fibre-matrix interface cracks. Assuming that the damage starts as small debonds originated by shear stresses at the position where their maximum values are reached, it has been found that the crack shows different morphologies at both tips: an open one and a closed one with a large contact zone. Then the interface crack grows unstably in mixed mode only on the open tip side until this growth changes to stable, once the crack closes at this tip, with the generation of a contact zone.     

Transmission electron microscopy of the fibre-matrix interface in SiC-SCS-6-fibre-reinforced IMI834 alloys

In SiC-SCS-6-fibre-reinforced IMI834 alloys, a high thermal stability of tensile properties and of the fibre-matrix interface for temperatures up to 700 °C was reported. In the present paper, the interface of these composites is investigated by analytical transmission electron microscopy in the as-processed condition and after a thermal treatment at 700 °C for 2000 h and at 800 °C for 500 h. The interface in (SiC-SCS-6)-IMI834 composites consists of two layers: the TiC reaction zone with a thickness of about 0.4 μm and a layer of a (Ti, Zr)xSiy phase with a thickness of about ∼0.1 μm. The energy-dispersive electron beam analysis of the (Ti, Zr)xSiy layer results in a (Ti, Zr)2Si phase with a Ti-to-Zr ratio of approximately 1.4. Electron beam diffraction of the (Ti, Zr)xSiy layer identifies it as S2 silicides present in near-α alloys. The thermal stability of the interface in the (SiC-SCS-6)-IMI834 composites is ensured by the continuous coating of the (Ti, Zr)xSiy phase. This is the case for a thermal treatment at temperatures up to 700 °C for 2000 h. After the treatment at 800 °C for 500 h, the thickness of the TiC reaction zone is increased, gaps in the (Ti, Zr)xSiy layer appear, titanium carbide grows further into the titanium matrix and the thermal stability of the interface is lost. This revised version was published online in November 2006 with corrections to the Cover Date.     

An XPS study of cation-exchanged Y-type zeolites

A study of Na-, Li-, and Cs-exchanged Y-type zeolites using conventional MgKα and high-energy monochromatic AgLα radiation is described. The use of an electron flood gun for charge neutralization in the potential range 0–10 V has revealed both physical and chemical instability of certain samples. Certain ions (e.g., sodium in the Na-exchanged samples) are mobilized by higher-energy (above 6 V) electron bombardment and migrate to the surface region, where they can be reduced to the metal state.     

Rubrene/TPD表面及其界面的AFM和XPS研究

在indium-tin-oxide(ITO)玻璃上采用多源蒸发的方法制备了红荧烯(Rubrene)/TPD样品。由原子力显微镜(AFM)得到的表面形貌图显示Rubrene膜具有良好的均匀性。利用X射线光电子能谱仪(XPS)研究了两种材料表面和界面的电子态。由Rubrene/TPD的表面分析可知,C1s精细谱中有3个峰最强的峰是由氧化造成的,而不是对应于芳香碳(284.55eV)。空气中的O2和H2O扩散到样品里形成O1s峰。用氩离子束溅射剥蚀表面,随着时间的增长,芳香碳对应的峰越来越强,282.45和289.62eV处的峰则迅速消失,当溅射时间超过4920s时,在2847eV处引入了一个新的峰,对应于C—N键。O1s峰迅速减弱是因为氧沾污的去除。N1s谱中的N—C键缓慢增强,到达界面附近时峰的强度变得稳定。C1sN1s和O1s谱中都有峰发生化学位移。     

The effect of hydrostatic pressure on the fibre-matrix bond in a steel-resin model composite

The shear strength of the adhesive bond between a steel wire and an epoxy resin has been measured and it has been found that when the composite is under pressure the strength corresponds closely to the shear yield stress of the resin determined in a plane strain compression test. Discrepancies at low pressure may be due to the nucleation of stress-concentrating cracks. Measurements of the friction stress between the wire and the resin as a function of pressure indicated that the coefficient of friction was 0·5 and that the compressive stress at the interface due to resin shrinkage was 7 Nmm^–2 (1000 psi).     

Scanning tunnelling microscopy study of activated carbon fibres

Scanning tunnelling microscopy (STM) has been used to study isotropic pitch-based carbon fibres before and after steam activation. The results show that the present carbon fibre precursor exhibits a particulate surface which is very favourable for the formation of activated carbon fibre. After activation, the carbon fibre surface becomes much more porous and rougher, and the mesopores are evidently present on the surface. Because the scale is down to atomic resolution, the STM observations offer direct evidence for the existence of micropores on the surface of the activated carbon fibres. In addition, the surface textures of both fibres are presented and discussed.     

Some aspects of interface adhesion of electrolytically oxidized carbon fibres in an epoxy-resin matrix

A comprehensive investigation of the adhesion at the interface of a carbon fibre in an epoxy resin was made. The fibre surfaces were modified, to increase their adhesion to resin, by an electrolytic surface treatment which was applied at various current densities. Subsequent changes in the fibre properties relating to possible mechanical, physical and chemical contributions to adhesion were monitored. Tensile tests on single fibres indicated that the treatment altered the strengths of the fibres, which were found to have their highest values and to be least variable at an optimum adhesion level. A method was developed to estimate the strength of the fibres in the resin, this confirmed the single-fibre data. A novel method of labelling the acidic sites by producing adsorption isotherms was developed to identify surface functionality. Surface acidity correlated well with adhesion levels. Single-fibre pull-out tests, modelled using a new combination failure criterion and fragmentation tests, indicated that the optimum adhesion level for this fibre/resin system was achieved with an electrolytic treatment at 25 C m^–2. The principal effects of this treatment were considered to be due to chemical modification of the fibre surface coupled with the removal of a loosely adherent surface layer.     

An XPS and reduction study of PrCoO3

The perovskite-type oxide PrCoO₃ has been studied by means of X-ray photoelectron spectroscopy (XPS), reduction in H₂ and X-ray diffraction. Two types of oxygen were detected: lattice oxygen (binding energy = 528.4 eV) and adsorbed oxygen (binding energy = 530.9 eV). The increase in relative intensity of the peak corresponding to the latter species after reduction of PrCo0₃ to 3e^– per molecule is assigned to the formation of hydroxyl groups. Temperature-programmed reduction (TPR) results showed two reduction steps: to 1 e^– per molecule (Co^3.1 Co²⁺) at 475 to 635 K, and to 3e^– per molecule (Co²¹ Co⁰) at 725 to 815 K. Reduction in the first and second steps occurs according to the contracting sphere model and the nucleation mechanism, respectively. Reduction of Co³⁺ to Co²⁺ causes minimal structural changes in the perovskite. Reduction to 3e^– per molecule yielded Pr₂O₃ and metallic cobalt. After this reduction and reoxidation at 973 K, the perovskite structure was regained. By XPS and TPR it was shown that PrCo0₃ is more easily reducible than LaCo0₃. It is concluded that the cation in the A position of the structure plays a significant role in the bulk and surface properties of LnCo0₃ (Ln, lanthanide elements) oxides.     

An experimental investigation of modified and unmodified flax fibres with XPS, ToF-SIMS and ATR-FTIR

Natural fibres are envisaged today as potential candidates for replacing glass fibres in composite materials. Although natural fibres have a number of advantages over glass fibres, the strong polar character of their surface is a limiting factor as, compatibility with strongly apolar thermoplastic matrices is very low. Such problems of incompatibility may be overcome with fibre pre-treatments, which can enhance compatibility although having a negative impact on the economics of using such materials. In this study two fibre pre-treatment methods, acetylation and stearic acid treatments, have been applied on flax fibres. The effect of these two pre-treatments has been examined by use of XPS, ToF-SIMS and FTIR spectroscopic methods. It was found that the fibre surface before treatment is very different to what may have been expected for cellulose materials. There is an appreciable coverage of the flax fibre surface with hydrocarbon compounds, possibly waxy substances, but no aromatic compounds were detected. All three spectroscopic methods revealed that the fibre surface chemistry has been altered after the treatments, and especially for acetylation it was found that ester bonds are present on the fibre surface after treatment. For the stearic acid treatment the situation still remains less conclusive. Finally, ToF-SIMS experiments revealed that the coverage of the fibre surface with acetyl groups and stearic acid is highly heterogeneous.