Matrix fractography of fibre-reinforced epoxy composites

This paper describes the origin and development of fractographic features found in the matrix of carbon fibre-reinforced epoxy composites and their significance in the analysis of the failure of structures fabricated from composite material.     

Tailoring of the interfacial properties of polymeric single fibre-reinforced epoxy composites by non-oxidative plasma treatments

The interfacial properties of epoxy composites reinforced with a single, plasma-treated fibre of either poly(p-phenyleneterephthalamide) (PPTA) or poly(p-phenylenebenzobisoxazole) (PBO) have been investigated with a focus on evaluating the effect of two non-oxidative (He and N₂) microwave plasma treatments on interfacial adhesion properties. Tensile testing of single filaments revealed that their tensile strength does not diminish with the plasma treatments, despite the fact that their surface properties have been both physically and chemically modified. Interfacial characterisation by Raman spectroscopy indicated that the quality of adhesion was substantially enhanced following exposure of the fibres to microwave plasma treatment in either pure helium or pure nitrogen flows for just one minute. Such improvement was higher than that attained when O₂ was used for blowing the plasma, under the same operational conditions. Moreover, no swelling effect was observed by AFM after exposure of the He or N₂ plasma-treated fibres to ambient conditions for as much as 24 h.     

Failure mechanism discrimination in carbon fibre-reinforced epoxy composites

To examine the possibility of discriminating between the various failure mechanisms which occur in carbon fibre-reinforced epoxy composites, unidirectional and crossplied specimens have been loaded to failure at various angles with respect to the fibre direction. The mechanisms which dominate the failure of unidirectional specimens under these loading conditions are known and the results have been compared with the amplitude of the acoustic signals recorded during loading. High amplitude signals were seen to originate from cracking of the matrix parallel to the fibres, whereas low amplitude signals were recorded from fibre breaks. These results have permitted an interpretation of the failure processes occuring in crossplied specimens.     

Electron paramagnetic resonance and optical absorption studies of Cu2+ ion doped polyvinyl alcohol films

Electron paramagnetic resonance (EPR) and optical absorption studies have been carried out on Cu²⁺ ions doped in polyvinyl alcohol films (PVA). EPR spectrum at room temperature exhibits hyperfine structure characteristic of Cu²⁺ ions in tetragonal symmetry. The EPR spectra have also been recorded at various temperatures. The number of spins participating in the resonance is measured as a function of temperature and the activation energy is calculated. The paramagnetic susceptibility (χ) is calculated from the EPR data at various temperatures and the Curie constant is evaluated from 1/χ versus T graph. The optical absorption spectrum exhibits a broad band which has been assigned to the transition ²B_1g → ²B_2g.     

Energy absorption mechanisms during dynamic fracturing of fibre-reinforced composites

Dynamic photoelastic experiments were conducted to study crack propagation in fibrereinforced materials and, in particular, to determine the energy losses occurring during the crack growth and arrest process. This study utilized modified compact tension specimens which were fabricated from polyester matrix and different reinforcing fibres. The effect of the fibre-matrix interface on energy absorbed was also studied. The energy absorbed was partitioned into two parts: that absorbed in the fracture process zone associated with the crack tip, and the energy lost outside this zone. Results show that fibre reinforcement reduces the energy absorbed in the fracture process zone by about 10% for well-bonded and 15% for partly debonded fibres. For the same initial strain energy, this reduction in fracture energy manifests itself in reduced K _ID and lower crack-jump distance as compared to monolithic specimens. Reinforced specimens are found to retain a higher strain energy after crack arrest. The energy absorbed outside the fracture process zone for monolithic and well-bonded fibres is about 45% of the initial strain energy, while for partly debonded fibres it is about 55%.     

Electron paramagnetic resonance (a review)

For the purposes of this review, the termelectron paramagnetic resonance is taken to refer to the resonant absorption of electromagnetic radiation by electronic systems which possesspermanent magnetic moments (due, in general, to the orbital as well as to the spin angular momentums of electrons) and which are therefore paramagnetic. The termparamagnetic resonance, which is frequently encountered, refers strictly to the magnetic resonance of permanent magnetic dipoles of any type, including nuclear magnetic resonance. On the other hand, the termelectron resonance includes cyclotron resonance (diamagnetic resonance).In a short article of this nature, it is clearly impossible to discuss, in detail, all aspects of such a wide field. Rather, the intention is to discuss the basic principles of the electron paramagnetic resonance technique together with some results in certain fields which illustrate its range, applications, and potentialities.     

Post-fire flexural properties of fibre-reinforced polyester, epoxy and phenolic composites

The effect of fire damage on the flexural properties of fibre-reinforced polymer (FRP) composites is investigated. The FRP composites studied contained glass, carbon or Kevlar fibres with a polyester, epoxy or phenolic resin matrix. Artificial fire tests were performed on the composites using a cone calorimeter. The residual flexural modulus and strength of the burnt composites were determined at room temperature after the fire tests. The post-fire flexural properties of all the composites decreased rapidly with increasing heating time. Even the properties of the fibre-reinforced phenolic materials were severely degraded despite their low flammability and excellent fire resistance. The flexural properties of the phenolic-based composites were reduced due to thermal degradation and cracking of the resin matrix. In comparison, the properties of the polyester- and epoxy-based composites were reduced by combustion of the resin and formation of delamination cracks. A model is presented for determining the post-fire flexural properties of FRP composites with good accuracy.     

The interfacial properties of aramid/epoxy model composites

Many attempts have been made to measure, evaluate and improve the level of interfacial adhesion in aramid/epoxy composites. Different surface treatments have been developed in order to promote chemical bonding between the fibre and the matrix but it is found that most of the surface treatments developed have shown little or no improvement in the level of interfacial adhesion. The interfacial properties of a model composite are often determined by measuring the interfacial shear strength using micromechanical test methods that employ different loading configurations. However, the values of interfacial shear strength determined using different test methods are found to be dependent upon the variation of localized stress in the samples due to the different loading configurations and often give different results. Using Raman spectroscopy it is shown that the strain-dependent shift of the 1610 cm^–1 aramid Raman band can be used to determine the point-to-point variation of axial fibre strain along aramid fibres embedded in epoxy resin matrices from which the interfacial properties can be derived. The interfacial properties of aramid/epoxy model composites have been determined using Raman spectroscopy where the properties of the fibre, including different surface treatments, and the matrix have been changed systematically. The results are reviewed here and compared to those obtained using conventional micromechanical test methods. It is also demonstrated that the Raman technique can be used to characterize the interfacial properties of aramid/epoxy model composites deformed using different micromechanical test methods. In this way the interfacial properties can be determined at different loading levels enabling the progressive failure of the fibre/matrix interface to be monitored and defined accurately.     

Electron paramagnetic resonance of some ABX3 compounds

Electron paramagnetic resonance studies of NaMnCl₃, KMnCl₃, KCuCl₃, NH₄CuCl₃ were made between 400 and 5 K. The peak-to-peak derivative linewidths of these substances were found to be 14, 70, 10 and 5 mT, respectively. The line splitting below 15 K in NaMnCl₃ was attributed to hyperfine interaction with the Mn²⁺ nucleus. The g values and the peak-to-peak derivative linewidths of KMnCl₃, KCuCl₃ and NH₄CuCl₃ were found to be temperature independent within the limits of experimental errors. The g values of NaMnCl₃ and KMnCl₃ are isotropic, however, of KCuCl₃ and NH₄CuCl₃ are slightly anisotropic at room temperature and their principal values were determined. Hyperfineless structure of the spectra and the broad linewidths seem to be mainly due to exchange interactions of Mn²⁺ and Cu²⁺ ions among themselves in the respective compounds.     

Electron paramagnetic resonance: recent developments and trends

Major recent advances:High-field multi-frequency EPR improves our knowledge of magnetic materials, conductive polymers, and spin systems with large zero-field splittings. High-field electron nuclear double resonance provides better access to subtle details of the electronic structure of materials. Materials structure on length scales between 1.5 and 8 nm can now be characterized more precisely.