G. Marom

Hybrid effects in composites: conditions for positive or negative effects versus rule-of-mixtures behaviour

A positive or negative hybrid effect in hybrid composites is defined as a positive or negative deviation of a certain mechanical property from the rule-of-mixtures behaviour. The question of hybrid effects is first examined with special hybrids which have been chosen so that the effect of the fibre-matrix interface is minimized. The hybrids examined consisted of two types of carbon fibres with different mechanical properties but similar surface treatments. The results of all the mechanical properties examined (modulus, strength, stress intensity factor, fracture energies) under quasi-static and fast testing conditions do not show any synergism. In view of these results a second hybrid system of E-glass fibre/AS carbon fibre-reinforced epoxy has been chosen. In this system both the mechanical properties of the fibres and the interface which they form with the resin are entirely different. None of the mechanical properties, excluding the fracture energies, show any signs of a hybrid effect. The fracture energy results, however, show the existence of a negative hybrid effect. A theory which sets upper and lower bounds for the hybrid effect is proposed, and the conditions for the occurrence of either a positive or a negative effect are discussed.

Amorphous and crystalline morphologies in glycolic acid and lactic acid polymers

Abstract This paper describes an investigation of a number of parameters which affect the physical structures of glycolic acid (GA) and lactic acid (LA). It has been found that the cooling rate of a quenching process determines the amorphous-crystalline morphology balance, and that the effect of a similar quenching process will vary with the molecular weight of the polymer. At very high molecular weights, even very rapid quenching does not produce higher degrees of amorphous phase. Copolymerization of PGA or of PLA with poly(ethylene oxide) results in either phase blending or phase separation, depending on the copolymer composition and the segmental chain length. The degree of crystallinity of the PGA or PLA component in the copolymer is mostly affected by copolymerization in a state of phase blending.

The kinetics of α and β transcrystallization in fibre-reinforced polypropylene

Abstract The kinetics of α (monoclinic) and β (hexagonal) transcrystallization of isotactic polypropylene on aramid Kevlar 149 fibres, glass fibres and high modulus carbon fibres was investigated under isothermal and gradient cooling conditions. No difference was found between growth rates of bulk spherulites and transcrystalline layers, and Hoffmans theory led to the same results in both cases. Regarding α transcrystallization, a transition between regimes II and III occurred near 137°C and the ratio of the slopes of the two regimes was close to the theoretical value of 2. Regarding β transcrystallization, only regime II was exhibited in the temperature range studied. However, the induction time for transcrystallization was strongly influenced by the type of fibre, which in turn—based on Ishidas approach—resulted in variations in free energy differences at the fibre–crystallite interface for various fibres and bulk polypropylene. The respective values were 1.3, 1.5 and 2.1×10 −3 J m −2 for Kevlar 149 fibres, high modulus carbon fibres than in polypropylene, showing that α crystallization is more likely to occur in Kevlar 149 fibres and high modulus carbon fibres and bulk polypropylene. Gradient-thermal measurements were performed for α transcrystallinity which allowed estimation of the activation energy of transcrystallization for the different composites. Activation energies of transcrystallinity promoted on Kevlar 149 and high modulus carbon fibres were found higher than the activation energy for bulk crystallization.

Determining the role of interfacial transcrystallinity in composite materials by dynamic mechanical thermal analysis

The ability of this research group to obtain specimens of isolated transcrystalline layer by microtoming, reported in this paper for the first time, has opened a window for a range of characterization techniques. Here, the interfacial transcrystallinity in aramid fibre-reinforced nylon 66 microcomposites is studied using dynamic mechanical thermal analysis. The results show that the viscoelastic energy damping of the transcrystalline layer, tanδ tc = 0.064, is smaller while the elastic modulus, E ′ tc = 4.6GPa, is higher compared with the crystallized matrix. Moreover, the magnitude of the energy damping by the transcrystalline layer could be used in a rule-of-mixtures expression to calculate the energy damping of an aramid fibrereinforced nylon 66 microcomposite. It is also shown that the activation energy for relaxation, corresponding to the energy barrier for polymer chain movement, increases in the presence of reinforcement and transcrystallinity.

Relaxation processes in polyethylene fibre-reinforced polyethylene composites

Abstract The dynamic mechanical properties of filament-wound composites comprising ultrahigh-molecular-weight polyethylene (UHMWPE) extended-chain fibres in matrices of linear low-density polyethylene (LDPE), high-density polyethylene (HDPE), and thermally treated HDPE have been studied in tensile mode over a wide frequency range. The study focused on the additional effects of the fibres, for three winding angles of 26°, 34° and 45°, on the dynamic properties of the polyethylene-fibre-reinforced polyethylene composites. These effects were expected to result from transcrystallinity, which is induced in the matrix and which may invade a significant proportion of the composites, and from the extra restraint imposed by the reinforcement. The effects of the fibres are expressed by the real and imaginary moduli and the loss tangent and are analysed in terms of frequency dependence and activation energies for the α, β and γ transition processes. The effects of the fibres and of the crystallinity are evident in significantly higher moduli and activation energies of the relaxation processes.

Microstructure of nylon 66 transcrystalline layers in carbon and aramid fibre reinforced composites

The effect of the transcrystalline layer on the performance of fibre reinforced composite materials has been attributed to its better elastic/mechanical properties, which in turn result from a higher degree of crystalline order in the transcrystalline phase. For the aramid and carbon fibre reinforced nylon 66 composites, atomic force microscopy reveals radial regularity in the transcrystalline layer relative to the fibre, and X-ray diffraction investigations of the isolated layer suggest that the polymer chain is oriented predominantly perpendicular to the fibre axis.

Fretting-Wear Performance of Glass-Carbon- and Aramid-Fibre/Epoxy and PEEK-Composites.

Abstract This paper deals with the wear behaviour of continuous-fibre-reinforced plastics under oscillatory sliding against aluminium counterparts. Firstly, the influence of the loading parameters such as amplitude, frequency, nominal contact pressure and environmental temperature on the fretting wear of a carbon-fibre/epoxy-resin (CF/EP) laminate was studied. The amplitude, the frequency and the contact pressure were found to have a critical influence on the fretting wear rate of the CF/EP. Furthermore, at constant testing conditions the effects of material parameters (fibre orientation relative to the sliding direction, fibre and matrix material) were investigated. The incorporation of Aramid fibres affected the wear resistance of the polymers only little, while carbon and glass fibres resulted in an increase in the wear rates. Epoxy resin composites were slightly superior to polyetheretherketone composites.

The Role of Water Transport in Composite Materials

The last few years have witnessed a tremendous growth in the use of composite materials in various fields, spanning the whole range from sporting goods to structural materials for the aerospace industry. Such applications almost invariably entail contacts with liquids or vapours—either aqueous or organic—bearing on both the immediate and the long-term performance of the material. Hence, problems pertinent to the role of permeability of composite materials must be of prime consideration as limiting factors. This applies particularly to the most common phenomenon of moisture penetration.

The influence of thermal history on the mechanical properties of poly(ether ether ketone) matrix composite materials

Abstract The purpose of this study is to provide insight into the microstructural factors that affect the flexural fatigue performance of carbon-fibre-reinforced poly(ether ehter ketone) (PEEK) composites. Specifically, the effect of the degree of crystallinity on the mechanical properties is examined at two crystallinity levels of the as-received composites (35%) and of quenched composites (10%). Higher static flexural strength and modulus as well as longer fatigue life are observed for the higher crystallinity level. By varying the loading angle with respect to the fibre direction it is shown that the crystallinity effect is not matrix dependent alone. Rather, a strong effect is evident in the fibre direction, which is attributed to the influence of the transcrystalline layer formed on the fibre surface in the high-crystallinity material. As a result, the longitudinal fatigue life at 1·7GPa of the 35% crystallinity material is three orders of magnitude higher than that of the 10% crystallinity composite.

The mechanical role of the fibre/matrix transcrystalline interphase in carbon fibre reinforced j-polymer microcomposites

Abstract Microcomposites of single-pitch-based carbon fibre reinforced J-Polymer are employed to investigate the mechanical role of the fibre matrix transcrystalline interphase. The transcrystalline interphase in this semicrystalline thermoplastic system is varied by changing the crystallization kinetics, as determined by the thermal history. The kinetics of transcrystallization under isothermal conditions are presented as an example, and are used to form different transcrystalline interphase thickness. These affect the fibre fragmentation process that occurs while the specimens are cooled from the crystallization temperature to room temperature. This fragmentation process is attributed to residual thermal stresses, which can be calculated by assuming that it is controlled by Weibull statistics. Tensile loading of either longitudinal or transverse microcomposite specimens results in additional fragmentation, the extent of which is determined jointly by the thickness of the transcrystalline layer and by the yield strain of the matrix.