Carbon Fibre PEEK Research Articles

The crystallinity of PEEK composites

Measurement of the degree of crystallinity of the polymer matrix in a composite is complicated by the presence of the reinforcing additive. This is particularly the case in poly(ether ether ketone) carbon fibre composite, APC-2, which may contain as much as 70% carbon fibre. A First Law procedure, developed for determining the degree of crystallinity of poly(ether ether ketone) (PEEK), has been outlined previously. It involved direct measurement of the enthalpy changes associated with melting, crystallization and heat capacity changes, and was found to be an effective method for determining the crystallinity of the PEEK matrix. The procedure has been applied to carbon fibre and glass fibre PEEK composites.

An evaluation of the interlaminar fracture toughness of a thermoplastic composite with offset centre flies

The offset DCB specimen has been used to characterize the influence of cooling rate and loading rate on the interlaminar fracture properties of carbon fibre reinforced PEEK. By offsetting the mid-plane fibres by several degrees, the amount of fibre bridging occurring during fracture has been reduced considerably. It has been shown that IM6 carbon fibre PEEK is quite sensitive to the cooling conditions employed after consolidation at 380 °C. Low rates of cooling yield a high level of crystallinity and a reduced fracture toughness. The modified DCB specimen has been successfully applied to highlight a distinct interlaminar fracture rate sensitivity. The high rate properties of this material still leave cause for concern and more work is required before these materials will find widespread use.

Study of the carbon fiber–Poly(Ether–Ether–Ketone) (PEEK) interfaces, 3: influence and properties of interphases

The aim of this third part is to analyze the structure and properties of the interfacial region between carbon fibers and PEEK as a function of different thermal conditioning treatments. First, it is shown by means of optical microscopy that the interfacial zone is not different from the bulk matrix when standard cooling conditions are used. On the contrary, a transcrystalline interphase is formed near the carbon fiber surface in systems that have been subjected to isothermal treatments. By comparison with previous results concerning the mechanical properties of the fiber–matrix interface, it appears that the interfacial shear strength decreases in the presence of a transcrystalline interphase or when the crystallization rate of PEEK increases. Moreover, it seems that the “constraint state” of the amorphous phase of PEEK near the fiber surface could also play a role in the interfacial shear strength. Secondly, a method is proposed in order to estimate the elastic modulus of crystalline interphases. It seems that this modulus is strongly dependent on the crystallization rate of the polymer. Finally, the determination of the stress-free temperature, defined as the temperature at which a longitudinal compressive stress just appears on the carbon fiber during the processing of the composites, is performed by recording the acoustic events corresponding to the fragmentation process in single-fiber composites. The results confirm that the crystallization rate and the “constraint state” of the amorphous phase of the matrix play an important role in the mechanical behavior of carbon fiber–PEEK interfaces.

Study of the carbon fiber–poly(ether–ether–ketone) (PEEK) interfaces, 1: surface characterization of fibers and matrices, and interfacial adhesion energy

AbstractThe aim of this general study is to determine the physicochemical characteristics and mechanical properties of carbon fiber–PEEK interfaces. In the first part, the dispersive component of the surface energy and the electron acceptor–donor (acid–base) characteristics of PEEK polymer and different types of untreated and surface‐treated carbon fibers are determined by means of inverse gas–solid chromatography at infinite dilution. It appears, in particular, that the acid–base surface properties of PEEK and, consequently, the orientation of macromolecules near the surface, depend on the processing of this polymer. Moreover, according to previous work, an estimation of the adhesion energy, corresponding to physical interactions (London and acid–base interactions) at carbon fiber–PEEK interfaces is proposed. Whatever the surface characteristics of PEEK, the highest level of carbon fiber–PEEK adhesion is achieved in systems involving oxidized or sized carbon fibers.

Study of the carbon fiber–poly(ether–ether–ketone) (PEEK) interfaces, 2: relationship between interfacial shear strength and adhesion energy

AbstractIn the second part of this general study, the carbon fiber–PEEK interfacial shear strength is measured by means of a fragmentation test on single‐fiber composites. Different thermal treatments (continuous cooling from the melt, isothermal treatments and long melting temperature time) are applied to these model composites prior to testing. The results are systematically compared with the previously determined reversible work of adhesion between carbon fiber and PEEK. It is shown that physical interactions at the interface determine, to a large extent, the magnitude of the interfacial shear strength between both materials. However, it appears that the magnitude of the stress transfer from the matrix to the fiber is affected either by the existence of an interfacial layer or by a preferential orientation of the polymer chains near the fiber surface. The results obtained on systems that have been subjected to isothermal treatments (isothermal crystallization of PEEK) seem to confirm the existence of a transcrystalline interphase, the properties of which are dependent upon the crystallization rate of the matrix and the interfacial adhesion energy.

Impact behaviour and damage tolerance of woven carbon fibre-reinforced thermoplastic composites

Instrumented drop weight impact and compression after impact test methods were used to assess the impact performance and damage tolerance of firm-stacked, woven carbon fibre-reinforced PEEK and woven carbon fibre-reinforced PPS composite systems. The tests were performed at three energy levels with a constant velocity, and at three velocity levels with a constant energy. Extreme energy of the levels were also determined. While the effect of the impact velocity was found to be significant within the range of the velocities used, the impact energy significantly affected the impact performance of the panels. Carbon fibre PPS panels evinced a high resistance to perforation by way of extensive delamination. On the other hand, carbon fibre PEEK panels showed an ability to confine the damage zone and hence, to markedly increase the damage tolerance of the panels. A set of material selection criteria for impact applications, based on the results of this study, has bee proposed

Properties and economics of reclaimed long fibre thermoplastic composites

Results are reported of investigations into the mechanical properties and forming behaviour of composite materials produced by the reconstruction of off-cuts from the pattern cutting of prepregs and other sources derived from apc-2 (long carbon fibre peek composite — Fiberite Europe) manufacturing processes. The approach adopted was to cut the prepreg into particles of controlled size and shape and to reconstitute them into sheets by compression moulding, with careful control of particle orientation. The tensile and flexural moduli, compressive, tensile and flexural strengths were measured and related to particle size and orientation. Properties are compared with those of apc-2 with various lay-up geometries. The forming properties of the reconstituted materials have been assessed using double curvature test mouldings. Forming properties are related to the various particle sizes employed. An economic assessment of this route to reclamation of long fibre thermoplastic composites is presented and possible applications of the material are suggested.

Impact behaviour and damage tolerance of woven carbon fibre-reinforced thermoplastic composites

Instrumented drop weight impact and compression after impact test methods were used to assess the impact performance and damage tolerance of film-stacked, woven carbon fibre-reinforced PEEK and woven carbon fibre-reinforced PPS composite systems. The tests were performed at three energy levels with a constant velocity, and at three velocity levels with a constant energy. Extreme energy levels were also determined. While the effect of the impact velocity was found to be insignificant within the range of the velocities used, the impact energy significantly affected the impact performance of the panels. Carbon fibre PPS panels evinced a high resistance to perforation byway of extensive delamination. On the other hand, carbon fibre PEEK panels showed an ability to confine the damage zone and hence, to markedly increase the damage tolerance of the panels. A set of material selection criteria for impact applications, based on the results of this study, has been proposed.

Healing of cracks in carbon fibre-PEEK composites

Etude de la recuperation de la resistance au delaminage en mode I de doubles poutres cantilever PEEK/CF soumises a un traitement thermique entre 320 et 380 o C. Essais preliminaires de reparation des composites par soudage ultrasonique