Interlaminar fracture morphology of carbon fibre/PEEK composites

The interlaminar fracture morphology of a carbon fibre/poly(ether-ether-ketone) composite (Aromatic Polymer Composite, APC-2) has been examined. The techniques used included scanning electron microscopy on fracture surfaces and on polished and etched sections. Two types of interlaminar fracture are observed: stable and unstable fracture. Both fracture surfaces exhibit microductility but it is more extensive for stable fracture. The fracture surfaces are not planar but have surface roughness. Fibre breakage and peeling are also observed and a quantitative examination enables the fracture energy contributions from the various processes to be calculated. The use of an etching technique reveals the spherulite texture and the presence of a deformation zone which extends into the bulk of the composite from the fracture surface. The extent of this zone is greater in the stable fracture region than in the unstable region and its presence indicates that the volume of composite which can be brought into the energy absorbing process extends well beyond the interlaminar region. The size of the zone has also been calculated using the fracture energy contributions and there is moderate agreement between calculated and observed zone size. Patterns of microductility on the fracture surface are seen to be due to spherulite texture, however the spherulite boundaries do not influence the fracture path.     

Mode I interlaminar fracture toughness of commingled carbon fibre/PEEK composites

Carbon fibre/poly (ether-ether-ketone) (PEEK) composites were fabricated from plain weave cloth using the commingled yarn of carbon fibres with PEEK filaments. The undirectional specimen was made from the warp of commingled yarn and the weft of PEEK yarn, while the two-dimensional specimen was made from commingled yarns both of the warp and the weft. During the hot-pressing process, PEEK filaments melt to form the matrix of the composite. The interlaminar fracture toughness of the commingled composite was measured and compared with that of the prepreg composite. The critical strain energy release rates,/'G _Ics, obtained for the commingled composites were higher than the prepreg composite. In particular, the two-dimensional composite exhibited higherG _Ic than the unidirectional commingled composite. Factors increasing the fracture toughness of commingled composites have also been investigated by SEM observation of the fractured surface.     

Interface morphology of carbon fibre/PEEK composites

The morphology of high-performance thermoplastic composites (APC-2) based on continuous carbon fibres embedded in a poly-ether-ether-ketone matrix is studied by means of scanning electron microscopy. Samples with different degrees of crystallinity obtained using different thermal treatments are investigated. The effect of the crystallinity content seems to be crucial for fibre/matrix adhesion, as can be detected by SEM analysis.     

Cooling rate influences in carbon fibre/PEEK composites. Part II: interlaminar fracture toughness

A study has been made of the effect of cooling rate on interlaminar fracture toughness of unidirectional carbon fibre–polyetheretherketone (PEEK) matrix composites. It is shown that the propagation values of both mode I and II propagation interlaminar fracture toughness increased with increasing cooling rate towards a saturation level for the range of cooling rate studied. The cooling rate dependency of the composite interlaminar fracture toughness is the result of complex interactions between two important properties, namely the matrix ductility and fibre–matrix interface bond strength. These two properties varied totally in an opposite manner against cooling rate through its effect on crystallinity: matrix ductility varied directly proportional to cooling rate while the converse is true for interface bond strength. The extent of plastic deformation of PEEK matrix contributed a predominant part to composite toughness, while an adequate interface bond is required to allow matrix deformation to take place to a full degree. A practical implication is that these two properties need to be optimised using an appropriate cooling rate to produce composites possessing high interlaminar fracture resistance.     

Compressive strength of unidirectional and crossply carbon fibre/PEEK composites

The commonly accepted production methods of composite systems generally result in departure of the plies properties from transverse isotropy due to stresses acting during fibre—matrix bond formation. This anisotropy coupled with the composite structure affects compressive loading; the ultimate stresses as well as the direction, in- or out-of-plane, of kink propagation. A unidirectional and a crossply carbon fibre/PEEK composites were compression tested at ambient and elevated temperature as well as exposed to various chemical environments. Significant disruptions in fibre—matrix interface in the crossply composite were indicated. The compression tests showed that failure occurred through in-plane and out-of-plane fibre bucking and kinking in the unidirectional and crossply composites, respectively. Failure of the longitudinal plies in the crossply laminate occurred at significantly higher compression stress than for the unidirectional composite. Compressive failure mechanisms in unidirectional and multi-directional laminates are considered.     

Mechanisms for rate effects on interlaminar fracture toughness of carbon/epoxy and carbon/PEEK composites

The objective of this study was to investigate strain-rate dependent energy absorption mechanisms during interlaminar fracture of thermosetting (epoxy) and thermoplastic (PEEK) uni directional carbon fibre (CF) composites. A simple model addressing the translation of matrix toughness to mode I and mode II interlaminar toughness of the composite is presented, in conjunction with a fractographic examination of the fracture surfaces and the fracture process. The observed rate dependency of composite fracture toughness is attributed to the rate dependent toughness of the viscoelastic matrix and the size of the process zone around the crack tip. Other important factors identified are the roughness of the fracture surface and fibre bridging.     

Spherulitic morphology of the matrix of thermoplastic PEEK/carbon fibre aromatic polymer composites

PEEK/carbon fibre composites (derived from APC-2) have been examined with a permanganic etching technique in order to reveal the crystalline, spherulitic morphology of the thermoplastic PEEK polymer matrix. The locations of the nucleation sites for spherulite growth have been categorized. Nucleation can occur both within the matrix and from the carbon fibres. Crystallization at lower temperatures favours matrix nucleation. Nucleation from fibres is dominated by sites associated with fibre-fibre contact. There is no evidence of “transcrystalline” growth. The study also identifies two types of crystal orientation effect in the polymer matrix. The first is a slight orientation that can occur in standard mouldings and is the result of the fibres constraining the shape of the volume into which spherulite growth can occur. The second effect produces abnormally high crystal orientation and is the result of improper processing at too low a melt temperature. Such conditions cause self-seeding during consolidation of laminates which, coupled with flow-induced orientation, can lead to directionally arranged spherulite precursors in different stages of morphological development.     

Surface oxidation of carbon fibre on tribological properties of PEEK composites

Abstract

In this work, ozone modification method and air oxidation were used for the surface treatment of polyacrylonitrile (PAN) based carbon fibre. The surface characteristics of carbon fibres were characterised by X-ray photoelectron spectroscopy. The interfacial properties of carbon fibre reinforced PEEK (CF/PEEK) composites were investigated by means of the single fibre pull-out tests. As a result, it was found that IFSS values of the composites with ozone treated carbon fibre are increased by 60% compared with that without treatment. X-ray photoelectron spectroscopy results show that ozone treatment increases the amount of carboxyl groups on carbon fibre surface, thus the interfacial adhesion between carbon fibre and PEEK matrix is effectively promoted. The effect of surface treatment of carbon fibres on the tribological properties of CF/PEEK composites was comparatively investigated. Experimental results revealed that surface treatment can effectively improve the interfacial adhesion between carbon fibre and PEEK matrix. Thus the wear resistance was significantly improved.     

Some mechanical property and fracture studies of carbon fibre/nickel composites

A carbon fibre reinforced nickel composite has been fabricated, and some of the mechanical properties investigated. The composite contains some misaligned and broken fibres, and a poor bond exists between the carbon fibre and the nickel. The mechanical properties are, to a very large extent, influenced by these factors. The oxidation resistance was found to be poor, and therefore a serious limitation for high temperature use.     

Cooling rate influences in carbon fibre/PEEK composites. Part 1. Crystallinity and interface adhesion

The effect of cooling rate on the fibre–matrix interface adhesion for a carbon fibre/semicrystalline polyetheretherketone (PEEK) composite was characterised based on the fibre fragmentation, fibre pullout and short beam shear tests. The interface adhesion was correlated to the degree of crystallinity and the crystalline morphology, as well as the bulk mechanical properties of neat PEEK resin, all of which were in turn controlled by cooling rate. It was shown that the interface bond strength decreased with increasing cooling rate; the tensile strength and elastic modulus of PEEK resin decreased, while the ductility increased with increasing cooling rate through its dominant effect on crystallinity and spherullite size. The improvement of crystalline perfection and flattened lamella chains with high crystallinity at the interphase region were mainly responsible for the strong interface bond in composites processed at a low cooling rate. The interphase failure was characterised by brittle debonding in slow-cooled composites, whereas the amorphous PEEK-rich interphase introduced in fast cooled specimens failed in a ductile manner with extensive plastic yielding.     

Cooling rate influences in carbon fibre/PEEK composites. Part III: impact damage performance

The effect of cooling rate on impact damage performance of carbon fibre/polyether ether ketone (PEEK) matrix composite is characterised based on the instrumented drop-weight impact test, scanning acoustic microscopy (SAM) damage evaluation and compression-after-impact (CAI) test. Judging from the incipient impact load, incipient impact energy and total damage area, the ability to resist damage initiation upon impact was higher in the order of fast-cooled carbon/PEEK, slow-cooled carbon/PEEK and carbon/epoxy laminates. Furthermore, the threshold impact energy was higher and the CAI strength reduction rate was lower for the fast-cooled specimen than the slow-cooled counterpart, strongly indicating higher impact damage tolerance of the former system. The present study demonstrates that the impact damage performance and other important properties of carbon/PEEK composites can be optimised, if not maximised, by proper control of processing conditions, especially the cooling rate.     

Strength and fracture toughness of carbon fibre polyester composites

Fracturing of carbon fibre/polyester composites has been studied by means of mechanical testing and scanning electron microscopy. Carbon fibres were surface-treated in several ways so as to vary the interlaminar shear strength of the composites, and the effect of this variation on the work of fracture was determined by means of Charpy V-notch impact tests and slow three-point bend tests on notched specimens of triangular cross-section. The effect of moisture on the fracture toughness was also studied by measuring toughness and interlaminar shear strength after exposure to steam. Improvement of the fibre/resin bond results, as expected, in an increase in the brittleness of composites and it appears that a purely mechanical bond, such as might be obtained by acid-etching the fibre surface, is less proof against deterioration in humid atmospheres than a chemical bond, such as can be obtained by the use of coupling agents. Estimates of the magnitude of various contributions to the fracture toughness show that in carbon-fibre-reinforced resins the effect of increasing the stiffness or load-bearing ability of the matrix and the work done against friction in pulling broken fibres out of the matrix contribute approximately one fifth and four fifths, respectively, of the total work of fracture.     

Stacking sequence effects on fracture of notched carbon fibre/epoxy composites

The purpose of this study was to investigate the ability of the so-called damage zone model (DZM) to predict the influence of stacking sequence on the strength of notched carbon fibre/epoxy composites. The DZM is in essence based on the unnotched tensile strength, σ₀, and the apparent fracture energy, G_c^*, and the damage zone is modelled as a crack with cohesive forces acting on the crack surfaces. The DZM predicts fracture loads for three-point bend (TPB) specimens and specimens with circular holes quite accurately. As an attempt to explain the difference in strengths, the damage zone extension in the TPB specimens with different stacking sequence was examined.