The mechanical performance and impact behaviour of carbon-fibre reinforced PEEK

The mechanical performance and impact behaviour of carbon-fibre reinforced polyether-ether ketone (PEEK) with a (0, ±45) lay-up has been compared with that of a similar carbon fibre/epoxy laminate. Differences occurred because of the greater shear strength and lower shear modulus of the carbon-fibre reinforced PEEK. When compared with the carbon fibre/epoxy laminate, carbon-fibre reinforced PEEK was more notch sensitive in tension and had a lower undamaged compressive strength. However, after impact, the residual compressive strength was significantly greater for carbon-fibre reinforced PEEK because delamination was less extensive.     

Compressional behaviour of carbon fibres

The axial compressive strain to failure of various types of PAN-based carbon fibres was measured by applying small and defined compressive loads to single filaments which have been bonded to a rectangular polymer cantilever beam and parallel to its long edge. By monitoring the Raman frequencies along the fibre with the 2 m laser probe of a Raman microscope, the critical compressive strain required for first fibre failure could be assessed and the residual load that each type of fibre supported after first failure, could be measured. Estimates of the compressive moduli for all fibres could, also, be obtained by considering the dependence of the Raman frequency upon compressive strain in the elastic region. The critical compressive strain to failure was found to decrease with fibre modulus and its value was, approximately, equal to 50% of the tensile fracture strain. However, for some low-modulus fibres the compressive strain to failure was found to approach the tensile fracture strain. The initial compressive moduli of high-modulus fibres were estimated to decrease up to a maximum of 10% with respect to their tensile moduli, whereas more significant reductions were found in the case of intermediate and low-modulus fibres.     

On the impact performance of carbon fibre laminates with epoxy and PEEK matrices

A study was made of the mechanical properties and impact performance of carbon fibre/PEEK (0,90), ($\text{±45}$) and ($\text{±45,0}$) laminates and comparisons were made with similar carbon fibre/epoxy laminatesFibre dominated properties such as plain tensile strength were similar to those of epoxy laminates with similar fibres and volume fractions. Because of the increased toughness of PEEK there was less extensive matrix cracking, even though there was fibre debonding, and this gave increased transverse and shear cracking strains, increased shear strengths but decreased notched tensile strengths. The lower modulus and yield stress of PEEK caused lower compressive strengths, but PEEK absorbed little moisture and at 120°C moisture had little effect on mechanical properties.Dropweight impact produced less extensive damage in carbon fibre/PEEK laminates. Residual tensile strengths were similar but, because of the less extensive damage and greater delamination fracture energy, the residual compressive strengths were significantly greater with a PEEK matrix.Microscopic examination showed less matrix cracking and more fibre buckling in the carbon fibre/PEEK and this is discussed in relation to mechanical properties.     

Compressional behaviour of carbon fibres

Spectroscopic-mechanical studies have been conducted on a range of carbon fibres by bonding single filaments on the top surface of a cantilever beam. Such a loading configuration allows the acquisition of the Raman spectrum of carbon fibres and the derivation of the Raman frequency strain dependence in tension and compression. Strain hardening phenomena in tension and strain softening phenomena in compression were closely observed. The differences in the slopes of the Raman frequency versus applied strain curves in tension and compression respectively, have been used to obtain good estimates of the compression moduli. A method of converting the fibre Raman frequency versus strain data into stress-strain curves in both tension and compression, is demonstrated. Values of fibre stress and fibre modulus at failure in compression compare exceptionally well with corresponding estimates deduced from full composite data. The mode of failure in compression has been found to depend upon the carbon fibre structure. It is demonstrated that certain modifications in the manufacturing technology of PAN-based fibres can lead to fibres which show resistance to catastrophic compressive failure without significant losses in the fibre compressive modulus.     

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.     

Interfacial studies on carbon/thermoplastic model composites using laser Raman spectroscopy

Interfacial studies were carried out on a model composite system consisting of a short carbon fibre embedded in a polycarbonate matrix. While the composite was being strained, the local strain along the fibre was monitored using a Raman spectroscopic technique. The residual compressive strain in the fibre due to fabrication was found to be –0.45%. Subsequent loading of the composite up to 0.55% in tension resulted in a complex stress field consisting of tension at the fibre ends and compression in the middle of the fibre. The fibre strain at different levels of applied load was converted to interfacial shear stress (ISS) distribution along the fibre by employing a simple equilibrium analysis. The shape of the ISS profiles indicated a predominantly frictional type of load transfer from the matrix to the fibre. Finally, the maximum ISS value of 15 MPa was found to be unaffected by the amount of strain experienced by the composite.     

Quantitative microstructural analysis of a continuous fibre composite

A micrograph of a carbon fibre-reinforced PEEK prepreg has been analysed to determine the fibre distribution and the results compared with the probability density of the theoretical distribution obtained using a spatial statistical model. The close agreement indicates that the model may be used with confidence for the predictive modelling of high performance continuous fibre composites of around 50% volume fraction.     

The fatigue damage mechanics of notched carbon fibre/PEEK laminates

A model is presented for the strength, post-fatigue residual strength and damage propagation in notched, cross-ply carbon fibre/polyetheretherketone (PEEK) laminates. Fracture mechanics principles are used to predict quasi-static damage growth, and the application of a Paris law permits extension to fatigue damage. Strength is predicted by applying a failure criterion based on the tensile stress distribution in the 0° plies, as modified by damage (either quasi-static or fatigue). The volume dependence of strength is included by using a simple Weibull distribution. The parameters of the model are determined from independent experiments. Good agreement with experimental results is obtained. Comparisons are made with previous results from carbon fibre/epoxy laminates. The behaviour of the carbon fibre/PEEK is similar, although the extent of delamination and matrix cracking is reduced owing to the higher inherent toughness of the matrix.     

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.     

Recycled carbon fibre for high performance energy absorption

This paper compares the mechanical properties of virgin and recycled woven carbon fibre prepreg and goes on to assess the potential for recycled carbon fibre reinforced plastic (rCFRP) to be used in high performance energy absorption structures. Three sets of material were examined: fresh containing virgin fibres and resin, aged which was an out of life but otherwise identical roll and recycled which contained recycled fibre and new resin. The compressive strength and modulus of rCFRP were approximately 94% of the values for fresh material. This correlated directly with the results from impact testing where rCFRP conical impact structures were found to have a specific energy absorption of 32.7 kJ/kg versus 34.8 kJ/kg for fresh material. The tensile and flexural strength of rCFRP were 65% of the value for fresh material. Tensile and flexural moduli of rCFRP were within 90% of fresh material and ILSS of rCFRP was 75% that of fresh.     

Compressive properties of single-filament carbon fibres

Compressive properties of mesophase pitch-based carbon fibres (NT-20, NT-40 and NT-60) were measured using the tensile recoil test and the elastica loop test. The NT-40 fibre with a 400 GPa tensile modulus showed a smaller loop compressive yield strain and a larger recoil compressive strength compared to these values obtained from the longitudinal compression test on its unidirectional composites. Further, the recoil compressive strength of this fibre was higher than that of PAN-based carbon fibre with a corresponding modulus. Under the ideal conditions in the tensile recoil test, the strain energy was conserved before and after recoil, and the initial tensile stress and the recoil compressive stress do not coincide when fibre stress-strain behaviour is non-linear, and the non-linearity in compression and in tension is different. The difference between the composite compressive strength and the recoil compressive strength of NT-40 was quantitatively explained by taking account of the fibre compressive stress-strain non-linear relation. The difference between the loop compressive yield strain and the composite compressive strain to failure was also explained by this non-linearity.     

Compression failure of carbon fibre-reinforced coupons containing central delaminations

As part of a study on the tolerance of carbon fibre-reinforced composites to impact, the effect of delaminations between the plies of laminates was investigated. Experiments were carried out on carbon fibre/PEEK and carbon fibre/epoxy coupons with artificially-introduced central delaminations to determine the effect on compressive strength. Delaminations in carbon fibre/epoxy grew prior to failure, those in carbon fibre/PEEK did not. A finite element method was developed to predict the strength reduction and delamination growth. It was found that the predictions matched experimental results, provided large displacement effects were included.     

Fatigue behaviour of carbon fibre-reinforced aluminium laminates

A new hybrid composite (CARALL), consisting of thin layers of carbon fibre/ epoxy prepreg sandwiched between aluminium sheets, has been developed. It is shown that this class of materials offers higher modulus, higher tensile strength and lower density than 2024-T3 alloy in the longitudinal direction. Under tension-tension fatigue loading, the hybrid laminates showed superior fatigue crack propagation resistance in the longitudinal direction, which may be attributed to the bridging effect imposed by the intact fibres in the crack wake. It has also been shown that the effectiveness of fatigue crack growth reduction increases with the thickness of the carbon fibre/epoxy layer. The resistance to fatigue crack propagation can be further improved by introducing compressive residual stresses in the aluminium layer by postcure stretching the laminate in the plastic region of the aluminium alloy.     

Effect of hot water immersion on the performance of carbon reinforced unidirectional poly(ether ether ketone) (PEEK) composites: Stress rupture under end-loaded bending

This paper describes the behaviour of AS4 and T700SC reinforced PEEK composites (SUPreM™ and ACP-2) under applied compressive bending strain. The effect of an increased molecular weight of the polymer matrix on the residual time under endloaded compression bending conditions is studied. Generally for a given composite material, the higher the testing temperature and the applied strain the faster the failure occurs. At test temperatures exceeding the glass transition temperature or at high strain ratios the time-to-failure for CF/PEEK composites follows a master curve. The residual times under endloaded compression bending conditions increase with increasing toughness of the PEEK matrix but decrease with increasing tensile strength of the reinforcing fibres. It seems that the better the fibre/matrix adhesion the lower is the time to failure of an endloaded composite, because more load is transferred from the matrix into the fibres.In order to simulate composite applications under ‘harsh’ conditions the CF/PEEK composites have been exposed to boiling water. PEEK is known to be highly resistant to environmental effects, but water uptake significantly influences the overall performance of CF/PEEK composites under endloaded compression bending conditions. The tensile properties of the composites have been measured as function of exposure time in boiling water. The fibre dominated uniaxial tensile strength is not/or only slightly affected by the boiling water conditioning even after extended exposure times but the transverse tensile strength decreases significantly after exposure to boiling water. The performance of SUPreM™ CF/PEEK-150 and 450 composites under endloaded compression bending conditions are positively affected by water conditioning whereas APC-2 fails at shorter residual times. The fracture behaviour under endloaded conditions is also affected by the ingress of water into the composite.The obtained results show clearly that applications of thermoplastic composites leading to large out of plane deformations can only be ‘safe’ if the maximum service temperatures of the finished part will be well below the glass transition temperature of the polymer matrix otherwise even at low bending radii a dramatic failure of the material cannot be excluded.     

碳纤维/环氧树脂预浸带铺放应力场及制品微观结构的模拟与实验

碳纤维/环氧树脂预浸带复合材料在铺放成型时,由于树脂基体与碳纤维之间的热膨胀系数存在差异以及成型时热-力参数作用下由于纤维的变形而导致纤维与基体接触处产生应力集中等原因,在制品材料中会产生热残余应力。针对碳纤维/环氧树脂预浸带复合材料的实际结构特点,利用ABAQUS有限元软件建立含有界面的碳纤维/环氧树脂预浸带复合材料的细观代表性体积单元(Representative volume element, RVE)有限元模型,采用实验研究和有限元仿真分析的方法,研究在温度-压力参数作用下预浸带铺放制品残余应力的分布规律及影响机理。首先,建立预浸带铺放时的温度和压力模型,研究不同温度和压力参数条件下碳纤维/环氧树脂预浸带铺放制品残余应力的分布情况。其次,采用耦合降温法模拟碳纤维/环氧树脂预浸带残余应力随纤维体积含量、铺放压力以及铺放温度的变化规律,并采用扫描电镜对不同工艺参数条件下预浸带铺放制品的微观结构进行分析。通过对模拟结果进行分析比较得到各因素对制品残余应力的基本影响规律;最后进行不同温度和压力等铺放参数对预浸带铺放成型时残余应力影响的实验测试研究。      

碳纤维/聚醚醚酮(PEEK)复合材料拉拔制管工艺设计和模拟

从太空制造角度出发,设计了一种碳纤维/聚醚醚酮(PEEK)预浸复合板料拉拔连续制管工艺.综合考虑了预浸复合板料的供料放卷过程、冲压、拉拔成形工步和超声焊接工序,并创造性的提出了卷曲拉挤成型方式.利用Johnson-Cook和Holzapfel-Gasser-Ogden模型构建了一种以PEEK为基体,以碳纤维编织布为增强体相互叠加的材料模型,通过实验数据确定材料参数.采用商业软件ABAQUS对各工序分步进行了地面条件下的有限元数值模拟,分析了预浸复合材料板材在供料放卷、冲压拉拔成形过程中的应力分布,并采用蔡-希尔最大变形能理论证明了本文设计的放卷模具和卷曲成形模具可以进行连续制管.在焊接过程中,分析了预浸料基体PEEK在焊接区域产生的Mises应力分布,证明了超声波焊接方案对管材表面质量的影响较小.模拟结果表明,所设计的连续拉拔制管工艺能够快速有效地生产出表面良好的管材.仿真结果可为后续复合材料在轨拉拔连续制管的工艺设计和制造提供借鉴.     

Comparing single-walled carbon nanotubes and samarium oxide as strain sensors for model glass-fibre/epoxy composites

The study of the interfacial stress transfer for glass fibres in polymer composites through the fragmentation test requires certain assumptions, such as a constant interfacial shear stress. In order to map the local interfacial properties of a composite, both Raman spectroscopy and luminescence spectroscopy have been independently used. Unlike other polymer fibre composites, the local strain state of a glass fibre cannot be obtained using Raman spectroscopy, since only very broad and weak peaks are obtainable. This study shows that when single-walled carbon nanotubes (SWNTs) are added to the silane sizing as a strain sensor, it becomes possible to map the local fibre strain in glass fibres using Raman spectroscopy. Moreover, if this model glass fibre contains a small amount of Sm₂O₃, as one of the components, luminescence spectroscopy can be simultaneously used to confirm this local fibre strain. A combined micromechanical properties study of stress transfer at the fibre–matrix interface using luminescence spectroscopy, together with Raman spectroscopy, is therefore reported. The local strain behaviour of both Sm³⁺ doped glass and SWNTs in the silane coating are shown to be consistent with a shear-lag model. This indicates that Sm³⁺ dopants and SWNTs are excellent sensors for the local deformation of glass fibre composites.     

Compression testing of pultruded carbon fibre-epoxy cylindrical rods

The fibre waviness inherent in conventional prepreg laminatessignificantly reduces their compressive strength. This waviness canbe reduced through the use of unidirectional fibre rods. In thiswork, the development of a new test procedure and specimen design isreported that was used to determine the compressive properties ofpultruted T300/828 and IM7/828 carbon fibre-epoxy unidirectionalrods at room temperature. The IM7/828 system demonstrates a highercompressive strength than the T300/828 composite due to strongerfibres used and fewer manufacturing defects. Since the fibres as intension primarily carry the compressive load, the final fracture ofthe rods occurs when the fibres fail. Post-failure examinationreveals that failure of the fibres is microbuckling-induced. This isa bending failure as a consequence of buckling. Other events such asfibre-matrix debonding (splitting) and matrix yielding do not bythemselves cause the final failure, but they facilitate fibrebuckling by reducing the lateral support for the fibres.Microbuckling failure models are used to predict the compressivestrength of the carbon fibre rods; agreement between theory andexperiment is acceptable.     

Studies on interface structure and crystal texture of poly(ether-ether-ketone)-carbon fibre composite

The interface structure of poly(ether-ether-ketone) (PEEK)-carbon fibre composite and the crystal texture of PEEK matrix have been studied by scanning electron microscopy (SEM) after the samples were properly etched by argon plasma. We find that most of the PEEK'S crystals are induced by nucleating carbon fibre and then they developed the transcrystalline or spherulite shape, depending on the crystallization conditions. Because the volume fraction of the carbon fibre is 60%, and the space distance between two adjacent carbon fibres is about 10 m–20 m, the crystal size of PEEK matrix is mainly controlled by the space distance between adjacent carbon fibres, and the crystallization temperature has little effect on it. The detail of the oriented crystal growth of the PEEK relative to carbon fibre was first observed by SEM. Our results show that the nucleation process of PEEK-carbon fibre composite is as follows: The first lamella is formed orientationally in the carbon fibre surface in the way that thea crystallographic direction is parallel to the radial direction of carbon fibre, theb direction parallel to the tangent line of cross section of carbon fibre, and thec direction parallel to the axis of the carbon fibre. Therefore, the crystals finally formed are oriented.     

Strength improvement by fibre steering around a pin loaded hole

A fibre steering technique has been applied around boltholes in carbon fibre reinforced epoxy composite laminates to locally enhance the bearing strength of bolted joints. The procedure can precisely place dry tows of fibre on a prepreg fabric following both the tensile and compressive principal stress trajectories around the hole. The bearing test results indicate that fibre steering improved the peak load of the composite bolted joints approximately in linear proportion to fibre addition by weight. The best result achieved an increase for the peak load by a factor of 2.69. The best improvement of bearing strength was by a factor of 1.36 for a specimen reinforced by 3 k fibre tows in tensile principal stress patterns and 6 k fibre tows in compressive principal stress patterns. The bearing strength improved due to significant increase in peak load and moderate change in thickness.