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.     

Fibrous composite matrix characterisation using nanoindentation: The effect of fibre constraint and the evolution from bulk to in-situ matrix properties

The variation of the in-situ matrix properties of a carbon-fibre composite has been investigated using nanoindentation. The aerospace carbon-fibre composite material (HTA/6376) and the bulk matrix (6376) have been co-cured to produce specimens ideal for matrix characterisation. The in-situ matrix has been characterised using fifty indentations in matrix pockets of many different sizes. The fibre constraint effect on in-situ matrix indentations has been characterised experimentally using the continuous stiffness measurement (CSM) technique, showing good correlation with finite element results from a previous study. The co-cured specimens allow the evolution of property change in the matrix material to be observed. The in-situ matrix modulus increases with decreasing matrix pocket size, and is up to 19% greater than the bulk matrix. This property change occurs outside the normal range of the interphase region for CFRP materials, and is statistically significant relative to the experimental scatter associated with the nanoindentation technique.     

Process investigation of a liquid PA-12/carbon fibre moulding system

A new RTM-type process has been developed to process complex geometry components utilising a thermoplastic matrix. The matrix is an anionically polymerised liquid PA-12 (APLC-12) system which may be injected into a fibre preform, with polymerisation occurring in-situ. The initially low melt viscosity can be utilised to good effect for the impregnation of all types of composite fabrics, yielding fibre volume fractions as high as 60%. Large, complex-shaped components can be manufactured using low injection pressures. A melt processing system was established and a test mould was constructed. Different geometries were used to investigate the processing characteristics and residual stress build-up effects of the unreinforced PA-12, composite plates and composite sandwiches. An industrial-scale dosing/mixing unit was also designed and developed, and used to produce carbon-fibre/PA-12 laminates for measurement of mechanical properties. Mechanical properties were tested and the results obtained were found to be comparable to those obtained from commingled carbon-fibre/PA-12 laminates.     

Effect of carbon-fibre powder on friction and wear properties of copper-matrix composites

ABSTRACT

The copper-matrix composite contains carbon-fibre powder which is prepared using a powder metallurgy sintering process in an effort to examine the effects of 50-mesh (270?μm) carbon-fibre powder on the microstructure and, friction and wear properties of copper matrix composites at high speed. When carbon-fibre powder is added within a certain range, the friction coefficient of the composite material is increased and the amount of wear is greatly reduced, When the content of carbon-fibre powder is 0.2wt-%, the hardness, density, friction and wear properties of the copper-based composite material has the best combination.     

Cracking orientation and induced anisotropy of a ceramic matrix composite under off-axis loading

The effect of matrix microcracking on the stiffnesses of a carbon-fibre/SiC-matrix woven composite is studied by means of an ultrasonic method. It provides the whole set of the stiffness tensor coefficients which are inaccessible by classical strain measurements and which are required to identify anisotropic damage. The induced anisotropy depends on the loading direction. If a tensile solicitation in a fibre direction leads to stiffnesses decreases without any rotation of principal axes, a tensile solicitation of 45° from a fibre direction creates microcracks with a predominant orientation that does not coincide with the elastic symmetry axes, and induce a fully anisotropic elastic degradation.     

The elastic constants of carbon-fibre composites

Data on the elastic constants of carbon-fibre composites are presented. It is shown that provided the elastic constants of both the fibre and the matrix are known it is possible to predict the elastic constants of the composite parallel to the fibre axis using either the reduced equations of Halpin and Tsai, based on a self-consistent model, or the exact calculations of Heaton. The properties of the composite perpendicular to the fibres cannot be explained on the basis of the theoretical models considered.     

Measurement of the static compressive strength of carbon-fibre/epoxy laminates

This paper describes an experimental study of the compressive failure of T800/924C carbon-fibre/efoxy composite laminates. Undirectional laminates loaded parallel to the fibres have compressive strengths that are 70% of the tensile strength and fail by fibre-microbuckling. During microbuckling the fibre debonds from the matrix, and the fibres break in bending. Multidirectional [(±45/0₂)₃]_sm laminates were also tested in compression, and the critical failure mechanism observed was microbuckling of the 0° plies. The failure strain was almost the same as for the undirectional laminate, The failure strain was almost the same as for the unidirectional laminate, which indicated that the ±45° plies have no significant influence on the failure strength of the 0° plies.     

The effect of fibre misalignment on the compressive strength of unidirectional carbon fibre/epoxy

The effect of a misalignment angle between the fibres and loading axis of a unidirectional composite is analysed by considering the shear strains induced by the misalignment. It is shown that shear instability in the matrix drastically reduces the predicted compressive strength even for very small misalignments. The same trend is predicted for composites with initial fibre curvatures due to the misalignment angle associated with the curvature. The reduction in compressive strength often attributed to initial fibre curvature may therefore actually be due to fibre misalignment angles. Small misalignments are hard to avoid during the manufacture and testing of unidirectional composites and so these results cast serious doubts on the possibility of measuring a true ultimate compressive strength for this kind of material.     

Influence of fibre architecture on the tensile,compressive and flexural behaviour of 3D woven composites

This paper presents a comprehensive study on the tensile, compressive, and flexural performance of six types of 3D woven carbon-fibre/epoxy composites which were manufactured using a traditional narrow fabric weaving loom and resin transfer moulding. Four orthogonal and two angle-interlock weaves were tested with the primary loading direction parallel to the warp direction. The mechanical performance was found to be affected by the distribution of resin rich regions and the waviness of the load-carrying fibres, which were determined by the fibre architectures. The binding points within the resin rich regions were found to be the damage initiation sites in all weave types under all loading conditions, which were confirmed with both visual observation and digital image correlation strain maps. Among all weave types, the angle interlock weave W-3 exhibited the highest properties under all loading conditions.     

Fracture mechanism of unidirectional carbon-fibre reinforced epoxy resin composite

The object of this study was to investigate the fracture mechanism of unidirectional carbon-fibre reinforced epoxy resin composite. For this purpose, the failure process of the composite under load was observedin situ by scanning electron microscopy and the matrix deformation around the broken fibre tip was examined by polarized transmission optical microscopy using a thin section of the composite. The failure process was shown to proceed through the following four stages: (1) fibre breakage began to occur at a load of about 60% of the failure load; (2) as the applied load was increased, plastic deformation occurred first from the broken fibre tip along the fibre sides, followed by final matrix cracking in the plastic region; (3) just before failure, partial delamination occurred, originating from fibre breakage and matrix cracking; (4) finally, a catastrophic crack propagation occurred from the delamination, leading to composite failure. Acoustic emission monitoring was also carried out for non-destructive evaluation, which indicated that internal failure began to occur at a load of 60% of the failure load and propagated remarkably before composite failure. A close correspondence between the acoustic signal and crack formation was obtained. The acoustic signal at lower amplitude, occurring over whole load range, corresponded to fibre breakage and matrix cracking while that at higher amplitude, occurring only just before failure, corresponded to partial delamination. From these experimental studies, the fracture mechanism of the composite has been clarified.     

A fibre direction compressive failure criterion for long fibre laminates at ply scale,including stacking sequence and laminate thickness effects

Long fibre laminate compressive failure is due to a microbuckling instability which leads to a kink band and a brittle failure of the fibres. This failure mechanism is well known, but more or less pertinently explained in the literature. Some references also showed that local microbuckling instability depends on parameters that belong to the scale of the elementary ply, like thickness and corresponding lay-up. The compressive strength of the unidirectional ply is therefore no more an intrinsic material property, but results from a structural effect of the design. In this paper, the so-called “structure effect” is included in a simple way as an analytical formula in the phenomenological compressive failure criterion which was initially presented by Budiansky and Fleck works. The criterion presented is expressed analytically for unidirectional composite and stands for the local compressive failure strength at ply scale in fibres direction.     

平纹机织玻璃纤维增强复合材料面内压缩力学行为及破坏机制

为了研究平纹机织玻璃纤维复合材料SW200/LWR-2 的面内压缩力学性能并建立其本构模型, 对其进行了应变率为0. 001 s-1 、0. 1 s-1 、500 s-1 , 温度从- 55 ℃到100 ℃范围内的面内压缩实验研究。动态压缩实验在SHPB 装置上进行, 通过波形整形器实现了恒定应变率加载, 且经过验证试样两端应力平衡。实验结果表明, SW200/ LWR-2 复合材料性能具有明显的应变率敏感性及温度敏感性, 其强度随着应变率的升高而增大, 随着温度的升高而减小。对破坏后试样进行宏观及微观观察发现, 准静态加载时试样为剪切破坏, 伴随大量纤维束内脱粘和纤维拔出; 动态加载时试样为剪切破坏与分层破坏并存, 并出现大量碎屑, 纤维束为整束剪断, 束内脱粘受到抑制。根据损伤力学理论, 建立了SW200/ LWR-2 复合材料应变率及温度相关面内压缩损伤统计本构模型, 本构模型结果与实验结果吻合较好。      

Experimental testing and phenomenological modelling of the fragmentation process of braided carbon/epoxy composite tubes under axial and oblique impact

A new simulation technique is presented for the phenomenological modelling of stable fragmentation in fibre reinforced composite structures under dynamic compressive loading. An explicit crash code is used for implementation of a hybrid modelling technique, in which two distinct material models act simultaneously. The first model is implemented in a multi-layered shell element and uses a unidirectional composites fracture criterion to predict potential ply fracture mechanisms on a macroscopic scale. This model is, however, unable to represent the complex localised fracture mechanisms that occur on a meso (sub-ply) scale under compression fragmentation loading. Therefore, a second constitutive model is added to capture the energy absorbing process within the fragmentation zone, utilising an Energy Absorbing Contact (EAC) formulation between the composite structure and the impacting body. The essential benefits of the procedure are that it requires minimal input data that can be obtained from simple fragmentation tests, and that the procedure is computationally efficient enabling application to large scale industrial structures. The EAC theory is discussed, together with the required material model parameters. A series of dynamic axial and oblique impact tests and simulations of cylindrical continuous carbon fibre reinforced composite tubes have been performed to validate the approach. Furthermore, the application to more complex load cases including combinations of fragmentation and global structural fracture have also shown a good correlation with test results.     

Mechanical behaviour of electrospun fibre-reinforced hydrogels

Mechanically robust and biomimicking scaffolds are needed for structural engineering of tissues such as the intervertebral disc, which are prone to failure and incapable of natural healing. Here, the formation of thick, randomly aligned polycaprolactone electrospun fibre structures infiltrated with alginate is reported. The composites are characterised using both indentation and tensile testing and demonstrate substantially different tensile and compressive moduli. The composites are mechanically robust and exhibit large strains-to-failure, exhibiting toughening mechanisms observed in other composite material systems. The method presented here provides a way to create large-scale biomimetic scaffolds that more closely mimic the composite structure of natural tissue, with tuneable tensile and compressive properties via the fibre and matrix phases, respectively.     

Rate-dependent fatigue of aramid-fibre/carbon-fibre hybrids

A previously observed hybrid effect in the flexural fatigue behaviour of aramid-fibre (A)/carbon-fibre (C) reinforced hybrid is re-examined as far as this effect can be attributed to the loading-rate dependence of the hybrids. This research comprises an investigation of the fatigue behaviour of composites distinguished by its combination of experimental conditions, including the materials, loading mode and rate of loading. Unidirectional carbon-fibre/aramidfibre reinforced hybrids were tested in flexure under a range of strain rates, in order to investigate the hybridization effect on the rate dependent fatigue behaviour. The ACA sandwich hybrid, whose fatigue performance is far better than that of the CAC hybrid, and which exhibits an improvement even with respect to the aramid parent composite, exhibits a clear strain-rate dependence. The different performances of the two hybrids are ascribed to the different rate dependences of the compressive and tensile strength of the parent C and A composites.     

The failure of composite tubes due to combined compression and torsion

The strength of unidirectional carbon-fibre epoxy laminates has been measured for combined compression and shear loading. Failure was by plastic microbuckling. The axial compressive strength decreased linearly to zero as the shear stress parallel to the fibres was increased from zero to the shear strength. These experimental results support the predictions of Budiansky and Fleck and suggest an average fibre misalignment angle of 2°–3°.     

Compression fracture of unidirectional carbon fibre-reinforced plastics

The effect of volume fraction and tensile strength of fibres, temperature and stress concentrators on the compression strength and fracture mode of unidirectional CFRP was studied. The cause of kinking is different for composites reinforced by low-(<3 GPa) and high-strength fibres. If fibre strength is high, the kink is initiated by composite splitting followed by fibre bend fracture in the tip of the split. In the case of low-strength fibres, kinking is initiated by compressive fracture of the fibres. The effect of stress concentrators on the CFRP compressive strength is described by linear fracture mechanics. In the presence of defects, fracture is a result of the emergence of splits near a hole. As the critical stress of splitting growth initiation reduces in proportion to the square root of the defect size, the Griffith criterion describes the composite compressive fracture. At elevated temperature, failure is caused by fibre buckling. The fracture band in this case is oriented perpendicular to the fibre direction. Carbon fibre compressive strength may be measured by the loop method. Bending a strand of carbon fibres glued to the elastic beam gives a fibre-controlled upper limit of the composite compressive strength.     

The in-plane shear modulus of fibre reinforced composites as defined by the concept of interphase

In this paper we describe a model to find the approximate equations for determining the in-plane shear modulus of a unidirectional fibre reinforced composite from the constituent material properties. Classical elasticity theory has been applied to the simplified model of a composite unit cell in which the concept of an interphase between fibre and matrix is taken into account. Thus the model considers that the composite material consists of three phases, that is the fibre, the matrix, and the interphase which is the part of the polymer matrix lying close to the fibre surface which possesses different physico-chemical properties from those of the main constituents. Thermal analysis was used for the determination of the thickness and volume fraction of the interphase. The theoretical results are compared with other theoretical expressions and with experimental data. The model introduced in this paper seems to be an improvement for the shear modulus.     

Time-dependent resistivity in carbon fibre sheets

The electrical properties of sheets of short carbon fibres in resin, glass-fibre and wood-pulp materials have been investigated. For carbon fibre in wood-pulp, a conductor-to-insulator transition was observed at 3 wt % (0.6 vol %) carbon fibre above which conductivity varied linearly with weight fraction. This result is interpreted in terms of a percolation threshold in a system of high aspect ratio. The data agree well with previous measurements on carbon-fibre in polymer composites, and satisfactorily with two-dimensional Monte Carlo calculations. At high concentrations of carbon fibre in all materials, the in-plane resistivity was found to be strongly time-dependent, the fractional change being proportional to Int. A theoretical model is presented which assumes a continuous increase in the number of interconnecting pathways as fibres physically move together under electrostatic attractive forces. Thermal activation over a continous spectrum of energy barriers leads to logarithmic time dependence as observed experimentally. Studies of the effect of external compression support the model for the time dependence. Shell (UK) Ltd Research Fellow in Materials Science.     

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.