Non-linear compression stress-strain curve of pitch-based high modulus carbon fibre composites and structural responses

The longitudinal compression behaviour of unidirectional composites is studied to understand the role of the fibre compressive property in deformation and failure by systematically varying the tensile modulus of reinforcing high modulus carbon fibre. As the composites deform, their softness increases with increasing compressive strain, and the loading path is traced back when the load is removed. The intensity of softening is correlated to the fibre's tensile modulus and possible softening mechanisms are discussed in conjunction with fibre and matrix properties. Further, it is investigated how the non-linear stress-strain relation affects the stress and strain distributions and deformation when plates fabricated from these fibres are tested by the three-point bending test.     

On the mechanical properties,deformation and fracture of a natural fibre/recycled polymer composite

A composite laminate based on natural flax fibre and recycled high density polyethylene was manufactured by a hand lay-up and compression moulding technique. The mechanical properties of the composite were assessed under tensile and impact loading. Changes in the stress–strain characteristics, of yield stress, tensile strength, and tensile (Young's) modulus, of ductility and toughness, all as a function of fibre content were determined experimentally. A significant enhancement of toughness of the composite can be qualitatively explained in terms of the principal deformation and failure mechanisms identified by optical microscopy and scanning electron microscopy. These mechanisms were dominated by delamination cracking, by crack bridging processes, and by extensive plastic flow of polymer-rich layers and matrix deformation around fibres. Improvements in strength and stiffness combined with high toughness can be achieved by varying the fibre volume fraction and controlling the bonding between layers of the composite.     

Compression test of a single carbon fiber in a scanning electron microscope and its evaluation via finite element analysis

A longitudinal compression test for a single polyacrylonitrile-based carbon fiber (T300) was performed using a scanning electron microscope. The compressive stress/strain behavior was initially linear, but subsequently became nonlinear. The longitudinal tangent modulus decreased with increasing compressive strain. A cyclic compression test revealed that the T300 carbon fiber deformed elastically up to ~90% compressive strength. The variability in the compressive strength was evaluated using Weibull analysis. The representative compressive strength of the T300 carbon fiber was nearly the same as the tensile strength. The compressive strength of the T300 carbon fiber was almost same as that of the high-tensile strength T800S carbon fiber. Finite element analysis was performed to investigate the validity of the test method. The results showed that the longitudinal compressive stress on the carbon fiber varied during longitudinal compressive loading. The maximum longitudinal compressive stress in the carbon fiber was slightly higher than the average compressive strength applied at the end. However, the variability in the measured compressive strength was much higher than that in the longitudinal compressive stress on the carbon fiber, which does not affect the former.     

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 recoil compressive strength of pitch-based carbon fibres

The recoil compressive strengths of pitch-based carbon fibres with folded-radial and flat-layer textures were compared. Using the recoil test method, it was shown that the compressive strengths of pitch-based carbon fibres with folded-radial textures are superior to pitch-based carbon fibres with flat-layer textures at all modulus levels. Analysis of the failed fibre ends revealed that the folded-radial texture appeared to inhibit shearing of the basal planes. This may account for the superior compressive strengths of pitch-based carbon fibres with folded-radial textures. Procedural differences in the recoil test were shown to influence the calculated recoil compressive strength of pitch-based carbon fibres. Microscopic analysis of the recoil test specimens revealed that some energy is absorbed in the area where the fibre is secured to the support tab.     

Tensile recoil measurement of compressive strength for polymeric high performance fibres

Recoil forces acting on the broken ends of a fibre after tensile failure are known to cause substantial damage to polymeric high performance fibres. This damage is the result of compressive stresses developed during snap-back, or recoil, whose magnitude exceeds the compressive strength of the fibre. An analysis describing the axial stress history experienced by a fibre following a tensile failure has been performed and the results have led to the development of a simple, single filament, recoil technique for measuring fibre compressive strength. A number of polymeric high performance fibres were examined using the technique and compressive strengths measured are in excellent agreement with values obtained from composite tests.     

The influence of non-linear elasticity on the determination of Weibull parameters using the fibre bundle tensile test

We address the influence of individual fibre stress–strain non-linearity on the extraction of Weibull-parameters from fibre bundle tensile tests. We extend the statistical theory of fibre bundle strength to include the non-linear elastic behaviour observed in many technically important fibres, e.g. glass-, carbon-, and alumina-fibres. It is shown that neglecting this non-linearity may lead to significant errors in determining the shape and scale parameters of the fibre fracture strength Weibull-distribution. A refinement of the existing extraction technique, accounting for this effect, is presented. The error resulting from neglecting the non-linear behaviour is assessed through a parametric study of the Weibull parameters for different levels of non-linearity. Explicit calculations are performed for two fibres of technical importance, namely Nextel 610™ α-alumina fibre and a T300 carbon fibre.     

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.     

The use of a torsion machine to measure the shear strength and modulus of unidirectional carbon fibre reinforced plastic composites

A torsion apparatus, in which a solid rod specimen is subjected to a shear stress field only, has been used to measure the shear modulus and strength of unidirectional carbon fibre reinforced plastics. Because of the absence of tensile and compressive forces, a more accurate value of the shear strength is obtained than from a test such as the short beam shear test. The shear strength is calculated allowing for the non-linear nature of the shear stress-strain characteristic. For type 2 treated fibre the shear modulus is found to increase rapidly with fibre volume loading, in reasonable agreement with the micromechanical theory of Heaton. For type 2 untreated and type 1 treated fibre composites, a slightly less rapid increase in shear modulus is noted. Results for type 1 untreated fibre composites increase with volume loading but are below both the other results and the theoretical curve. The shear strength of composite materials made from type 2 treated fibre is greater than that of the pure resin, and has a maximum for about 50% volume of fibre. For type 1 and untreated carbon fibres the shear strength decreases with increasing volume loading. By using the concepts of fracture mechanics and assuming that the bond between type 2 treated fibre and resin is completely effective, so that failure starts in the matrix, it is possible to give a plausible explanation of the shear strength results. The shear modulus, but not the shear strength, can be measured accurately, using either square or circular cross-section specimens.     

Transverse loading of monofilament reinforced microcomposites: a novel fragmentation technique for measuring the fibre compressive strength

Carbon/J-polymer single fibre composite samples were tested under tensile conditions with the fibre direction perpendicular to the tensile loading axis. The Poisson ratio effect induced a compression strain field in the fibre, resulting in a fragmentation phenomenon similar to that observed in a fibre subjected to tensile loading. This observation introduces a novel technique for the measurement of the compressive strength of single fibres, calculated either from the stress at first break, or from the Weibull scale parameter obtained from the fragmentation data produced at various stress levels. The special sample loading configuration used here also provides the first measurement of the effect of the length of the fibre on its compressive strength value.     

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.     

Development of damage in a 2D woven C/SiC composite under mechanical loading: I. Mechanical characterization

An investigation has been undertaken to determine the damage mechanisms and the associated mechanical response of a 2D reinforced composite of carbon fibers in an SiC CVI-processed matrix subjected to uniaxial tensile and compressive loadings at room temperature. Under tension loading, an extended non-linear stress/strain response was evidenced and related to a multi-stage development of damage involving transverse matrix microcracking, bundle/matrix and inter-bundle debonding as well as thermal residual stress release. This tensile behavior proved to be damageable-elastic with respect to a fictitious thermalstress-free origin of the stress/strain axis lying in the compression domain. In compression, after an initial stage involving closure of the thermal microcracks present from processing, the composite displayed a linear-elastic behavior until failure. The extent of damage over the material was characterized quantitatively at the microscale by the decrease of the average transverse microcrack spacing and at the macroscale by the decrease of both the longitudinal Young's modulus and the in-plane Poisson's ratio.     

Nextel™ 610 alumina fibre reinforced aluminium: influence of matrix and process on flow stress

Continuous alumina fibre reinforced aluminium matrix composites are produced using two different liquid metal infiltration methods, namely direct squeeze casting and gas pressure infiltration. Net-shape fibre performs for longitudinal parallel tensile bars are prepared by winding the Nextel™ 610 alumina fibre (3M, St Paul, MN) into graphite moulds. High purity aluminium, two binary (Al–6% Zn and Al–1% Mg) and one ternary (Al–6% Zn–0.5% Mg) aluminium alloys are used as matrix materials. The composite is tested in uniaxial tension–compression, using unload–reload loops to monitor the evolution of Young's modulus. A linear dependence between Young's modulus and strain is observed; this is attributed, by deduction, to intrinsic elastic non-linearity of the alumina fibre. This conclusion is then used to compare on the basis of the in situ matrix flow curve the influence of matrix composition and infiltration process on the composite stress–strain behaviour.     

Modeling and testing strain rate-dependent compressive strength of carbon/epoxy composites

A technique for testing high modulus fiber-reinforced composites in compression at different strain rates is investigated. The rate-dependent compressive behavior of unidirectional AS4/3501-6 carbon/epoxy composite is characterized by using off-axis specimens. It is found that, in the compression test, a titanium coating applied at the contact ends of the off-axis specimen can greatly reduce contact frictions, allowing a fully developed extension–shear coupling so that a state of uniform stress in the specimen can be achieved. A rate-dependent nonlinear constitutive model and a dynamic compressive strength model (fiber microbuckling model) for the unidirectional AS4/3501-6 composite are established based on the low strain rate off-axis test data. Model predictions and experimental data including high strain rate data are in very good agreement indicating that the constitutive model and compressive strength model obtained with low strain rate data are valid for high strain rates as well. A technique is also developed to extract the longitudinal compressive strength of the composite from those of the off-axis specimens.     

High-performance aromatic polyimide fibres

A new polyimide has been synthesized from 3,3,4,4-biphenyltetracarboxylic dianhydride (BPDA) and 2,2-dimethyl-4,4-diaminobiphenyl (DMB). A high-strength, high-modulus, high-temperature fibre has been developed from this polyimide via a dry-jet wet spinning method. The tensile strength of BPDA-DMB fibres is 3.3 GPa and the tensile modulus is around 130 GPa. The compressive strength of the fibres has been investigated through a tensile recoil test (TRT), while the fibre morphology after compression has been studied via polarized light microscopy (PLM) and scanning electron microscopy (SEM). From the TRT measurements, we have observed that the compressive strength of this fibre is 665 (±5) MPa, which is higher than those of other aromatic polymer fibres. The effect of fibre diameter on the compressive strength of BPDA-DMB fibres is not substantial. The critical compressive strain for this fibre at which the kink bands start appearing under the observation of PLM is at 0.51–0.54%. Subglass relaxation processes have been observed and the measure of an apparent relaxation strength may serve as one of the factors which significantly affect the compressive strength of the fibres. Tensile tests of pre-compressed fibres reveal a continuous loss in tensile strength (up to 30%) with increasing the compressive strain (up to 2.6%). PLM and SEM observations show that during the compression BPDA-DMB fibres form regularly-spaced kink bands at ±60 ° (±2 °) with respect to the fibre axis. The kink band density initially increases with the compressive strain, and reaches a maximum at around 1.1%. Further increase of the compressive strain decreases this density due to the merge of the neighbouring bands. The size of kink bands also correspondingly increases within this compressive strain region. The morphological observation via SEM implies the existence of a skin-core structure and microfibrillar texture which are common features in polymer fibres.     

Carbon fibre composites with improved fatigue resistance due to the addition of tin-lead alloy particles

The addition of tin-lead (60 wt% Sn) alloy particles (about 21–25 μm in diameter, in an amount up to 37 wt% or 7.2 vol%) between continuous unidirectional carbon fibre layers in an epoxy-matrix composite was found to improve the fatigue life by over 100 times. The alloy addition had little effect on the tensile strength, tensile modulus, compressive strength, compressive modulus (with the compressive force parallel and perpendicular to the fibres), but increased the electrical resistivity. The composites were fabricated by compression moulding at 185–200°C and 1 MPa for 30 min. The heating allowed the alloy to melt while the epoxy cured. The fatigue life enhancement is probably due to the hindering of the fatigue crack growth by the alloy particles.     

三维机织碳/碳复合材料双轴压缩载荷下的力学行为

基于三维机织碳/碳复合材料的细观结构特征, 设计平板十字形试样, 在材料双轴力学性能试验机上开展了复合材料单轴、 双轴加载压缩试验, 对比分析了三维机织碳/碳复合材料在双轴压缩载荷下的力学行为。研究表明: 三维机织碳/碳复合材料的压缩行为表现为非线性、 脆性断裂; 双轴载荷作用下非线性特征更为显著, 压缩模量随应力的增加而增大, 强度与模量相较于单轴有较大幅度增加, 双轴压缩载荷作用下材料的强化效应显著; 试样破坏位置并未出现在试样中心区, 而是发生在试样的加载端部或十字形试样的加载分枝根部, 主要表现为基体开裂、 纤维断裂和层间脱粘, 碳布及其层间界面剪切强度的强弱直接影响材料的压缩强度。     

Predicting the compressive engineering performance of carbon fibre-reinforced plastics

This paper examines the compressive strength data of a recent experimental study [Smith FC. The effect of constituents’ properties on the mechanical performance of fibre-reinforced plastics. PhD thesis. Centre for Composite Materials, Imperial College, April 1998] concerned with the evaluation of a range of engineering properties of continuous carbon fibre/epoxy composites subjected to static tensile and compressive loading. A plastic fibre kinking analysis [Budiansky B. Micromechanics. Comput Struct 1983;16(1):3–12] and a linear softening cohesive zone model (CZM) [Soutis C. Compressive failure of notched carbon fibre–epoxy panels. PhD thesis. Cambridge University Engineering Department, UK, 1989; Soutis C, Fleck NA, Smith PA. Failure prediction technique for compression loaded carbon fibre–epoxy laminates with an open hole. J Comp Mat 1991;25(5):1476–1498] are used for the prediction of the unnotched and open hole compressive strength (OHC) of unidirectional and multidirectional laminates made of six different commercially available CFRP prepregs. Damage introduced by drop-weight (low-velocity) impact is modelled as an equivalent open hole and the cohesive zone model [Soutis C. Compressive failure of notched carbon fibre–epoxy panels. PhD thesis. Cambridge University Engineering Department, UK, 1989; Soutis C, Fleck NA, Smith PA. Failure prediction technique for compression loaded carbon fibre–epoxy laminates with an open hole. J Comp Mat 1991;25(5):1476–1498] is applied to estimate compression-after-impact (CAI) strength. The unnotched strength is accurately predicted from the knowledge of initial fibre misalignment and the shear yield stress of the composite, while the difference between the theoretical and experimental OHC and CAI strength results in most cases is less than 10%.     

The effect of nanostructure upon the compressive strength of carbon fibres

The relationship between the structure and the compressive strength of carbon fibres has been studied in detail. In order to determine the compressive strength, a combination of single-fibre composite tests and Raman spectroscopy was employed. It was found that the compressive stress–strain curves showed nonlinear behaviour, with modulus softening in compression. The compressive strengths for the fibres with a modulus ≥400 GPa were measured as ≤2 GPa and those with a modulus <400 GPa were >2 GPa. We have introduced a model to explain this behaviour that assumes that the fibres behave as composites consisting of both crystallites and amorphous carbon. It is suggested that the compressive strength is controlled by the critical stress for kinking the crystallites in the fibres. Hence, the compressive strength of carbon fibres is found to depend upon the shear modulus of the fibres and the orientation of the crystallites within them.     

Hybrid specimens eliminating stress concentrations in tensile and compressive testing of unidirectional composites

Two novel approaches are proposed for elimination of stress concentrations in tensile and compressive testing of unidirectional carbon/epoxy composites. An interlayer hybrid specimen type is proposed for tensile testing. The presented finite element study indicated that the outer continuous glass/epoxy plies suppress the stress concentrations at the grips and protect the central carbon/epoxy plies from premature failure, eliminating the need for end-tabs. The test results confirmed the benefits of the hybrid specimens by generating consistent gauge-section failures in tension. The developed hybrid four point bending specimen type and strain evaluation method were verified and applied successfully to determine the compressive failure strain of three different grade carbon/epoxy composite prepregs. Stable failure and fragmentation of the high and ultra-high modulus unidirectional carbon/epoxy plies were reported. The high strength carbon/epoxy plies exhibited catastrophic failure at a significantly higher compressive strain than normally observed.