Effect of off-axis ply orientation on 0°-fibre microbuckling

The aim of the present work is to study both experimentally and theoretically the compression failure mechanisms in multi-directional composite laminates, and especially the effect of the off-axis ply orientation on fibre microbuckling in the 0°-plies. The critical mechanism in the compressive fracture of unidirectional polymer matrix composites is plastic microbuckling/kinking. In multi-directional composites with internal 0°-plies, catastrophic failure also initiates by kinking of 0°-plies at the free-edges or manufacturing defects, followed by delamination. When 0°-plies are located at the outside, or in the case of cross-ply laminates, failure rather tends to occur by out-of-plane buckling of the 0°-plies. T800/924C carbon-fibre–epoxy laminates with a [(±θ/0₂)₂]_s lay-up are used here to study the effect of the supporting ply angle θ on the stress initiation of 0°-fibre microbuckling. Experimental data on the compressive strength of laminates with θ equal to 30, 45, 60 or 75° are compared to theoretical predictions obtained from a fibre kinking model that incorporates interlaminar shear stresses developed at the free edges at (0/θ) interfaces. Initial misalignment of the fibres and non-linear shear behaviour of the matrix are also included in the analysis.     

Effects of fibre content on mechanical properties and fracture behaviour of short carbon fibre reinforced geopolymer matrix composites

Geopolymer matrix composites reinforced with different volume fractions of short carbon fibres (C_f/geopolymer composites) were prepared and the mechanical properties, fracture behaviour and microstructure of as-prepared composites were studied and correlated with fibre content. The results show that short carbon fibres have a great strengthening and toughening effect at low volume percentages of fibres (3·5 and 4·5 vol.%). With the increase of fibre content, the strengthening and toughening effect of short carbon fibres reduce, possibly due to fibre damage, formation of high shear stresses at intersect between fibres and strong interface cohesion of fibre/matrix under higher forming pressure. The property improvements are primarily based on the network structure of short carbon fibre preform and the predominant strengthening and toughening mechanisms are attributed to the apparent fibre bridging and pulling-out effect.     

Compressive fracture behavior of 3D needle-punched carbon/carbon composites

Compressive fracture behavior under transverse and longitudinal compressive loading are determined for 3D needle-punched carbon/carbon (C/C) composites with single rough laminar (RL) pyrocarbon matrix or dual matrix of RL pyrocarbon and resin carbon. The results of Weibull statistics analysis indicate that scale parameter σ₀ of transverse and longitudinal compression of the composites with single matrix are 153.41 and 94.26 MPa, and σ₀ of the composites with dual matrix are 205.16 and 105.33 MPa, respectively. The mean compressive strength of both composites is nearly equal to σ₀ under each experimental condition. Failure modes of both composites under transverse and longitudinal compressive loading are shear and extension, respectively. Both composites exhibit quasi-ductile fracture behavior under transverse compression. Many small fragments of fibers and matrix carbon on the fracture surface of the composites are observed for single matrix composites. And the fiber bundle breakage with extensive debonding occurs for dual matrix composites. Under longitudinal loading, the composites with single matrix show quasi-ductile fracture behavior and delamination and splitting of non-woven long carbon fiber cloth layers are observed. The composites with dual matrix exhibit catastrophic failure behavior and crack runs through the composites along compressive loading direction.     

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.     

Mechanical behaviour and microstructural characterization of 3D four‐directional braided SiO2f/SiO2 composites

In this paper, SiO_2f/SiO₂ composites reinforced by 3D four‐directional braided quartz preform were prepared by the silica sol‐infiltration‐sintering method in a relatively low sintering temperature (450 °C). To characterize the mechanical properties of the composites, mechanical testing was carried out under various loading conditions, including tensile, flexural and shear loading. The microstructure and the fracture behaviour of the 3D four‐directional braided SiO_2f/SiO₂ composites were studied. The tensile strength, flexural strength and the in‐plane shear strength were 30.8 MPa, 64.0 MPa and 22.0 MPa, respectively. The as‐fabricated composite exhibited highly nonlinear stress–strain behaviour under all the three types of loading. The tensile and flexural fracture mechanisms were fully discussed. The fracture mode of the 3D four‐directional braided SiO_2f/SiO₂ composite in the Iosipescu shear testing was based on a mixed mechanism because of the multi‐directivity of the composite. Owing to low sintered temperature, the fibre/matrix interfacial strength was weak. The SiO_2f/SiO₂ composites showed non‐catastrophic behaviour resulting from extensive fibre pull‐out during the failure process.     

Anorganische Fasern. Herstellung,Eigenschaften und Verwendung

Inorganic Fibres – Fabrication, Properties and Application Glass- and carbon fibres are preferred reinforcement materials for composites with polymer matrix. Basing on an analysis of their properties it is shown that other inorganic fibres can combine the advantages of both, and avoid their disadvantages. Boron-, siliconcarbide- and alumina-fibres are discussed in detail. The boron fibre has a YOUNG's modulus up to 45 MN/m² and a strength of 3000–4000 MN/m² as well as high compressive and shear strength. Therefore the boron fibres are superior to the carbon fibres as high modulus reinforcement material. The disadvantages of the boron fibres are their complicated fabrication process (chemical vapour deposition on a tungsten monofilament), and their only availability in form of monofilaments with diameters of at least 60 μm. The boron fibre recristallizes at 6000 °C and reacts also with the tungsten substrat. Thus, its application at elevated temperatures is limited. The SiC-fibre shows the same mechanical properties as the boron fibre but it can be fabricated by chemical vapour deposition also on a carbon monofilament. The advantages are the chemical compatibility with carbon substrat and the resistance against oxidation. The disadvantage is the higher density compared with that of boron (3,5 against 2,6 · 10³ kg/m³) Carbon yarns (with 10 000 monofilaments of 10 μm diameter) with SiC coatings of 0,5 μm can be seen as an alternative to the relatively thick SiC-monofilaments with 60 μm diameter. The advantage of such coated carbon yarn is a better applicability in fibre reinforced composite materials. There exists a further alternative preparation process for SiC-yarn, namely the spinning of polycarbosilanes with subsequent formation of SiC by pyrolysis treatment. Al₂O₃-fibres are chemically inert against most oxidic and metallic matrix materials, and promises to be candidate reinforcement materials for aluminium. They can be prepared by melt-spinning process as well as by a hydrolysis-process starting from aluminium organic compounds with subsequent heat treatment for thermal decomposition. The properties of all these fibre materials are compared with those of glass-, polyamid- and carbon-fibres as well as with metal wires.     

Mg–3Zn–0.5Zr/HA nanocomposites fabricated by high shear solidification and equal channel angular extrusion

Magnesium alloy matrix and hydroxyapatite (HA) nanoparticle reinforced composites for biomedical applications were fabricated by combined high shear solidification and equal channel angular extrusion (ECAE). The high shear treatment was performed immediately prior to casting at 680°C using a rotor–stator mechanism. The as-cast composite ingots were processed by ECAE at 300°C to various strains. The high shear treatment effectively reduced HA particle agglomeration and produced a fine grain structure for all HA contents. ECAE processing resulted in further grain refinement and an improved HA particle distribution, with the formation of a desirable HA dispersion. The composites with 3–5 wt-% HA displayed an optimum combination of strength and ductility, with a yield strength of 150–210?MPa and compressive reductions of 9～13% before fracture.     

The impact toughness of discontinuous boron-reinforced epoxy composites

Previous theories for the impact strength of discontinuously-reinforced composites predict that the toughness is a maximum when critical transfer length fibres are used. Experiments utilizing mini-Charpy specimens of unidirectional boron-fibre-reinforced epoxy composites have been conducted which corroborate this prediction. However, calculations of the fracture energy, based on a uniform interfacial shear stress during fibre pull-out, proved inadequate for the reinforced epoxy composites. Revisions to existing theories are presented to take into account the non-uniformity of the interfacial shear stress distribution along the fibre length and catastrophic failure of the interfacial bond.Nomenclature A _f fibre cross-sectional area - E _f fibre Young's modulus - G _m matrix shear modulus - l fibre length - L fibre pull-out length - l _c fibre critical length - r fibre radius - R half fibre centre-to-centre spacing - V _f fibre volume fraction - W mean work of fracture per unit area of specimen cross-section - x distance from fibre end - y dummy variable of integration - surface energy - strain in composite - tensile stress on fibre - _f fibre fracture strength - interfacial shear stress     

Investigation of the effect of surface modifications on the mechanical properties of basalt fibre reinforced polymer composites

Unsaturated polyester-based polymer composites were developed by reinforcing basalt fabric into an unsaturated polyester matrix using the hand layup technique at room temperature. This study describes basalt fibre reinforced unsaturated polyester composites both with and without acid and alkali treatments of the fabrics. The objective of this investigation was to study the effect of surface modifications (NaOH & H₂SO₄) on mechanical properties, including tensile, shear and impact strengths. Variations in mechanical properties such as the tensile strength, the inter-laminar shear strength and the impact strength of various specimens were calculated using a computer-assisted universal testing machine and an Izod Impact testing machine. Scanning Electron Microscope (SEM) observations of the fracture surface of the composites showed surface modifications to the fibre and improved fibre–matrix adhesion. The result of the investigation shows that the mechanical properties of basalt fibre reinforced composites are superior to glass fibre reinforced composites. This work confirms the applicability of basalt fibre as a reinforcing agent in polymer composites.     

Thermal and impact study of PP/PET fibre composites compatibilized with Glycidyl Methacrylate and Maleic Anhydride

In this paper, two grafted copolymers, Glycidyl Methacrylate grafted polypropylene (PP) (PP-g-GMA) and Maleic Anhydride grafted PP (PP-g-MA) were used in PP reinforced with short poly(ethylene terephthalate) (PET) fibre composites. Transcrystallization (TC) of PP on PET fibres was investigated using a polarized optical microscope, which revealed no TC for either of the modified composites at the fibre–matrix interface. Heat deflection temperature (HDT) results of GMA modified composites revealed more enhancement than HDT of MA modified samples. The composite strength results showed enhancement for both modified composites up to 10 wt.%, and this growth was bigger for GMA modified composites. The morphological analysis of GMA modified PP/PET composites pointed out a marked improvement of fibre dispersion and interfacial adhesion as compared to non-compatibilized PP/PET composites. The results of impact strength showed about 43% enhancement for 15 wt.% PET fibre composites. It was found that at low fibre percentages, using either of the modifiers reduces the impact strength a little in comparison to impact strength of the unmodified samples. According to linear elastic fracture mechanics LEFM, impact fracture toughness (G_c) and critical stress intensity factor (K_c) were evaluated for these composites based on the fracture energy obtained from impact tests.     

Plasma surface modification of advanced organic fibres

The interlaminar shear strength, interlaminar fracture energy, flexural strength and modulus of extended-chain polyethylene/epoxy composites are improved substantially when the fibres are pretreated in an ammonia plasma to introduce amine groups on to the fibre surface. These property changes are examined in terms of the microscopic properties of the fibre/matrix interface. Fracture surface micrographs show clean interfacial tensile and shear fracture in composites made from untreated fibres, indicative of a weak interfacial bond. In contrast, fracture surfaces of composites made from ammonia plasma-treated fibres exhibit fibre fibrillation and internal shear failure as well as matrix cracking, suggesting stronger fibre/matrix bonding, in accord with the observed increase in interlaminar fracture energy and shear strength. Failure of flexural test specimens occurs exclusively in compression, and the enhanced flexural strength and modulus of composites containing plasma-treated fibres result mainly from reduced compressive fibre buckling and debonding due to stronger interfacial bonding. Fibre treatment by ammonia plasma also causes an appreciable loss in the transverse ballistic impact properties of the composite, in accord with a higher fibre/matrix interfacial bond strength.     

Strength and fracture properties of asbestos-cement mortar composites

The strength and fracture properties of random asbestos fibre-reinforced cement mortar composites are reported in this paper. The fibre content varies between 5% and 20% by weight. Both the ultimate tensile strength ( _t) and the modulus of rupture ( _b) increase with increasing fibre-volume fraction. These results are shown to agree satisfactorily with the law of mixtures modified for randomly oriented short fibre-reinforced composites. The critical stress intensity factor (K _c) and the specific work of fracture (R) have been determined using three-point bend edge-notched beams and grooved double-cantilever-beam (DCB) specimens. There is generally good agreement between these two physical quantities estimated from the two testpiece geometries. It is shown that the fibre pull-out mechanism is dominant in the fracture of asbestos cements and that the specific work of fracture can be reasonably well predicted by considering the energies absorbed in both the pull-out and the fibre/matrix interfacial debonding processes.     

Fabrication and properties of SiC fibre-reinforced Li2O·Al2O3·6SiO2 glass-ceramic composites

Ta₂O₅, Nb₂O₅ and TiO₂ were used separately as additives to a Li₂O·Al₂O₃·6SiO₂ glass-ceramic composition, to act as nucleating dopants and to aid the formation of an interfacial carbide layer (TaC and NbC) between the fibre and matrix in SiC fibre uniaxially reinforced glass-ceramic composites, The composites exhibited high modulus of rupture (>800 MPa) and fracture toughness (K _IC > 15 MPam^1/2). The interfacial amorphous carbon rich layer and carbide layer were responsible for lowered interfacial shear strength but permitted high composite fracture toughness. The composite with the TiO₂ additive in the matrix showed a lower flexural strength (<500MPa) and a smaller K _IC (-11 MPam^1/2) which resulted from the high interfacial shear strength between the SiC fibre and the matrix due to the formation of the interfacial TiC layer.     

Shear strength of polymers and fibre composites: 2. carbon/epoxy pultrusions

Pultrusions were made with carbon fibres and an epoxy resin. Three different curing agents were used, so that the matrices were resins with different glass transition temperatures. The composites were tested for shear strength at different temperatures, so that the effect of the resin shear strength on composite shear strength could be observed, with a fixed fibre architecture. It was found that the composite was always much stronger than the resin both for the 0 and 90° fracture modes. The 90° fracture surfaces contained many broken fibres, and shear hackles were observed in the resin-rich regions. These suggested that shear failure (rather than tensile failure) took place in the Iosipescu test for the 90° specimens. It was concluded that the fibre architecture played a dominant role in the composite shear strength, with interphase effects being involved also.     

Compressive failure of unidirectional hybrid fibre-reinforced epoxy composites containing carbon and silicon carbide fibres

The compressive failure of unidirectional hybrid fibre-reinforced epoxy matrix composites containing carbon (C) and silicon carbide (SiC) fibres has been investigated. In contrast to the case of flexural testing previously investigated by the authors, no significant increase in compressive strength, elastic modulus, or work of fracture was noted for the case of composites containing a mixture of C and SiC fibres. The specific compressive strength and elastic modulus generally decreased with increasing SiC fibre content due to the higher density of these fibres. Failure modes of tested specimens were classified into two main groups, namely compressive shear and compressive crushing, with the presence of fibre kinking and longitudinal splitting being noted in both cases.     

碳纤维/TiO2改性树脂复合材料的制备及力学性能

针对环氧树脂脆性大、与碳纤维形成的界面性能较差等问题,本文选用纳米TiO₂对5284环氧树脂进行改性,并以角联锁机织物为增强体制备了碳纤维/环氧树脂复合材料。使用FT-IR、旋转流变仪、表面张力仪等设备对TiO₂/环氧树脂进行表征,并研究了树脂改性对复合材料压缩与层间剪切性能的影响。研究表明：TiO₂的羟基与环氧树脂的环氧基和羟基发生了反应;经1wt.%TiO₂改性的树脂复数黏度为0.066 Pa·s,纤维与树脂间接触角为28.85°,浸润效果较好;相较于未改性复合材料,树脂改性的复合材料纵向压缩强度与模量分别提高了7.46%和11.03%,横向压缩强度与模量分别提高了6.99%和4.96%,纵向、横向的剪切强度分别提高了6.88%和4.65%。TiO₂改性环氧树脂提高了复合材料的承载能力,改善了界面结合强度。     

Thermomechanical fatigue of short fibre reinforced aluminium matrix piston alloy

Abstract

The technical potential of short fibre reinforced aluminium matrix composites lies in their higher stiffness and higher strength at elevated temperatures compared with unreinforced matrix alloys. In the present investigation, thermal cycling creep tests were conducted on the piston alloy AlSi12CuMgNi reinforced with 20% Saffil (Al₂O₃) short fibres, to simulate the cold start conditions of combustion engines. After processing of the metal matrix composite (MMC) by direct squeeze casting, four heat treatment conditions were produced. Specimens under constant load were thermally cycled between 50 and 300°C, whereby a heating and cooling speed of 12.5 K s1 was achieved. Series of up to 5000 cycles at tensile stresses between 20 and 80 MPa were executed, comparing reinforced specimens and unreinforced matrix material. The results of these experiments showed that the creep properties of the alloy, especially minimum creep rate and lifetime to fracture, were improved by the reinforcement. Furthermore, the creep rate of the MMC was essentially independent of the heat treatment condition, whereas the minimum creep rate was increased significantly for the matrix material by overaging. It can be concluded that precipitation strengthening influenced the creep properties of unreinforced specimens only, which is in good agreement with theoretical considerations. An analysis of fibre length revealed that the majority of the fibres broke at between 50 and 75% of the lifetime, just before the beginning of tertiary creep. Metallographic investigations using a scanning electron microscope did not show fibre pullout, but multiple fracture of fibres along the whole specimen. Micromechanical models for isothermal creep in short fibre reinforced aluminium alloys confirm the above results, since tertiary creep is assumed to be a consequence of fibre fracture.     

Analyses of the mechanical properties and microstructure of bamboo-epoxy composites

Bamboo reinforced epoxy possesses reasonably good properties to waarrant its use as a structural material, and is fabricated by utilizing bamboo, an abundant material resource, in the technology of fibre composites. Literature on bamboo-plastics composites is rare. This work is an experimental study of unidirectional bamboo-epoxy laminates of varying laminae number, in which tensile, compressive, flexural and interlaminar shear properties are evaluated. Further, the disposition of bamboo fibre, the parenchymatous tissue, and the resin matrix under different loading conditions are examined. Our results show that the specific strength and specific modulus of bamboo-epoxy laminates are adequate, the former being 3 to 4 times that of mild steel. Its mechanical properties are generally comparable to those of ordinary glass-fibre composites. The fracture behaviour of bamboo-epoxy under different loading conditions were observed using both acoustic emission techniques and scanning electron microscopy. The fracture mode varied with load, the fracture mechanism being similar to glass and carbon reinforced composites. Microstructural analyses revealed that natural bamboo is eligibly a fibre composite in itself; its inclusion in a plastic matrix will help solve the problems of cracking due to desiccation and bioerosion caused by insect pests. Furthermore, the thickness and shape of the composite can be tailored during fabrication to meet specific requirements, thereby enabling a wide spectrum of applications.     

Anodic oxidation of pitch-precursor carbon fibres in ammonium sulphate solutions: the effect of fibre surface treatment on composite mechanical properties

Tonen HM pitch-based carbon fibres were electrochemically treated in solutions of ammonium sulphate using a pilot plant surface treatment apparatus. Embedded single fibre and short beam shear specimens prepared with these treated fibres exhibited strengths over 300% greater than those made with untreated fibres. The surface treatment did not, however, result in improvements in the longitudinal compressive strength. This suggests that the compressive strength is not limited by the shear strength of the fibre/matrix interface.     

The compression strength of composites with kinked,misaligned and poorly adhering fibres

Experiments carried out on pultruded fibre reinforced polyester resins show that, at moderate fibre volume fractions, the compressive strength of aligned fibre composites depends linearly on the volume fraction. The strength falls off when the fibre volume fraction,V _f=0.4 with Kevlar and high strength carbon fibres. The effective fibre strength atV _f<0.4 is much less than the tensile strength but it is close to the tensile strength with E-glass fibres and high modulus carbon fibres. Poor adhesion between fibres and matrix reduces the compressive strength, as does kinking the fibres when the fibre radius of curvature is reduced to below 5 mm. Misalignment of the fibres reduces the compressive strength when the average angle of misalignment exceeds about 10° for glass and carbon fibres. However, with Kevlar no such reduction is observed because the compression strength of Kevlar reinforced resin is only a very little better than that of the unreinforced resin.