Carbon fibre compressive strength and its dependence on structure and morphology

The axial compressive strength of carbon fibres varies with the fibre tensile modulus and precursor material. While the development of tensile modulus and strength in carbon fibres has been the subject of numerous investigations, increasing attention is now being paid to the fibre and the composite compressive strength. In the present investigation, pitch- and PAN-based carbon fibres with wide-ranging moduli and compressive strengths were chosen for a study of fibre structure and morphology. A rayon-based carbon fibre was also included in this study. Structural parameters (L _c, L_a(0), L _a(90), orientation parameter Z, and the spacing between graphitic planes d(00, 2)) were determined from wide angle X-ray spectroscopy (WAXS). Fibre morphology was characterized using high-resolution scanning electron microscopy (HRSEM) of fractured fibre cross-sections. The mechanical properties of the fibres, including compressive strength, the structural parameters from WAXS, and the morphology determined from HRSEM are reported. The influence of structure and morphology on the fibre compressive strength is discussed. This study suggests that the width of the graphitic sheets, the crystallite size perpendicular to the fibre axis (L _c and L _a(0)), and crystal anisotropy play significant roles in accounting for the large differences in compressive strengths of various carbon fibres.     

Comparing the surface and internal structure of polypropylene fibres using advanced microscopy techniques

This paper highlights the importance of both surface and internal (bulk) structure of polypropylene (PP) melt extruded monofilament fibres and the dependence of structure on processing conditions. Gravity spun and as-spun fibres showed similar spherulitic surface structure but Wide Angle X-ray Scattering (WAXS) results indicated that the overall fibre crystallinity was contrasting for the two fibre types. From analysis of longitudinal and transverse fibre cross sections using Scanning Probe Microscopy (SPM) and Environmental Scanning Electron Microscopy (ESEM) it was found that gravity spun fibres showed a shish-kebab type structure in contrast to the macrofibrillar internal structure of the as-spun variant. In situ tensile testing gave powerful evidence to suggest that deformation in the necking region for the gravity spun fibres was due to the composite behaviour of the spherulitic surface and the internal shish-kebab structure.     

Copper coating on coir fibres

Conditions for obtaining continuous coatings of copper (thickness range 1.5 to 5μm) on coir fibres have been reported. Activation of the surface of coir fibres was achieved by treating the surface of the fibres with NaOH-HCHO/ammoniacal AgNO₃ solution. Copper was deposited on the activated surface of coir fibres from Fehling-formaldehyde solution. The effects of variation in formaldehyde and sodium hydroxide concentrations and pressures inside the coating vessel on deposition rates were determined. The minimum concentrations of NaOH and HCHO required for maintaining a maximum rate of deposition of copper from a solution contaning 10g l⁻¹ copper sulphate were found to be 6.6 g l⁻¹ and 2.5 to 3.5 g l⁻¹, respectively. Optical and scanning electron microscope studies show that relatively more uniform and non-porous copper coatings were obtained when deposition was carried out under reduced pressures. A 5μm thick copper coating on coir fibre prevents the propagation of flame as was shown by flammability tests. Copper coating on coir fibre decreases its electrical resistivity from 2.55×10⁶Ωcm to 4.68×10⁻³Ωcm with 1.5μm thick coating and 3.76×10⁻⁵Ωcm with 5μm thick coating. Reinforcement of polyester with copper-coated coir fibre leads to an increase of about 25% in tensile strength and flexural strength as compared to polyester reinforced with plain coir fibre.     

TEM characterization of some crude or air heat-treated SiC Nicalon fibres

Commercial Nicalon fibres were prepared by thin transverse sectioning and studied by transmission electron microscopy. A progressive tilting of the incident beam allows us to explore the selected-area diffraction (SAD) pattern along two orthogonal directions, increasing the tilting angle (dark-field (DF) imaging). The lattice fringes technique was also used. The samples were Nicalon 001, 101 and 201 fibres, the latter also being studied after heat treatment in air at 1300° C for 48 h. The SAD pattern of the 001 fibre only shows the SiC, 1 1 intense halo whereas the other samples show all the SiC (1 1 1, 2 2 0 and 31 1) strongly scattered beams, indicating a microcrystalline state. Correspondingly, DF imaging does not indicate any localized measurable scattering domain for 001. Only bright dots can be seen, less than 1 nm in size. The other fibres show SiC microcrystals respectively 2 nm (1 01 ), 3 nm (201 ) and up to 7 nm (heat-treated 201) in extent. Free aromatic carbon, shaped in small units less than 1 nm in size fills up the interstices between SiC. These units tend to lie flat on SiC. In heat-treated fibres, they form incomplete layers around the edges. In addition, the heat-treated 2 01 fibre show a 1m thick layer of cristobalite at the fibre surface. These crystals are polytypes.     

Fibre strength selection and the mechanical resistance of fibre-reinforced metal matrix composites

For predicting the strength of fibre-reinforced metal matrix composites, the in situ fibre strength value has to be introduced in the calculations. Tension tests series have been conducted on SiC fibres (SCS0 and SCS2 TEXTRON) before and after chemical interaction with a pure liquid aluminium bath and the reacted fibres have been tested before and after dissolution of the aluminium coating simulating the metallic matrix around the fibres. The results obtained for the different fibre batches show that the in situ fibre resistance may differ significantly from the strength of as-received or extracted fibres that is usually adopted in the models.     

An investigation into the relationship between processing,structure and properties for high-modulus PBO fibres: part 3: analysis of fibre microstructure using transmission electron microscopy

A detailed morphological study of the microstructure of poly(p-phenylene benzobisoxazole) (PBO) fibres (HM and HM+) and a polypyridobisimidazole (PIPD) (HT) fibre has been undertaken using transmission electron microscopy. Both PBO and PIPD fibres are composed of rigid-rod polymers having p-phenylene rings in the molecular backbone and show high modulus (280–360 GPa) and high strength (4–6 GPa). It is found that the PBO HM+ fibre has the highest degree of molecular orientation of the three fibres and the longest crystal length along the fibre axis, while the PIPD fibre shows a lower degree of orientation and a shorter crystal length than the PBO fibres. To understand the effect of crystalline size and fine structure of the fibres upon mechanical properties, dark-field and high-resolution lattice images were obtained and analysed in detail.     

TEM study of the microstructure of carbon-carbon composite

A detailed study of the structure of carbon-carbon composite was performed using transmission electron microscopy (TEM) techniques. Structural characterization is essential in order to understand fully the mechanical and physical properties of these materials. The use of an atom mill in sample preparation made it possible to perform TEM analysis on the fibres, matrix, and the fibre-matrix interface. Full characterization of the fibres was performed across the fibre diameter. Structural heterogeneity was characterized in terms of preferred orientationZ, stack heightL _c, stack widthL _a, and d₀₀₂ spacing. The use of 002 dark field imaging showed the orientation and size of turbostratic and graphitic crystallites in the fibre and the matrix. Amorphous resin was located in the interbundle matrix.     

Elevated temperature tensile properties and failure of a copper-chromium in situ composite

A Cu-10 vol% Cr in situ composite was produced by melt processing and deformed by swaging to form rods with a total deformation true strain of 3.15. Scanning electron microscopy showed that the composite microstructure consisted of Cr fibres aligned with their long axes parallel to the rod axis. X-ray diffraction indicated that the Cr fibres had a strong <110> fibre texture. The mechanical properties of the composite were measured by tensile testing over the temperature range –70 to 600°C. Examination of fibre fracture and fibre-matrix debonding at and near the tensile test fracture surfaces indicated that a transition from localised to global damage occurred between 300 and 400°C.     

The matrix fatigue behaviour of fibre composites subjected to repeated tensile loads

In a fibre/metal matrix composite the mechanical properties of the matrix itself are changed by the presence of the reinforcing fibres. This changed behaviour of the metal is referred to asin situ behaviour, and a phenomenological model is developed to evaluate thein situ plastic stress-strain properties of a metallic matrix containing fibres, from a study of the properties of the composite. The model is based upon the idealised behaviour of the two components of the system. The application of the model to B/Al alloy composites shows that the plastic stress-strain behaviour of the matrix containing fibres varies strongly with the fibre volume content, and also that the matrixin situ cyclic stress-strain behaviour can be approximately described by a power law of the type: where the strength coefficient and the exponent increase with the fibre volume fraction. It also predicts that in the steady state fatigue behaviour of the composites, the fraction of load amplitude carried by the fibres decreases with increasing applied stress amplitude, and is also dependent on the fibre volume fraction. The effect of the applied stress on the damping capacity is established through expressions derived from the basic ideas involved in the model.     

The structure and deformation behaviour of poly(p-phenylene benzobisoxazole) fibres

The relationship between structure and mechanical properties in as-spun and heat-treated high modulus poly (p-phenylene benzobisoxazole) (PBO) fibres has been examined using a combination of electron microscopy, mechanical testing, and Raman microscopy. The structure of the fibres has been determined by obtaining longitudinal sections, and electron diffraction has shown that skin regions are significantly more oriented than the fibre cores. Heat treatment of the fibres at elevated temperatures produces an improvement in the level of crystallinity especially in core regions. Heat treatment also produces an increase in fibre modulus but for fibres heat treated at 650° C there is a significant decrease in strength compared with ones heat treated at 600° C. Well-defined intense Raman spectra were obtained from individual fibres and three main bands at 1280, 1540 and 1615cm^–1 have been identified. All three bands are sensitive to the level of applied strain with the 1280cm^–1 being the most sensitive, shifting by-7.9cm^–1% strain for PBO fibres heat treated at 600° C. The dependence of the sensitivity of the position of the 1615cm^–1 band to strain upon fibre structure has been examined in detail. The rate of shift of band position with strain increases with fibre modulus. It is shown that these shifts in Raman bands are a direct reflection of molecular deformation within the fibres.     

The mechanical properties and fracture behaviour of unidirectionally reinforced Nylon 6

The tensile and shear properties of Nylon 6 polymerizedin situ around unidirectionally aligned carbon and glass fibres have been investigated and the fracture behaviour characterized by optical and scanning electron microscopy. The tensile strengths are found to lie within the limits predicted by the law of mixtures and deviations from the predicted strengths have been correlated with fibre type and surface treatment. The shear strength values follow the same trend and an important mode of fracture in bending is shown to be the compressive failure which accompanies a yield drop in the load deflection curve. Depending upon the fibre type and the properties of the matrix this compressive damage need not lead to catastrophic failure of the composite as, in certain cases, the matrix can undergo substantial deformation before failure.     

Thermoforming woodfibre–polypropylene composite sheets

Thermoforming of woodfibre–polypropylene composite sheets made without any modification of the fibres or the polymer is the focus of this paper, the emphasis being on their formability and the associated issues. Both the degree to which a material conforms to the desired part geometry after deformation and the extent to which a sheet material may be deformed before unacceptable defects occur are considered. Four thermoforming processes such as V-bending, die-match forming, air pressure forming and deep drawing have been utilised to examine both single-curvature and double-curvature deformation conditions. The technique of Grid Strain Analysis (GSA) has been applied to quantify differences in strain distributions during sheet deformation. The effects of thermoforming process parameters and sheet composition on sheet formability are also discussed. Notably, this study considers composite sheets reinforced with wood fibres rather than woodflour, enabling the study of fibre layup and fibre interlocking effects. While the tensile strengths of the composite sheets increase marginally the stiffnesses increase significantly compared to those of unreinforced polypropylene. The key deformation mechanism for layered woodfibre–polypropylene composite sheets is inter-ply shear while intra-ply shear dominates the deformation of homogeneous sheets. Forming temperature and blank size have the most pronounced effects on the formability of these composite sheets.     

An acoustic emission study of failure by stress corrosion in bundles of E-glass fibres

Fibre failures in bundles of parallel E-glass fibres loaded from their ends and immersed in aqueous HCl solutions or in water are detected using acoustic emission. In constant load and constant stress tests the times to failure of the fibres and the bundles are accounted for on the basis of a theory which assumes a Weibull distribution of initial strengths, and that the velocity (v) of subcritical stress-corrosion cracks in the fibres depends on the stress intensity (K _l) according to a power law (VK _l ⁿ ). The crack growth parameter (n) is measured by three methods, and its variation with acid strength is correlated with other corrosion studies. The stress corrosion measurements also allow a determination of the Weibull modulus of the fibres.     

Cladding and characteristics of LiNbO3 single crystal fibre

Abstract

A magnesium ion indiffusion process is applied to LiNbO₃ single crystal fibres deposited with MgO thin film on their surface. A core–cladding waveguide structure with different refractive index profile is formed in LiNbO₃ single crystal fibres by selecting suitable diffusion parameters such as diffusion temperature, diffusion time and MgO film thickness. An electron probe microanalysis was used to measure the Mg-ion concentration distribution of the Mg diffused layer. The propagation loss of the clad crystal fibre (Mg-diffused crystal fibre) is measured to be 14 times lower than that of the unclad crystal fibre and the transmission modes were observed. The microstructural characteristics of the Mg diffused crystal fibres were also characterized by using Laue X-ray diffraction and scanning electron microscopy. It was shown that a good starting single crystal of the Mg diffused crystal fibres can be preserved by controlling suitable diffusion parameters and matching suitable crystal fibre diameter.     

Studies on void coalescence analysis of nanocrystalline cryorolled commercially pure aluminium formed under different stress conditions

Cryorolling was performed on commercially pure aluminium sheet from an initial thickness of 7 mm to 0.25 mm with a total true strain of around 3.33. Cryorolling was performed in many passes with only 5% reduction in each pass to avoid adiabatic heating during rolling process. Detailed Transmission Electron Microscopic studies showed increased dislocation density and distributed dislocation cell structure. Streaks along with ring pattern in selected area electron diffraction of transmission electron microscopy evidenced the existence of texture component on the surface of rolled sheet along with nanocrystalline sub-structure. Studies on fracture behavior of the cryorolled sheets were performed using specimen of double edge-notch tensile geometry. Compared to the conventionally rolled sheet metal, the strain triaxiality ratio value in the case of cryorolled sheets is insensitive to the void growth analysis due to the presence of nano-sized grains. The ratio of length to the width of voids varies from 1.6 to 2.4 in the case of conventionally rolled sheet. In contrast, length to width ratio is very close to 1.0 in the case of cryorolled sheet. Thus no oblate or prolate voids were observed during cryorolling compared with conventionally rolled sheets and it is observed the formation of equiaxed nanostructured grains. In the case of cryorolled sheets, minimal variation in length to width ratio was observed with the variation in the shear strains, due to the presence of nanostructured grains. Whereas, in the case of conventionally rolled material, wide variation in the length to width ratio with the variations in the shear strains was observed.     

An analytical model for number of fibre–fibre contacts in paper and expressions for relative bonded area (RBA)

This paper proposes an analytical model for calculating the number of fibre–fibre contacts per unit fibre length from the cross-sectional dimensions of the fibres in a sheet and sheet density. This model has been verified with data of the number of fibre–fibre contacts measured directly in handsheets. The measured fibre–fibre contacts in the handsheets were classified into full and partial contacts. The model best fits this data when 1.5 partial contacts are equated to one full contact. A plot of measured versus predicted equivalent full contacts produced a linear correlation with a slope of 0.99 and correlation coefficient of R ²  = 0.93. The model was used to derive expressions for the fraction of the available fibre surface, which is bonded to other fibres, a quantity called the Relative Bonded Area (RBA). The validity of the model was checked using experimental data for RBA. RBA was determined both by nitrogen adsorption ( ) and by scattering coefficient (RBA_sc). The extrapolation method of Ingmanson and Thode to determine S ₀, the scattering coefficient of an unbonded sheet, proved to be inaccurate. We estimated S ₀ for some samples from their linear relationship between scattering coefficient and nitrogen adsorption. The new model accurately predicted both and RBA_sc.     

Microstructure and mechanical properties ofin-situ network composite fibres of PBZT with nylon

A method for preparing composite fibres by infiltrating nylon into swollen poly(p-phenylene benzobisthiazole) (PBZT) fibre is described. PBZT fibre forms a microfibrillar network structure during the coagulation process.In-situ network composite (IC) fibres may be prepared by exchanging the coagulant with a solution containing the desired matrix material. These new composite fibres exhibit nearly identical mechanical properties and similar thermomechanical properties to those of so-called molecular composite (MC) fibres prepared from isotropic solutions of PBZT and nylon in methane sulphonic acid (MSA), The mechanical properties of these fibres were determined before and after heat treatment under tension. The structure of pure PBZT and its composite fibres (ICs' and MCs') were characterized using nitrogen adsorption (Brunauer-Emmett-Teller (BET) experiments), small-angle X-ray scattering, and scanning and transmission electron microscopy (SEM and TEM, respectively). The structure of both composite fibres was found to be a microfibrillar network of PBZT in a matrix of amorphous nylon. The average diameters of the PBZT microfibrils were in the range of 10 to 20 nm forin-situ network composites and approximately 4 nm for molecular composites.     

Damage tolerance behaviour of cloisite 15A incorporated recycled polypropylene nanocomposites and bionanocomposites

In the present investigation, recycled polypropylene (rPP) used as a matrix was modified by incorporating nanofillers through melt blending technique to prepare a masterbatch of nanocomposites. Untreated sisal fibre and mercerised sisal fibres were further incorporated into the nanocomposites for the preparation of bionanocomposites. Bionanocomposites containing 40 wt% of UT fibre and 5 wt% of MA-g-PP revealed an increase in the tensile strength and modulus to the tune of 27% and 370%, respectively, compared to rPP. The flexural strength and modulus also increased to the tune of 129% and 269%, respectively, compared to rPP. Further, the surface treatment of the fibre slightly increased the mechanical properties and stiffness of bionanocomposites. Interfacial strength between fibre and matrix was also evaluated by using Turcsanyi and Sato–Furukawa models. Damage tolerance of rPP nanocomposites and its bionanocomposites was evaluated using single-edge-notch specimens. The notch length ‘a’ to width ‘W’ ratios, a/W, were chosen as 0.3, 0.45 and 0.6. The nanocomposites showed better damage tolerance as compared to the rPP matrix. The corrugated structure with increased fractured surface area was observed in scanning electron microscopy. Better dispersion of clay in the nanocomposites was observed in transmission electron microscopy.     

Mechanisms of nucleation and crystallization of lithium disilicate in Li-Zn-silicate glass-ceramic fibres

The preferred crystal orientation of Li₂Si₂O₅ crystallites has been evaluated in several partially crystallized glass fibres with the following chemical composition in mol %: 66.75 SiO₂; 23.45 Li₂O; 8.00 ZnO; 1.00 K₂O and 0.80 P₂O₅. The crystallites were shown to be grown radially with thec-axis directed from the surface towards the core of the fibre. When glass fibres have been subjected to a nucleation treatment step, this preferred orientation slightly increases in the temperature range 590 to 620° C, by increasing the crystallinity content. This behaviour was attributed to the presence of active nuclei. A kinetic study on the crystalline volume coarsening, carried out according to the Johnson-Mehl-Avrami equations, showed a mechanism of linear growth for the lithium disilicate phase with the (00/) growing planes moved inward from the surface to the core of the fibre. The small-angle X-ray scattering results were obtained by using a masking liquid with an electron density close to that of the glass fibres investigated, in order to eliminate the total external reflection and surface scattering. Results indicated that two distinct particle fractions are precipitated inside the fibres after glass-in glass phase separation and nucleation at 500° C for various periods of time. The fraction of the largest particles,13 to20 nm in size, is probably constituted with vitreous SiO₂-rich droplets, whereas the smallest particles, 2.5 to 6.5 nm in size, are probably Li₂O-rich clusters containing P₂O₅.     

Fibre orientation of novel dynamically sheet formed discontinuous natural fibre PLA composites

This paper describes work carried out to fabricate and to assess the fibre orientation in PLA reinforced by aligned discontinuous harakeke and hemp fibre mats produced using a dynamic sheet former (DSF). These mats were combined with PLA sheets to make composites with fibre contents of 5–40 wt% using a hot press. It was found that the fibre orientation factors (K_θ) for both harakeke and hemp fibre composites were higher than those values seen in the literature for composites prepared using injection moulding and hot pressed using randomly oriented fibre mats, but slightly lower than the highest values obtained with aligned fibre nonwoven preform composites utilising more processing stages. The highest K_θ values for harakeke and hemp fibres in this work were found to be 0.58 and 0.44 respectively. K_θ decreased, reflecting increased fibre misalignment as fibre content increased, believed to be due to fibre agglomeration and the higher pressure required during processing.