Fragmentation in alumina fibre reinforced epoxy model composites monitored using fluorescence spectroscopy

It has been found that well-defined fluorescence R₁ and R₂ lines can be obtained from PRD-166 alumina-zirconia fibres and that the fluorescence R lines shift with applied stress. They are found to shift to higher wavenumber when subjected to tensile deformation and to lower wavenumber in compression. The stress-sensitive fluorescence R₂ line has been used to map the distribution of stress along PRD-166 fibres embedded in an epoxy resin matrix cured under different conditions. It has been shown that the distributions of stress along the PRD-166 fibres at different levels of matrix strain are consistent with those predicted by conventional shear-lag analysis. The interfacial shear stress has been derived from the point-to-point variation of stress along the fibre. The fluorescence technique has also been used to map the stress distribution along a PRD-166 fragment in an epoxy matrix during a single-fibre fragmentation test where it is found that debonded regions propagate along the fibre fragments during loading, after initial fragmentation has occurred.     

氧化铝纤维的结构和力学性能

报道氧化铝－氧化锆纤维和纯氧化铝纤维的结构和力学性能及相互间关系。使用Ｘ射线衍射和电子显微术表征结构，力学性能的测定除去常规的应力应变关系外，主要使用激光拉曼光谱和荧光光谱术研究纤维的拉伸形变特性。     

Microstructure and mechanical properties of pitch-based carbon fibres

The microstructure of a series of mesophase pitch-based carbon fibres have been examined using X-ray diffraction, electron microscopy and Raman spectroscopy. It has been shown that the mechanical properties of the fibres are related directly to the response of this microstructure to deformation and, in particular, that the Young's modulus and tensile strength of the fibres are controlled directly by the fibre microstructure. It has also been shown that Raman spectroscopy can be a useful technique for not only characterizing the microstructure of the fibres but also for following molecular deformation in the fibres. It was found that the position of the 1580 cm^–1 Raman band for the fibres shifted with the application of stress and that the rate of shift per unit strain was proportional to the Young's modulus of the fibres. It was also shown that this reflected the higher degree of stressing of the graphite plane in the higher modulus fibres, consistent with recently developed theories which attempt to explain the dependence of the mechanical properties of carbon fibres upon the degree of orientation of the graphite planes.     

Relationship between structure and mechanical properties for aramid fibres

The relationship between structure and mechanical properties for a series of twelve wellcharacterized aramid fibres has been determined. The fibres were produced under a variety of processing conditions and the fibre structure has been characterized using transmission electron microscopy. In particular, both the overall degree of molecular orientation in the fibres and the difference in structure between the fibre skin and core regions have been investigated in detail. The mechanical properties of the fibres have been evaluated using conventional mechanical testing and molecular deformation followed using Raman microscopy to monitor strain-induced band shifts. It has been shown that the mechanical properties of the fibres are controlled by the fibre structure. In particular, it is shown that the fibre modulus is governed by the overall degree of molecular orientation. It is also demonstrated that the fibre strength is controlled principally by the overall molecular orientation but may also be reduced by the presence of a highly-oriented skin region. It has been found that the rate of shift of the Raman bands per unit strain is proportional to the fibre modulus except for fibres with large differences in molecular orientation between fibre skin and core regions. For these fibres the rate of shift reflects the higher orientation of the skin.     

Evaluation of interface fracture energy for single-fibre composites

Raman and luminescence spectroscopy have been used for the first time to determine the interface fracture energy for single-fibre composites. By using the measured fibre stress distributions in single-fibre fragmentation composite specimens and a simple energy-balance scheme, the energy for the initiation of interfacial debonding has been estimated for carbon (T50) and α-alumina (PRD-166 and Nextel 610) fibres embedded in epoxy resins. It has been found that the interface fracture energy shows good sensitivity to changes in the level of fibre/matrix adhesion due to surface treatment and sizing of the fibres. It is also found that the values of interface fracture energy correlate well with measured values of interfacial shear strength determined for the same fibre/matrix systems.     

Production of carbon fibres from a pyrolysed and graphitised liquid crystalline cellulose fibre precursor

Regenerated cellulose fibres, spun from a liquid crystalline precursor, were pyrolysed at temperatures in the range 400–2,500?°C. Raman spectroscopy and X-ray diffraction showed that the degree of graphitisation of the fibre increased with increasing temperature. Electron microscopy, however, suggested that the fibres have a skin–core structure. This observation was confirmed by micro-Raman analysis, whereupon the ratio of the intensities of the D and G bands shows that the skin consists of a graphitised structure, whereas the core consists of significantly less graphitised material. The contributions of the graphitised skin and the inner core to the potential mechanical properties of the fibres were also assessed by following the position of the 2D Raman band during tensile deformation of the fibre. The Raman band shift rate against strain was used to evaluate the fibre modulus, which suggested a modulus of ~140 GPa for the skin and 40?GPa for the core, respectively. If this incomplete graphitisation could be overcome, then there is potential to produce carbon fibres from these novel precursor materials.     

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.     

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 relation of dynamic elastic moduli,mechanical damping and mass density to the microstructure of some glass-matrix composites

Dynamic elastic moduli and mechanical damping were measured with the PUCOT (piezoelectric ultrasonic composite oscillator) technique at room temperature for ceramic-matrix composites (CMCs) of the following compositions: PRD-166 (fibres)/N51A glass (matrix), PRD-166 fibres coated with SnO₂/glass, Nextel 480 fibres/glass, Nextel 480 fibres coated with SnO₂/glass, and Nextel 480 fibres coated with BN/glass. The fibres were continuous, and the volume fractions varied from 0.24 to 0.43. Some of the mechanical-property measurements correlated with the thickness of one of the coating materials, and with microstructural observations of the misorientation angle of the fibres and normalized fibre length. With increasing volume fractions of fibres, the fraction of broken fibres increased. For the PRD-166/glass and PRD-166/SnO₂/glass, a substantial fraction of the fibres were misoriented by angles of up to 15 °. Assessments were made of the measured properties in terms of the rule of mixtures and other theoretical estimations.     

The microstructure of a Nicalon/SiC composite and fibre deformation in the composite

The microstructure of a Nicalon/SiC composite has been examined in detail using electron microscopy. The microstructure of the fibres does not change significantly during fabrication of the composite. It was found that there were two kinds of SiC grains in the matrix, polyhedral-shaped ones near to the fibres with a size ranging from 10–100 nm and columnar grains, further away from the fibres and up to 500 nm in length. It was also found that there is an interfacial layer of thickness about 100 nm between the fibres and the matrix. Fibre deformation was followed using Raman spectroscopy. Well-defined Raman spectra could be obtained from free-standing fibres and the fibres and the matrix in the composite. It was found that for free-standing fibres, the 1350 cm^–1 Raman band shifts to lower frequency during tensile deformation. In the composite, the same band shifts to lower frequency during axial compressive deformation indicating that the fibres are in tension while the composite is subjected to overall compression. It is suggested that this behaviour is consistent with the woven arrangement of Nicalon fibres in the composite.     

A microstructural study of silicon carbide fibres through the use of Raman microscopy

The microstructures of three different silicon carbide (SiC) fibres produced by CVD (chemical vapour deposition) have been examined in detail using Raman microscopy. Raman spectra were mapped out across the entire cross-sections of these silicon carbide fibres using an automated x-y stage with a spatial resolution of 1 m. The Raman maps clearly illustrate the variations in microstructure in such types of silicon carbide fibres. It appears that the SCS-type fibres contain carbon as well as SiC whereas the Sigma 1140+ fibre also contains free silicon. Furthermore, the differences in the detailed structures of the carbon and silicon carbide present in the fibres can also be investigated. Raman microscopy is demonstrated to be a very sensitive technique for characterising the composition and microstructure of CVD silicon carbide fibres prepared using different processing conditions.     

Analysis of the deformation of gel-spun polyethylene fibres using Raman spectroscopy

Raman spectroscopy has been used to investigate molecular deformation in a number of high-performance gel-spun polyethylene (PE) fibres. Well-defined Raman spectra can be obtained for the fibres and the study has concentrated upon the symmetric C-C stretching mode (1128 cm^–1). During mechanical deformation, the Raman spectra show the existence of a bimodal molecular stress distribution in the crystalline phase resulting in splitting of the Raman band. The changes in the Raman band peak position and area with strain for different modes of deformation, including stress relaxation and creep, have indicated the molecular response of the material to stress is highly complicated. This information, however, is particularly useful in analysing the molecular deformation both quantitatively and qualitatively and it is shown that the Young's modulus of the fibres is related to both the relative areas of the two Raman bands and their rate of shift per unit strain.     

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.     

Molecular deformation processes in gel-spun polyethylene fibres

The structure/property relationships in the PE fibres have been interpreted quantitatively using a microfibrillar model and the low-strain mechanical properties have been analysed using the Takayanagi models. Information obtained from Raman spectroscopy in the previous paper has been analysed to determine the molecular deformation behaviour of the gel-spun polyethylene (PE) fibres. It is demonstrated that there is a bimodal distribution of stress in the crystalline regions due to the two-phase microstructure of the fibres and it has been shown that the molecular deformation behaviour can be interpreted quantitatively using a parallel-series model. It is found that the Young's modulus of the crystalline regions increases with the degree of chain extension and for the highest-modulus fibres may be close to the theoretical modulus of polyethylene. The fibre modulus is reduced by the presence of low-modulus non-crystalline material in parallel with the crystals.     

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.     

The effective Young's modulus of carbon nanotubes in composites

A detailed study has been undertaken of the efficiency of reinforcement in nanocomposites consisting of single-walled carbon nanotubes (SWNTs) in poly(vinyl alcohol) (PVA). Nanocomposite fibers have been prepared by electrospinning and their behavior has been compared with nanocomposite films of the same composition. Stress transfer from the polymer matrix to the nanotubes has been followed from stress-induced Raman band shifts, which are shown to be controlled by both geometric factors such as the angles between the nanotube axis, the stressing direction and the direction of laser polarization, and by finite length effects and bundling. A theory has been developed that takes into account all of these factors and enables the behavior of the different forms of nanocomposite, both fibers and films, to be compared in different polarization configurations. The effective modulus of the SWNTs has been found to be in the range 530-700 GPa which, while being impressive, is lower than the generally accepted value of around 1000 GPa as a result of factors such as finite length effects and nanotube bundling. This value of effective modulus has, however, been shown to be consistent with the contribution of nanotubes to the 20% increase in Young's modulus found for the nanocomposite films with a loading of only 0.2% of SWNTs. Hence a self-consistent method has been developed which enables the efficiency of reinforcement by nanotubes, and potentially other high-aspect-ratio nanoparticles, to be evaluated from stress-induced Raman bands shifts in nanocomposites independent of the specimen geometry and laser polarization configuration.     

Analysis of the fragmentation test for carbon-fibre/epoxy model composites by means of Raman spectroscopy

The interfaces between high-modulus PAN-(T50) and mesophase pitch-based (P55) carbon fibres and an epoxy matrix have been studied by using the conventional fragmentation test in conjunction with polarised-light optical microscopy. Raman spectroscopy has also been used to follow stress transfer from the matrix to the fibres for the same fragmentation geometries. The level of fibre/matrix adhesion and mechanisms by which the stress is transfered from the matrix to the fibres has been determined from both the stress birefringence patterns and strain-induced Raman band shifts in the fibres. The values of interfacial shear strength have been determined by means of both the conventional analysis and the Raman technique. It is found that the Raman method gives a much more detailed picture of stress transfer in the test specimens and that the two methods give somewhat different values of the interfacial shear strength. The values of interfacial shear stress have been discussed with respect to fibre surface energy, surface chemistry and surface morphology. It was found that the surface chemical functional groups appear to have no direct correlation with interfacial shear strength. Furthermore, it appears that mechanical interlocking due to surface roughness could contribute to the higher values of interfacial shear strength determined for the PAN-based fibre.     

Deposition and characterization of diamond-like nanocomposite coatings grown by plasma enhanced chemical vapour deposition over different substrate materials

Diamond-like nanocomposite (DLN) coatings have been deposited over different substrates used for biomedical applications by plasma-enhanced chemical vapour deposition (PECVD). DLN has an interconnecting network of amorphous hydrogenated carbon and quartz-like oxygenated silicon. Raman spectroscopy, Fourier transform–infra red (FT–IR) spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD) have been used for structural characterization. Typical DLN growth rate is about 1  ${\upmu} $ m/h, measured by stylus profilometer. Due to the presence of quartz-like Si:O in the structure, it is found to have very good adhesive property with all the substrates. The adhesion strength found to be as high as 0·6 N on SS 316 L steel substrates by scratch testing method. The Young’s modulus and hardness have found to be 132 GPa and 14· 4 GPa, respectively. DLN coatings have wear factor in the order of 1 × 10^???7 mm ³ /N-m. This coating has found to be compatible with all important biomedical substrate materials and has successfully been deposited over Co–Cr alloy based knee implant of complex shape.     

Investigation of the graphitic microstructure in flake and spheroidal cast irons using Raman spectroscopy

It has been demonstrated that Raman spectroscopy is an excellent technique for the characterisation of the graphite in flake and spheroidal cast irons. The Raman spectrum of graphite is highly sensitive to both structural ordering and residual stresses as can be seen from the position, intensity and shape of the Raman bands. The relative intensities and widths of the G Raman bands for flake and spheroidal graphite have been compared and found to correspond to the degree of graphitisation across the graphite. The two Raman bands of the G doublet (G₁ and G₂) change in intensity and width with distance across a graphite spheroid, although their positions remain approximately constant. In contrast, the change in intensity and width of the two Raman bands of the G doublet for flake graphite shows no discernible pattern leading to the conclusion that there is no systematic change of graphitic ordering along a graphite flake. Finally, the fact that the positions of the G₁ and G₂ Raman bands for both flake and spheroidal graphite remain relatively constant with distance implies that there are no residual stresses in the graphite in flake and spheroidal cast iron.     

Nanoscale uniaxial pressure effect of a carbon nanotube bundle on tip-enhanced near-field Raman spectra

In situ measurement of tip-enhanced near-field Raman spectra of an isolated single-wall carbon nanotube (SWNT) bundle has been demonstrated by applying a uniaxial pressure up to approximately 2 GPa to the bundle via a metal-coated atomic force microscope tip. We investigated the pressure dependences of Raman frequencies and the intensity of the radial breathing mode bands, the D-band and the G-band, which were related to deformation of SWNTs caused by the tip pressure.