N. Melanitis

Characterization of PAN-based carbon fibres with laser Raman spectroscopy

Laser Raman spectroscopy (LRS) has been employed to characterize the structure and morphology of a series of carbon fibres, to assess the combined effects of ultimate firing temperature (UFT) and pre-graphitization drawing during manufacture and, finally, to investigate the influence of oxidative treatment upon the integrity of the fibre surface. Ten types of PAN-based carbon fibres of varying modulus, diameter and manufacturing method, were examined. All their spectral features were recorded and analysed in terms of position, bandwidth and band intensity. Low-modulus fibres, produced at low graphitization temperatures, exhibit weak and broad Raman bands in the 1200–1700 cm−1 frequency region. With the increase of the firing temperature, the spectral features sharpen and new lines appear at higher frequencies. The observed changes in the Raman spectra are discussed in detail and related to alterations in the conditions of manufacture.

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.

Monitoring the micromechanics of reinforcement in carbon fibre/epoxy resin systems

The technique of laser Raman spectroscopy (LRS) was employed to obtain the interfacial shear stress (ISS) distribution along a short high-modulus carbon fibre embedded in epoxy resin at different levels of applied stress. Up to 0.6% applied strain, the ISS reached a maximum at the bonded fibre ends and decayed to zero at the middle of the fibre. At higher applied strains, the maximum value of the ISS distribution shifted away from the fibre ends towards the middle of the fibre. At the point of fibre fracture, fibre/matrix debonding was found to initiate at the fibre breaks. Further increase of applied strain resulted also in debonding initiation at the fibre ends. Current analytical stress-transfer models are reviewed in the light of the experimental data.

Interfacial studies on carbon/thermoplastic model composites using laser Raman spectroscopy

Interfacial studies were carried out on a model composite system consisting of a short carbon fibre embedded in a polycarbonate matrix. While the composite was being strained, the local strain along the fibre was monitored using a Raman spectroscopic technique. The residual compressive strain in the fibre due to fabrication was found to be −0.45%. Subsequent loading of the composite up to 0.55% in tension resulted in a complex stress field consisting of tension at the fibre ends and compression in the middle of the fibre. The fibre strain at different levels of applied load was converted to interfacial shear stress (ISS) distribution along the fibre by employing a simple equilibrium analysis. The shape of the ISS profiles indicated a predominantly frictional type of load transfer from the matrix to the fibre. Finally, the maximum ISS value of 15 MPa was found to be unaffected by the amount of strain experienced by the composite.

Residual strain mapping in carbon fibre/PEEK composites

Abstract The strain dependency of the Raman frequencies of carbon fibres has been used in order to map the residual fibre strains in a HMS4/PEEK composite tape. The fibres have been found to be subjected to residual compressive strains of the order of 0.2%. Such compressive strains are thought to be developed principally during manufacture.

Interfacial shear stress distribution in model composites: the effect of fibre modulus

Abstract The micromechanics of reinforcement have been investigated for a continuous intermediate-modulus (IM) carbon fibre embedded in an epoxy resin (MY-750). The embedded single-fibre (fragmentation) geometry was employed as the loading configuration. A laser Raman spectroscopic method was used to obtain the fibre strain distribution along the embedded fibre fragments, at various levels of applied strain. The interfacial shear stress distribution along the fibre was derived through a balance of forces analysis. A number of parameters, such as the maximum interfacial shear stress at each level of applied strain and the fibre debonded length, were evaluated. The maximum interfacial shear stress of the IM fibre system was found to increase by 80%, compared with the high-modulus fibre system examined previously, while the distance from the fibre end where the interfacial shear stress maximizes was significantly shorter. The debonded length was found to increase only marginally up to an applied strain of 1.8%, followed by a dramatic rate of increase between 1.8% and 2.5% of applied strain.

FRAGMENTATION STUDIES ON CARBON FIBRE / EPOXY SYSTEMS, USING LASER RAMAN SPECTROSCOPY.

Keywords: Interfacial Shear Stress (ISS); Interfacial Shear Strength (IFSS); Fragmentation test; Laser Raman Spectroscopy; Carbon Fibres