Dieter Loidl

Texture of PAN- and pitch-based carbon fibers

The new technique of scanning microbeam X-ray diffraction is used to get information about the axial as well as the cross-sectional crystallographic texture of single PAN- and mesophase-pitch-based carbon fibers. The resulting preferred orientation in the axial direction is considerably higher than the values obtained from conventional X-ray diffraction measurements on fiber bundles. A change in azimuthal width of the 002 reflection was observed across some of the fibers, which can be attributed to a radial folded cross-sectional texture for pitch-based fibers, and to a different preferred orientation of skin and core layers for PAN-based fibers.

Elastic moduli of nanocrystallites in carbon fibers measured by in-situ X-ray microbeam diffraction

Abstract The in-plane Young’s modulus and the shear modulus of carbon nanocrystallites were investigated during in-situ tension tests of single carbon fibers by X-ray diffraction using the shift of the 10 band in the meridional direction and the change in the azimuthal width of the 002 reflection. The limiting value for the Young’s modulus was found to be 1140 GPa, which is higher than the value for graphite obtained from macroscopic specimens, but coincides with recent measurements on nanotubes. Furthermore, the shear modulus was evaluated using a uniform stress approach and was found to increase with increasing misorientation of the crystallites. It turns out that both the in-plane Young’s modulus and the shear modulus are not constant, but dependent on the orientation parameter.

The internal structure of single carbon fibers determined by simultaneous small- and wide-angle X-ray scattering

Simultaneous small-angle scattering and wide-angle diffraction using a synchrotron radiation microbeam was applied for the first time to investigate single carbon fibers in a position-resolved way. Taking into account the exact X-ray beam profile and examining the fibers in two scattering geometries allowed a discrimination between different models for the internal arrangement of carbon layers and pores. For a fiber based on polyacrylnitrile the carbon layers were randomly oriented within the fiber cross section, whereas in a mesophase-pitch based fiber the layers were arranged in a radial structure.

Bimodal strength distributions and flaw populations of ceramics and fibres

Abstract For the evaluation of the fracture strength data of brittle materials such as ceramics or ceramic fibres, the Weibull statistics has been widely applied. If the materials show a bimodal distribution of fracture strength values, different models (multiplicative bimodal, additive bimodal Weibull distributions and Gamma distributions) were used. With the preliminary assumption of the validity of weakest link and Griffith’s law, this work intends to show the different flaw probability and flaw probability density distributions, which are described by the respective models. These model descriptions are compared to two experimental examples, a ceramic material (recrystallised silicon carbide) and a carbon fibre.

Cross-sectional texture of carbon fibres analysed by scanning microbeam X-ray diffraction

Scanning microbeam X-ray diffraction analysis of single carbon fibres allows the cross-sectional orientation distribution (texture) of the carbon layers to be determined, even when the fibre axis is oriented perpendicular to the X-ray beam (fibre geometry). The fibre is scanned across a microbeam with a diameter significantly smaller than the fibre diameter, and fibre diffraction patterns are recorded for every scanning step. The cross-sectional texture information is obtained from the integrated intensities of two different equatorial reflections as a function of the position on the fibre. As an example, results from two different types of carbon fibres are presented: a polyacrylonitrile-based fibre, with random cross-sectional texture, and a fibre based on mesophase pitch, which exhibits a radially folded cross-sectional texture. Detailed modelling of the diffraction data allows a quantitative description of the radial folded texture.

Non-contacting strain measurements of ceramic and carbon single fibres by using the laser-speckle method

A non-contacting laser correlation sensor is used to determine the strain of different ceramic (alumina, silicon carbide) and carbon fibres in a single fibre tension test. The diameter of these fibres varies from 15 to only 5 mm, which makes the application of conventional strain sensors impossible. The advantage of the non-contacting method is that the strain is directly measured and end-effects from the gripping system need not to be considered. Results for different ceramic and carbon fibres with linear and non-linear stress ‐strain curves are presented as an example. q 2003 Elsevier Ltd. All rights reserved.

Hutchinson's shear coefficient for anisotropic beams

Timoshenkos theory of vibrating beams requires a shear correction factor to correctly take into account the effects of shear deformation for different beam cross-sections. This correction is crucial for a precise determination of the shear modulus from the resonant frequencies. Hutchinsons beam theory is used to derive a new shear correction coefficient for anisotropic materials. A comparison is made with other shear coefficients for anisotropic materials published in the literature. Computer-simulated spectra are used to validate the new anisotropic shear correction coefficient.

Structure and mechanical properties of carbon fibres: a review of recent microbeam diffraction studies with synchrotron radiation

Hard X-ray beams with beam sizes in the sub-micrometre range are frequently available at third-generation synchrotron radiation sources, enabling new insights into the structure of materials at different levels of hierarchy by applying novel techniques such as scanning microbeam diffraction or scanning small-angle scattering. The high brilliance of the radiation from wiggler and undulator sources allows in situ experiments such as mechanical testing of single fibres to be performed, and even the combination of microbeam scanning with in situ testing is feasible. Three different experiments on single carbon fibres are presented: an in situ tensile test using a 10 microm collimated beam, a scanning experiment applying a 3 microm beam from a tapered glass capillary, and a scanning experiment with simultaneous in situ bending with a 0.1 microm beam provided by a waveguide structure.

ON THE DETERMINATION OF ELASTIC MODULI WITH BRITTLE MATRIX COMPOSITES

ABSTRACT Composite materials behave in general elastically anisotropic. For the purpose of designing constructive parts under load by finite element calculations, the elastic moduli in dependence on orientation and on temperature have to be known. A short survey on practicable procedures for the determination of the elastic moduli, their applicability to composites and their extension to measurements at high temperatures are given. A new procedure, based on the Resonant Beam Technique, and its application to anisotropic composites at high temperatures is presented together with results gained from a 2,5D carbon fibre-reinforced carbon composite up to 1800 °C. Finally an outlook on a new procedure, a combination of Resonant Ultrasound Spectroscopy and Resonant Beam Technique is given.

Determination of elastic modules in dependence on orientation by the resonant beam technique

A novel procedure, based on the Resonant Beam Technique, and its application to anisotropic composites is presented. The evaluation of the elastic modules of anisotropic materials from the measurement of the transverse eigenfrequency spectra of resonant beams is performed by a two step process: firstly the beams cut out from the test material in different directions are evaluated in-dependently of each other under the assumption, that they are isotropic, solving Timoshenko´s equations using an isotropic correction factor for shear. Secondly the beams are evaluated together as representatives of one anisotropic material, using an anisotropic correction factor for shear. The equipment, developed for such measurements is presented. Finally, the procedure is applied to a transversely isotropic carbon fibre-reinforced carbon composite and the relevance of the results is discussed.