Papers by Author: Dieter Loidl

Abstract: 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.

Abstract: Unidirectionally reinforced ceramic matrix composites (CMC) with matrix derived by pyrolysis of polymethylsiloxane or polymethylphenylsiloxane resins and reinforced with Nicalon or Nextel fibres were investigated. In order to assess and mutually compare their mechanical properties and stability when exposed to hot air up to 1200°C their elasticity moduli were measured prior to and after this exposure. The oxidation - induced damage of the material was monitored also in terms of microstructural features. Moreover, an attempt was undertaken to assess the bonding level by monitoring the push - through of macroscopic bundles of fibres using a flat indentor.

Abstract: Creep tests have been performed for carbon fibre bundles (Toray, HTA5131) in a temperature range from 1500 °C to 1800 °C and at various stress levels in a tensile testing machine in vacuum (pressure was kept below 10-3 mbar). Creep parameters have been obtained from the diagrams elongation versus time. X-rays (SAXS and WAXD) have been used for accompanying structural investigations. An increase in the alignment of the graphene sheets with respect to the fibre (and thus the loading) axis was observed with increasing test temperature. For fibres stabilised by heat treatment (2 hours, 2100 °C, without load) no creep and no structural change could be observed.