Mechanical properties of alumina Nextel™ 720 fibres at room and elevated temperatures:: tensile bundle testing

Tensile tests on alumina Nextel 720(TM) fibre bundles were performed and results were analysed using Weibull statistics to calculate the tensile strength and elastic modulus of the fibres as a function of temperature and gauge length. A compliance-correction technique was used to deduce the net fibre strain from the recorded experimental data separately for room and high-temperature tests; the latter tests were performed under inert (argon) gas environments. The Weibull parameters, calculated in terms of strain values, yielded the mean failure strain and the initial compliances yielded the elastic moduli. Both parameters allowed to obtain the mean fibre strength. Statistical analysis was performed to provide a high and low confidence limit for both the Weibull modulus and the strength. Fibre strength was projected to a reference gauge length of 25-mm using the Weibull gauge length correction to allow for comparison with results from single-fibre tests. Room-temperature tests showed that gauge lengths larger than 75 mm are associated with a non-smooth fibre failure regime that can be explained by the larger amount of energy stored in the tested volume at the onset of failure. Results from relevant tests yielded an elastic modulus of 260 GPa and a Weibull modulus of 5.6, 20% less than the value reported by the manufacturer. Fibre strength was substantially less than the value quoted by the manufacturer for both room and high-temperatures; the room-temperature value corresponding to only 53% of the value reported. The observed divergence is a direct effect of the fundamental characteristic of the fibre bundle testing technique which, contrary to the single-fibre test used in the reference studies, is associated with minimal fibre handling and damage and, as such, is believed to fully assess the fibre strength distribution by not eliminating weaker fibres from the population before testing. (C) 2002 Elsevier Science B.V. All rights reserved.