Quantitative multi-scale characterization of single basalt fibres: Insights into strength loss mechanisms after thermal conditioning

This article presents an experimental investigation to quantify the effects of high temperature exposure (400-600 degrees C) on the mechanical properties of single basalt fibres. To this purpose, a combination of single edge notch tension and nanoindentation micro-pillar splitting methods was used to provide an assessment of the fracture toughness of as-received and thermally treated basalt fibres. Similar values were obtained by the two different methods, and interestingly both highlighted an increase in K-Ic after heat treatment, up to 22% after exposure at 600 degrees C for 1h (1.59 +/- 0.06 MPa root M). The increase in K-Ic suggests that microstructural changes occur in the fibres, as confirmed by high-speed nanoindentation mapping. Local radial heterogeneity in the fibre structure and elastic modulus and, possibly, the loss of defect orientation originally induced during the fibre drawing process are envisaged to control the decay of basalt fibres tensile strength during high temperature exposure, mimicking a thermal recycling process for composites.