We have also performed a series of fracture toughness measurements on heat-treated 60 mm diameter short-rod specimens of Icelandic basalt using the ISRM recommended methodology. The results are presented in Figure 21.
The fracture toughness value of 2.71 for the samples that were not heat-treated compares well with previously published data for very similar materials . The fracture toughness remains essentially constant up to 400C, but there is then a very rapid decrease in fracture resistance between 400C and 600C, with relatively little change between 600C and the highest heat-treatment temperature of 900C. This pattern of behaviour is entirely consistent with the wave velocity data. These experiments were conducted in air with rapid loading, so that environmentally-assisted subcritical crack growth was not a significant factor. Any reduction of fracture resistance must, therefore, be due to changes in the microstructure of the rock due to thermal cracking.
We have observed very similar trends in measurements of the indirect Brazilian tensile strength and Young's modulus for heat-treated basalt samples. These other results are not reported here due to space constraints.