Fracture toughness of heat-treated Icelandic basalt

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.

  

Figure 21: Variation in tensile fracture toughness for Icelandic basalt.

The fracture toughness value of 2.71 $MPa/m^\frac{1}{2}$ for the samples that were not heat-treated compares well with previously published data for very similar materials [39]. The fracture toughness remains essentially constant up to 400$^\circ$C, but there is then a very rapid decrease in fracture resistance between 400$^\circ$C and 600$^\circ$C, with relatively little change between 600$^\circ$C and the highest heat-treatment temperature of 900$^\circ$C. 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.