In this paper, the feasibility of a thermally assisted drilling method is investigated. The working principle of this method is based on the weakening effect of a flame-jet to enhance the drilling performance of conventional, mechanical drilling. To investigate its effectiveness, we study rock weakening after rapid, localized flame-jet heating of Rorschach sandstone and Central Aare granite. We perform experiments on rock strength after flame treatments in comparison to oven heating, for temperatures up to 650 • C and heating rates from 0.17 to 20 • C/s. The material hardening, commonly observed at moderate temperatures after oven treatments, can be suppressed by flame heating the material at high heating rates. Our study highlights the influence of the heating rate on the mechanism of thermal microcracking. High heating rate, flame treatments appear to mostly induce cracks at the grain boundaries, opposed to slow oven treatments, where also a considerable number of intragranular cracks are found. Herewith, we postulate that at low heating rates, thermal expansion stresses cause the observed thermal cracking. In contrast, at higher heating rates, thermal cracking is dominated by the stress concentrations caused by high thermal gradients.

Thermal diffusivity, heat capacity, and thermal conductivity of Central Aare granite are reported in the temperature range from 25°C to 500°C. Each rock sample underwent three consecutive heating and cooling cycles. Significant irreversible changes in the properties due to thermal crack formation could be observed. After the first thermal cycle, both thermal diffusivity and conductivity dropped to about 75% of the initial value at room temperature, whereas the heat capacity did not show an irreversible decay. For subsequent thermal cycles, no further permanent changes of the investigated properties could be observed. From the conducted measurements, accurate correlations are derived, offering a platform for precise high‐temperature experiments and other research on Central Aare granite and similar granitic rocks. The report shows that the assumption of constant thermal properties leads to significant inaccuracies at elevated temperatures, especially if thermal cycles are present.

Thermal cracking of rocks is an intensively studied topic in different research areas and for various engineering problems. In this context, we present a modeling approach which describes thermal spalling of rocks with a focus on thermal spallation drilling. This drilling technology uses high thermal loads to locally destruct the surface of the rock formation. With the presented model, the operating conditions which are required to initiate spalling of rocks can be estimated. Additionally, a Spallability number is introduced allowing a categorization of rocks according to their ability to spall. The presented model is based on linear fracture mechanics and the stress intensity concept. It evaluates if a crack with a certain geometry embedded in a rock with specific properties propagates during exposure to different heat loads and external pressures. Thereby, rapid heat transfer processes are coupled with induced thermal stresses and fracture mechanics in rocks.

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