Effects of microstructural features, thermal shocks and strain rate on the mechanical response of granitic rocks

Abstract

Percussive drilling is regarded as the most effective method for excavation, tunneling, and shallow well boring in the hard rock such as granite. However, its efficiency has been questioned in some specific environments and applications such as drilling for geothermal energy, where bores as deep as 5000 m are needed to reach the desired temperature zone. It is therefore understandable that attempts to drill bores that deep can face significant difficulties, and even though these difficulties have already been overcome by developing new techniques for deep drilling, there still are no replacement for the percussive drilling technique. The reason for this situation can be found in the shortage of technological readiness and in the nature of the rock and its behavior itself. However, in the previous attempts to find a replacement for percussive drilling, not enough of attention has been paid to altering the rock’s properties before drilling for example by using a thermal shock.<br/><br/>In this work, the mechanical behavior of the rocks before and after applying heat shocks was studied in quasi-static and dynamic loading conditions. Two different heat shocks were applied on the two studied rocks, one using a flame torch and one using a plasma gun. The heat shocks using the flame torch were applied on the Brazilian disc samples with durations of 10, 30, or 60 seconds. The thermal shocks using the plasma gun were applied on the Brazilian disc samples and on the bulk of the rock for dynamic indentation tests. Three different plasma gun heat shocks were applied on Brazilian disc samples with durations of 0.40, 0.55, or 0.80 second. The heat shocks applied on the bulk of the rock had a duration of 3, 4, and 6 seconds.<br/><br/>A methodology was developed to analyze and characterize the damage caused by the heat shocks on the surface of the specimens. In this method, a liquid penetrant was applied on the surface of the samples before and after applying the heat shocks with images taken from the specimens’ surface under an ultraviolet light. Later on, the fractal dimension of the surface crack patterns was calculated using the box counting method. The results indicate that the fractal dimension of the samples increases by increasing the duration of the thermal shock and there is a relationship between the relative increase of the fractal dimension and the mechanical response of the rock material. Even though the fractal dimension analysis is limited to the surface of the samples, the computed tomography results suggest that the effects of the heat shocks are also limited to the very surface of the specimens. Therefore, the fractal dimension analysis provides a fast and accurate enough estimation of the mechanical response of the rock.<br/><br/>The mechanical behavior of rock was studied at low and high strain rates using the Brazilian disc samples. The results indicate that by increasing the duration of the thermal shock, increasing the fractal dimension, the strength of the rock decreases in the studied strain rate range. Nonetheless, there are some differences in the rock mechanical behavior at low and high strain rates. The dynamic strength of the rock decreases considerably faster with increase of the fractal dimensions than the quasi-static strength. Therefore, the strain rate sensitivity of the rock decreases with the increasing fractal dimension.<br/><br/>The dynamic indentation tests were performed to study the effects of heat shocks in situations similar to percussive drilling. The tests were performed using both single and triple button indenters. Even though the direct measurements of the bit-rock interactions obtained from the stress waves are useful, they do not provide any information about the side chipping and chipping between the indenters. Therefore, optical profilometry was used to study the craters formed during the impacts, and the concept of destruction work was used to characterize the effects of the heat shocks on the material removal during dynamic indentation. The results imply that after applying the heat shock, the extent of material removal increases even though the force levels are not affected much. This means that the efficiency of the indentation processes cannot be evaluated only by using the force-displacement curves but additional analysis such as the ones used in this work are needed.