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Research sheds light on the effects of thermal shock and strain rate on the properties of granite

Percussive drilling is the most effective method for excavating, tunnelling and shallow well boring through hard rocks. However, even more robust methods are needed to perform demanding operations, such as drill 5,000-metre geothermal wells.

The doctoral dissertation of MSc Ahmad Mardoukhi explores potential solutions for enhancing the efficiency of the percussive drilling method.

“The biggest problem with improving percussive drilling is that there are too many variables affecting the process,” says Ahmad Mardoukhi.

Previous research on the area has placed much emphasis on drilling equipment and largely ignored the rock material itself. However, Mardoukhi’s research focuses on altering the properties of the rock material. For this reason, he developed a new method for estimating the mechanical properties of rocks before and after applying thermal shocks.

Understanding the behaviour of rocks

Rocks and especially igneous rocks are very hard materials. Breaking them is costly, time consuming, and extremely challenging. The current drilling technology is based on rotary and impact drilling. According to Mr. Mardoukhi, to design better drilling tools, we need to know the strength of the intended rock and the drilling conditions. Since experimental testing under deep drilling conditions is highly complicated due to both scientific and practical reasons, various numerical simulations and constitutive models are currently being used to better understand the rock-tool interactions.

“However, the development of these models and the validation of simulation results rely both on a sound scientific understanding of the material behaviour and on the gathering of reliable experimental data to calibrate the parameters of the material models,” Mardoukhi emphasizes.

“What is needed is a considerable amount of data to develop a model that includes all the affecting parameters and a reliable testing method to achieve consistent results.”

Mardoukhi applied thermal shocks on the rock samples using a flame torch and plasma gun. These thermal shocks lead to the cracking of the rock because of the rapid heating and cooling, which results in a weaker rock structure. The application of thermal shocks, especially using the plasma gun, proved to be an efficient method to decrease the strength of the rock. With a plasma heat-shock, the strength of the rock decreases by about 20% in less than a second. Additionally, an imaged based method was developed based on the fractal dimension analysis of the cracks. The results suggest that there is a correlation between the strength of the rock and the change in the fractal dimension of the rock before and after applying the heat shock.

“And finally we developed a technique to characterize the mechanical behaviour of the rock by quantitative image-based analysis of the microcracks of the material,” Ahmad Mardoukhi says.

Public defence of a doctoral dissertation on Friday, 6 October

MSc Ahmad Mardoukhi will publicly defend his doctoral thesis titled “Effects of microstructural features, thermal shocks and strain rate on the mechanical response of granitic rocks” on Friday 6 October 2017 at 12:00 at Tampere University of Technology in the Konetalo lecture hall 1702. Professor Leopold Kruszka (Military University of Technology of Warsaw, Poland) and Professor Ezio Cadoni (University of Applied Sciences and Arts of Southern Switzerland, Switzerland) will act as opponents. Professor Veli-Tapani Kuokkala from the Laboratory of Materials Science will act as Chairman.

Ahmad Mardoukhi comes from Tehran, Iran, and works as a doctoral student in the Laboratory of Materials Science at Tampere University of Technology.

The dissertation is available online at:

Further information: Ahmad Mardoukhi, tel. +358 46 627 8477,

News submitted by: Sanna Kähkönen
Keywords: science and research, materials science