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Geomechanics and Engineering
  Volume 19, Number 1, September20 2019 , pages 49-60

Characteristics of EMR emitted by coal and rock with prefabricated cracks under uniaxial compression
Dazhao Song, Qiuju You, Enyuan Wang, Xiaoyan Song, Zhonghui Li, Liming Qiu and Sida Wang

    Crack instability propagation during coal and rock mass failure is the main reason for electromagnetic radiation (EMR) generation. However, original cracks on coal and rock mass are hard to study, making it complex to reveal EMR laws and mechanisms. In this paper, we prefabricated cracks of different inclinations in coal and rock samples as the analogues of the native cracks, carried out uniaxial compression experiments using these coal and rock samples, explored, the effects of the prefabricated cracks on EMR laws, and verified these laws by measuring the surface potential signals. The results show that prefabricated cracks are the main factor leading to the failure of coal and rock samples. When the inclination between the prefabricated crack and axial stress is smaller, the wing cracks occur first from the two tips of the prefabricated crack and expand to shear cracks or coplanar secondary cracks whose advance directions are coplanar or nearly coplanar with the prefabricated crack\'s direction. The sample failure is mainly due to the composited tensile and shear destructions of the wing cracks. When the inclination becomes bigger, the wing cracks appear at the early stage, extend to the direction of the maximum principal stress, and eventually run through both ends of the sample, resulting in the sample\'s tensile failure. The effect of prefabricated cracks of different inclinations on electromagnetic (EM) signals is different. For samples with prefabricated cracks of smaller inclination, EMR is mainly generated due to the variable motion of free charges generated due to crushing, friction, and slippage between the crack walls. For samples with larger inclination, EMR is generated due to friction and slippage in between the crack walls as well as the charge separation caused by tensile extension at the cracks\' tips before sample failure. These conclusions are further verified by the surface potential distribution during the loading process.
Key Words
    coal and rock; prefabricated crack; failure mode; electromagnetic radiation; surface potential
Dazhao Song,Liming Qiu and Sida Wang: School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China

Qiuju You: Beijing Research Center of Urban Systems Engineering, Beijing 100035, China

Enyuan Wang and Zhonghui Li: School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China

Xiaoyan Song: College of Applied Science and Technology, China University of Mining and Technology, Xuzhou 221116,

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