Experimental Investigation of Microstructure-Related Scale Effect on Tensile Failure of Coal
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Original Paper
Experimental Investigation of Microstructure-Related Scale Effect on Tensile Failure of Coal Honghua Song,1 Yixin Zhao ,1,3,5 Jiehao Wang,4 and Yaodong Jiang2,3 Received 17 August 2020; accepted 31 October 2020
We explored the microstructure-related scale effect on the tensile failure of coal. Disc specimens with different diameters (25 mm, 38 mm, 50 mm, and 75 mm) were processed for the Brazil split test. The influence of microstructures on fracture initiation and propagation was detected and analysed by the combination of X-ray computed tomography scanning, digital image correlation approach, and acoustic emission monitoring. The investigation indicated that tensile strength decreased with increasing specimen diameter, and the relationship can be described by an exponential equation. Four tensile failure modes were observed in specimens with different diameters, namely central, non-central, central edge, and central multiple pattern. Larger specimens exhibited more complicated failure patterns (central edge and central multiple failure patterns) and a greater percentage of shear failure fractures. Strain concentration and strain reversal were observed, respectively, in mineral—inclusion-rich regions and pre-existing discontinuities regions. This possibly contributed to fracture initiation, propagation, and coalescence. Fractures tended to grow along with the interface of mineral inclusion and coal matrix, and they could have propagated into and connect the pre-existing discontinuities. The greater volume of microstructure in larger specimens resulted in fractures that were more complex and may have increased the number of shear failure cracks and led to failure modes that were more complicated. KEY WORDS: Microstructure, Scale effect, Tensile failure, X-ray CT, Digital image correlation, Acoustic emission.
INTRODUCTION Tensile strength, which controls fracture initiation and failure development (Diederichs and Kaiser 1999; Perras and Diederichs 2014), is a critical 1
School of Energy and Mining Engineering, China University of Mining and Technology, Beijing 100083, China. 2 School of Mechanics and Civil Engineering, China University of Mining and Technology, Beijing 100083, China. 3 State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, China. 4 Energy and Mineral Engineering, G3 Center, EMS Energy Institute, Pennsylvania State University, University Park, PA 16802, USA. 5 To whom correspondence should be addressed; e-mail: [email protected]
parameter for the analysis of stability of coal and rock mass (Minh Phong Luong 1990; Dai and Xia 2010; Ko¨ken 2020). As an indispensable support material in mining activities (Jaiswal and Shrivastva 2009; Poulsen et al. 2014; Shaojie et al. 2016), coal mass has attracted extensive researches whereby its strength and failure character were measured largely under laboratory conditions with small-scale samples (Gonzatti et al. 2014). However, the application of these laboratory-measured properties
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