Abstract
To investigate the effects of preexisting flaws with different geometries, including flaw inclination angle and ligament angle on dynamic strength, deformation properties, and fracture evolution of rock materials, a series of dynamic impact tests were conducted on green sandstone specimens containing double elliptical flaws using a 75 mm diameter split Hopkinson pressure bar (SHPB) testing device with a high-speed camera recording in real time. The experimental results show that dynamic strength of specimens with different flaw angles is reduced between 5.91% and 39.92% but from 18.50% to 28.44% for specimens with different ligament angles, indicating that the effect of the flaw angle on the dynamic strength is more significant than that of the ligament angle. However, the dynamic deformation properties are influenced greatly by the ligament angle. Macroscale cracks mostly initiate at or near the flaw tips and then propagate in different paths with varying flaw geometries, leading to the ultimate failure in five typical modes based on the crack coalescence. Shear crack coalescence and tensile crack coalescence are identified through both macroscopic fracturing photos taken by the high-speed camera and microscopic surface morphology obtained by the scanning electron microscope (SEM).
Highlights
Rock mass is a complex natural medium containing various defects, such as flaws, fissures, pores, and holes, where new cracks always initiate, propagate, and coalesce with each other into macroscale fractures, leading to the rapid damage and failure to lower the stability of rock structures such as underground mining craven and tunneling [1,2,3,4].erefore, the investigation of mechanical properties and fracturing process of brittle rock materials with preexisting defects is helpful to guide the underground excavation scientifically.ere have been a large number of experimental and simulative works carried out to study the crack initiation, propagation, and coalescence of rock and rock-like materials with preexisting flaws under static loading
For a single crack-like flaw, wing cracks first generate at the flaw tips and grow along the axial stress direction, and subsequently, secondary cracks emerge around the flaw tip and propagate in the coplanar or oblique direction with the preexisting flaw [5,6,7,8,9,10]
Bobet and Einstein [15], Wong and Chau [16], Sagong and Bobet [17], Yang et al [18, 19], Park and Bobet [20], and Gratchev et al [21] investigated the influence of flaw geometries, including flaw number, length, width, frictional coefficient, inclination angle, and ligament angle, on mechanical properties, crack initiation, fracturing process, coalescence, and ultimate failure
Summary
Rock mass is a complex natural medium containing various defects, such as flaws, fissures, pores, and holes, where new cracks always initiate, propagate, and coalesce with each other into macroscale fractures, leading to the rapid damage and failure to lower the stability of rock structures such as underground mining craven and tunneling [1,2,3,4]. Ere have been a large number of experimental and simulative works carried out to study the crack initiation, propagation, and coalescence of rock and rock-like materials with preexisting flaws under static loading. AE measuring method, photographic monitoring technique, and PFC numerical simulation were conducted by Huang et al [31] to study cracking process and failure modes by using granite specimens containing three preexisting holes, showing that they were all in very good agreement with each other. Shear cracks initiate earlier and dominate the propagation process of single crack-like awed specimens under a high loading rate by numerical simulation [33] and experimental tests [10]. Dynamic impact tests are performed using prismatic green sandstone specimens to investigate the e ect of aw inclination angle and ligament angle on dynamic mechanic properties and fracture mechanisms with a high-speed camera recording in real time
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