Experimental Study On Propagation And Coalescence Of Open Cracks Under Uniaxial Compressive Loads

Fractured rock mass is a kind of complicated engineering geological body,studyon the initiation,propagation and coalescence of pre-existing flaws has importantpractical significance to the engineering construction. In the past of model test study,the geometrical distribution of fissures in specimens are often designed as singlecollinear or echelon type, however, specimens containing other spatial distribution offissures have not been studied comprehensively.In order to realize the diversification of the spatial distribution of the original cracks,three different thickness of steel were used to simulate the crack aperture, while thethird fissure was changed by45°increasing each conversion; and then completed thecrack samples preparation and maintenance. The compression test system (MTS) isadopted to accomplish the uniaxial compression of fissure specimens and obtained thecorresponding mechanics data, while the crack initiation and coalescence dynamicinformation was recorded by HG-100K high speed dynamic analysis system. Byanalyzing the experimental data and image information, mainly obtained the followingresults：⑴Despite the geometric distribution and the aperture of fissures are constantlychanging, it was found that the wing-crack initiation still was the mainly crack initiationmode,but the type,location and number of the crack initiated from the tips ofprefabricated fissures were greater influenced by the varied fracture aperture and thechanging geometric distribution.⑵For the coalescence mode between fissures, there were three mainlycoalescence modes-tensile mode,shear mode and tensile-shear mixing mode occurredin the specimens fissure containing,but the aperture and layout of fissures wereprimarily affecting the number of coalescence cracks,where they initiated and whattype of them;we also found the pure shear coalescence mode only appeared inspecimens containing collinear fissures.⑶As the fissure aperture was constant, specimens whether contained echelonfisssures or collinear fisssures, the peak strength was showed a wave changing;and thepeak strength was showed a smaller value when the fissure angle β was45°and135°,and often in the angle of135°reached the minimum value; while the peak strain and thepeak stress was almost one-to-one correspondence. ⑷When the fissure angle β is constant,the specimens’ peak strength decreasedwith the increase of fissure aperture,and the change trend of the peak strain wasconsistent with the peak stress.⑸For the crack iniation stress of specimens,with the increase of crack aperture,the smaller the crack initiation stress level, which is more close to its correspondingpeak stress,while the changing of crack inclination effected on initiation stress was notvery obvious.