Experimental Study On Determination Of Dynamic Fracture Toughness Of Marble Using P-CCNBD Specimens

Deep mining,tunnel excavation and other projects are related to the dynamic fracture of rock,the existence of dynamic crack initiation,rapid expansion and crack arrest.Rock dynamic fracture toughness is one of basic mechanical parameters of rock dynamic mechanics,which is composed of dynamic initiation toughness and dynamic propagation toughness correspondingly used to describe the capability of rock resisting the crack initiation and propagation subjected to dynamic loading respectively.Consequently the accurate determination of rock dynamic fracture toughness has become a hot-but-hard problem in this field.The large-diameter(160 mm)Pre-Cracked Chevron Notched Brazilian Disc(P-CCNBD)specimens of Nanyang marble were impacted by large-diameter(100 mm)split Hopkinson pressure bar.The crack initiation time and crack propagating velocity were sequentially measured with strain gauge and crack propagation gauge glued near the crack tip of the specimen.The results show that the crack initiation time near the incident bar is earlier than the crack initiation time near the transmission bar.And the specimens are fractured by multiple stress wave loading,under the low air-pressure loading condition(0.12~0.13MPa)for the projectile of SHPB,and the striking velocity of the projectile is 3.0~3.5 m/s.Meanwhile the crack arrest phenomenon of high speed crack is captured.The dynamic stress intensity factor at both ends of P-CCNBD specimen was calculated by ANSYS.The results show that the dynamic stress intensity factor of P-CCNBD increases gradually along the center of crack tip.The dynamic stress intensity factor increases greatly in the region of the crack near the edge of the disk.It is suggested that when the fracture toughness is calculated the average of the remaining values as the dynamic fracture toughness of P-CCNBD specimens,the values of the edge crack should be canceled.The P-CCNBD specimen overcomes the shortcomings of the dynamic crack initiation and propagation mode of the CCNBD specimen.It is more convenient to measure the dynamic fracture initiation and propagation toughness of Mode I.The large-size specimens do not satisfy the stress equilibrium condition in the dynamic test,so,we adopted the experimental-numerical-analytical method.The experimental data,including dynamic loads applied to the specimen by Hopkinson pressure bar,were inputted to three dimensional finite element analysis program for dynamic numerical simulation of P-CCNBD,in order to obtain the time history of dynamic stress intensity factor was obtained.A semi-analytical modification with the universal function was applied.The results show that the dynamic propagation toughness of P-CCNBD marble specimen was slightly lower than the dynamic initiation toughness,and the dynamic propagation toughness increased with crack propagation velocity.In addition,crack propagation velocity oscillation and propagation path deflection were analyzed,and the possibility of achieving crack arrest using P-CCNBD was discussed.