Dynamic Fracture Of Sandstone:Crack Velocity,Acceleration,Dynamic Propagation Toughness

“The concrete structure design is based on the tensile strength,which is more reasonable than that based on the compression strength”,said by Karihaloo,an international famous expert in the field of fracture mechanics.It was published on the50^th anniversary issue of International Journal of Fracture.Both rock and concrete are the quasi-brittle material,by which it is difficult to study the tensile strength.The test methods about rock tensile strength and fracture toughness,especially dynamic tensile strength and dynamic fracture toughness,are hot topic in solid mechanics and other natural sciences.In addition,the dynamic crack extension process at the varying speed is a hot issue at the world.However,it is still difficult to study accelerated velocity of crack propagation due to unmature precise measurement technology of high-speed crack propagation.Especially for the transparent materials including rock,there are still huge challenges in rock crack propagation at varying speed.This paper focused on the dynamic fracture behavior of mode-I?opening mode?in suchmaterials as rock,and explored the effective sample of mode-II?shearing mode?preliminarily.High attention was paid to the movement of rapidly expanding crack tip,such as dynamic crack initiation,dynamic crack propagation,acceleration and deceleration in the process of crack propagation.Abandoning the usual method of constant-speed crack expanding,the changes in velocity and acceleration during crack propagation were studied.The stress wave affecting the crack propagation speed was monitored.The particles near rapidly expanding crack tip were affected by the stress concentration zone.This paper was divided into two parts:?1?The research of rock fracture mainly focus on mode-I?opening mode?.However,the rock failure which is closely related to the great disasters in nature is mainly shear failure.The current research achievements is far less than mode-I.Based on the previous researches,the crack location of square plate specimen was improved,and the bias single cleavage drilled compression?BSCDC?was proposed.Finite element method was used to analyze stress distribution for the different geometric parameters of BSCDC specimen,and BSCDC specimen with different crack deviation distance was calibrated using dimensionless stress intensity factor.The suggested size of BSCDC specimens were offered for the dynamic fracture experiments of mixed Mode I-II and pure shear fracture.?2?The crack velocity of dynamic fracture is oscillating or changing.Many scholars regard this phenomenon as the accuracy error of measuring instruments,and still regard crack propagation process as a constant-velocity expansion.Although some results of mechanical properties can be obtained simply,key influential factors are neglected.In order to study the mode-I dynamic fracture process comprehensively,particularly the relationship among velocity,acceleration and dynamic fracture toughness during crack propagation,a new specimen—a Semi-Circular Hole Disk Bend?SCHDB?was developed.The Hopkinson pressure bar was used to impact the SCHDB to achieve mode-I dynamic fracture.The detailed crack propagation process was monitored using the strain gauge and crack extension meter guled on the specimen's surface.In the experiments,after crack initiation,the crack propagation speed increased to a critical value,and then oscillated up and down affected by the reflection stress wave.The experimental-numerical-analytical method was employed to obtain the dynamic stress intensity factor K_I^d?t?of the extended cracks.The time history of K_I^d?t?increased first,and decreased,then increased.It was found that the dynamic propagation toughness during deceleration stagewas greater than the dynamic propagation toughness during acceleration stage even if the crack growth speed was the same.Based on the theoretical findings of Freund and Fineberg,we proposed the energy release rate equation affected by the crack propagation velocity and acceleration,and verified the relationship between the observed acceleration and dynamic propagation toughness in the experiments.