A Direct Identification Of The Cohesive Crack In Sandstone By Acoustic Emission And Imaging Technology

Influences of the fracture process zone(FPZ)pose a challenge to accurately determine the FPZ and traction-free crack during fracture process in a rock structure.In this study,an engineering approach based on concepts of cohesive zone model,is proposed to identify them in a sandstone rock beam under three-point bending.Two types of specimens were tested including center notch and smooth boundary beams.Acoustic emission(AE)and digital image correlation(DIC)were used to monitor local material behaviors,which involve two crucial parameters: AE energy and DIC opening displacement pattern.(1)AE energy has been classified into three levels,where AE energy of detected events within Level 1 is 1 or 2 order larger than Level 2 and 3.Thus,only AE locations within Level 1 are considered to be inside the FPZ because other AE events release small AE energy and pose little influences on fracture process.(2)There are 4 types of DIC opening displacement patterns: pattern(I)is related to tensile loading of the material;pattern(II)suggests the possible development of material softening within the FPZ;pattern(III)indicates the possible onset of a traction-free crack;pattern(IV)confirms the material softening and unloading.These patterns together with different reference loadings are used to identify the FPZ and traction-free crack.With the help of two parameters,the precise kinematics of the FPZ and traction-free crack in the specimen are determined,and the fracture properties such as the FPZ length and critical opening displacement are also reported.Finally,experimental results provide insights to understand opening-mode fracture in sandstone.