Acoustic Emission Study On Initiation And Propagation Of Mode Ⅰ And Mixed Mode Fractures In Sandstone

A nonlinear zone ahead of the crack tip,called the fracture process zone(FPZ),dominates the fracture initiation and propagation.There are two charactistics to understand the fracture process: the fully-develop FPZ length and its corresponding loading stage.However,due to the limitations of observation techniques and processing approaches,it is rare to have the complete curve of the FPZ development.Thus,it is a challenge to have the accurate measurments about the fully-develop FPZ length and its corresponding loading stage.In this study,a series of sandstone specimens were tested by three-point bending with the helps from acoustic emission.By processing the acoustic emission signal to locate the acoustic emission source,a series of the acoustic emission events with respect to the load stage is obtained.With the improved approach to analyze the AE cluster and determine the FPZ boundary,the complete curves of the FPZ development for mode I and mixed-mode fracture are obtained.The fully-develop FPZ length and its corresponding loading stage can be accuaretly measured.The main conclusions are:(1)The measured curves of the FPZ development match the theoretical prediction well,and the height and eccentricity of precast cracks have no effect on the development of FPZ,indicating the FPZ is a property of the material.(2)Based on the complete curve,the FPZ in the mode I fracture reaches its fulldeveloped stage when the external load passes the peak(at the post90%,meaning the load is at the 90% of peak in the post-peak regime).The observation confirms the declaration from Prof.Bazant at Northwestern University in USA: when the external load reaches its peak,there are residual stresses at the fracture tip such that the FPZ does not fully develop.Of course,the claim of Prof.Bazant is based on numerical inversion of global loading,and our observation is based on laboratory measurement of AE testing.(3)Based on the complete curve,the FPZ in the mixed-mode fracture reaches its full-developed stage when the external load is at the post98%.It indicates there may be also residual stresses at the fracture tip.By comparison of the mode I and mixed-mode fractures,it can claim that the local shear stress can cause the fully-deveoped FPZ to occur earlier.(4)The length of the mode I fully-developed FPZ is determined to be 17 mm,and the length of the mixed-mode fully-developed FPZ is determined to be 20 mm approximately.It indicates that the local shear stress may influence the length of the fullydevelop FPZ.However,because the difference of two measured values are quite small,it requires more experiments to help understand them.