Experimental Study Of Fracture Characteristics Of Dense Sandstone Via Acoustic Emission And Digital Image Correlation

The fracture process zone（FPZ）,an inelastic region near the fracture tip,causes the fracture behavior to deviate from the linear-elastic fracture mechanics theory after damage occurs in rocky quasi-brittle materials.The study of FPZ is essential for the evaluation of fracture problems and the failure forecast.The complete and clear monitoring of macroscopic fracture process zones and the precise determination of fracture characteristics are challenging challenges in laboratory research both nationally and internationally.The softening behaviour of the cracked tips of sandstone materials at laboratory scales makes experimental loading of sandstone under uniaxial tensile conditions difficult.Therefore,in this study,a reasonable three-point bending experimental scheme is designed instead of uniaxial tensile experiments,and three-point bending loading experiments are implemented on sandstone specimens to obtain pure mode I cracks and mixed-mode cracks under experimental conditions.The fracture model of the crack tip was adopted as a cohesive model,based on which the crack was assumed to be a cohesive model containing the elastic deformation zone,the fracture process zone and the traction-free crack zone;the softening phenomenon of the crack tip was described by a bilinear TSL curve,and the fracture characteristic parameters of the curve included the critical opening displacement w_cand the fracture dissipation energy G_F.The length evolution pattern of the FPZ is difficult to obtain accurately by conventional measurement techniques,therefore,while experimentally loaded,acoustic and optical monitoring techniques are combined to achieve the location and energy capture of microcrack locations in the local damage area of the specimen using acoustic emission（AE）techniques,and the continuous real-time change curve of the length of the FPZ during the whole loading process of each specimen is obtained by an improved acoustic emission cluster analysis algorithm;based on the moment of full development of the FPZ determined by the length change curve,accurate measurement results are obtained for the local opening displacement of the material crack tip via digital image correlation（DIC）method.The results of the study show that the evolution of FPZ under the monitoring of acoustic emission technique goes through four stages:sprouting,development,upward translation away from the prefabricated cracks,stopping development and forming traction-free cracks.The FPZ in mode I cracks is fully developed at around 90%of the post-peak load,with a stable length of 17 mm-19 mm,while the FPZ in mixed-mode composite cracks is fully developed at around 95%of the post-peak load,with an elongated length of 20 mm-23 mm.wc shows a difference of about 15μm.Evaluation of the numerical results of the G_Fderived from the opening displacements against the total acoustic emission energy E_aeshows that the bilinear TSL model is applicable to both mode I and mixed-mode cracks.