Microscopic Experiments On Fracture Process Of Mode ? Crack In Rock Under Different Loading Rates

Loading rate is one of the important factors affecting sandstone fracture characteristics.It is of great significance to study the sandstone mode I fracture process at different loading rates in meso scale in view of the failure form of sandstone mode I fault.In this paper,the digital speckle correlation method is used to calculate and analyze the obtained mesoscopic images by means of the three-point bending loading test of red sandstone rock samples and the comprehensive use of acoustic emission monitoring and high-speed microscopic observation.Acoustic emission activity and mesoscopic fracture characteristics of red sandstone during mode I fracture under different loading rates are studied in depth.The results are as follows:Using the loading system of sandstone mechanics test,the high speed micro monitoring system(long working distance microscope and high speed camera)and the acoustic emission monitoring system,the three point bending mesoscopic test of sandstone mode I fracture at different loading rates was carried out.The characteristics of intensity of mode I fracture,the characteristics of acoustic emission ringing counting and acoustic emission energy under different loading rates are obtained,and the stage of acoustic emission activity is divided under different loading rates.In this paper,the initial crack loading law determined by acoustic emission at different loading rates is studied,and the mechanical and acoustic emission characteristics of sandstone mode I fracture under different loading rates are revealed.Based on the results of three-point bending test of sandstone mode I fracture at different loading rates,the fracture parameters of sandstone mode I fracture were measured by using integrated master control system(KH-8700)with built-in solidification software.The characteristics of crack length,crack area,crack width and crack propagation angle in prefabricated notch tip region under different loading rates are systematically analyzed.The characteristics of crack micromorphology at different loading rates under peak loading state are compared,and the crack propagation characteristics of red sandstone mode I fracture at different loading rates at meso scale are revealed.Based on the DIC technique,the mesoscopic images of the mode I fracture process of red sandstone under different loading rates are calculated.The horizontal displacement and strain cloud images are obtained.It is considered that the dense intersection of the contour lines of horizontal displacement means the initiation of cracks at this location.The fracture initiation load law of sandstone mode I fracture determined by acoustic emission technique and DIC technique at different loading rates is compared and analyzed,and the effect of loading rate on the initiation rate of sandstone fracture at mesoscale scale is further studied.The results show that the lower the loading rate is,the slower the initial load is determined by the DIC technique,and the closer the initial crack load is determined with the increase of loading rate,when the loading rate is larger,the lower the loading rate is,the higher the loading rate is.It can be concluded that the initiation loads are basically the same,and the pattern I fracture initiation characteristics of red sandstone under different loading rates are deeply revealed.