Tensile fracture propagation and acoustic emission activity in sandstone: The effect of loading rate

摘要

In three-point bending tests on sandstone cylinders, critical stress intensities and critical energy release rates for tensile fracturing are constant for a variation in COD between 5 X 10~(-6) to 5 X 10~(-10) m/s. Resulting macroscopic fracture propagation velocities range from 2 X 10~(-4) to 3 X 10~(-8) m/s. Macroscopic tensile fracture propagation in Flechtingen sandstone in the CB-test occurs at critical stress intensities even at very low loading rates. At low loading rates stress corrosion microcracking may contribute to fracture propagation. AE activity increases with increasing loading rates, suggesting that dynamic crack propagation on the grain scale is important. The FPZ width and the crack density remain constant in the range of applied loading rates, in good agreement with constant stress intensities. The shape of the process zone is elliptical with a length of about 25 mm and a width of about 5 mm. The source mechanisms of acoustic events indicate tensile- and shear-type events suggesting that tensile and shear microcracks contribute to macroscopic fracture propagation in Mode I loading.