Mechanical damage characteristics of granite with different cooling methods

The physical and mechanical properties, acoustic emission characteristics, mineral evolution and microstructure changes of high-temperature granite are studied by the acoustic emission monitoring in Brazil splitting test under two conditions: natural cooling and water cooling.The results show that the tensile strength, P-wave velocity, elastic modulus and Poisson's ratio of granite under the two cooling methods all decrease with the increase of temperature, and the decrease is greater when the granite is cooled by water, and the physical and mechanical properties of granite change significantly at 500-600 ℃ due to the phase transition of quartz.The AE(acoustic emission) characteristics(AE counts, AE energy evolution and variation of AE b-value) in the failure process of granite are basically consistent with each stage of stress-time curve.The AE activity is stronger when the granite is cooled by water and the b-value of AE is lower than that of the sample under natural cooling, which shows that the granite is mainly damaged by large-scale cracks when it is cooled by water.The granite shows tensile failure at low temperature and high temperature, tensile and shear failure at medium temperature.Moreover, the failure mode of sample is more complex when it is cooled by water.After granite heated at high temperature, the main mineral composition of granite do not change obviously, among which the content of feldspar and quartz change greatly.With the increase of temperature, the number, opening width, extension range, connectivity and extension length of the internal micro-cracks including intergranular crack, transgranular crack, and detachment defect under the two cooling methods all change significantly, and the samples cooled by water at high temperature suffer the most serious damage.The damage degree of granite gradually increases with the increase of internal cracks.The damage mechanism of rapid cooling of high-temperature granite is mainly the combined action of dehydration, mineral phase transition and thermal stress, and the essential characteristic is the weakening effect of micro-cracks caused by thermal stress.