Damage Characteristics And Dynamic Response Of Sandstone In Coal Fire Area Under Impact Loading

The coal fire burning area is formed by self-combustion of unmined underground coal seams,which is widely existed in Ningxia,Gansu,Xinjiang and other regions.Due to the complex geological structure and immature fire extinguishing conditions,there are still some potential safety problems of rock in Rujigou coalfield fire area of Ningxia.For the safety of blasting,water injection method is usually used to extinguish and cool the fire at the coal fire area,and the rock characteristics around the coal fire area will inevitably change.If the design and construction are carried out according to the original mechanical parameters,it is easy to cause safety accidents.Therefore,it is very important to understand and master the physical and mechanical properties of rocks in coal fire area,especially the dynamic mechanical properties.In this paper,the sandstone collected from the fire area of Rujigou coalfield in Ningxia is taken as the research object.The damage characteristics and dynamic response of sandstone in coal fire area under impact loading are systematically studied by using split Hopkinson pressure bar(SHPB),X-ray diffraction(XRD)test technology,high-speed photography and digital image correlation(DIC)technology.The following is a summary of the main works finished.(1)The physical and mechanical properties of sandstone in coal fire area were studied.The density,longitudinal wave velocity,porosity and mineral composition were measured.The effects of rock integrity and heat treatment temperature on the static properties of sandstone were studied.The results show that when the longitudinal wave velocity is basically the same,the static peak stress of high-temperature sandstone decreases with the increase of heat treatment temperature.When the heat treatment temperature is consistent,the better the integrity and quality of the rock,the greater the static peak stress.(2)The SHPB with the spindle-shaped punch dynamic impact test of sandstone in coal fire area was carried out when the impact loading changed(the impact pressure is 0.4MPa,0.8MPa and 1.2MPa).The dynamic characteristics were analyzed,and the dynamic constitutive models at different strain rates were studied.The SHPB dynamic impact test of sandstone under different impact loads was verified by threedimensional numerical simulation.The results show that the dynamic peak stress and dynamic elastic modulus of sandstone increase with the increase of impact pressure.When sandstone is loaded with different strain rates at room temperature,its dynamic peak stress increases exponentially with the increase of strain rate.The failure mode of sandstone in coal fire area is analyzed.It is concluded that when the impact pressure is small,the failure of sandstone is mainly splitting,’X’ shear type or conical failure.When the impact pressure is large,the failure is mainly sporadic fragments.The energy consumption law of sandstone in coal fire area is analyzed.It is concluded that the incident energy of sandstone in coal fire area increases with the increase of impact pressure,and the reflection energy,rock crushing absorption energy and dynamic peak stress increase with the increase of incident energy.In addition,it is also found that the unit volume absorption energy of sandstone specimens after different high temperatures increases with the increase of strain rate during dynamic impact.(3)The dynamic and damage characteristics of sandstone in coal fire area were studied by high-speed photography and the SHPB with cylindrical punch when the impact loading changed(the impact pressure is 0.58MPa,0.60MPa and 0.62MPa).The results show that the strain rate of sandstone increases with the increase of temperature under the same impact pressure.When the sandstone is at the same heat treatment temperature,the strain rate of sandstone increases with the increase of impact pressure.The impact pressure is set to 0.6MPa,when the room temperature and the heat treatment temperature is 200℃,400℃,600℃,the damage cracks of the specimens appear on the side surface;when the heat treatment temperature of the specimen is 800℃ and 1000℃,the end face of the specimen is broken.Based on DIC technology,it can be concluded that the evolution law of strain field of sandstone specimens after different high temperatures is different in the loading process.When the temperature is below 800℃,with the increase of heat treatment temperature,the initial damage of sandstone gradually increases,and the dynamic characteristics of rock specimen have a close correlation with the crack initiation point and extreme point.When the temperature exceeds 800℃,the rock specimen is seriously damaged,the brittleness is large,the overall strain is small,and the stress transfer efficiency is low.(4)SHPB dynamic impact test of sandstone in coal fire area under different cooling methods(natural cooling and watering cooling)were carried out to study its dynamic characteristics,and the dynamic constitutive models at different high temperatures were studied.The results show that under the two cooling conditions,the dynamic peak strength of sandstone increases with the increase of longitudinal wave velocity(Vp),and decreases with the increase of heat treatment temperature,but the damage degree increases with the increase of heat treatment temperature.The dynamic compressive strength of naturally cooled sandstone is higher than that of water-cooled sandstone.The failure of natural cooling hightemperature sandstone mainly occurs "X-shape",transverse splitting failure or breaks into pieces,but the watering cooling high-temperature sandstone presents transverse or longitudinal splitting failure.Based on the damage factor defined by the elastic modulus,the analysis of its damage characteristics shows that with the increase of heat treatment temperature,the static and dynamic thermal damage factors will also increase,indicating that the higher the heat treatment temperature,the more serious the damage of sandstone caused by high temperature.(5)The SHPB dynamic impact test of sandstone in coal fire area under cyclic impact loading was carried out,and its constitutive model,dynamic characteristics and energy consumption law were studied.The results show that the peak stress and elastic modulus of sandstone decrease with the increase of impact times,and the peak strain increases with the increase of impact times at the same heat treatment temperature.The failure modes of sandstone specimens at different heat treatment temperatures are similar,that is,the macroscopic cracks on the surface of the specimen first appear on the side surface of the specimen,and the longitudinal cracks expand with the increase of impact times,and the degree of fragmentation increases with the increase of impact times.In addition,the dissipated energy ratio of sandstone specimens increases with the increase of impact times.