Study On Dynamic Mechanical Properties And Damage Mechanism Of Green Sandstone After High Temperature

In recent years,with the development of major national-level projects such as deep resource exploitation,The change of dynamic mechanical properties of rock after high temperature has become a hot topic for many scholars.The problem of changes in physical and mechanical properties of rocks after high temperatures has become a hot topic for researchers in rock mechanics.However,most of the results at home and abroad seldom combine the impact of dynamic load and microscopic morphology of rock to analyze its damage and failure characteristics.Therefore,ultrasonic test,SHPB test and SEM scanning test are carried out on the green sandstone specimens after high temperature to study the damage variables,The dynamic mechanical properties and fracture morphology of green sandstone under temperature and impact load were studied.Dynamic damage and failure mechanism of high temperature green sandstone are analyzed.The main research results and conclusions are as follows:(1)The P-wave velocity and density of green sandstone samples treated at different temperatures were measured by ultrasonic testing.The influence of temperature on the elastic modulus and damage variable of samples was analyzed.With the increase of heating temperature,the average density,average wave velocity and average elastic modulus of green sandstone samples showed a decreasing trend,however,the damage amount of green sandstone increases gradually.After that,with the increase of temperature again,the damage of green sandstone continues to increase,but its growth trend gradually slows down.(2)Hopkinson(SHPB)equipment was used to test the dynamic mechanical properties of green sandstone,samples after different high temperature treatments.The stress-strain curves of the samples were obtained,and the failure characteristics of the samples after impact were analyzed.The effects of temperature and impact pressure on the peak stress,The whole process of stress-strain curve changes slightly with temperature and impact pressure,and basically follows compaction stage,elastic stage,yield stage and failure stage;With the increase of temperature,the average peak stress of green sandstone samples decreases,and the peak stress decreases fastest in the range of 600??1000?.The temperature and impact load have great influence on the dynamic mechanical properties of green sandstone.The failure degree of specimens increases with the increase of temperature and inpact load.Both temperature and impact air pressure have a significant impact on the dynamic mechanical properties and damage degree of green sandstone.With the increase of temperature,the influence degree increases.Under different impact pressures,the dynamic mechanical properties of blue-green sandstone show obvious strain rate effect after high temperature.(3)The fracture morphology of green sandstone under different high temperature and impact load was observed by scanning electron microscope(SEM),the micro level failure mechanism of green sandstone was analyzed and studied: under impact load,the fracture morphology of high temperature green sandstone can be divided into cleavage fracture and intergranular fracture,in which cleavage fracture can be subdivided into river pattern,step pattern and tongue pattern In addition,there are also some secondary cracks.Under 400?,the separation of adsorbed water and interlayer water and the thermal expansion of clay minerals lead to the loss of part of the bearing structure,and the thermal expansion of clay minerals leads to the appearance of some microcracks in the samples,which together lead to the decrease of the strength of the samples,but the decrease is not significant;above600?,the strength of the samples decreases slightly The thermal expansion,structural dehydration,recrystallization and phase transformation of clay minerals are the main factors in the rock samples,and a large number of secondary fissures are produced in the samples,resulting in serious deterioration and significant decline of bearing capacity.(4)Combined with the physical parameters of specimen failure in macroscopic view and the observation and analysis of specimen fracture in microscopic view,the influence of impact load and temperature on cohesion in rock,damage degree of cement and mineral particles is discussed.With the increase of temperature,the cement and mineral particles may be connected due to phase transformation,which leads to the increase of cohesion in sandstone to some extent.Therefore,when the impact load is small(0.3MPa),the damage degree of green sandstone after high temperature is light,and the macro fracture characteristics are not obvious;But when the impact load is large enough(0.6MPa),the green sandstone will fracture and break into pieces,or even become powdery due to insufficient strength growth.