Experimental Study On Compressive Properties And Damage Fracture Of Red Sandstone Under Different Water Cut Processes

Due to the heavy rainfall,the rise and fall of reservoirs and rivers,rocks have different water content in different periods,and different water content parts may also have the same water content.At present,there are more researches on the mechanical properties of rocks in different water content states,and less on the mechanical laws of rocks in different parts,especially under the coupling of water content,strain rate and water content parts.Based on the National Natural Science Foundation of China(no.52064015),this paper studies the static and dynamic compression tests of red sandstone samples with different water contents and water bearing parts were carried out by using nuclear magnetic resonance(NMR),RMT-150C rock press and split Hopkinson pressure bar(SHPB).The mechanical responses of red sandstone under different water content,different strain rate and different water content parts are compared and analyzed,and the dynamic constitutive equations of red sandstone under different water content and strain rate coupling are established from the perspective of damage mechanics.The main conclusions are as follows:(1)In this paper,the composition of red sandstone samples was obtained by XRD technology,and the T₂ spectrum,water cut curve and NMR imaging of red sandstone samples with different water cut processes were obtained by NMR.It is found that the change of water content of red sandstone samples with time is in the form of logarithmic function,whether in the process of saturation or drying;for different samples,NMR has similar response,and the pore size distribution map contains two obvious peaks,one with pore radius of 0.1?m and the other with pore radius of 3.5?m The pore radius between 0.1?m accounts for 95%of the total pore;The results show that red sandstone samples with the same water content and different water content can be prepared after two different water content processes.(2)Compared with the drying process,the compaction time of saturated red sandstone is shorter under static compression,and the elastic stage becomes smaller.The failure mechanism of the dry sample in the compression process is as follows:directly cutting the mineral particles,because the strength of the mineral particles is far greater than that of the cementitious materials filled,the failure stress is high.The fracture mechanism of saturated samples under uniaxial compression is dominated by initial micro crack fracture and mineral boundary fracture;the softening effect of water reduces the cementation between mineral particles,and the crack propagation directly destroys the pore structure without shearing the mineral particles.(3)Under the water rock coupling,the softening effect of water is the main reason for the strength change of the sample.With the increase of strain rate,the incident energy increases fastest,the absorption energy and reflection energy increase slowly,and the transmission energy decreases gradually;there are two obvious inflection points between strain rate and energy ratio.When the strain rate is greater than 80 s^-1,the increase of absorptivity and reflectance increases and the decrease of trans missivity increases;when the strain rate is greater than 108 s^-1,the increase of absorptivity and reflectance slows down and the decrease of transmissivity slows down;under the same strain rate,the higher the water content,the higher the degree of rock fragmentation;under the same water content,with the increase of strain rate,the internal fracture surface of the sample is more and more,and the fragmentation increases.(4)A rate effect model based on saturation time is proposed,which can better describe the change trend of strength of red sandstone under different water content and strain rate.The calculation is more accurate under low strain rate and high strain rate.The higher the water content of sample is,the closer the calculation result is to the test value.The theoretical and experimental data fit well,which proves the applicability of the model.(5)From the point of view of damage mechanics,the damage variable under water bearing condition is defined by the change of elastic modulus in the stress-strain curve obtained from the test,and then the water bearing damage and load damage are regarded as macro damage and micro damage respectively.On the basis of Drucker Prager criterion,the composite damage variable based on the coupling of water content and strain rate is obtained.The dynamic constitutive equation of red sandstone under different water content and strain rate coupling is established,and compared with the actual stress-strain curve,it is found that the model can better describe the stress-strain curve of red sandstone under dynamic compression,and the calculation of elastic stage and elastic-plastic deformation stage is more accurate,which shows the effectiveness of the model.