Research On Mechanical Properties Of Damage Sandstone Under "Air Dry-Immersion" Circulation Function

Rock failure process is a development process of its internal cumulative damage. Inthe process of dynamic action, a lot of damage formed in the "shattering mountain" and"shook loosening mountain", which will failure in the later action of rainfall andreservoir level fluctuation. It is necessary for the study of the mechanical propertiesdegradation law and degradation mechanism of damaged sample under the "airdry-immersion" cyclical action. On basis of theoretical analysis, a "air dry-immersion"cyclical test of damaged rock sample and direct soaked rock sample has been carried out,which considered the time effect as well as the fluctuation of reservoir level. In the test,a detailed study has been carried on the mechanical properties,“air dry-immersion”degradation mechanism and the constitutive model considering the initial dynamicdamage of the two groups of rock sample. Major results are as the following:(1) There is a good correlation between the compressive strength, density value ofthe sample and longitudinal wave velocity. The ultimate strength can be corrected byartificial method and wave velocity-density synthesis method to amend the initialstrength of the sample to the same level, which can control the discreteness of sampleitself effectively, so as to better reflect the test rules.(2) The initial damage of rock sample can be simulated through the cyclic loading.Three methods such as elastic modulus method, the modified elastic modulus methodand energy method have been used to calculate the damage variable, and the calculationresults shows that: the calculation result of elastic modulus method is quite small, thecalculation result of energy method is close to that of the modified elastic modulusmethod, however the calculate accuracy of the modified elastic modulus method isrelatively high.(3) Stress-strain curves of the two groups rock samples under different confiningpressures and different cycles are basically consistent, however with the increase ofcycle numbers, the compaction segment and yield failure period become longersignificantly, and the ductility of rock sample become bigger, as well as the soft trend.What’s more, compare with the direct soaked rock sample, the damaged rock samplesare softer under the same confining pressures and same cycles.(4)"Air dry-immersion" cyclical action has a cumulative effect to rock damage. The compressive strength, tensile strength, elastic modulus,c、φ, and dynamic elastic modulus of the two groups are falling with the "air dry-immersion" cyclical action, at the meanwhile the poisson’s ratio, damping coefficient, damping ratio, damage variable and the strain under different confining pressure increased gradually. The change rules of the previously mentioned parameters, which can be better fitted by y=y0-a×ln(1+b×Nc). At the same time, in the same condition all the mechanical parameters of damaged rock samples are weaker than the direct soaked rock sample.(5) The effect order of the "air dry-immersion" cyclical action on rock dynamic mechanics parameters is:damage variable> damping ratio> damping coefficient dynamic elasticity modulus; while the effect order of the "air dry-immersion" cyclical action on rock static mechanics parameters is:uniaxial compressive strength> tensile strength>triaxial compressive strength>elastic modulus>cohesive force> poisson’s ratio>friction angle.(6) Constitutive model which considered the initial dynamic damage and "air dry-immersion" cyclical action are in good agreement with the test curve. Both m and F0reduced with the increase of "air dry-immersion" cycles. At the meanwhile, both m and F0of damaged rock sample are less than that of the direct soaking rock sample.