| Shallow marine sedimentary quartz sandstone with mechanism of sedimentary is a kind of special soft rock which has complex constrLuction and mechanical property,it is wildly used in in transpotation,minging,hydraulic and many scientific and engineering problems come out.As a kind of the rock whose mechanical peoperty effect by many factor,shallow marine sedimentary quartz sandstone with high porosity and beding is primary specific in engineering and science.At the same time has a wide distribution in China and commonly used in engineering.It is sedimentary sandstone with beding from the view of microstructure and remarkable anisopropic.It is low strength and quasi-brittle soft rock from the view of lithology.The rock will begin to yield unter medium stresslevel,and mechanical property would be significantly changed.In this paper,we focuse on shallow marine sedimentary quartz sandstone,compression failure and tensile failure are studied that takes into account the anisotropy,stress path,yield condition and size effect.We got following result associate with failure mechanism comparison with the experimental test,mesoscopic optical observation and analysis techniques and dem numerical simulation:(1)To investigate the effect of bedding on fracture mode,elastic modulus and strength of rock with high porosity under different stress condition,5 different angles of beding with 9 stress path,totally 45 groups of experiment on different angle of specimen were tested with pre-hydrostatic compression and axial stress increasing mode.The result suggests that anisotropy quartz sandstone showed different characteristics of deformation damage under different stress condition.With 30MPa as dividing line of confining pressure,shear failure mode happened in the range of low confining pressure and as the confining pressure increased,fracture surface turned slope to horizontal and was influenced by the bedding.In range of high confining pressure,under the influence of bedding,fracture mode could be divided to shear yield and volumetric yield.As the confining pressure increased,the number of newly closed crack reduced,the effect of anisotropy to elastic modulus showed reducing trend and the average increasing rate of young’s modulus decreased.With 30MPa as dividing line of confining pressure,yonung’s modulus has different regular change.Feature coefficient of compressive strength for quartz sandstone with different angle of bedding decreases as the confining pressure increases,and keep constant as confining pressure was over 30MPa.(2)Based on test result,treat quartz sandstone with beding as transversely isotropic material.Consider the rock will begin to yield after 30MPa,coordinate transforming method were used to analyze the reason of regular changes as confining pressure and angles of beding change on young’s modulus and strength theoretically.The theoretical result fits test result very well.Deformation and failure characteristics and mechanism of fracture mechanics were studied in both theory and experiment.(3)According to this characteristic of high porosity,the conclusion of the rock will be volumetric yield and plastic deformation has been made,and confiremed by scanning electron microscope checking the rock microstructure.Based on above conclusion,yield condition of bedding weak surface and the most unfavorable position for waek surface were made by theoretical derivation.Discriminant result was made for whether rock or beding weak surface fall into plastic state under different stress path.(4)In the section of tesile fracture,improvements have been made for tensile test.Tensile test,brazilian test,monotonic and fatigue mode I fracture test were performed.Based on tensile test result,a group of geometrically similar sandstone specimens were tested in three point bending.The result shows that:specimen size has effect on fracture parameters,as the size of specimen increase,apparent tensile strength decrease,apparent fracture toughness and length of process zone decrease,three factors are the function of size.Based on test result,ELEFM was used for calculate fracture parameters such like length of process zone for rock material.The mechanism of fracture of sandstone was between perfectly plastic limit and perfectly brittle limit and size effect of quasi-brittle material were shown.In order to verify the correctness of ELEFM,the length of process zone were obtained by DIC test,experimental and theoretical results fits well.(5)For study the size of process zone which is the core index for judging size effect in mode I fracture problem,a two-dimensional hybrid discrete element program,CA2 specifically designed for geo-materials,was used to simulate the failure process.The model includes two kinds of bonded-particle contact and fit the rock fracture mechanism.Reasonable micro parameters were determined by compared with experimental test result.The effect of size of specimen on size of process zone was studied;Brittleness which can not be quantitative study was also investigated in numerical sumilation.The result of numerical sumilation fit the experimental well and shows that as the rock turns brittle to dunctile and the size of specimen increases,the size effect incrases.(6)The most well-known kineticse quation for fatigue crack growth is probably the Paris-Erdogan law,but this law fit on plastic material.It is found that a general Paris-Erdogan law could not be obtained throuth the mode I fatigue test result.For solve this peoblem,based on energy equivalence principle,an improved general Paris-Erdogan law was established by introduce a size parameter.The critical specimen size and limit process zone size value were obtained in case of fatigue fracture parameters which is not changes by the increases of the specimen size.Scaling of fatigue crack growth in rock was certified.At last,the correctness of improved Paris-Erdogan law was vetified by digital image correlation. |
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