Study On Fracture Process And Criterion Of Rocks Induced By Non-penetrating Flaw

In rock engineering,the sudden and unpredictability of rock rupture often cause serious casualties and economic losses.It has been shown that rock rupture is mainly caused by propagation and coalescence of cracks,which are usually non-penetrating.Therefore,the study on crack propagation and coalescence of non-penetrating flaws in rocks or rock-like materials has important theoretical significance and engineering value for revealing macroscopic nonlinear mechancal behaviour of rock fracture process,establishing effective failure criterion and explainng the engineering phenomenon caused by crack propagation and coalescence.This paper mainly investigated rock fracture process caused by propagtion and coalescence of nonpenetrating flaws,the features and characteristics of crack propagation and coalescence in fracture process was analyzed,and the failure criterion of rocks was studied,and the propagation and coalescence process of flaws and landslides of high slope of jointed rocks were simulated.The research contributions are as follow:(1)Based on linear elastic fracture theory,stress field of non-penetrating flaw tip in uniaxial compression was analyzed,critical condition of non-penetrating flaw propagation was obtained,and critical curved surface of three dimensional frature tensile stress criterion was discussed,and the curved surface equation was derived.Besides,the surface path equation of wing crack was also obtained through inverse proportional function.(2)In the physical test,two similar types of wing cracks and four types of coalsecence cracks were observed.From specimen surface,the patterns and features of crack propagation and coalescence of penetrating and non-penetrating falws are similar.In specimen inside,coalescence between the non-penetrating flaws is produced by wing cracks,secondary cracks and petal cracks.The curved surface of wing crack is smooth whereas the curved surface of secondary crack is uneven and roughness.In addition,inclination angle of pre-existing flaw has a certain influence on the initiation and propagation of wing and secondary cracks.When the pre-existing flaws are parallel,a small inclination angle is good for new crack initiation whereas a large inclination angle may lead to splitting failure of the specimen in uniaxial conmpression.(3)The flaw depth ratio(d/t,d is the depth of pre-existing flaw,and t is the thickness of specimen)and spacing of pre-existing flaws had an obvious influence on the peak load of specimen.When the flaw depth ratio is 1/3,the variance of specimen peak loads is 3.114,and the peak load decresed with the increase of spacing as a whole.When the flaw depth ratio is 2/3,the variance of specimen peak loads is 2.154,and the peak load fluctuated around 6.8 kN with the increase of spacing.When the flaw depth ratio is 1,the variance of specimen peak loads is 0.398,and the peak load increased with the increase of spacing.(4)Secondary crack usually initiated after the occurrence of wing crack.The drop of loaddisplacment curve caused by generation of tensile crack was large whereas the drop caused by initiation and propagation of secondary crack was small.In some cases,the curve rose with initiation and propagation of secondary crack.Besides,in softening stage,stepped decline of load-displacment curve occurred,which is consistent with the initiation and propagation of secondary crack.When specimen fractured completely,spalling at or near the tip of pre-existing flaw was observed,which is closely related to the propagation and coalescence of secondary crack,and on the specimen surface one or more tensile cracks were observed,which usually coalesced with secondary crack when specimen ruptured completely.(5)Relationship between residual load(RL)and peak load(PL)was obtained when specimen was failure completely.When the flaw depth ratios of pre-existing flaws(d/t)are same:1)when the flaw depth ratio is between 0 and 1/3,RL with PL less than 0.70 times;2)when the flaw depth ratio is between 1/3 and 2/3,RL with PL less than 0.55 times;3)when the flaw depth ratio is between 2/3 and 1,RL with PL less than 0.45 times;When the flaw depth ratios of pre-existing flaws(d/t)are different:RL with PL less than 0.50 times.Furthermore,the criterion of rock failure induced by non-penetrating flaws was proposed according to the residual bearing capacity and peak bearing capacity.(6)Mechanism of step-path failure in rock slopes was investigated,as well as stability analysis of heigh rock slopes induced by step-path failure.The results showed that step-path failure was usually caused by wing cracks(or tensile cracks)when the preexisting flaws were isolated or when two preexisting flaws were adjacent.With an increase in the number of flaws,step-path failure was more likely to be caused by mixed wing-secondary crack formation(or tensile-shear crack formation).In addition,the stability of hight slope landslides caused by steppath failure was analyzed.(7)Based on ABAQUS,Python script was written,and combined with the user subroutine,the damage evolution process was brought into the calculation,and simulation and analysis of crack propagation and coalescence during rock farcture were realized.The simulation results are in good agreement with the experimental results and previous simulation results.