The Infuence Of T-stress On Rock Fracture Toughness And Crack Initial Angle

It is a very important problem that how to avoid the crack growth in the engineering materials such as rock,concrete and so on.A large number of engineering practice shows that the exploitation of shale gas,oil,coal bed gas and other energy resources is closely related to the expansion of internal cracks.On the other hand,the instability failure of some rock engineering is closely related to the initiation and propagation of cracks.Therefore,the study of rock fracture resistance,crack initiation and expansion and so on,is of great significance to oil and gas engineering and rock engineering design and construction.The stress intensity factor was used to describe the singular stress field at the crack tip.Recent studies show that the high order singular crack tip field is also very important,especially the second non singular T stress,and Williams series constant in parallel to the crack on both sides of the stress,it is considered to be the significant influence of crack tip stress and strain field parameters.Therefore,the traditional fracture criterion can not be used as an accurate fracture criterion to predict the crack propagation,and the more reliable results can be obtained by using the T stress criterion.This paper first through three points to calculate the different support point spacing,crack angle,crack length by using the finite element software ABAQUS bending half disc specimen fracture parameters,and through the center of Brazil round straight crack numerical solution compared with the analytical solution,confirmed the reliability of numerical calculation;and then introduces the half disk processing process,processing size and three point bending device,using 50kN material test machine on disc specimens obtained shale,the initial fracture toughness of Longmaxi shale and crack initiation angle;finally,the maximum circumferential stress criterion and the traditional one to consider the generalized maximum stress T the results are compared with the experimental results to predict the stress criterion,verify the correctness of theoretical analysis.Through the research,the conclusions and results are as follows:(1)the specific support interval of S cases,the dimensionless stress intensity factor KI increases with the relative crack length;specific crack length,the dimensionless stress intensity factor KI increases with increasing the distance between support points;relative crack length,the dimensionless stress intensity factor increases more and if the relative crack length is too large,the error will increase the fracture toughness test results;this paper proposed the relative crack length is between 0.2～0.6.When the relative crack length is less than or equal to 0.5 a/R,the dimensionless T stress were negative;dimensionless T stress with the increase of relative crack length,decreases first and then increases;(2)the selection of shale compressional wave slowness distributed between 200 mu m/s～300 mu m/s,S-wave sonic distribution between 300 mu m/s～400 mu m/s;different crack angles of shale fracture load distribution in the range of 3.27kN～10.92kN,with the increase of crack angle,semicircular disk bearing shale sample maximum load increases gradually,Longmaxi shale of pure Ⅰ fracture toughness for 1.13～1.38 MPa v m,pure Ⅱ fracture toughness for 0.55～0.62 MPa root m,the variation range of different angle crack rock T stress-0.49～9.48MPa;(3)for pure Ⅰ type fracture,without considering the T stress condition,the maximum circumferential stress criterion of Ⅰ type fracture,crack propagation will occur and overlap fracture plane,considering the T stress conditions,this situation does not exist;the traditional MTS criterion did not take effect consider the T stress of the predicted initiation angle there is a big error,especially when more error;prediction of fracture toughness also has great error,especially dominant in KⅡ when the error is bigger;for the compound fracture toughness,crack angle and fracture toughness depend on T stress the size of negative T stress will make the expansion angle decreased,the fracture toughness will increase,on the contrary,the T stress will make the expansion angle increases,the fracture toughness will decrease;(4)the classical fracture criterion considers only the singular terms of the Williams expansion,neglecting the higher order coefficients,which leads to a large error between the predicted results and the measured values.In consideration of T stress on the basis of the establishment of the generalized maximum circumferential stress criterion(GMTS),and compared with the traditional MTS criterion,study the effect of T stress on crack propagation;then based on the GMTS criterion of different inclination of sandstone specimens crack initiation angle and fracture toughness were predicted,and the results are compared with the test value.The results showed that T stress on the crack stress intensity factor influence angle and the critical stress can not be ignored,especially the effect of Ⅱ type fracture is more significant,prediction of rock compound crack initial GMTS criterion can better spread angle and fracture toughness.According to the actual engineering problems,starting from the analysis of rock fracture toughness theory,seeking the best prediction method,numerical simulation of composite rock fracture toughness test and crack propagation angle,to promote the development of hydraulic fracturing of shale gas reservoirs and high quality.