The End Effect Of Rock Strength And The Experimental Determination Of Poisson's Ratio

The mechanical and deformation parameters of rock are the basic basis of engineering design,and the test measurement is the most basic technical problem.Although the relevant specifications prescribe or recommend methods for testing and determining general parameters,there are still many special problems in the actual project,which requires the use of special size test pieces,so it is impossible to apply the standard practices.In the multiaxial test(biaxial or true triaxial test)of rock,the end condition has a significant influence on the test strength of rock.In order to reveal the influence of the end conditions,the uniaxial compression tests of marble and red sandstone are carried out.Four different cushion layers and nine working conditions are used to study the influence of end conditions on rock test strength.The test results show that the cushion characteristics have a significant impact on the test strength and failure characteristics.The difference between the maximum test strength and the minimum test strength is more than 3 times.When the thickness is the same,the greater the rigidity of the cushion,the greater the test strength of the tested part;for the same cushion,when the cushion rigidity is large,the test strength has no great relationship with the thickness of the cushion.When the cushion transverse deformation modulus is small,the smaller the cushion thickness,the greater the test strength of the tested part;the cushion transverse deformation energy When the force decreases(stiffness increases or thickness decreases),the failure mode changes from cleavage failure to shear failure.In order to quantitatively analyze the end friction effect,a comparative test was carried out on a cylindrical specimen with a height diameter ratio of 2.5(it can be approximately considered that there is no end effect),and two dimensionless parameters,relative strength and equivalent lateral deformation capacity,were defined.Based on the relative strength and equivalent lateral deformation capacity,the influence rules of different end deformation conditions on the test strength were given.The static strain gauge was used to measure the strain of some specimens at different positions.The transverse deformation of specimens at different heights(2mm polytetrafluoroethylene)under typical cushion conditions was given through the analysis of digital image correlation(DIC)deformation field.The mechanism of eliminating the end friction effect is explained from the point of view of matching the lateral deformation characteristics of cushion and rock.The test results provide a reference for the reasonable evaluation of the reliability of the test results and a new solution for eliminating the end friction effect.Poisson's ratio is a very important deformation parameter.However,compared with the deformation modulus,strength and other indicators,there are only a few literatures on the measurement and value of Poisson's ratio of rock.In order to explore the change law of Poisson's ratio,the Poisson's ratio of various rocks is analyzed according to the code.It is found that both secant Poisson's ratio and average Poisson's ratio increase with the increase of stress level.It is questionable to regard Poisson's ratio as elastic constant.When the central point is fixed as stress of 50% uniaxial compressive strength,the size of the stress range has little effect on the average Poisson's ratio,and this value is not affected by the nonlinear deformation at the beginning of loading.Compared with the secant Poisson's ratio,it is more suitable to describe the axial and radial deformation relationship of hard rock.The average Poisson's ratio of soft rock often exceeds the theoretical limit value at a lower stress level,and its testing and value taking methods need to be further explored.The viewpoint of this paper has a strong reference significance for the test,value and use of Poisson's ratio.