Test Method For Tensile Strength And Fracture Toughness Of Sandstone

The test method of tensile strength and fracture toughness of rock has been the frontier research topic of natural sciences such as solid mechanics.As early as 1978,the International Society for Rock Mechanics(ISRM)determined the method for obtaining uniaxial tensile strength of rock in the form of a standard: Brazilian splitting test and direct tensile test,and corresponding Specimen size standards and calculation formulas.For the testing of static fracture toughness of rock materials,ISRM also proposed four recommended sample configurations and test methods.However,due to the complexity of sample preparation,the limitation of sample loading mode,and the lack of precision of monitoring instruments and equipment,relevant scholars are still making unremitting research on the ideal samples for testing tensile strength and fracture toughness of rock.Based on the analysis of Brazilian discs and subsequent improved specimens proposed by relevant scholars,this paper firstly proposes a new sample configuration for determining the tensile strength and type I static fracture toughness of brittle rocks in a simple uniaxial compression process.The research in this paper is mainly divided into two parts:(1)Since the focus of this paper is to propose a new configuration sample for testing the tensile strength and fracture toughness of brittle rock,this paper first studied three improved configuration of Brazilian discs.The platform was introduced into the Brazilian disc to obtain the flatten Brazilian disc.Based on the Griffith strength criterion,the effective range of the platform loading angle of the flatten Brazilian disc was re-determined,and the flatten Brazilian disc with the central straight crack of different platform angles was more accurate.The dimensionless stress intensity factor of the platform is calibrated,and the calculation formula of the tensile strength and type I static fracture toughness of the flatten Brazilian disk is given.The grooved was introduced into the Brazilian disc to obtain the grooved Brazilian disc,and the uniaxial compression experiment of the grooved Brazilian disc was carried out to verify the effectiveness of the configuration of the grooved Brazilian disc specimen proposed by the relevant scholars.The central hole is introduced into the platform Brazilian disc.The introduction of the platform prevents the end from being damaged.The introduction of the center hole causes the stress concentration at the center hole of the sample to ensure the cracking in the center of the sample.According to the Griffith strength criterion,the Griffith equivalent stress at the intersection of the sample loading diameter and the central hole of different loading platform angles is calculated,and the condition of central cracking is guaranteed again.The critical crack length and the largest dimensionless stress intensity factor of the holed Brazilian disc under different loading platform angles and aperture ratios are calibrated.The calculation formula of type I static fracture toughness and the calculation formula of tensile strength based on non-local stress method are given.Finally,uniaxial compression tests were carried out on three different configuration samples(FBD,GBD,HFBD),and the tensile strength and I-type static fracture toughness results of three configurations were obtained.(2)The introduction of the loading platform,the grooved and the hole are all aimed at changing the stress distribution inside the sample configuration,so as to obtain the tensile strength and fracture toughness of the brittle rock by simple uniaxial loading.On the basis of the holed Brazilian disc configuration,the arc is straightened and a new configuration is proposed: Drilled Compression(DC).The results of finite element stress analysis show that DC is firstly destroyed at the central hole under uniaxial compression,which verifies the effectiveness of the new specimen center cracking.The finite element software is used to numerically calibrate the type I dimensionless stress intensity factor Y of DC.The calculation formula for the determination of type I static fracture toughness of DC is given.Based on non-local stress method and TCD,two formulas for determining the tensile strength are given.The uniaxial compression test was carried out on DC sandstone specimens,and the fracture toughness and tensile strength of DC specimens were determined and compared with other configuration specimens to verify the effectiveness of newly proposed DC.Finally,based on the Bazant fracture toughness scale law formula,the author gives the geometric shape function of DC,and uses the function to obtain the true fracture toughness value of the test rock sample and the effective size of the fracture process zone,eliminating size effect of DC.The test results show that with the newly proposed DC configuration,reasonable brittle rock tensile strength and fracture toughness can be obtained from a simple and effective uniaxial compression experiment.