| Rock fracture toughness is a ductile property of rock resistance to crack initiation and propagation fracture and it is the most basic parameter in rock fracture mechanics.Therefore,accurately measuring the rock fracture toughness is of great significance for the design of geotechnical engineering structures.Based on the different radius of NSCB specimens which are recommended by the International Society for Rock Mechanics(ISRM)for static fracture toughness testing in 2014,the influence of different width and length of artificial prefabricated crack on fracture toughness were studied by means of numerical simulation and laboratory measurement.The fracture process of NSCB specimens were monitored by means of strain gauge,acoustic emission and digital speckle.The size effect of fracture toughness of NSCB specimens was studied by Bazant scale law.The test principle of fracture toughness of NSCB specimen is introduced.Based on Bazant’s size effect theory,the dimensionless energy release rate expression of NSCB specimen is deduced and the method for obtaining fracture toughness scaling law formula is given.Finally,several methods to solve the stress intensity factor are introduced.Using finite element software ABAQUS to analyze the influence of prefabricated crack width,length and specimen radius on the dimensionless stress intensity factor of NSCB specimens.The results show that with the increase of prefabricated crack width,the dimensionless stress intensity factor of NSCB specimens are continuously increasing.And the prefabricated crack length is more longer,the increasing speed is more faster.However,the test support spacing does not substantially affect the increasing speed.When the value of dimensionless prefabricated crack is in the range of 0.1~0.8,with the increase of prefabricated crack length,the non-dimensional stress intensity factor of NSCB specimens decreases at first and then increases.The results of three-point bending test of NSCB specimens show that the fracture toughness of NSCB specimens tend to increase with the increase of pre-crack width.The radius of NSCB specimens are more smaller,the larger extent of the increase.This means the smaller size NSCB specimens are more likely to be affected by the width of the prefabricated crack.With the increase of the dimensionless prefabricated crack length,the fracture toughness of the NSCB specimens show a clear downward trend.The fracture toughness of NSCB specimens also increases continuously with the increase of specimens radius.However,the increasing tendency of fracture toughness of NSCB specimens will appear different law with the change of prefabricated crack length.Based on the theory of size effect,the mathematical expression of non-dimensional energy release rate of NSCB specimens were deduced.And the rupture toughness scale formula of NSCB specimens were obtained and used it to predict the fracture toughness of large size sample.The results show that the fracture toughness scale formula of NSCB specimens can well predict the fracture toughness of large-size specimens with an error of no more than 3% and had a good agreement with the fracture toughness values obtained from other mechanical parameters of the specimens.The above research results provide a certain theoretical reference for the accurate measurement of the fracture toughness of rock by using NSCB sample.It will promote the suggestion of recommend measurement of NSCB. |
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