Comparison Study On Test Methods For Ⅰ And Ⅱ Mode Fracture Toughness Of Granite

In order to evaluate the mechanical properties of rocks,the mechanism of crack initiation and extension,and the fracture characteristics of rocks,fracture toughness,a key material parameter in rock fracture mechanics,is crucial.To acquire more reliable fracture toughness values for academic research and engineering practice,it is vital to select an appropriate fracture toughness testing method because the findings obtained by different testing methods are frequently varied.The type I and type II fracture toughness tests used in this study on granite samples included the Brazilian splitting test with herringbone cut grooves(CCNBD),the three-point bending test with a semicircular disc(NSCB),the straight cut grooves with the Brazilian splitting test(SNBD),and the asymmetric semicircular disc with the three-point bending test(ASCB).A good approach for assessing the fracture toughness of rocks is best chosen after multiple views,including acoustic emission(AE),digital image correlation(DIC),and roughness analysis,have disclosed the microscopic rupture mechanism of various tests.The following are the primary research findings:(1)The CCNBD,NSCB,SNBD,and ASCB tests of rock fracture toughness are introduced,and indoor mechanical testing is carried out.Acoustic emission and DIC systems are used to monitor the testing process,and mechanical characteristic curves,acoustic emission parameter curves,and DIC strain clouds are all examined.The crack compacting stage,the crack sprouting stage,the crack extension stage,and the post-peak damage stage are the four stages that make up the test damage process.(2)In comparison to the NSCB test,the type I fracture toughness value(KIC)of the CCNBD test was higher,and the results’ dispersion was also higher.The type II fracture toughness value(KIIC)of the SNBD test was higher than that of the ASCB test and was more stable in the type II fracture test.(3)To compare and analyze the acoustic emission characteristics throughout the various experiments,the load and acoustic emission parameter-time relationship curves were established.The findings indicate that,whereas the NSCB and ASCB test processes suffered instant damage,the CCNBD and SNBD test procedures sustained progressive damage.(4)Through the collection and statistical analysis of the AF-RA values of the acoustic emission characteristics,it was discovered that tension damage was the primary type of damage in both the NSCB and CCNBD tests and that the percentage of tension damage in the NSCB test was higher than that in the CCNBD test.For the type II fracture test,shear damage primarily occurred in the SNBD test,and shear-tension compound damage occurred in the ASCB test,in which the percentage of In comparison to the ASCB test,the SNBD test exhibits a higher percentage of shear damage.The results of counting the AF-RA values by the test’s various damage stages revealed that the NSCB test process was dominated by tensile damage,the CCNBD test process was under the control of both tensile and shear damage,and the SNBD and ASCB test processes in the type II fracture test were under the control of both tensile and shear damage.(5)The horizontal displacement clouds and roughness indices of the CCNBD and NSCB tests were obtained using DIC and section roughness scanning techniques.The findings also revealed that the percentage of shear damage in the post-peak phase of the CCNBD tests was higher than that of the NSCB tests,which was in good agreement with the acoustic emission results.(6)The relationship between rupture patterns and acoustic emission phenomena from the microscopic to the macroscopic was established through the synthesis and analysis of the acoustic emission and DIC phenomena.The findings demonstrate that the damage processes of various fracture toughness tests differ noticeably.The DIC phenomena of the CCNBD and SNBD tests lag behind the acoustic emission phenomena and exhibit the traits of progressive damage,whereas the DIC phenomena of the NSCB and ASCB tests adopt the loading mode of three-point bending and are in excellent agreement with the acoustic emission phenomena and are associated with transient damage.