Experimental study on two semi-circular bending methods for mode I fracture toughness determination in tuff

Abstract

Rock fracture mechanics has been widely applied to understand the deformation and failure processes of rocks. As a significant parameter in rock fracture mechanics, mode I fracture toughness can be easily measured using semi-circular bend specimens. The objective of this research is to experimentally study two typical semi-circular bending methods for determining mode I fracture toughness of tuff. Particularly, the semi-circular bend (SCB) specimen and cracked chevron notched semi-circular bend (CCNSCB) specimen are used to determine mode I fracture toughness of fresh lapilli-ash tuff. The typical axial load (N) versus load point displacement (mm) curves of the two methods have quite similar in pattern, while the actual failure modes of the two specimens are different in terms of the fracturing surfaces roughness. The mean mode I fracture toughness measured by the SCB method is found to be lower (24.8%) than that using the CCNSCB method. With comparable numbers of specimens, the CCNSCB method produces more scattered results as compared with the SCB method. Given the influence of fracture process zone (FPZ) in SCB test, the results are modified using the effective crack model. The modified SCB fracture toughness values are higher than the original SCB and CCNSCB fracture toughness values.