Prediction of Mode I Fracture Toughness of Shale Specimens by Different Fracture Theories Considering Size Effect

Abstract

In this study, mode I fracture tests on cracked straight-through Brazilian disc (CSTBD) and notched semi-circular bend (NSCB) shale specimens with different sizes were conducted to investigate the difference between maximum tangential stress fracture criterion and the size effect law (SEL) model in predicting apparent fracture toughness (Ka) of shale. In addition, the effects of specimen size and geometry on the Ka and the selection of fracture criterion on the prediction of the inherent fracture toughness (KIc) were also studied. The results show that the Ka increases with the increase of specimen size, and the difference between KIc of shale specimens with different sizes predicted by the fracture process zone length determined by the further improved maximum tangential stress (FIMTS) criterion is the smallest. For the prediction of Ka of NSCB specimen, the results predicted by the FIMTS criterion are the closest to the tested fracture toughness. However, the effect of SEL model applied to the prediction of Ka of NSCB specimens is poor. The effective establishment of SEL model requires high accuracy for test data, especially for the configuration with large variation of the dimensionless stress intensity factor (Y*) with normalized crack length (α).

Investigating the influence of various three-point bending specimen configurations on rock fracture parameters testing: an experimental study

“…This indicates that the distribution of tangential stresses at the crack tip front in the two specimen configurations studied in this research is independent of the specimen's configuration. Previous studies [7] have shown that when the specimen dimensions are sufficiently large or the ligament length is sufficiently long, the tangential stress at the crack tip front can be completely determined based on the stress intensity factor alone. Both configurations in this study satisfy this condition.…”

“…It is worth noting that these testing methods do not yield consistent fracture toughness results, and different fracture toughness testing methods produce significantly different results [6]. Many studies have indicated that this disparity may be attributed to the presence of a fracture process zone at the crack tip front in rock specimens [7,8].…”