A comparative study on stress intensity factor-based criteria for the prediction of mixed mode I-II crack propagation in concrete

Combined with the fictitious crack model, the stress intensity factor (SIF)-based criteria are widely adopted to determine the crack propagation of mixed mode I-II fracture in normal strength concrete. However, less research is reported on the applicability of the different SIF-based criteria when they are used to analyze the crack propagation process of concrete with different strength grades. With this objective in mind, three-point bending and four-point shear tests were conducted in this study on C20, C50 and C80 grade concrete to measure the initial fracture toughness, fracture energy, load-crack mouth opening/sliding displacement (CMOD/CMSD). Four SIF-based criteria, including two initial fracture toughness-based (with/without mode II component of SIF K-II) and two nil SIF-based (with/without K-II), were introduced to determine crack propagation and predict the P-CMOD/CMSD curves for the notched concrete beams under four-point shear loading. The results indicated that the difference between the peak loads from experiment and from the analysis based on the nil SIF criterion with K-II approximately increases with the increase of the concrete strength. By contrast, the predicted peak load and P-CMOD/CMSD curves adopting the initial fracture toughness-based criterion with K-II showed better agreement with experimental results for the different concrete strength. Meanwhile, in the case of the initial fracture toughness-based criteria, the predicted initial load was underestimated if the component of K-II was not considered. However, the fracture mode transformed from mixed mode I-II to mode I after the crack initiation, meaning the K-II component in the criterion had a less significant effect on the crack propagation process.