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 KII) and two nil SIF-based (with/without KII), 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 KII 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 KII 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 KII was not considered. However, the fracture mode transformed from mixed mode I-II to mode I after the crack initiation, meaning the KII component in the criterion had a less significant effect on the crack propagation process.