Imaging opening-mode fracture in sandstone under three-point bending: A direct identification of the fracture process zone and traction-free crack based on cohesive zone model

Influence of the fracture process zone (FPZ) poses a challenge to accurately determine the FPZ and traction-free crack during the fracture process in a rock structure. In this study, an engineering approach based on concepts of the cohesive zone model, is proposed to identify them in a sandstone beam under three-point bending. Two types of specimens were tested, including center notch and smooth boundary beams. Acoustic emission (AE) and digital image correlation (DIC) were used to monitor the local material behaviors, which involves two crucial parameters: AE energy and DIC opening displacement pattern. (i) Three levels of AE energy have been classified to "filter" the FPZ from AE events cluster. Based on AE energy levels, three groups of AE events can be obtained. For Level 1, AE events involves AE energy that is 1 or 2 order larger than that within Level 2 and 3, such that they occupy about 95% of total AE energy release. Thus, Level 1 is considered to represent the group of large AE energy. For Level 2 and 3, although many AE events are detected, these events have little influence on the fracture process. The FPZ consists of only AE events within Level 1. (ii) Four types of DIC opening displacement patterns have been observed on the specimen surface to identify the FPZ. Every displacement pattern represents a feature that is related to specific loadings in the cohesive zone model. Depending on the distribution of opening displacements and displacement gradients, displacement pattern can be decided and the individual pattern has its own special physical correlation: pattern (I) is related to elastic tension of the material; pattern (II) suggests the possible existence of the FPZ; pattern (III) indicates the possible onset of a traction-free crack; pattern (IV) confirms material softening within the FPZ and material unloading outside the FPZ. These patterns together with different reference loads are used to identify the FPZ and traction-free crack. With the help of two parameters, the precise kinematics of the FPZ and traction-free crack in the specimen are determined, and the fracture properties such as the FPZ length and critical opening displacement are also reported. Finally, experimental results provide insights to understand opening-mode fracture and related structural response, i.e., possible formation of the traction-free crack at peak under three-point bending, reasonable assumption of elastic material behavior before the softening, and the dynamic effect of fracture propagation on laboratory determination of fracture properties.