Fragmentation produced by explosive blasting and impact penetration of rock depends largely in the dynamic fracture process involving the nucleation and propagation of microcracks that finally coalesce, breaking the rock into fragments. Moreover, rock is an inhomogeneous material, which follows a stochastic process in fracturing. This causes the nonlinear behavior and strain rate dependency of the fracture properties under a dynamic load.

In this study, a modified compact test (MCT) was suggested to investigate the fracture process zone near the crack tip and fracture toughness of rock materials. By adopting the FEM analysis, the geometry design of MCT was determined. Furthermore, the stress intensity factor for MCT method with consideration of specimen geometry was also determined in this study. A dynamic measurement system was adapted to observe the dynamic fracture process zone around the running cracks. Fracture toughness obtained by J-integral-based technique were compared with the fracture toughness determined by the suggested stress intensity factor in this study.

1. Introduction

The fracture process zone (FPZ) is one of the significant issues in the fracture mechanics of rock materials. This nonlinear behavior region may influence the rock properties and the stability of structure. Furthermore, some researchers pointed out that one reason why linear elastic fracture mechanics (LEFM) could invalid for rock materials is the existence of the FPZ(Zhang and Wu, 1999). However, it is still not clear how the size of the FPZ, and whether it is an independent materials property or influenced by external conditions, even there is a general agreement on the presence of the FPZ. According to the precious studies, there are several major approaches that have been conducted to investigate the FPZ such as using acoustic emission technique, crack propagation gauge(CPG), interpretation of crack patterns and the micro-structures(Erarslan, 2016, Hu and Xu, 2016, Tarokh et al., 2017, Wu et al., 2018). The AE technique can provide the location of crack initiation, CPG also measure the crack behavior with gauge location with a function of time. In case of the investigation of crack pattern or micro-structure, it is seldom to deduce the fracture behavior or the FPZ because it has been performed after the fracturing. Consequently, a practical measurement technique that can estimate the ranges of FPZ and its stress or strain behavior with proper fracture testing method is required.

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