Comprehensive sandstone fracturing characterization: Integration of fiber Bragg grating, digital imaging correlation and acoustic emission measurements

Prominent characteristics of rock fracturing can be described mainly by crack opening displacement (COD), fracture process zone (FPZ) and fracture energy. In view of lack of real-time and comprehensive measurement techniques for characterizing rock fracturing in field applications, this paper developed a characterizing method of rock fracturing by utilizing and integrating Fiber Bragg Grating (FBG), Digital Imaging Correlation (DIC) and Acoustic Emission (AE). In this paper, mode I fracturing test was performed on sandstone using three-point bending, and the fracturing process was simultaneously detected and recorded by FBG, DIC and AE. The experimental results showed that (1) DIC-based measurement of horizontal displacements across the fracture presented a localized discontinuity, in accordance with FBG-based real-time measurement. At the onset of traction-free zone formation (peak load), the critical COD (δc) was found to be 76 μm as identified by DIC and FBG. (2) Moreover, the dissipated energy distribution and cohesive crack profile in FPZ characterized by the integrated measurement, indicated that 70%-90% of AE energy (dissipated energy) populated in the FPZ, little AE energy concentrated in the traction-free zone, and the position of δc delineated a boundary for specifying FPZ. The length of fully developed FPZ was 20 mm (±2 mm) in length. (3) The integrated measurement delineated the cohesive crack, which was not available previously. The integrated measurement revealed the softening curve followed a straight line; the relationship between FPZ length and crack tip opening displacement (CTOD) was linear, and the relationship between dissipated energy and FPZ length was quadratic. Based on the interrelations of cohesive crack characteristics identified by the integrated measurements, real-time fracturing can be captured by one technique (e.g. FBG) in field application.