Experimental investigation of progressive cracking processes in granite under uniaxial loading using digital imaging and AE techniques

For a full understanding of the cracking processes in granite, uniaxial compression tests are conducted on prismatic specimens with pre-existing flaws, and digital imaging and AE techniques are applied to monitor real-time rupture behaviors. The digital image correlation (DIC) method is used to detect the process zone nucleation characteristics in granite. A novel approach for representing AE data in terms of the inter-event time (IET) function F(τ) is employed to analyze fracture-related AE event rate characteristics. Based on the coupled analyses of the acousto-optic-mechanical characteristics, the complete cracking processes of granite are classified into six levels, which are distinguishable by five characteristic stresses. Process zone nucleation, which represents the clustering of micro-cracks, is incorporated into the classification for the first time. Process zone nucleation is visualized as a white patch and is either linear or diffuse. By DIC analysis, the maximum principal strain field reliably predicts a linear white patch, and the maximum shear strain field implies a diffuse white patch. The evolutionary rules of the IET function F(τ) strongly support the new classification of cracking levels in granite. For the identification of stress thresholds and cracking levels, a combined acousto-optic-mechanical methodology is established, in which the role of the IET function F(τ) is highlighted.