Uniaxial compression tests combined with nondestructive testing techniques are performed to explore the roles of non‐isometric flaws in crack developments in brittle rocks. The acoustic emission (AE) rate‐process theory is adopted to analyze fracture‐related AE event rate characteristics. The full‐field optical method is applied to detect cracking modes. Experimental results show that AE activity is quite active when the matrix microcracking is dominant, while after each macrocracking event, AE activity becomes inactive because of the stress release. Multiphysical data for each tested flaw configuration faithfully confirm the rupture progressivity. The larger the flaw length ratio, the lower the peak stress (also peak axial strain and elastic modulus), as well as the more progressive the cracking process. Moreover, ultimate failure is triggered by the shear fracturing from the relatively long flaw. The short flaw is conditionally involved in ultimate failure when the stress buildup effect dominates. Finally, the fracture mechanism of brittle rocks with non‐isometric flaws is revealed.

In this paper, uniaxial compression tests are conducted on fissured red sandstone specimens to predict fracture damage (large‐scale events). The acoustic mission (AE) coupled with digital image correlation (DIC) technologies are used to monitor and record the real‐time cracking process of tested specimens. The AE characteristics are analysed during the cracking process. Moreover, three types of b‐value methods based on the AE parameters are adopted to predict the occurrence of large‐scale events (macro‐cracking). The results show that every macro‐cracking leads to a rapid decrease in three types of b value. When the fissured specimens reach to ultimate failure, all three types of b value reach to the minimum. The b value based on the AE parameters can be used as a predictor of large‐scale events during the cracking process of fissured rocks.

To investigate the dynamic tensile behaviours of concrete and cement mortar, a 50‐mm split Hopkinson pressure bar was applied on Brazilian disc specimens for dynamic tensile experiments, in which strain rate varied from 10−5 to 20 s−1. The high‐speed camera testing technique was used to capture the dynamic fractured process of the specimens at relative high strain rate. The experimental results revealed that the dynamic tensile strength of concrete specimens has a stronger strain rate effect than that of cement mortar specimens. Then three typical failure patterns of the specimens were confirmed in dynamic experiments. In addition, one‐parameter semi‐empirical relation between dynamic tensile strength and strain rate was established. Finally, the limitation of dynamic splitting experiments on Brazilian disc specimens was discussed in detail at high strain rate, in which the crack initiates from the contact point between the incident bar and specimens rather than the centre of the specimens.

No abstract

No abstract