Abstract
The cracking process in rock or concrete is usually characterized by the formation of microcracks that eventually form a propagating macrocrack. A series of three-point bending experiments were performed on sandstone containing Mode I crack under different loading rates. The microscopic monitoring system was established to capture the cracking process at notch tip. The loading rate dependence of microcracking behaviour was analysed based on load-time curves, acoustic emission (AE), microscopic images, and micrograph-based digital image correlation (DIC) technology. Results showed that the specimens underwent a short period of compression and elastic deformation stage under high loading rate, and the peak loads increased with the increase in loading rate. The AE results revealed that the fracturing process can be divided into elastic stage, damage stage, and postpeak stage, and more extensive damage occurred before the peak under low loading rate. It can be observed from microscopic images that the crack was initiated during the elastic stage, which was earlier than that determined from the AE monitoring. In addition, the microcracks were initiated at multiple locations and were mainly located at the interfaces between dense grains under low loading rate, while microcracks were observed inside the grains under high loading rate. Furthermore, the DIC results showed that the crack opening displacement (COD) of 0.6 mm/min at the peak was almost twice than that of 3.0 mm/min. The COD under the same loading rate at the peak can be considered as the material property of sandstone.
Highlights
IntroductionE cracking process in quasibrittle materials such as rocks and concretes in geotechnical engineering is usually characterized by the formation of microcracks that eventually merge and form a propagating macrocrack
With increase in demand for resources and energy, the relevant rock engineering activities, such as deep mining and development of geothermal resources, have increased [1,2,3,4,5,6,7].e cracking process in quasibrittle materials such as rocks and concretes in geotechnical engineering is usually characterized by the formation of microcracks that eventually merge and form a propagating macrocrack
A DS2-8B acoustic emission (AE) monitoring system (Softland Times Scientific & Technology Co., Ltd, China; Figure 2(b)) was used to capture the AE activity inside the specimen during the loading process. e system consisted of AE host, AE sensors, and preamplifiers. e AE sensor had a diameter of 8 mm, and its monitoring frequency range was 100–900 kHz. e AE signals were amplified with the gain of 40 dB in a preamplifier, and the AE preamplifier had the advantages of low noise, wide bandwidth, impact resistance, and small volume. e time parameters for AE waveforms included peak definition time, hit definition time, and hit locking time and were set to be 50, 100, and 100 μs, respectively
Summary
E cracking process in quasibrittle materials such as rocks and concretes in geotechnical engineering is usually characterized by the formation of microcracks that eventually merge and form a propagating macrocrack. Erefore, the research focusing on the cracking process of rock under different loading rates on a microscale is of great significance to better understand rock fracture behaviour and establish fracture mechanical model. Previous studies have focused on the fracture behaviour of rock materials [12]. Muralidhara et al [13] and Ohno et al [14] studied the fracture characteristics of concrete and determined the length of fracture process zone and initiation stress using acoustic emission (AE) location or energy. Wang et al [18] studied the anisotropic shale tensile failure
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