Experimental study on influence of wetting-drying cycle on dynamic fracture and energy dissipation of red-sandstone
Introduction
Due to the periodic water changes caused by rainfall and groundwater level change, rock masses are generally in the wetting-drying cycles during the construction and operation of dams, slopes, mines, tunnels and underground engineering. Under the action of the wetting-drying cycle, the physical and mechanical properties of rock deteriorate in varying degrees, which seriously threatens the safety of rock engineering [[1], [2], [3], [4], [5]]. If the relevant evaluation and measures are not timely taken, geological disasters may be caused, including the landslide, collapse, karst collapse, instability of surrounding rock and foundation deformation [6]. Therefore, research on rock deterioration performance and mechanism under wetting-drying cycles has received much attention. In terms of physical properties, bulk density, mass loss, water absorption, effective porosity and longitudinal wave velocity are mainly studied as the research indexes [[7], [8], [9], [10], [11], [12], [13]]. The results showed that with the increase of the number of drying and wetting cycles, the bulk density and P-wave velocity of rock decrease, while the water absorption and effective porosity increase [14]. In terms of mechanical properties, compressive strength, tensile strength, shear performance, triaxial characteristics and fracture toughness are mainly studied [[15], [16], [17], [18], [19], [20], [21]]. All the studies showed that the mechanical properties of rock decrease in varying degrees after drying and wetting cycles. For example, Hale and Shakoor et al. [8] analyzed the uniaxial compressive strength of sandstone after 50 dry-wet cycles, and found that the deterioration effect of dry wet cycles was not obvious. Özbek et al. [10] explored the variation rules of uniaxial compressive strength with the number of wetting-drying cycles, and established the corresponding expressions. Deng et al. [15] studied the effect of water saturation-air drying cycles on the mechanical properties of sandstone, and confirmed that water pressure and water saturation-air drying cycles have cumulative effects on sandstone damage. Liu et al. [16] studied the physical and mechanical properties of argillaceous sandstone under different wetting-drying cycles, and discussed the change of microstructure characteristics with the increasing number of cycles. Hua et al. [20,21] investigated the influence of wetting-drying cycles on the dynamic fracture toughness of sandstone. It was found that the more the wetting-drying cycles, the lower the fracture toughness of the specimens.
In fact, the damage or breakage of the rock mass is mainly caused by the widespread existence of blasting, earthquake, impact and other dynamic loads in many rock engineering [22,23]. Under the dynamic loads, the mechanical response of rock is different from that of static loads [24,25]. However, the above studies mainly focus on the range of statics, and the wetting-drying cycle and dynamic load are rarely analyzed. Yuan et al. [26] carried out the uniaxial impact compression test on coal mine sandstone after wetting-drying cycles. It was found that the dynamic compressive strength of the specimen is the highest after one wetting-drying cycle, and then the compressive strength decreases exponentially with the increase of the number of cycles. Zhou et al. [14,27] systematically studied the dynamic compression and dynamic tensile properties of sandstone after different wetting-drying cycles. It was found that the mechanical properties of the rock are degraded by wetting-drying cycles, and the mechanical properties decrease after the wetting-drying cycles. As an important mechanical index of rock materials, fracture toughness can reflect the resistance ability of rock to the crack propagation [28,29]. At present, there is little research on the dynamic fracture behavior of rock considering the effect of the wetting-drying cycle and loading rate. For example, Peck and Gordon [30] studied the effect of water on the fracture energy of quartzite by wedge-shaped loading double cantilever beam method, and found that the fracture energy of quartzite in dry condition is higher than that in water. Zhou et al. [31,32] conducted dynamic notched semi-circular bending (NSCB) tests on dry and saturated sandstone specimens to understand combined effects of water saturation and loading rate on the fracture behavior of rock materials. They reported that the dynamic fracture initiation, propagation toughness and crack propagation velocity of saturated specimen were apparently lower than that of dry ones at the same loading rate. To effectively obtain the dynamic fracture behavior of rock after wetting-drying cycles, dynamic fracture tests on red sandstone specimens after different wetting-drying cycles were carried out by straight cut groove semicircle bending tension method (NSCB) in this paper. The change rule of dynamic fracture toughness with the number of wetting-drying cycles and loading rate was analyzed, and the energy evolution process of the specimen in the dynamic fracture process was discussed. This study provides a reference for the design and maintenance of related rock engineering.
Section snippets
Specimen preparation
Fine-grained red sandstone from Linyi, Shandong Province was used as the rock materials in this paper. The basic physical property parameters of red sandstone were as follows: density (2248 kg/m3), water absorption (3.24%), porosity (6.58%) and longitudinal wave velocity (2513.34 m/s). According to the results of X-ray diffraction tests, the mineral composition of red sandstone in the natural state mainly included quartz (58.6%), feldspar (21.9%), calcite (9.7%), hematite (6.3%), chlorite
Effect of loading rate on dynamic fracture toughness
Fig. 5 shows the change of dynamic fracture toughness of red sandstone with the loading rate after different wetting-drying cycles. It can be seen that under different wetting-drying cycles, with the increase of the loading rate, the dynamic fracture toughness generally presents an increasing trend, and the increasing speed is first fast and then slow. When a certain critical loading rate is exceeded, the dynamic fracture toughness does not increase, but fluctuates in a small range. With the
Conclusion
- (1)
Under a given number of wetting-drying cycles, the dynamic fracture toughness of red sandstone increases with the increase of the loading rate, and the increase rate is first fast and then slow; when the loading rate exceeds a certain critical value, the dynamic fracture toughness basically remains unchanged. The relationship between dynamic fracture toughness and loading rate can be expressed as follows:
Data availability
The data used to support the findings of this study are included within the article.
Author contribution
Du Bin: Writing – original draft preparation.
Cheng Qiangqiang: Sample preparation.
Miao Leigang: Formal analysis.
Wang Junqiang: Formulation of test plan and SEM Formal analysis.
Bai Haibo: Reviewing and Editing.
All authors read and contributed to the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work is supported by the National Natural Science Foundation of China (51323004, 51704281, 52004105), Xuzhou Science and Technology Project (KC19012, KC20199), General Projects of Natural Science Fund Project of Colleges in Jiangsu Province (19KJB130004, 20KJB410002, 20KJB170024).
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