Deterioration Characteristics and Energy Mechanism of Red-Bed Rocks Subjected to Drying-Wetting Cycles

: 25 The red-bed rocks were chosen and studied by using uniaxial compressive 26 experiment and scanning electron microscopy to investigate the effect of drying- 27 wetting (D-W) cycles on the mechanical properties and microstructural characteristics 28 of red-bed rock. Additionally, the energy mechanism of specimens subjected to drying- 29 wetting cycles was also explained. Experimental results showed that, the stress-strain 30 could be divided into four characteristic stages in the compression failure process. After 31 subjecting to cycles of D-W, the stress-strain curve gradually changed from softening 32 to hardening. At the same time, uniaxial compression strength (UCS) and elastic 33 modulus dropped obviously, while Poisson’s ratio gradually raised. Microstructural 34 analysis results indicated that the microstructure of the specimen surface was no longer 35 dense and uniform, and the porosity of tested specimens significantly increased with 36 D-W cycles increasing. As the porosity grew, UCS and elastic modulus gradually 37 declined. According to the first law of thermodynamics, the process of rock failure was 38 an event of energy transfer and conversion. As the number of D-W cycles increased, 39 the energy density of specimens all present linear fell. From the perspective of the 40 theory of energy dissipation, the dissipated energy was essential for rock failure, and 41 closely related to the strength of the specimen. With D-W cycles increasing, the 42 specimens were more prone to failure, and the dissipated energy required for failure 43 decreased gradually.

Abstract: 25 The red-bed rocks were chosen and studied by using uniaxial compressive 26 experiment and scanning electron microscopy to investigate the effect of drying-27 wetting (D-W) cycles on the mechanical properties and microstructural characteristics 28 of red-bed rock. Additionally, the energy mechanism of specimens subjected to drying-29 wetting cycles was also explained. Experimental results showed that, the stress-strain  114 The rock materials used in this study were the slightly weathered rock of red-bed 115 collected from the city of Tongcheng city Anhui province, China. Some essential 116 physical parameters were measured: water content (4.03%), density (2.33 g/cm 3 ), 117 porosity (12.50%) and specific gravity (2.77). In addition, the chemical composition of 118 the tested specimens was analyzed by X-ray fluorescence (XRF), and shown in Table   119 1. The XRD results of the tested rock are shown in Fig. 1 After being subjected to their designated numbers of D-W cycles, specimens were 148 cut into small pieces with an approximate size of 5 mm × 5 mm × 5 mm, and the surface 149 of tested specimens was cleaned using a hairbrush. Then, immersed in liquid nitrogen 150 and freeze-dried in the Alpha 1-4 LDplus Freeze Dryer for 24 h. In order to improve 151 the electrical conductivity, the specimens were vacuum metalized before the 152 examination. Finally, morphology observation was carried out using a JSM-6490LV 153 scanning electron microscope (SEM).

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All the experiments were performed at an ambient temperature of 25±0.1℃.

E=
(2) In these equations, where σ is the axial stress (MPa), P is the maximum load (kN),

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A is the cross-sectional area of the specimen (mm 2 ), E is the Elastic modulus (MPa), εy 197 is the axial strain (10 -2 ), μ-Poisson's ratio, and εx is the lateral strain (10 -2 ). calculated here using the following formula: Where DdUCS is the total degradation degree of UCS of specimens subjected to n 218 drying-wetting cycles, UCSn is the uniaxial compressive strength of specimens 219 subjected to n drying-wetting cycles, and UCS0 is the initial uniaxial compressive 220 strength of specimens.

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The UCS degenerated significantly in the initial stage, whereas with further increases curves of E showed a rapidly rising trend, then slowly developed, as n grew from 1 to Where DdE is the total degradation degree of elastic modulus, En is the elastic 252 modulus of specimens subjected to n D-W cycles, and E0 is the initial elastic modulus 253 of specimens.  Results in Fig. 7 showed that, these two evolutionary relationships were all linear. energy density (U d ) is illustrated in Fig. 8(b).  could be obtained (see Fig. 9). cycles increased, the dissipated energy density gradually fell (shown in Fig. 10 (b)), It can be seen from Fig. 9, the dissipated energy density remained unchanged in the initial compressive stage, and was mainly used for development and expansion of 407 internal fracture. With the increase of strain, the UCS and dissipated energy density 408 gradually increased. Once the stress reached the peak strength, the elastic energy 409 density was rapidly released, the dissipated energy density significantly raised, and then 410 the specimen failed. Furthermore, as given by Fig. 11, with the increase of D-W cycles,     Stress-strain curves of specimens subjected to different drying-wetting cycles Evolutionary relationship between the uniaxial compressive strength, elastic modulus and porosity  Please see the Manuscript PDF le for the complete gure caption.

Figure 11
Please see the Manuscript PDF le for the complete gure caption.

Figure 12
Relationship between dissipated energy and UCS and porosity Relationship between the dissipation energy ratio and the number of D-W cycles