3.1 Effect of wetting-drying cycles on tensile strength of rock
The stress-strain curves subject to different numbers of wetting-drying cycles are shown in Fig.2. Representative and average values are shown in Fig.3.. The basic schematic diagram of Brazil split can be seen in Fig1 (b). According to the analytical solution of elastic mechanics of plane stress problem, the compressive stress at the center of the disk is only three times of the tensile stress, so the tensile failure of the sample is not the compression failure, and then the splitting tensile strength of the sample is calculated.
It can be seen that as the number of wetting-drying cycles increases, the stress-strain curve becomes more ductile, especially after 10 cycles. This indicates that after the repeated wetting-drying process, the stiffness of sandstone is significantly lower than that in the initial state. Moreover, it can be seen from Fig.2 that after two wetting-drying cycles, the peak stress drops by about 50 % compared with the initial state. After that, while the peak stress changes little with increasing number of cycles, the strain significantly increased. This phenomenon indicates that the first few wetting-drying cycles have a greater impact on the strength of rock, while the latter mainly affects the stiffness of rock. Furthermore, the tensile strength of all samples under different wetting-dry cycles was shown in Fig.3 and Table 2. It demonstrates that the sandstone in natural state has the highest average tensile strength at 4.842 MPa with the largest variance value at 2.891. Due to the influence of water during the wetting-drying cycles, the tensile strength of the samples is reduced, and the variance value is lower than that in the natural state. Moreover, it can be found that the tensile strength of the sandstone is not only affected by the number of wetting-drying cycles, but also closely related to the clay mineral content in the sandstone. For example, the tensile strength of samples subjected to 20, 30 and 40 wetting-drying cycles is greater than the three groups of samples subjected to 10 cycles, which is mainly because of the low clay mineral content of these three groups. See Table 3 for specific clay mineral content. At the same sampling location, the clay mineral composition of the sample is different. When the sample is cut into the standard test sample size, the dry and wet cycles are carried out under the same conditions. The clay content of the first three groups is high, and that of the last three groups is relatively low.
In addition, reduction in tensile strength of sandstone subjected to different number of wetting-drying cycles was shown in Fig.4. Fig.4 (a) presents the reduction in tensile strength caused by cumulative wetting-drying cycles, and Fig.4 (b) presents the reduction in tensile strength caused by a single wetting-drying cycle. It can be seen from Fig.4 (a) that the reduction of tensile strength of sandstone with large clay content increases with the number of wetting-drying cycles, while that of sandstone with low clay mineral content is not. This is consistent with previous research results. For example, the number of wetting-drying cycles of sandstone with clay mineral content of less than 2% has little or negligible effect on tensile strength. However, for the sandstones containing clay minerals, the reduction in tensile strength increases with the number of wetting-drying cycles. Furthermore, the reduction in tensile strength caused by a single wetting-drying cycle is calculated using the following equation. That is, the reduction degree of average tensile strength of each time in 2 times, 4 times ,10 times,20 times,30 times and 40 times:
where σ is the reduction in tensile strength, is the average tensile strength in nature, is the average tensile strength of sandstone subjected to n times wetting-drying cycles. As shown in Fig.4 (b), the average cycle strength of each group of samples decreased under different cycles. As the number of wetting-drying cycle increases, the effect of single wetting-drying cycle on tensile strength is get reduced until it becomes constant. Moreover, the effect of a single wetting-drying cycle on tensile strength can be fitted by a quadratic curve.
3.2 Effect of wetting-drying cycles on crack propagation process
In order to analyze in detail the effect of the number of wetting-drying cycles on crack propagation, the images on the front surface of specimens were captured using high-speed camera. The crack propagation process of sandstone with different wetting-drying cycles was shown in Fig.5. From the perspective of qualitative analysis，it can be seen from Fig.5 that the crack opening and penetration time is prolonged with the increase of wetting-drying cycles. This further indicates the great influence of the number of wetting-drying cycles on the stiffness of sandstone. In addition, as the number of wetting-drying cycles increases, besides the primary crack, secondary cracks also appear in the crack process. Moreover, the crack initiation and penetration time of all samples are counted in Fig.6 which shows that as the number of wetting-drying cycle increases, the crack initiation time is delayed accordingly. Meanwhile, it can be seen in Fig.6 (b) that the crack penetration time increases with the increase of wetting-drying cycles, which indicates that the rate of crack propagation becomes slower as the number of wetting-drying cycles increases. This is the time from the beginning of crack shooting by high-speed camera to the conversion of frame rate corresponding to the picture. This result only shows the influence of drying- wetting cycles on crack growth rate, and does not consider clay mineral content.
In addition, the AE energy rate and cumulative AE energy during the entire experiment was monitored. The variation of AE characteristics during the loading process was shown in Fig.7. Similar to previous studies, the AE characteristics of sandstone throughout the experiment with respect to different wetting-drying cycles can be divided into four stages [42-44]. The first is the existing micro-crack or pore compaction stage. During this stage the corresponding acoustic emission energy is rare. As the number of wetting-drying cycle increases, this stage tends to be extended. The second is the elastic deformation stage, in which the corresponding acoustic emission energy increases gradually and slowly. This stage is also extended as the number of wetting-drying cycle increases. The third is the crack propagation stage, in which the corresponding acoustic emission event increases sharply due to rock rupture. The fourth is the post-peak stage. As the number of wetting-drying cycles increases, the acoustic emission energy of the sample at this stage also increases accordingly. This indicates that the wetting-drying cycle enhances the plasticity and ductility of sandstone.
3.3 Effect of wetting-drying cycle on microstructure and strength weakening mechanisms
During the wetting-drying cycle, the microstructure of the sample will be altered due to repeated water-rock interactions. Fig.8 presents the SEM images of the sandstone with different number of wetting-drying cycles. Table 3 is included with the mineralogy after cycling. Please refer to the group number. The right column in Fig.8 shows the overall microstructure changefor sandstone samples with clay minerals. It shows that as the number of wetting-dry cycle increases, there is no big change in large granular minerals such as quartz, but significant changes in clay mineral particles. Due to the action of water, the clay mineral particles gradually change from being massive, neat and dense to flat, muddy and honeycomb. The right side of Fig.8 is a partially enlarged picture. It can be seen that more inter granular cracks appear as the number of wetting-drying cycle increases. Especially when the number of wetting-drying cycles is above 10, there are micro-cracks between almost every two sandstone grains, and inter granular pore increases obviously. In summary, with the increase of the number of wetting-drying cycles, the secondary fractures and micro fractures increase, the morphological characteristics and internal structure of sandstone change to a certain degree, and the microstructure of sandstone gradually changes from being neat and dense to rough and disordered, until it becomes muddy, loose and slice particle structure. In addition, with the increase of inter granular porosity, some interstitial fillings become muddy or flaky, and the cementation between particles weakens. Bigger pores in the sample surface develop into micro-cracks. At the macro level, it is reflected in the lower strength parameters of the sandstone, which is similar to the prior experimental study .
Previous studies have shown that the weakening mechanism of water on rock strength mainly includes fracture energy reduction, capillary tension decrease, pore pressure increase, fractional reduction, and chemical and corrosive deterioration. Regarding sandstone specimens in this experiment, the main tensile strength weakening mechanism is fraction reduction and physicochemical corrosion. It can be seen from the SEM of samples subjected to different wetting-drying cycles that as the number of wetting-drying cycle increases, the cementation between the particles weakens and the pores between the granularities increase, which seriously weakens the fractional effect. Moreover, it can be seen from Table 3 that after the sample undergoes the wetting-drying cycle, the feldspar minerals containing soluble ions such as potassium and sodium decreased or even disappeared due to hydrolysis reaction. In addition, the most important reason is that clay minerals such as montmorillonite and illite swell when they encounter water, and the original structure of sample is destroyed during repeated expansion to reduce its tensile strength.