## 3 Results and Discussion
### Strain Field and Failure Mode  
The result obtained by the VIC-3D system showed that the strain fields of the selected samples under the axial load had a similar evolutionary trend and underwent a process from uniform distribution to inhomogeneous distribution. The strain field focused on in this study refers to the maximum principal strain field, which can effectively reflect the propagation and distribution of cracks on the sample surface during the loading process [28]. Figure 5 presents the strain field evolution of the sample subjected to 40 drying\(-\)wetting cycles in an acidic solution with pH = 3. Five representative points marked from point A to point E in the stress\(-\)strain curve were selected to display the strain field evolution. They corresponded to 70%, 80%, 90%, 100%, and 90% (in the post-peak stage) of the peak stress, respectively. It can be seen that the apparent strain of the sample showed little difference for a long time, and the strain field was uniformly distributed until point B. Afterward, the strain field transitioned to an inhomogeneous distribution, accompanied by the onset of a strain localization band on the right side of the sample. This suggests that a few micro-fractures were generated into the band. The maximal strain value exceeded 5.12 \(\times\) 10\({}^{-2}\) at point C. As the stress level was close to the peak stress, the strain localization band became more apparent and extended to the upper end of the sample progressively. Meanwhile, the area of apparent strain exceeding 5.12 \(\times\) 10\({}^{-2}\) increased significantly. After point D, the strain localization band ultimately evolved into a macro-crack, and the sample was destroyed at point E.  
The residual samples under the axial load also experienced the strain field evolution from homogenization to localization, as depicted above. It is worth noting that due to the coupling effect of drying\(-\)wetting cycles and acidic solutions, the development of strain localization bands and failure mode of the samples had obvious differences. The strain field nephograms at the peak stress and fractured images of the samples under different drying\(-\)wetting cycles and pH solutions are shown in Figure 6. There is a certain internal relationship between them, which is the basis for predicting the spatial distribution of rock failure from strain localization bands. However, the crack system of the latter was more \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Cycles**} & \multicolumn{3}{c}{**Elastic Modulus (GPa)**} & \multicolumn{3}{c}{**Uniaxial Compressive Strength (MPa)**} \\  & **pH = 7** & **pH = 5** & **pH = 3** & **pH = 7** & **pH = 5** & **pH = 3** \\ \hline
0 & 13.17 & 12.57 & 11.75 & 77.45 & 73.98 & 70.03 \\
10 & 8.92 & 8.32 & 7.22 & 66.87 & 62.10 & 56.05 \\
20 & 8.71 & 7.44 & 6.18 & 62.88 & 56.97 & 50.93 \\
30 & 7.97 & 6.76 & 4.85 & 58.91 & 53.81 & 44.85 \\
40 & 6.41 & 5.61 & 3.96 & 56.32 & 49.84 & 41.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical parameters of samples under different drying\(-\)wetting cycles and pH solutions. Figure 5: Stress\(-\)strain curve and strain field nephograms of the sample subjected to 40 drying\(-\)wetting cycles in the acidic solution with pH = 3.  
complex and abundant. The reason is that the massive release of elastic energy after the peak stress leads to the formation of secondary cracks [39]. Most of the strain localization bands initiated on the end of samples and extended to the middle or the other end with increasing axial stress. When the axial stress reached the peak stress, the strain localization bands were quite prominent. In addition, the apparent strain in the bands increased with increasing drying\(-\)wetting cycles and manifested a poor regularity with the solution pH, which is in agreement with the variation law of peak strains obtained by the compression testing machine.