## 3 Damage Characteristics
### Mesoscale Damage
In the propagation stage, the main cracks emerge and rapidly propagate, leading to a substantial increase in the number of cracks. In the stable stage, crack development reaches a plateau, and the crack system attains a dynamic equilibrium. The extent of crack development first increases and then stabilizes, and the rate of crack development initially accelerates and then slows down. Due to space constraints, this study focuses solely on analyzing the crack evolution pattern of a representative specimen. This case unequivocally illustrates the response relationship between the number of wet-dry cycles (\(N\)) and the extent of crack development, thereby providing a crucial foundation for the development of a universal damage evolution model. Cracks serve as prominent indicators of geotechnical material degradation. Figure 7a illustrates the crack propagation process in a representative sample subjected to wet-dry cycling. Before wet-dry cycling (\(N=0\)), the sample exhibited a dense structure, a smooth exterior, and high structural integrity, with no cracks visible on the surface. After the initial wet-dry cycle (\(N=1\)), significant annular pores emerged between the sample and the confining ring, accompanied by particle loss from the sample's base, resulting in a decrease in its structural integrity and concurrent mass loss. The sample's dimensions and shape underwent significant changes, with relatively narrow and limited in scale cracks manifesting on the surface. The second wet-dry cycle had the most pronounced influence on crack development, with peak growth rates observed for both crack density and crack width during this phase. Subsequently, as \(N\) increased, the rate of crack propagation diminished. The cracks on the sample surface progressively widened and coalesced into an extensive crack network, featuring several prominent wide cracks interspersed with finer cracks distributed irregularly around them. After four wet-dry cycles, crack propagation nearly ceased, but constant physical fissure creation as well as smaller crack healing. The \(r_{c}\), \(w_{max}\), and \(w_{avg}\) are effective indicators for quantitatively assessing the mesoscale response characteristics of cracks during wet-dry cycles. Figure 7b,c illustrate the relationship curves between the \(r_{c}\), \(w_{max}\), \(w_{avg}\), and \(N\) for the specimens. Significant changes in the \(r_{c}\), \(w_{max}\) and \(w_{avg}\) of the specimens are observed throughout the wet-dry cycle process. However, the response of crack development to each wet-dry cycle exhibits variability, and the sensitivity of crack indicators to the wet-dry cycles differs accordingly. The initial wet-dry cycle has a relatively minor influence on the cracks, with both the scale and size of the cracks being relatively small. The \(r_{c}\) is only 0.30%, and the maximum and mean crack widths are 0.515 mm and 0.184 mm, respectively. Subsequently, the specimens undergo a phase of rapid crack propagation. After the second wet-dry cycle, the main crack begins to emerge. Cracks continue to propagate along these primary cracks, and the growth rates of the \(r_{c}\), \(w_{max}\) and \(w_{avg}\) all reach their peaks, with increases of 300%, 298%, and 160%, respectively. During the third wet-dry cycle, while the growth rates of the \(r_{c}\) and \(w_{max}\) decline slightly, they remain significant. Cracks continue propagating along the established primary crack; however, the growth rate of the \(w_{avg}\) diminishes significantly, with subsequent wet-dry cycling exerting a reduced impact. As \(N\) increases, the growth rates of the \(r_{c}\), \(w_{max}\) and \(w_{avg}\) gradually decrease. When \(N\) reaches four, crack development enters a stable phase, and crack propagation reaches the bottleneck period. The crack density, crack length, and average crack width of the specimens all approach  
Figure 7: (**a**) Crack image; (**b**) relationship between crack width and \(N\); (**c**) relationship between crack ratio and \(N\).