Nucleation-percolation transition in desiccation cracking of clay

Crack formation plays a critical role in both natural and engineered materials, influencing the structural integrity and failure of materials from soils to buildings. In this study, the clay desiccation cracking is observed to exhibit a transition of cracking mode from nucleation through avalanche to percolation. This cracking mode transition is dictated by the strain field disorder, which is deliberately regulated by changing the pore structure heterogeneity, with nucleation dominating at low disorder and percolation emerging at higher disorder levels. These transitions are captured by the fractal dimension of the sample-spanning crack, with ∼0.99 for nucleation, ∼1.72 for percolation, and values in between for avalanche, maintaining universality among the four types of clay. Additionally, the fractal dimension increases with strain field disorder in avalanche mode, which can be well captured by a logarithmic relation derived via the fractal tree model. Moreover, this logarithmic relation is universal among both the experimental data of the four types of clay and the numerical data of the classical fuse model. Our findings enhance the understanding of how material heterogeneity governs crack propagation, providing valuable insights for predicting and controlling fractures in natural and industrial processes.