The loading mechanisms involved in blasting are assumed to consist of a stress wave propagation and a gas pressurization. Both mechanisms play significant roles in the resulting fragmentation. During the process of fracture, each mechanism contributes to the dynamic evolution of the state of stress in the material and, in this way, each influences the propagation of the load induced fractures and the resulting fragmentation. The problem is analyzed numerically for a material with spatially varying material properties using the Dynamic Lattice Network Model. In the numerical study, the effects of material damage and induced stress fields ahead of a moving crack on the overall fracture behavior is considered.
The detonation of an explosive in rock results in the propagation of an intense stress wave into the material and the release of high pressure gas inside the borehole. These phenomena result in a dynamic loading of the rock mass caused by the passage of the stress wave and a quasistatic loading due to the flow of expanding gas into the fractures. Though the loading mechanisms can be delineated distinctly, the material response to the loadings is coupled.
The coupling is manifest in the evolving state of stress and the growth of the process zone ahead of the crack tip, which profoundly influence the fracture behavior. For example, because the stress wave propagates at a higher velocity than the crack tip, the resulting state of stress due to the passing wave will influence the behavior of a gas driven crack.
Furthermore, the stress state, especially in the singularity region, can lead to incipient failure or process zone formation ahead of the crack tip, thus greatly affecting the fracture propagation. In order to understand the fracture propagation behavior and ultimately the resulting fragmentation produced by blasting, the dynamically evolving material state of stress must be examined. This examination not only involves the contributions of the stress wave and the gas pressurized crack to the stress state, but also the additional influence of the induced stress fields caused by the propagation of neighboring fractures. Through this approach, the growth and orientation of a progressing crack influenced by the presence of stress waves and neighboring fractures can be studied.
The problem of a running crack interacting with stress waves has been studied analytically by Rossmanith (1983). The problem has also been studied using photoelasticity by Rossmanith and Shulda (1981), Fourhey (1993), and Fourhey et al. (1993). These studies indicate the influence of stress waves on the crack propagation and branching. The development of the process zone and the influence of damage on crack growth has been studied by Taylor, Chen, and Kuszmaul (1986), Johnson (1993), and Li and Chudnovsky (1993). The analyses describe the change in material response due to the formation of the process zone and accumulating damage ahead of the crack tip. In this study, the evolution of the dynamic stress state ahead of the crack tip caused by blasting is examined numerically using the Dynamic Lattice Network Model. The simulations provide a qualitative explanation for the fracture behavior under the loading conditions imposed off [he material by blasting.
