Weakening of granite by alternating voltage excitation of dispersed quartz: A 2D numerical analysis based on cohesive interface elements with a fatigue damage model

This paper presents a numerical-experimental approach to investigate the weakening of granite caused by High Voltage Alternating Current (HV-AC) excitations. A two-dimensional, mesoscale finite element (FE) model was developed to simulate the HV-AC-induced damage. The microstructural features of the rock were obtained from Electron Back Scatter Diffraction (EBSD) data, and the micromechanical behavior of each mineral constituent is defined using nanoindentation tests. Cohesive elements account for load transfer between neighboring grains and simulate the damage accumulation during loading. A cohesive zone model was developed to describe the evolving weakening of grain boundaries during cyclic loading, featuring strain rate sensitivity and continuous time-evolving damage. The constitutive cohesive zone model parameters were identified in separate stages using multiple types of experimental data, that is quasistatic compression, dynamic indirect tension tests of Brazilian discs, as well as low speed and high speed (impact) fatigue tests, to isolate the contribution of individual mechanical processes. The model predictions match well with the experimental evidence under all loading conditions, including the resulting reductions of dynamic tensile strength caused by HV-AC excitations, which were around 17 % of the strength of the nontreated rock. The cracking patterns predicted by the model match well with the observed experimental patterns. This study provides a quantitative comparison of the simulated cracking patterns of treated and nontreated rocks using a novel image treatment method. The comparison revealed that the HV-AC treatment generates weak spots in the form of small flaws in the rock microstructure. These small flaws act as nucleation sites for fracture propagation during dynamic loading, leading to the coalescence of cracks and facilitating rock breakage.