Inverse characterization of shrinkage and fracture of bentonite buffer material for geological repositories of nuclear waste using an integrated DIC-FEM approach

This study presents an inverse method integrating the digital image correlation (DIC) and finite element modeling (FEM) to characterize the shrinkage and fracture of desiccating bentonite clay, considered a buffer material in engineered barrier systems for nuclear waste repositories. Rheology tests were conducted to measure elastic moduli of bentonite at various moisture contents, and a restrained ring test was designed and conducted to induce shrinkage followed by crack initiation and propagation due to the soil desiccation process. Due to the complex nature of characterizing multiphysical fracture parameters, especially for materials undergoing desiccation cracking, an inverse method was employed. The full-field displacement of the desiccating bentonite, captured by DIC, was used to calculate shrinkage-induced stresses. A boundary value problem representing the test conditions was modeled through the finite element method incorporated with cohesive zone elements to simulate the shrinkage-induced cracking. The moisture-dependent shrinkage and fracture parameters of bentonite were inversely characterized through an optimization process, where material parameters in FEM were iteratively calibrated to ensure that the simulated full-field displacements before the onset of cracking and subsequent crack dimensions agreed well with the experiment. The novel experimental-computational method can identify evolving material characteristics subjected to complex hygro-mechanical conditions with fracture damage.