Mechanical response and strain localization in coal under uniaxial loading, using digital volume correlation on X-ray tomography images

A comprehensive analysis of crack initiation and propagation in coal is critical for understanding its mechanical behavior and its impact on permeability. The finite element-based digital volume correlation (DVC) of time-resolved X-ray tomography images under cleat-parallel uniaxial compression provides 3D incremental strain fields to understand the evolution and propagation of cracks. In this study, DVC coupled with quantitative image analysis was used to understand the role of cleats in guiding and initiating cracks in coal. Simulation of fluid flow through coal at different loading segments provides dynamic quantification of evolution in permeability. After each loading stage, quantification of strain eigen vectors and image analysis indicates appearance, coexistence, and coalescence of tensile and shear cracks guided by the organic matter in coal. We established that constructive interference between tensile and shear crack systems enhances permeability and decreases tortuosity by almost 3x and 7x respectively in coal by widening crack aperture, however, destructive interference is likely to lead to matrix pulverization and clogging of larger crack apertures. During compression, while one part of the coal matrix shows extension perpendicular to the loading direction, another part shows slippage parallel to the loading direction. Based on the variation in crack propagation behavior, four distinct crack morphology zones were identified and discussed. We found that the tortuosity of the cracks decreases exponentially, while permeability and crack volume fraction increase exponentially up to 2.8 mu m(2) and 28.6% respectively with increasing load. The box-counting-based fractal dimension for the cracks shows a steady increase from 1.58 to 1.78 with loading due to the evolution of existing cracks, and the formation of new cracks.