This study explores the interaction between crack initiation and nanomechanical properties in the crack tip process zone (zone of microcracking at the tip of a propagating crack) of a brittle material. Samples of Carrara marble with pre-existing cracks (“flaws”) were loaded in a uniaxial testing machine until the process zone appeared at the tips of the pre-existing cracks in the form of “white patching”. Two techniques were then used to obtain nanomechanical properties of the process zone and relate them to macroscale crack initiation: digital photography, to visually assess the macrostructure and crack formation, and nanoindentation, to yield nanomechanical properties and assess nano/microheterogeneities. Nanoindentation testing was comprised of lines and grids of single nanoindentations located both near and far from the process zone. The purpose of nanoindentation testing is to investigate the underlying trend in nanomechanical property change between intact and process zone marble. Analysis of nanoindentation testing results showed a decrease of both modulus and hardness (a) near grain boundaries in intact material, and (b) with closeness to the process zone. Ultimately, the study confirms that the crack tip process zone manifests itself as an area of reduced nanoindentation hardness and nanoindentation modulus in marble.
The study of geomaterial cracking at its most fundamental scale, nano and microscales, is critical to predicting crack propagation. With this in mind, this study explores the interaction between crack initiation and nanomechanical properties in the crack tip "process zone" of Carrara marble. Nanomechanical properties of material in the fracture process zone (FPZ), i.e. the material within the zone of microcracking around the crack tip, are compared with nanomechanical properties of the "intact material". The differences in nanomechanical properties between intact and process zone material reveal the existence of nanoscale damage in the FPZ that may well contribute to the fracture propagation. The literature contains extensive information on the three defining aspects of this investigation - process zone, nanomechanical properties, and geomaterials - but this study represents the first occasion to bring these aspects together in such a way. Many theories exist regarding the process zone, but the experimental investigation of this theoretical region in rock is a relatively recent development. In Linear Elastic Fracture Mechanics (LEFM) the process zone is defined as the core region of plastic-behaving material. Rice investigated the process zone region from a theoretical perspective to ultimately configure the J-Integral, which expresses the energy release rate. However, the integral does not consider the interplay between nano/micromechanical properties and the energy release rate or fracture energy [2]. The Dugdale model for the size of the process zone, or “inelastic zone,” has been applied even at the fault scale (several kilometers), but does not consider the variability of properties in the region [3]. Application of the closely related Cowie and Scholz model to existing faults reveals a logarithmically decreasing microfracture density within and moving away from the “cohesion zone” (a low-strength area surrounding a displaced fault), and a finite scale-independent limiting microfracture density [4].
