Microcracks in rock

In general, microcracks in rock can be subdivided into four groups: File:Grain boundary crack123.png|Grain boundary crack File:Intragranular crack.png|Intragranular crack File:Intergranular crack.png|Intergranular crack File:Transgranular crack.png|Transgranular crack == Characteristics ==

The characteristics of microcracks are orientation, length, width, aspect ratio, number, and density. The aspect ratio is the ratio of width to length. Densities of microcracks near a fault are dramatically high, but they decrease rapidly within a few mineral grains away from a fault. == Formation mechanism ==

Microcracks in rock can be induced by the applied stress or temperature. The intrinsic properties of rock such as mineralogical heterogeneity give diverse types of mechanically induced microcracking. The following mechanisms have strong correlations to the locations that allow stress concentration in grain-scale. • Twin induced microcracking: stresses are concentrated at twin lamella. • Kink band and deformation lamellae associated microcracking: kink bands and deformation lamella can become a zone for stored strain energy to be concentrated. • Cleavage separations: cleavage planes are the weaknesses in crystals. Therefore, stresses are likely to be concentrated on these weakness planes first. • Microcracking from stress concentrations at grain boundaries: the contacts between grain boundaries provide space for stresses to be concentrated, especially tensile stresses. • Microcracking from stress concentrations around cavities: pre-existing cracks and pores within a grain allow stress concentration. This kind of stress concentration depends on the orientation and geometry of these pre-existing microcavity, as well as the mechanical properties of the surrounding material. • Elastic mismatches induced microcracking: each mineral type has its own elastic property. When two distinct minerals have a good contact between their boundaries, the applied stress will pull the stiffer mineral's boundary away from the contact. Therefore, the formed microcracks in the stiffer mineral are extensional cracks. • Grain translations and rotations: in crystalline rock, sliding along grain boundaries can be induced from deviatoric stresses, resulting grain boundary cracks. In clastic rock, the grains may be rotated by neighbor grains, forming cracks in the cement or along the grain boundary. Thermally induced Thermally induced microcracking refers to microcrack formation due to thermal effects. Heating or cooling can cause thermal expansion or contraction between grains, respectively. Minerals with different thermo-elastic properties have different reactions to cooling or heating, resulting in microcrack formation. Also, thermal gradients at internal boundaries of grains may also allow stress concentration, thus forming microcracks. == Evolution ==

The evolution of microcracks has been studied through experiment. When force is applied to a rock sample, microcracks initially form randomly in space. • The formation of microcracks starts at pre-existing microcracks. • The newly formed microcracks grow in size individually. • The number of growing microcracks also increases. • The growing microcracks starts interaction as more and more cracks form and grow. • The growth of the microcracks suddenly becomes intense and localized, leading to macroscopic failure. After failure, the overall microcrack density increases near the fault and decreases rapidly away from the fault. It is a region of microcracks near the tip of a rock failure. It is associated with the crack localization and related to energy dissipation. The size of a fracture process zone is related to the specimen size. The larger the specimen size, the large the size of the fracture process zone. This relationship no longer exists when the specimen size is larger than a certain size. The heterogeneity of rock makes the microcracking behavior much more complicated than other simple materials. Factors controlling microcracking behavior still have been identified and studied: • Rock type and composition: rock types can be classified into crystalline rocks including igneous rocks and metamorphic rocks, as well as sedimentary rocks including clastic and chemical sedimentary rocks. For example, many studies show that quartz content of a rock has a great impact on the number of microcracks. • Pre-existing weaknesses: they are already in rock, for example, cleavage planes of minerals, pores, and cracks. • Stress state: the state of a rock experiencing the stresses. == Recovery ==

In addition to microcracks formation, microcracks in rock can be recovered either by microcrack closure or microcrack healing. Microcrack recovery will directly cause a decrease in permeability of rock. For example, microcracks perpendicular to the maximum stress direction will close. However, in nature, parts of a microcrack can be in different directions. For this reason, it will result in incomplete closure that some parts of the microcrack are closed while some parts are still open. Microcrack healing It is driven by transportation of chemical fluid in microcracks. For example, healing of microcracks in quartz is activated by temperature. Healing in quartz becomes fast when the temperature is above 400 °C. The rate of healing also depends on the crack sizes. The smaller the cracks, the faster the healing. == Influence ==

Microcracks affect the properties of rock including stiffness, strength, elastic modulus, permeability, fracture toughness, and elastic wave velocity. == Methodology to study microcracks ==

Studies of microcracks are focused on their distributions of the characteristics and microcracking behavior. Many experiments to study microcracks in rock have been conducted in the past decades, whereas numerical study also has been widely used to study microcracks in recent years because of the technology development. Specimen configuration Specimen configuration refers to the dimensions of a specimen and its man-made crack. Rock samples are usually obtained from rock cores. Therefore, cylinder shape, chevron-bend shape, and semi-circular-bend shape (SCB) are the common specimen shapes used in experimental study. It is generated from microcrack formations, Four types of models using in modelling microcracks in rock are particle-based models, block-based models, grain-based models, and node-based models. Since grain-based models can consider all types of microcrack, they are good at understanding microcracking behavior. == Geological implication ==

Experimental study of microcracks provides insights into faulting and microcracks formation in nature. Microcracks studies with CL and fluid-inclusion studies are able to reconstruct the growth of fractures from microcracks. Population of microcracks is useful to distinguish whether the detachment is due to landslide or tectonic in origin. The fracture process zone can be used to understand the permeability of fault zones which controls fluid flow. Therefore, microcracks can be useful for assessing the stress history or fluid movement history of rock. Acoustic emission from microcrack growth may help to understand earthquakes. == Implications of underground engineering problems ==

Microcracks can affect the thermal and transport properties of rock. Studies of microcracks in rock provide an important insights into underground engineering problems as follows used in actual practice in the design, operation, and organization of nondestructive testing and monitoring of industrial mining facilities that include these interfaces It is at depth of hundred metres in a stable rock mass. Deep geological repositories are all over the world, such as the United States (WIPP) and Finland (Olkiluoto Nuclear Power Plant). It is a porous and permeable rock mass so that convection of trapped hot water and steam and recharge of heat supply can occur. Reservoir rocks have high porosity and permeability while the surrounding rocks that act as barriers have low permeability. It is composed of porous rocks surrounded by nonporous rocks so that it can trap the CO2 for a long time. A depleted oil and gas reservoir that is out of energy source is one of the examples used for underground storage. == See also ==