Microcracks in rock, also known as microfractures and cracks, are spaces in rock with the longest length of 1000 μm and the other two dimensions of 10 μm. In general, the ratio of width to length of microcracks is between 10−3 to 10−5. Due to the scale, microcracks are observed using microscope to obtain their basic characteristics. Microcrack formation provides insights into the strength and deformation behavior of rocks. Experimental and numerical results both play an important role in studying microcracks, especially their kinematics and dynamics. Microcracks in rock have been studied to understand geologic problems such as the early stage of earthquakes and fault formation. In engineering, microcracks in rock have been linked to underground engineering problems, such as deep geological repository.
Types In general, microcracks in rock can be subdivided into four groups:
Grain boundary cracks: microcracks are along the grain boundary. Intragranular cracks: microcracks are within a grain. In addition, intragranular cracks along a cleavage plane are cleavage cracks. Intergranular cracks: microcracks are along the boundaries of two or more grains. Transgranular cracks: microcracks are across the grains or are across the grains from a grain boundary. They are the most abundant in rock specimens in the experiment.
Characteristics The characteristics of microcracks are orientation, length, width, aspect ratio, number, and density. These characteristics have been tried to be explained by mathematical functions. For example, distribution of microcrack lengths away from the fault has been described by lognormal or exponential distributions.
Orientation The orientations of microcracks are random in unstressed rock. Once a rock has been stressed, the microcracks will have a trend of orientations more or less parallel to the maximum applied stress or the fault strike. For example, the average orientation of microcracks of stressed Westerly granite is 30° to the fault strike.
Length, width, and aspect ratio In a thin section, the observed length and width may not necessarily be the true length and width of a microcrack in three dimensions. The aspect ratio is the ratio of width to length. It is generally10−3 to 10−5. The crack length increases with increasing maximum applied stress, resulting in a decrease in the aspect ratio.
Number and density Density of microcracks can be either the number of microcracks per unit area or per grain or the microcrack length per unit area. 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.
Mechanically induced A microcrack is formed when the stresses exceed the local strength of grains. The strength of materials is the ability to resist an applied load so that failure will not occur. 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. They then become more and more localized and intense with continuous loading. This phenomenon is called crack localization. A theory of failure helps to explain the evolution of microcracks with increased loading:
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