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Bridges connecting opposite sides of the crack surface are normally found near the crack tip (e.g. in the hydraulic fracture); their extension and breakage are considered to constitute the mechanism of formation of the fracture process zone. Bridges however may be distributed all over the crack. It primarily relates to natural fractures in rocks and rock masses, which are usually non-persistent. This paper reports series of laboratory experiments showing the presence of distributed bridges in the hydraulic fractures induced in mortar and three different types of rock samples. This is confirmed by direct observations via X-ray micro-CT scans as well as by tensile tests of the samples which remain in one piece after hydraulic fracturing. A model of hydraulic fractures with bridge-produced constricted opening is proposed and the effect of bridges is investigated. It is shown that there exists a characteristic length associated with the combined action of bridges such that the effect of bridges is considerable only for the cracks whose dimensions exceed the characteristic length.