Study of fracture processes in sandstone subjected to four-point bending by means of 4D X-ray computed micro-tomography

High-resolution X-ray computed micro-tomography (CT) is a powerful technique for studying the processes of crack propagation in nonhomogenous quasi-brittle materials such as rocks. To obtain all the significant information about the deformation behaviour and fracture characteristics of the studied rocks, the use of a highly specialised loading device suitable for the integration into existing tomographic setups is crucial. Since no adequate commercial solution is currently available, a completely newly-designed loading device with a fourpoint bending setup and vertically-oriented scanned samples was used. This design of the loading procedure, coupled with the high stiffness of the loading frame, allows the loading process to be interrupted at any time and for CT scanning to be performed without the risk of the sudden destruction of the scanned sample. This article deals with the use of the 4D CT for the visualisation of crack initiation and propagation in clastic sedimentary rocks. Two types of quartz-rich sandstones of Czech provenance were used for tomographic observations during the four-point bending loading performed on chevron notched test specimens. It was found that the crack begins to propagate from the moment that ca. 80 % of the maximum loading force is applied.