R-curve behavior in notched beam tests of rocks

The R-curve for sandstone is obtained from the load-crack mouth opening response of notched specimens subjected to three-point-bending. This approach is used to analyze the fracture behavior under monotonic and cyclic loading. The asymptotic limit of the R-curve compares well with the fracture toughness determined through an effective crack model. The analysis of the relaxation observed before the unloading–reloading cycles in the cyclic tests leads to the conclusion that the fracture toughness remains practically constant while the crack propagates slightly during the load drop.