The semi-circular bending (SCB) test is one of the useful testing methods for determining mode-I fracture toughness of rocks. A SCB specimen with an artificial notch is loaded at three points including lower two points and upper single point in the test. In general, there are two types of geometry in artificial notch: straight notch and chevron notch. The straight notch in the SCB test is adopted in the suggested method for estimating of mode-I fracture toughness of rocks in ISRM. On the other hand, the cracked chevron notch SCB (CCNSCB) test using a specimen with a chevron notch has been proposed. However, the stress intensity factor on the CCNSCB test has not been made clear, sufficiently considering cracking such as initiation/propagation and geometry of critical crack front at maximum load.
In this paper, by means of software ABAQUS, a cracking behavior from the tip of artificial notch on CCNSCB test is analyzed by XFEM in order to clarify the geometry of crack front in a process of cracking. Analyzing stress intensity factors of specimen with the calculated geometries of crack front during the cracking process by FEM, the relation between crack length and stress intensity factor is obtained precisely. Using this relation, the minimum stress intensity factor at a critical crack length is determined for estimating mode-I fracture toughness of rocks on CCNSCB test.
High-level nuclear waste disposal and caprock in carbon capture storage must be designed considering its long-term stability. For this purpose, it is important to consider the strength of the bedrock around these rockmass structures. For the design of such rockmass structures uniaxial compressive and tensile strengths are used as a rock strength. These are macroscopic mechanical properties. But it is necessary to understand the fracture behavior of rockmass for the design and stability evaluation. In fracture process of rock, fracture toughness, which is microscopic mechanical property and represents resistance to the crack propagation, should be considered.
In order to measure the fracture toughness of rock, several methods have been proposed by the International Society for Rock Mechanics (ISRM): Chevron bend (CB) test and Short Rod (SR) test by Ouchterlony (1988); Cracked Chevron Notched Brazilian Disc (CCNBD) test by Fowell (1995); Semi-circular Bend (SCB) test by Kuruppu, Obara et al. (2014) and so on. The SCB testing method which was proposed originally by Chong and Kuruppu (1984) has recently received much attention by researchers. This specimen is a semi-circular disk and has a straight notch (Cracked Straight Through SCB: CSTSCB) or chevron notch (Cracked Chevron Notched SCB: CCNSCB).
Among above methods, the chevron notch has an advantage over the straight notch. By concentrating stress on the chevron notch tip, it can be fractured at a relatively low load. Then, the propagated crack during the loading is natural rather than artificial. However, the preparation of the chevron notch is relatively difficult compared to that of the straight notch. If the fracture toughness obtained from CSTSCB test is the same as that from CCNSCB test, it is unnecessary to use CCNSCB specimen. However, in the case of CCNSCB specimen, the normalized stress intensity factor used for fracture toughness evaluation has not yet be determined, because that behavior of crack initiation/propagation from the artificial notch and geometry of critical crack front at maximum load have not been made clear.
In this study, firstly, using software ABAQUS, a cracking behavior from the tip of the artificial notch on CCNSCB test is analyzed by an extend finite element method (XFEM) in order to clarify the geometry of crack front in a process of cracking. Secondly using FEM, the stress intensity factor is analyzed for the 3D models with various geometries of the crack front during loading obtained by the XFEM analyses. Then, the relation between the crack length and the stress intensity factor is obtained precisely. From these analyses, the geometry of the critical crack front at the maximum load as well as the minimum normalized stress intensity factor is determined for estimating the mode-I fracture toughness of rock specimens for CCNSCB test.
