Abstract
Mode I fracture toughness (KIC) is an intrinsic material property that quantifies its resistance to tensile fracture propagation. The International Society for Rock Mechanics has endorsed four methods to determine the KIC of rock, namely, the short rod, chevron bend, cracked chevron notched Brazilian disc, and semi-circular bend (SCB) methods. In this study, we compare the results of the SCB technique with those of the recently proposed pseudo-compact tension (pCT) test, which has proven to be convenient for the assessment of KIC in both fragile and ductile rocks. We select the SCB as a benchmark method due to its popularity, simplicity, and straightforward testing configuration. We discuss the results of 146 tests performed with different lithologies (Arcera, Pinacas and Corvio sandstones, and Blanco Mera granite), different sample sizes (100, 50 and 38 mm diameter), and a range of notch lengths. We also assess test repeatability and intercomparability of the results obtained using the two techniques. Compared with the SCB test, the pCT test allows for improved control of the specimen behaviour after the peak load, which provides a greater wealth of fracture mechanics information. pCT specimens yield results with higher repeatability than SCB samples. Although we observe that KIC tends to decrease with an increase in the notch length ratio, this effect appears to be non-significant based on statistical assessments. Accordingly, the corresponding mean KIC values are comparable for medium- and large specimens. The influence of specimen size is more pronounced in the SCB tests, while the pCT tests show less dependence for harder lithologies. Therefore, to set up minimum specimen requirements for fracture toughness testing, in addition to geometrical constraints, some key lithology-dependent properties (strength, mineralogy, grain size, etc.) should also be considered. Further methodological considerations related to test execution are also discussed.
Highlights
Fracture toughness (KC) represents the ability of a material to resist crack initiation and propagation
Acknowledging that further experimental efforts are still needed to fill some critical gaps to better understand the variability in the results of fracture toughness testing methods with rocks, in this work, we present the results of 146 KIC determinations performed with four different rock types and the pseudo-compact tension (pCT) and semi-circular bend (SCB) testing approaches
We experimentally investigated the mode I fracture toughness (KIC) of three sedimentary rocks (Arcera, Pinacas and Corvio sandstones) and one igneous rock (Blanco Mera granite) by means of two different testing methods: the ISRM-suggested SCB method and the pCT technique
Summary
Fracture toughness (KC) represents the ability of a material to resist crack initiation and propagation. The International Society for Rock Mechanics (ISRM) endorses four testing methods: the short rod (SR), chevron bend (CB), cracked chevron notched Brazilian disc (CCNBD) and semi-circular bend (SCB) methods.[1,2,3,4] Due to the simple sample preparation and straightforward methodology, SCB is perhaps the most popular testing method in rock fracture mechanics. Other advantages are the b) simpler sample preparation (straight groove and thin starter notch), in comparison with that of the CB, SR, and CCNBD tests (chevron notch); c) enhanced control of crack propagation, in comparison with CCNBD and SCB tests, in which the post-peak behaviour cannot be recorded; and d) application of pure tensile loading, in comparison with the loadings of the CB (three-point bending), CCNBD (compressive loading) and SCB (three-point bending) tests. The increased ligament area is advantageous in the case of coarse-grained rocks and small specimens
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