Size effect and other effects on mode I fracture toughness using two testing methods

Mode I fracture toughness (K-IC) 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 K-IC 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 K-IC 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 K-IC 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 K-IC 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.

Automated summary

In this study, the SCB technique was compared with the pCT test for assessing fracture toughness in rocks. Results showed that the pCT test allows for better control of specimen behavior after peak load and yields more repeatable results compared to the SCB test. The influence of notch length ratio on K-IC values was found to be non-significant for medium and large specimens.