Size effects on the tensile strength and fracture toughness of granitic rock in different tests

This study investigates the tensile failure mechanisms in granitic rock samples at different scales by means of different types of tests. To do that, we have selected a granitic rock type and obtained samples of different sizes with the diameter ranging from 30 mm to 84 mm. The samples have been subjected to direct tensile strength (DTS) tests, indirect Brazilian tensile strength (BTS) tests and to two fracture toughness testing approaches. Whereas DTS and fracture toughness were found to consistently grow with sample size, this trend was not clearly identified for BTS, where after an initial grow, a plateau of results was observed. This is a rather complete database of tensile related properties of a single rock type. Even if similar databases are rare, the obtained trends are generally consistent with previous scatter and partial experimental programs. However, different observations apply to different types of rocks and experimental approaches. The differences in variability and mean values of the measured parameters at different scales are critically analysed based on the heterogeneity, granular structure and fracture mechanics approaches. Some potential relations between parameters are revised and an indication is given on potential sample sizes for obtaining reliable results. Extending this database with different types of rocks is thought to be convenient to advance towards a better understanding of the tensile strength of rock materials.