Experimental Investigation on the Size Effects of K_IC in Selected Rock Types Available to Purchase

In this study, two specimen geometries were used to determine the mode-I fracture toughness (K_IC) of rock: Semi-Circular Bend (SCB) specimens subjected to three-point bending, and pseudo-Compact Tension (pCT) specimens loaded in pure tension. We discuss the results of a number of tests performed on four different types of rock (Arcera, Pinacas and Corvio sandstones, and Blanco Mera granite), which served as a basis for comparison between both testing methods. In addition, specimens of 100, 50 and 38 mm in diameter were used in order to assess eventual size effects on K_IC. The effect of notch length was also analyzed by testing specimens with varying notch length ratios. Our experimental results suggest that both specimen size and notch length have an effect on K_IC. K_IC was found to be dependent on the specimen geometry and testing method, with pCT specimens yielding more consistent values than SCB specimens.

Fracture toughness represents the ability of a material to resist crack initiation and propagation. In rock-mechanics related fields, it is used as a parameter for classification of rock material, as an index for fragmentation processes, or as a material property in stability analysis. Developing appropriate testing methods, with the resulting improvement in accuracy of fracture toughness values, has attracted a significant interest in the last years. Worth mentioning among them are the recommendations issued by the International Society for Rock Mechanics (Ouchterlony, 1988; Fowell et al., 1995; Kuruppu et al., 2014).

Since fracture toughness represents a material property, experimental results should be reasonably independent of specimen size. However, a number of authors have reported a size effect in mode-I fracture toughness (K_IC) (Iqbal & Monhanty, 2007; Ueno et al., 2013; Brevik, 2016). For large-enough specimens, the conditions for linear elastic mechanics should prevail and fracture toughness would remain constant. However, these conditions are not met when decreasing the diameter of the specimen below 50 mm (Ouchterlony, 1988). In this contribution, we study the effect of specimen size and notch length on K_IC. Among the published experimental approaches, the Semi-Circular Bend (SCB) test (Kuruppu et al., 2014), as per suggested by the ISRM was selected due to its simplicity in terms of specimen geometry, sample preparation, loading configuration and testing procedure. K_IC values derived with this method were compared with results obtained using an alternative methodology, namely the pseudo-Compact Tension (pCT), in which disc-shaped samples are loaded in pure tension (Muñoz-Ibáñez et al., 2019).