Pure tensile fracture modelling and toughness measurements on brazilian discs of andesite and marble

Özdoğan, Cansın
In fracture mechanics, mode I loading type is characterized by opening of cracks in tension. Considering the convenient availability samples from drilling work of the site investigations, core-based specimen geometries are commonly used in rock fracture testing. Three- or four-point bending loading and compressive loading are major loading configurations for fracture toughness testing. For the investigation of geometric parameters in tensile fracturing of Brazilian disk type geometries, Flattened Brazilian disc (FBD) method is employed for mode I fracture toughness testing. This method is chosen due to simple specimen preparation procedure and loading configuration. Brazilian disc geometry is constructed with opposite flat ends corresponding to specific loading angles from the specimen center. Testing with this core-based geometry can be conducted without pre-cracking. Disc with flattened ends is subjected to compressive loading along vertical diametric line. Compressive loading generates a centerline crack under tensile opening mode. Numerical modeling is conducted to compute mode I stress intensity factors for the centerline crack initiation. Parametric relations are proposed to express stress intensity factor in terms of the geometric entities of the samples. Improved equations established the relations between the stress intensity factor and loading angle for a wider range of loading angles than those of the previous work. The range was expanded from 2° to 50°. Two rock types as Ankara andesite and Afyon marble were included in the testing work. Average mode I fracture toughness was measured as 2.58±0.34 MPa√m and 3.43±0.27 MPa√m, respectively for 75 mm core specimens of andesite and marble. For 100 mm diameter samples, mode I toughness values were 3.34±0.15 MPa√m for andesite and 3.04±0.70 MPa√m for marble. Specimen size effect on the toughness was investigated by employing two different diameters of 75 mm and 100 mm for andesite and marble core samples. The size effect is observed to be around 30% (3.34/2.58=1.29) for andesite having 75 mm and 100 mm diameter.