Investigation of proper specimen geometry for mode I fracture toughness testing with flattened Brazilian disc method

Abstract

Investigation of geometrical parameters for flattened Brazilian disc method is important, since this is a simple and attractive method for mode I fracture toughness testing on rock cores. Evaluating numerical modeling results, a parametric equation in terms of principal stresses at the center of the disc and the loading angle of the flattened end was developed. An equation was proposed for maximum stress intensity factors at critical crack lengths around stable to unstable crack propagation. Comparing fracture toughness results of flattened Brazilian disc method to the results of the suggested cracked chevron notched Brazilian disc method, geometrical parameters for flattened Brazilian discs were investigated. Diameter, loading angle of flattened ends, and thickness of andesite rock core specimens were changed to obtain comparable results to the suggested method. The closest results to the suggested method were obtained by 54 mm diameter discs with loading angles larger than 32 • , and thicknesses between 19 and 34 mm. Results were confirmed by the flattened Brazilian disc tests on a marble rock. In flattened Brazilian disc tests with smaller loading angles and larger diameters, larger fracture toughness values than the results of the suggested cracked chevron notched were obtained. However, excluding tests with large loading angles over 27 • ; specimen size was less effective on the results of these tests. Critical crack C. Keles (B) · L. Tutluoglu length parameters computed from modeling and experiments were close to each other for the flattened Brazilian disc specimens with smaller loading angles around 20 • and thickness/radius ratio equal or less than 1.1.

An experimental and theoretical comparison of CCNBD and CCNSCB specimens for determining mode I fracture toughness of rocks

“…Chevron bending (CB) method 16,17 Three-point bending Chevron-notched short rod (SR) method 16,17 Direct tension Cracked chevron notched Brazilian disc (CCNBD) method [18][19][20][21][22][23][24][25] Brazilian-type compression Cracked straight-through Brazilian disc method [26][27][28][29][30][31] Brazilian-type compression Modified ring method 32 Brazilian-type compression Hollow centre cracked disc method 33 Brazilian-type compression Cracked chevron notched semicircular bending (CCNSCB) method [34][35][36] Three-point bending Straight-crack semicircular bending (SCB) method [37][38][39][40] Three-point bending Flattened Brazilian disc method 41 Brazilian-type compression Edge-cracked triangular method 42 Three-point bending Four point bend rectangular beam method 5…”

Influences of the diameter and position of the inner hole on the strength and failure of disc specimens of sandstone determined using the Brazilian split test

“…Because the vertical load is applied in a narrow range of the disc using the above methods, crack initiation may occur at the position of the loading points, which may produce inaccurate results (Fairhurst, 1964;Mellor and Hawkes, 1971). To determine the tensile strength of brittle materials more precisely, discs with different shapes (Fowell, 1995;Lambert and Ross, 2000;Tong et al, 2007;Dai et al, 2010;Keles and Tutluoglu, 2011;Cai, 2013;Surendra, 2013;Hua et al, 2015;Riazi et al, 2015;Lin et al, 2015Lin et al, , 2016 have been proposed, including ring specimens that are placed under a pair of radial loads (Hobbs, 1964;Hudson, 1969). Significant stress and steep stress gradients appear in the specimens, which causes initiation and propagation of the resulting cracks (Wang et al, 2014).…”

Numerical and experimental studies on the effect of loading angle on the validity of flattened Brazilian disc test

“…FBD method is the most convenient method among other methods in terms of specimen preparation, loading type and testing procedures (Keles and Tutluoglu, 2011). FBD specimen and related geometries are illustrated in Figure 1.…”

“…Among compressive tests, flattened brazilian disc (FBD) test is one of the most convenient method for determining fracture toughness of rocks and rock like specimens (Keles and Tutluoglu;2011). However, the validity of FBD test depends on the location of crack initiation, and location of crack initiation is a function of loading angle (2α) ( Figure 1) (Wang and Xing, 1999).…”