Evaluation of Mode I Fracture Toughness Assisted by the Numerical Determination of K-Resistance

Abstract

The fracture toughness of a rock often varies depending on the specimen shape and the loading type used to measure it. To investigate the mode I fracture toughness using semi-circular bend (SCB) specimens, we experimentally studied the fracture toughness using SCB and chevron bend (CB) specimens, the latter being one of the specimens used extensively as an International Society for Rock Mechanics (ISRM) suggested method, for comparison. The mode I fracture toughness measured using SCB specimens is lower than both the level I and level II fracture toughness values measured using CB specimens. A numerical study based on discontinuum mechanics was conducted using a two-dimensional distinct element method (DEM) for evaluating crack propagation in the SCB specimen during loading. The numerical results indicate subcritical crack growth as well as sudden crack propagation when the load reaches the maximum. A Kresistance curve is drawn using the crack extension and the load at the point of evaluation. The fracture toughness evaluated by the K-resistance curve is in agreement with the level II fracture toughness measured using CB specimens. Therefore, the SCB specimen yields an improved value for fracture toughness when the increase of K-resistance with stable crack propagation is considered.Keywords Fracture toughness Á SCB specimen Á Fracture process zone Á DEM Á K-resistance curve List of Symbols

Effect of High Temperature (600°C) on Mechanical Properties, Mineral Composition, and Microfracture Characteristics of Sandstone

“…Advances in Materials Science and Engineering toughness value of the rock will be underestimated. e test results of many SCB specimens have also shown that the fracture toughness value is low [53]. erefore, it is not appropriate to directly apply linear elastic fracture theory to analyze rocks.…”

Combined numerical and experimental mechanical characterization of a calcium phosphate ceramic using modified Brazilian disc and SCB specimen

“…This method was recently used by the International Society for Rock Mechanics (ISRM) in determining fracture mode I toughness of brittle materials because of its inherently favourable characteristics, such as simplicity, minimal machining requirements and easy testability through the application of three-point compressive loading using a standard test frame [39], [46], [30][31][32][33][34][35][36]. The fracture toughness K IC values of SCB with straight crack are calculated with the following formula:…”

“…The cracked straight through Brazilian disc (CSTBD) specimen (Fig.1c) were employed to study the stress intensity factor K I under tensile stress (mode I rupture) and stress intensity factor K II under shear stress (mode II rupture) [25][26][27][28] Semi-circular bend (SCB) (Fig.1b) specimens were used to experimentally determine the maximum flexural strength "σ f" [29] and tensile strength "σ t" [30]. Mode I fracture toughness K IC was determined by adding a crack to the semi-disc SCB [30][31][32][33][34][35][36] (Fig.1a). Compression tests were conducted on cylindrical blocks to determine compressive strength "σ c "and compressive modulus "E c " [17,23].…”

Experimental and numerical investigation of cracked chevron notched Brazilian disc specimen for fracture toughness testing of rock

“…For rock materials, crack propagation can be easily induced by mode I loading; thus, most studies of rock fracture toughness focus on mode I . Particularly, four different laboratory test methods have been successively recommended by the International Society for Rock Mechanics (ISRM) to quantify K Ic of rock, including chevron bend (CB) method, short rod (SR) method, cracked chevron notched Brazilian disc (CCNBD) method, and semicircular bend (SCB) method . Compared with the CB and SR methods, the CCNBD has simpler specimen preparation process, fewer restrictions on the testing apparatus, easier testing operations and no need of a special fixture.…”

“…[1][2][3][4][5] Particularly, four different laboratory test methods have been successively recommended by the International Society for Rock Mechanics (ISRM) to quantify K Ic of rock, including chevron bend (CB) method, 6 short rod (SR) method, [6][7][8] cracked chevron notched Brazilian disc (CCNBD) method, [9][10][11][12][13][14][15] and semicircular bend (SCB) method. [16][17][18][19][20][21][22][23][24][25] Compared with the CB and SR methods, the CCNBD has simpler specimen preparation process, fewer restrictions on the testing apparatus, easier testing operations and no need of a special fixture. Furthermore, as a chevron notched specimen, the CCNBD can avoid the difficulty of precracking the specimen or fabricating a sharp crack.…”