Tensile opening mode fracture toughness measurements and size effect investigations with Brazilian disc type rock specimens

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2020
Şar, Elifnaz .
The flattened Brazilian disc(FBD)geometrieswere subjected to compressive loads at the loading ends. Tensile stresses were generated indirectly at the center of the disc specimens. Cracks were initiated, and they propagated stably to the onset of unstable fracture. Loads recorded at this stage were used in measuring the tensile opening mode fracture toughness.The specimen size effect onmode Ifracture toughnessof Gölbaşı andesite was investigated by changing specimen diameters.The boundary influence on fracture toughness was examinedbymachining flat compressive loading ends of various widthson specimens.Numerical modeling wasperformed with the ABAQUS finite element program to compute mode I stress intensity factorsof FBD geometriesofvarious sizes. With the evaluation of numerical results,two different equations in terms of loading anglehave been developed, one of which isused to calculatethe critical dimensionless crack length and the other to determine themaximum dimensionless stress intensity factor.For examining size effect phenomena for FBD geometry, the fracture toughness tests were carried out on Gölbaşıandesite specimensof different sizes. The specimen diameters were 54 mm,75 mm, 100 mm, and 125 mm.The loading angles ranged vifrom 16° to 42°. The thickness to diameter ratio {u1D461}/{u1D437}was kept constantataround0.6 in all fracture toughness tests.The average mode I fracture toughnessvaluesof Gölbaşıandesite were 2.03±0.20 MPa√{u1D45A}, 2.33±0.13 MPa√{u1D45A}, 2.85±0.16 MPa√{u1D45A}, and 3.26±0.39MPa√{u1D45A}for 54 mm, 75 mm, 100 mm and 125 mm diameters, respectively.The test results indicated thatan increase in specimen diameter from 54 mm to 125 mm led to an approximately 60.6%(3.26/2.03=1.606)increase in mode I fracture toughness of Gölbaşı andesite.