Üç noktalı eğme örnek geometrileri için çekme modu çatlak tokluğu üzerinde destek aralık mesafesinin etkisi

Straight notched disk bending specimen geometry has a potential to find size independent mode I fracture toughness, since the thickness of the disk loaded by threepoint bending can be increased. Increasing the thickness and decreasing the loading span of bending reduces the severe bending action at the notch tip and a pure tensile mode stress state can be achieved at and ahead of the crack tip. 3D finite element program (ABAQUS) was used for computing the stress intensity factors by numerical modeling of circular disk geometry (SNDB). Modeling procedure and stress intensity computation work were verified by 3D modeling of square plate and semi-circular bend geometries under three-point bending loads and comparing the SIF results to the relevant analytical solutions of these two geometries. Tensile mode fracture toughness tests were conducted on notched circular plate type disk and semi-circular disk andesite rock specimens. Tests were carried out under three-point bending loads. 0.5 mm wide notches providing a crack length/radius ratio of a/R=0.20 were machined to the specimens by a saw. Mode I fracture toughness tests were performed on straight notched disk bending (SNDB) disk specimens of thicknesses 50 mm and 60 mm. Loading spans were varied between 40 mm and 90 mm corresponding to span/radius ratios of S/R=0.40 to 0.90. Using the same a/R and S/R configurations, three-point bend tests were conducted with the ISRM suggested semi-circular bending (SCB) core specimen geometry for comparison purposes. SCB tests were conducted with 50 mm thickness as suggested. Ankara Gölbaşı Andesite rock type was used in both test series. Mode I fracture toughness values of Ankara Gölbaşı Andesite were found as KIc=1.39 𝑀𝑃𝑎√𝑚 and KIc=1.23 𝑀𝑃𝑎√𝑚 for 60 mm and 50 mm thickness SNDB test specimens, respectively. In comparison, mode I fracture toughness value was found as KIc=0.93 𝑀𝑃𝑎√𝑚 for the tests with SCB specimen. The significant observation is KIc increases with decreasing span length for SNDB and SCB specimens. Another observation is the mode I fracture toughness value increases with increasing specimen thickness for SNDB. A tendency to the size independent fracture toughness value of 1.53 𝑀𝑃𝑎√𝑚 was observed as the result of the curve fitting process on the available test data.