Date

2015

Author

Alkan, Uğur

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In rock engineering applications inherent cracks and other type of impurities are seldom under the effect of loads acting along principal directions. Dominant loading states mostly consist of mixed mode type of loads. Mode I loading state has been studied by researchers for a long time. Therefore, common principles have been established for mode I loading state. Shear type (mode II) loading state is still an active subject to investigate in fracture mechanics. Although, numerous test methods have been suggested to determine the mode II fracture toughness K_IIc of a rock, common opinion for mode II loading state is not well-established yet. Beams are well-known structural elements. Mechanical foundations for beams under bending loads are solid. Dimensions like beam length, loading span, thickness, and beam depth can be varied easily to make adjustments to generate pure opening and pure shear mode states around preliminary notches for fracture toughness testing. These advantages make beams appropriate for fracture tests. Four-point asymmetric bending test specimen (FPAB) has a rectangular shaped geometry. Shear mode (mode II) fracture toughness investigations were conducted on rectangular shaped rock specimens under asymmetric bending loads. Tests were carried out under four-point asymmetric bending loads. In order to assure generation of pure mode II stress intensity factor state for FPAB test geometry, numerical modeling with ABAQUS Finite Element Software was conducted. Different sized rectangular shaped rock specimens were prepared to investigate size effect phenomena for FPAB test geometry. Numerical and experimental studies were conducted for three main beam depth groups having different notch lengths. The generic FPAB test geometry which was 120 mm long and 50 mm thick consisted of three different beam depths 40-50-60 mm and included a preliminary single edge notch at the bottom center. Results of pure shear mode fracture toughness values from FPAB test geometry were compared to the ones from SNDB (Straight Notched Disk bending) method testing. The same rock type, namely Ankara Gölbaşı Andesite was used in both. In the models, stress paths were created to analyze potential plastic regions or fracture process zones ahead of the preliminary notch. Von Mises plasticity in the vicinity of notch tip was examined along the potential crack propagation directions of mode I and mode II loading states. Stress paths were beginning from the notch tip and expanding to the outmost contour integral region. Stress paths for mode I and mode II stress intensity factor were compared. Boundary influence effect in rectangular shaped rock specimens under mode I and mode II loading states were compared. Mode II fracture toughness value of Ankara Gölbaşı Andesite was found as K_IIc=0.61 MPa√m for FPAB test geometry. In comparison, mode II fracture toughness value of Ankara Gölbaşı Andesite was found as K_IIc=0.62 MPa√m for the tests with SNDB geometry. Size of the beam specimens was changed by applying three different beam depths. Close results were achieved for mode II fracture toughness values for test geometries with different beam depths. No size effect was observed in shear mode fracture toughness values of tests with different beam depths of FPAB geometry.

Subject Keywords

Rock mechanics., Geotechnical engineering.

URI

http://etd.lib.metu.edu.tr/upload/12619153/index.pdf
https://hdl.handle.net/11511/24988

Collections

Graduate School of Natural and Applied Sciences, Thesis