Shear mode fracture toughness of rock with different core-based specimen geometries

Het, Kıvanç
Shear mode fracture toughness tests on core specimens of gabbro rock were conducted. Tests were conducted under three-point bending and Brazilian type indirect compression load. In order to investigate the reasons for having different fracture toughness values with the different testing methods and geometries, modeling with ABAQUS software was conducted. Stress distributions around the mode II notches were analyzed. In order to assess which specimen geometry results in similar stress distributions as in the stress state of a plate under pure mode II, stress distributions along some paths were compared. Effect of the thickness of the notches on the stress intensity factor was analyzed. In experiments, core specimens had 100 mm diameter and 60 mm thickness, were used. Mode II fracture toughness results were KIIc(CSTBD)=2.35MPa√m and KIIc(CSTBD)=1.76 MPa√m for the Brazilian type CSTBD specimens with notches of 32 mm and 72 mm in length, respectively. Mode II fracture toughness decreased as the notch length increased. This was attributed to the effect of boundary influence on the fracture toughness. Mode I and mode II fracture toughness values for the three-point bend SCB geometry were lower with KIc(SCB)=1.77 MPa√m and KIIc(SCB)=0.92 MPa√m. Another three-point bend circular plate type SNDB geometry method produced closer results to the CSTBD method with KIc(SNDB)=2.10 MPa√m and KIIc(SNDB)=1.18 MPa√m. Reasons for lower fracture toughness with three-point bending loading were explained in terms of having larger yield zones ahead of the notch and cracks, and the boundary influence issue.