Investigation of Beam Specimen Geometries under Four Point Asymmetric Bending for Shear Mode Fracture Toughness Measurements of Rocks

Four-point asymmetric bending beam geometry can conveniently be used in shear mode fracture testing of rock and concrete-like materials. Stress intensity factors are computed by contour integration procedure embedded in a finite element program package. To achieve pure shear mode state, loading configuration and parameters are varied in models of rectangular beams. Beam depth is varied as 40, 50, and 60 millimeters. Initial crack length/beam depth ratio (a/W) is varied between 0.15 and 0.60. Pure shear conditions at crack are generated by changing asymmetric loading spans at upper and lower boundaries of the beam geometry. Computations showed that as beam depth was increased, mode II stress intensity factor decreased. Mathematical relations for pure shear mode stress intensity factor were proposed with varying asymmetric loading span and a/W to assure pure shear mode state at crack front. Proper loading and beam geometric conditions were identified for pure mode II fracture toughness KIIc testing. From four-point asymmetric bending tests on Ankara andesite, average mode II fracture toughness value was found as 0.61 MPa√m. Results of shear mode fracture toughness tests showed that variation of beam depth had no significant effect on mode II fracture toughness of beam type rock specimens