Geometry factors for Mode I stress intensity factor of a cylindrical specimen with spiral crack subjected to torsion

•A geometry factor is proposed for extracting the Mode I stress intensity factor from experimental data obtained during torsional loading of spirally-cracked solid and tubular cylindrical specimen.•The proposed spirally cracked specimen can be used to study the plane strain fracture toughness of all engineering materials, including in the case when there is no sufficient material volume to make thick, standard samples required to approximately establish plane strain conditions.•The method works for both shallow and deep surface cracks so that one can obtain nominally plane strain Mode I fracture toughness results for a broad range of industrially relevant fracture geometries. A geometry factor is proposed for extracting the Mode I stress intensity factor from experimental data obtained during torsional loading of solid and tubular cylindrical specimens with a spiral crack on the surface. Using torque at fracture and specimens geometry as an input, the stress intensity factor at the corresponding fracture load was determined using a finite element analysis based on interaction integral method. The computed Mode-I stress intensity factor and the measured fracture load are used to quantify the geometry factor for different depths of spiral crack in cylindrical specimens following Benthem’s circumferential crack solution approach. The proposed model was validated by testing a polycarbonate specimen and compared it with a conventional three-point bending method. The difference between results from the proposed formula and the standard method was about 1.7% and 4.1% for solid and tubular specimens respectively. Furthermore, the fracture toughness value of different materials in the open literature was compared to the results recalculated by using the proposed formula. The result is in good agreement with different materials considered with a maximum difference of less than 6%.