Mode-I dynamic fracture initiation toughness using torsion load

Abstract

An experimental and numerical approach is proposed to determine the dynamic fracture initiation toughness of materials from a cylindrical specimen with spiral surface crack subjected to dynamic torsional load using a torsional Hopkinson bar apparatus. The torsion load creates predominantly tensile stress perpendicular to the spiral crack of the specimen, resulting in nominally Mode I conditions. The torque applied to the specimen is measured by strain gages attached to the bar and the time at which the crack propagation initiated is measured using stereo imaging and stereo digital image correlation. Using the measured torque and the time of fracture as input, a commercial FE package, ABAQUS, is utilized to analyze the spiral crack and numerically extract the dynamic fracture parameters. A 3D format of the dynamic interaction integral method is utilized to calculate the three components of the applied dynamic stress intensity factors. The result demonstrates that the spiral crack-torsional loading configuration indeed generates nominally Mode I conditions and can be used to measure the dynamic fracture initiation toughness. To demonstrate the proposed method, three aluminum alloys; Al 7050-T6, Al 2024-T3, and Al 6061-T6, were experimentally studied. The results are consistent, repeatable and in good agreement with literature data.