26. A General Approach to Evaluate the Dynamic Fracture Toughness of Materials

In this paper a combined experimental and numerical approach is proposed to evaluate the dynamic fracture toughness of materials. A circular tube specimen, made of Aluminum alloy, 7050-T7651, having a spiral crack on its outer surface is used to demonstrate the technique. A torsional Hopkinson bar is used to generate a dynamic torsion pulse, which in turn creates predominantly a tensile load along the crack line of the specimen. The torque applied on the sample is obtained from strain gages attached on the bars using one dimensional wave equation. Commercial FE package, ABAQUS, is used to simulate the dynamic fracture parameters. In this case the subspace projection method and standard implicit integration in ABAQUS with time increment are used, assuming the system is linear. The angle of twist associated to the torque measured on the specimen is used as input in the model. The dynamic stress intensity factor is determined using minimum strain energy theory. Digital image correlation is used to measure the deformation field near the crack tip. The measured strain/displacement fields are used to determine the exact time at which the crack initiated. The result show that, all the three classical modes of fracture are existing, but mode I is at least one order magnitude higher than the others. Also the dynamic SIF of the Aluminum alloy 7075-T6571 is higher than the quasi-static SIF (i.e K_Ic ^d=1.36K_Ic), The value obtained in this experiment is in well agreement with the values documented in the literature.