Preliminary investigations on dynamic fracture of ductile metals by using electromagnetically driven expanding ring

Experiments referring to dynamic fracture of ductile metals, which is strongly dependent on the loading path, have been performed on the platform of an electromagnetically driven expanding ring. The time of the onset of fracture and the time at which the first complete fracture surface is formed were accurately measured by incorporating a B dot probe. The resistive voltage and current of the ring were measured in the experiment as a function of time, and the ratio between them subsequently yields time dependence of the ring resistance, which manifests itself as a quantity extraordinarily sensitive to localized nucleation and growth of fracture and can be used to verify the related theoretical model of dynamic fracture. The temperature of the specimen was determined by Joule heating and plastic work under an assumption of adiabatic process. Hence, the time of initiation of fracture and the loading path including stress, strain, strain rate, and temperature were determined at the same time. The correlation between the initiation of fracture and the loading path can be used to establish a fracture model. A binary model of fracture was proposed to depict the thermodynamic character of fracture, and a phenomenological fracture criterion was established based on experimental observations. The binary model implies that the fraction of fractured atoms obeys a Fermi–Dirac-like distribution. The key point of the proposed fracture criterion lies in that fracture grows when the departure velocity of neighboring instable points due to expansion of the ring is larger than the velocity of energy transport.