Rapid crack propagation in a high strength steel

The relation between fracture velocity and the energy dissipated by unstable fractures in high strength 12.7 mm-thick plates of SAE 4340 steel has been measured using the wedgeloaded double-cantilever-beam (DCB) specimen. The experiments are analyzed using the dynamic beam-on-elastic-foundation model. In agreement with the model, steady-state crack velocities are attained. In addition, the theoretical velocity-arrest length relation is closely obeyed. Statically calculated values of the stress intensity at arrest,K a, are relatively invariant, but in view of the kinetic energy contribution, are not regarded as a materials property. Increases in crack velocity up to ∼860 ms-1 are accompanied by a 2-fold increase in dynamic toughness (a 4-fold increase in the dynamic fracture energy) and by corresponding increases in the size of the shear lips. Measurements of the plastic work associated with the shear lips show that the per-unit-volume shear lip fracture energy, φSL = 0.21 J/mm3, is essentially constant over this range of velocity. These agreements imply that kinetic energy imparted to the test piece during propagation is substantially recovered and makes a significant contribution to the crack driving force.