Microstructures in front of crack tips for various propagation rates and re-evolution of the dislocation under changing propagation crack rates in pure polycrystalline copper with a large grain size

The dislocation structure in front of crack tips for various crack propagation rates and the redevelopment of the dislocation structure after reducing the fatigue load (i.e., after decreasing the crack propagation rate from approximately 5×10−5 to 5×10−7 mm/cycle) are investigated in this study. Pure polycrystalline copper with large grains was used in this study. The materials were annealed at 800 °C for 4 hours in a 10−5 Torr vacuum and then cooled in a furnace. The specimen preparations followed the ASTM E647 instructions for compact tension (CT) specimens. Fatigue crack propagation was achieved using a computerised Instron 8801 hydraulic testing machine at R=0.1 at a frequency of approximately 20 Hz. The results are as follows: (1) The dislocation structure includes misoriented cells next to the crack tips for propagation rates greater than 5×10−5 mm/cycle. When the crack propagation rate is less than 5×10−7 mm/cycle, the dislocation structure next to the crack tips is dominated by a loop patch structure. (2) When crack tips have the same steady crack propagation rate, the dislocation structures are similar regardless of the load history. (3) The dislocation evolution presents an anti-sequence between positive and negative.