Fatigue Crack Propagation Rates for Duralumin in Simple Bending

Flat sheet stock of 2024-T3 Duralumin was tested for fatigue crack propagation in simple bending. Transcrystalline cracking was observed at a rate proportional to stress intensity factor squared at low levels where the plastic zone is less than grain size. A mixture of trans-and-intercrystalline cracking was observed at a rate proportional to stress intensity factor to the fourth power at high levels where the plastic zone is larger than grain size. A strain redistribution mechanism giving constant stress intensity factor through the thickness is postulated. The combination of strain redistribution in bending and change of mechanism with plastic zone size leads to a macroscopic behavior which is different from that previously reported for simple tension, particularly in mild steel.