MICROSTRUCTURAL ASPECTS OF FATIGUE-CRACK PROPAGATION IN AL-ZN-MG ALLOYS

Fatigue studies, performed at 35 and 100 Hz on peak-aged Al-Zn-Mg sing le crystals in vacuum, have shown that crystallographic crack initiati on and propagation dominate under all circumstances. Crack closure mea surements undertaken with these specimens have shown that crystallogra phic cracks propagate in the near-threshold region even in a ''closed manner'', i.e. the effective stress intensity factor DELTAK(eff) appro aches zero. Obviously, the propagation of these cracks cannot be suffi ciently described by a DELTAK(eff) concept any more. Normally, a persi stent slip band is initiated by cyclic deformation on the specimen sur face and acts as seed for a crystallographic crack. The formation is d etermined by mode II which takes an important contribution to the stre ss intensity factor DELTAK. The crystallographic crack will be denoted as an active slip band crack and is able to propagate even in a close d manner. To describe the propagation of these cracks it is proposed t o use a K concept where mode I and II are incorporated.