SHORT FATIGUE-CRACK GROWTH IN PLAIN AND NOTCHED SPECIMENS OF AN 8090 AL-LI ALLOY

Fatigue tests on notched and plain specimens were carried out under a combined acoustic and optical microscope. Crack initiation in plain sp ecimens occurs invariably at inclusions which act as sites of stress c oncentration. Short crack propagation behaviour shows the usual patter n of accelerations and retardations associated with microstructural va riations. Growth can only be sustained if the applied stress is suffic iently high to overcome the microstructural barriers. For the notched specimens the extent of the notch tip and crack tip plastic zone sizes control crack propagation. A short crack will continue to propagate o nly if its own plastic zone can sustain growth as the crack tip extend s beyond the notch zone, otherwise it becomes non-propagating. Notch p lastic zones are calculated using both approximate and accurate soluti ons. Some microstructural effects are discussed.