ENVIRONMENTAL-EFFECTS ON CRACK CLOSURE IN THE CORROSION-FATIGUE OF AN ALUMINUM - LITHIUM ALLOY

The corrosion fatigue cracking of an aluminium-lithium alloy has been investigated in dry air, 3.5 wt% NaCl and 0.5 M Na2SO4 solutions by measuring the crack length and closure, simultaneously during the crack propagation with a personal computer-aided unloading compliance technique. Fatigue tests were carried out on centre-cracked tension peak-aged alloy specimens (L-T orientation). In the low DELTAK ranges, apparent fatigue crack propagation rates in aqueous solutions were similar to those in dry air. However, the plots of da/dN vs DELTAK(eff) indicated that in the low DELTAK(eff) ranges intrinsic environment-induced fatigue crack propagation rates significantly exceeded intrinsic fatigue crack propagation rates in dry air. The high crack closures appearing in the low DELTAK ranges under dry air and environments were characterized with the tortuous fractured surface and the occurrence of various crack paths such as rolling plane delamination etc., respectively. The rise in intrinsic environmental crack propagation rates under the active conditions was accompanied by the increment of the environmental crack closure relative to the passive conditions. The difference between environmental crack closures under active and passive conditions has been discussed in terms of the combined action of the localized corrosion and environment-assisted crack-tip damage processes.