Fatigue and fracture behaviour of (Al-Cu-Li) AA2198 under different ageing conditions

Abstract

The effect of artificial ageing on tensile and fatigue properties along with resistance to fracture of aluminium alloy 2198-T3 was investigated. The fatigue and damage tolerance capability of the alloy was studied under different artificial ageing conditions, simulating the natural ageing process which takes place during an aircraft lifespan. Four (4) different ageing conditions corresponding to under-aged (UA), peak-aged (PA) and over-aged (OA) conditions based on the initial, T3 temper were considered. In the PA condition, the conventional yield stress Rp0.2% increased by more than 50 % with a simultaneous 46 % decrease in tensile ductility Af and 44 % decrease in the critical stress intensity factor Kcr. The high cycle fatigue results showed that with increasing artificial ageing time, the fatigue life continuously decreased until PA condition, and substantially increased in the OA. Normalized fatigue stress-life diagram over conventional yield stress underlined the suitability of the T3 temper in terms of constant amplitude fatigue testing. Fatigue crack propagation (FCP) characteristics at low ΔΚ levels were superior in T3 temper, while at medium to high ΔΚ values the UA condition was more damage tolerant, due to the presence of higher precipitates volume fraction and crack propagation promotion by precipitate bypassing mechanism. On the other hand, precipitates shearing at the crack path was observed in the PA condition and it was linked with higher FCP rates. Resistance to fracture evaluated by quasi-static Kcr tests, showed similar behaviour to the FCP tests, and was found to be worst in the PA condition. A small recovery of all damage tolerance properties was noticed for the OA condition.