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A B S T R A C T A controlled overaging process is proposed to increase region II fatigue crack propagation resistance of 2024-T3 aluminium alloy. Overaging was achieved by subjecting the material from the initial T3 state to heat treatment at specific aging temperatures resulting in substantial reduction in hardness. Fatigue crack growth tests were subsequently performed in the intermediate ΔΚ region to assess the influence of aging treatment on fatigue crack propagation rate. The experimental results showed that overaging at high temperatures enhances the fatigue crack growth resistance of the material with regard to initial T3 state. Fatigue crack growth rates were found to decrease with increasing overaging temperature. Cyclic stress strain tests were performed to assess the impact of the performed overaging on cyclic behaviour. The results revealed that cyclic strain hardening is enhanced in the overaged material, contributing to increased fatigue crack closure levels.A 25 = elongation at fracture b = fatigue strength exponent C = parameter of Paris equation c = fatigue ductility exponent E = Young's modulus h = cyclic hardening capacity as a percentage increase of yield strength H = strength coefficient H' = cyclic strength coefficient m = Paris equation exponent n = strain hardening exponent n' = cyclic strain hardening exponent N f = fatigue life p = cumulated plastic strain P max = maximum applied load P min = minimum applied load P op = crack opening load Q = maximum amount of hardening R = stress/strain ratio V = crack opening displacement at points A,B in C(T) specimen V max = crack opening displacement at points A,B in C(T) specimen corresponding at P max V min = crack opening displacement at points A,B in C(T) specimen corresponding at P min V op = crack opening displacement at points A,B in C(T) specimen when crack surfaces come into contact β = driving rate of hardening ΔV = crack opening displacement range at points A,B in C(T) specimen Δε = total strain range Δε p = plastic strain range ΔΚ = stress intensity factor range Correspondence: A. Tzamtzis.

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A B S T R A C TThe present work aims to provide evidence of corrosion-induced hydrogen embrittlement of the aircraft aluminium alloy 2024. An extensive experimental investigation involving metallographic and fractographic analyses as well as mechanical testing was performed. The corrosion exposure led to a moderate reduction in yield and ultimate tensile stress and a dramatic reduction in tensile ductility. Metallographic investigation of the specimens revealed a hydrogen-rich embrittled zone just below the corrosion layer. Furthermore, fractographic analyses showed an intergranular fracture at the specimen surface followed by a zone of quasi-cleavage fracture and further below an entirely ductile fracture. Mechanical removal of the corroded layers restored the yield and ultimate stress almost to their initial values but not the tensile ductility. The tensile ductility was restored to the level of the uncorroded material only after heat treatment at 495 • C. Measurement of hydrogen evolution with temperature showed that by heating the corroded alloy at 495 • C, the trapped hydrogen is released.

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