Mechanisms of fatigue crack growth in 7050-T6 aluminium alloy

• da / dN -Δ K curves in AA7050-T6 shows three linear regions in the Paris law regime. • Ductile striation is observed at high Δ K , while cleavage occurs at low Δ K. • A lower boundary for the dominance of ductile FCG is proposed. • Ductile propagation fails when the cyclic plastic zone is similar in size to the grain size. Fatigue crack growth (FCG) is usually associated with cyclic plastic deformation at the crack tip. However, this damage mechanism loses dominance for relatively low stress intensity factor ranges ( ΔK ), and a lower boundary must be considered in numerical modelling. So far, the definition of this lower boundary and its variation with the stress ratio remain unclear This paper studies FCG in CT specimens made of 7050-T6 aluminium alloy, submitted to different load ratios. A multi-linear behavior was observed in Paris law regime, independently of stress ratio. SEM analysis showed that ductile striations are observed at high ΔK , while at low ΔK a cleavage mechanism is observed. A numerical analysis was developed, assuming the dominance of cyclic plastic deformation, which overestimated the experimental results obtained for low Δ K . The comparison of the size of cyclic plastic zone, r pc , with the grain size, ρ, indicated that plastic deformation loses dominance when r pc < ρ .