Abstract
The thresholds of effective stress intensity range ΔK eff th of a recrystallized and cold-rolled aluminium (pure), and aluminium alloys 6061, 7020 and 7075 were studied. A special technique to determine ΔK eff th very accurately was applied. The measured ΔK eff th value was about 0.85 MPa m1/2 in all alloys. The significant difference in the yield stress (it varies between 30 and 630 MPa) and the chemical composition does not influence ΔK eff th.