AbstractThe behaviour of small fatigue cracks in the A1‐Li‐Cu‐Mg‐Zr alloy 8090 has been studied. Growth rates of small fatigue cracks in these alloys were found to be proportional to ΔK2. This relationship is compared to that suggested by Murakami where crack growth rate was proportional to ε2pa. The difference between the equations is attributed to the relative importance of plastic strain for the two sets of data. It is suggested that the two equations are equivalent if crack growth rate is related to total strain. The common observation of a Paris law exponent of two in different alloys suggests that there is a single common mechanism for small fatigue crack growth. This contrasts with long fatigue crack growth, where a variety of Paris law exponents are observed. Other characteristics of this fatigue mechanism include crack propagation on {111} slip bands, insensitivity of crack propagation rate to precipitate distribution and a plastic zone comparable in size to the cra