Abstract: Closure and propagation properties of fatigue small cracks in double-edge-notched LY12CZ Al-alloy plate specimens were measured. The initiation and growth of small cracks are observed bv using optical microscope. The varying distance between the microhardness indentations at the blanks of the cracks as a function of applied loads could be monitored to determine the crack opening displacements near the crack tip, which were used to obtain the crack opening load. Δk was replaced by ΔKeff = Kmax-Kop in Paris formula to account for the closure effects of small cracks using the experimental values and the Newman's prediction values σop/σmax. The da/dN values thus predicted were in good agreement with the tested values.

Key words: small crack, closure, opening load