Fatigue crack closure and closure development in a high strength aluminium alloy

Abstract

Fatigue crack closure characteristics were studied in a 70i7 aluminium alloy in the as received (AR) and heat treated (HT) conditions with the view of establishing the general closure trends and how well the modified closure parameter AKeff (- K^ax ' K0_) was capable of characterising fatigue crack growth. The results of this work indicated that a significant amount of strain intensification occurred below K0p which implied that AKe££ generally underestimated the stress intensity range experienced at the crack tip during cyclic loading. The major objective of this dissertation was to determin' whether a relationship exists between the distance over which closure develops from zero to a steady state jlcsure value, and the distance over which short crack growth behaviour occurs. Two techniques were used to eliminate closure for a fatigue crack in a compact temion specimen. Closure development was then measured as a function crack length and a steady state closure value was approached tier about 0.19 mm in the AR material and 0.?8 mm in the HT material irrespective of AK. This distance compared well with that distance over which short cracks exhibited "anomalous" behaviour. This implies that anomalous short ctack behaviour is dependent on the distance over which closure develops in this alloy, although microstructural influences may play a rola.