Effect of Pre-Strain on Fatigue Crack Growth in AA2024-T351

The objective here is to study the effect of pre-strain on fatigue crack growth (FCG) in 2024-T351 aluminium alloy. Three pre-strain conditions were considered: without pre-strain, compressive and tensile permanent pre-strains of 4% . A numerical approach based on cumulative plastic strain at the crack tip was followed to predict FCG rate. The compressive pre-strain increased FCG rate, while the tensile pre-strain reduced the da/dN relatively to the conditions without pre-strain. A strong link was established between the influence of pre-strain and plasticity induced crack closure. This relevant conclusion was based on the linear trend obtained between da/dN and Keff for three crack lengths (a=16.184; a=15.048 mm and a=15.152 mm) and three the pre-strain conditions. The increase of stress ratio from R=0.1 to R=0.5 and the elimination of the contact of crack flanks reduced significantly the effect of pre-strain, also pointing for the huge relevance of crack closure in this context. The effect of pre-strain on da/dN after an overload was also explained by crack closure variations.