Corrosion pit size distributions and fatigue lives—a study of the EIFS technique for fatigue design in the presence of corrosion

Abstract

Samples of Alclad 7475 aluminium alloy containing a 3.18 mm radius semi-circular edge notch were subjected to artificial and natural corroding environments, producing a distribution of pit depths in the notch from 20 to 150 μm. After corrosion exposure, 20 samples were subjected to constant and variable amplitude loading. The pits reduced fatigue life by 40–50% when compared with lives of uncorroded samples. Measurements of fatigue crack growth rates on cracks greater than 50 μm in length showed similar rates in both corroded and uncorroded samples. The effect of the pits was to greatly reduce the number of cycles to grow the crack to 100 μm. Multiple crack initiation at pits occurred in almost all tests, resulting in irregular growth as either crack shielding or crack coalescence occurred. Equivalent initial flaw size distributions (EIFSD) were calculated from the life data. Life predictions made from the EIFS distributions were non-conservative. The errors are shown to result from multiple initiation and crack coalescence behaviour. The applicability of EIFS approaches to design against pitting corrosion is discussed.