2019 Volume 60 Issue 11 Pages 2346-2352
Three aluminum alloys in the T6 state (6061, 6082, and 7075) were subjected to static and cyclic loading conditions in humid and dry air environments and the corresponding crack growth characteristics were investigated. The dominant crack growth mechanisms, i.e., stress corrosion cracking (SCC) and fatigue crack growth (FCG), were identified. Three types of testing machines were used to apply the cyclic loading at high stress ratio and frequencies ranging from 0.02 Hz to 20 kHz. Crack growth mechanisms are discussed based on the traditional model proposed by McEvily and Wei. Various test results revealed that SCC is the dominant crack growth mechanism at 0.02 Hz, whereas FCG is dominant at 20 Hz and 20 kHz. These results suggested that the crack growth behavior is controlled by either SCC or FCG with no mutual interaction. Very slow crack growth (rate: <10−13 m/cycle) occurred during ultrasonic fatigue tests performed at 20 kHz. For each alloy and stress ratio considered, this slow growth occurred only in humid air and at low values of the stress intensity factor. Scanning electron microscopy observations suggested that, owing to the numerous cyclic loads, the very slow crack growth behavior in the humid environment occurs via peeling-off of the oxide film near the crack tip.
