Chemical and metallurgical aspects of environmentally assisted fatigue crack growth in 7075-T651 aluminum alloy

Effect of Frequency, Environment, and Temperature on Fatigue Behavior of E319 Cast-Aluminum Alloy: Small-Crack Propagation

“…Therefore, migration of water molecules is the rate-limiting step. This is consistent with the results by Wei et al, [15,29] i.e., for highly reactive gas-metal systems, such as water vapor and aluminum, hydrogenassisted fatigue-crack growth is controlled by the rate of transport of water molecules to the crack tip.…”

“…It can be seen that crack-growth rates increase with water exposure and then become essentially independent of water exposure, indicating a saturated environmental effect. Similar dependence of crack-growth rate on water exposure was observed for a 7075-T651 aluminum alloy [15] and some Al-Cu-Mg/Li alloys. [25] The calculated water exposure in each environmental condition is summarized in Table II.…”

“…In the model proposed by Wei et al, [15,29] the processes for the increase of fatigue-crack growth rates in the presence of water vapor are described in three steps: first, water molecules migrate to the crack tip by impeded molecular transport, i.e., Kundsen flow; [30] second, water reacts with aluminum on the freshly formed fatigue-crack surface and hydrogen atoms form from the dissociation reaction (Eq. [3]); third, hydrogen atoms diffuse from the crack surface to the crack-tip plastic zone, causing hydrogen-assisted cracking as follows:…”

“…It has been observed that the fatigue-crack growth threshold decreased and fatigue-crack growth rate increased in the presence of water vapor in atmospheric air for aluminum alloys. [10][11][12][13][14][15][16][17] Because the duration of crack-tip opening under ultrasonic-frequency loading at 20 kHz is an order of magnitude shorter for each cycle than for conventional fatigue experiments, any environmentally assisted increase in fatigue-crack growth rate is generally presumed to be less pronounced at 20 kHz, leading to lower fatigue-crack growth rate at this frequency. Holper et al [9] studied the influence of frequency on fatiguecrack growth of aluminum alloys and reported that fatigue cracks propagated at lower growth rates at 20 kHz than at 20 Hz in ambient air if cycled above threshold; however, the test frequency had no influence on the fatigue-crack growth threshold itself.…”

Investigation on some factors affecting crack formation in high resistance aluminum alloys

“…This assumption is confirmed by the analyses of the fracture surfaces that highlighted the presence of pit having a particular morphology. These pits can form on fracture surfaces by means of acidic solution etching [13][14][15]: in the considered case they formed during anodization on the existing fracture surface. Usually the morphology of pits originated on fracture surfaces by an ad hoc etching give information about the fracture propagation plane.…”