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Effects of Very Low Pressures on Fatigue Fracture Surface Features in Ti-6Al-4V
Summary
This study investigated how hydrogen content and cyclic loading frequency affect fatigue fracture surface features in Ti-6Al-4V titanium alloy tested in vacuum, finding that quasi-cleavage and crack growth rates both increase with hydrogen. Precise matching of fracture surface features suggests that certain pseudo-striations are not true fatigue markings, and that quasi-cleavage proceeds by combined micro-cleavage and microplastic crack coalescence.
The amount and quality of quasi-cleavage and possibly the crack growth rates caused by fatigue of Ti-6Al-4V in vacuum were enhanced by increasing hydrogen content and decreasing cyclic loading frequency. Precision matching studies of pseudo-striations on mating vacuum fatigue fracture surfaces lead to the tentative conclusion that these do not mark successive positions of the crack front, hence are not fatigue striations. Similar studies of quasi-cleavage areas show that tear ridges mate exactly, suggesting that such cracking proceeds by a combination of micro-cleavage crack initiation and microplastic coalescence of the micro-cracks.