| The study fruits in relation to fatigue theory put forward by former researchers were introduced in this paper, especially, the theoretical and practical developments in aspect of effects of hydrogen on fatigue characteristics of titanium alloys. From the basis, the influences of hydrogen on static tensile properties and fatigue properties for two new types of titanium alloys-Ti-2Al-2.5Zr and Ti-4Al-2V were investigated, and the laws of influences of frequency and load ratio on the two types of materials were audited. Additionally, comparison was made to evaluate the intensity of hydrogen effects on the crack propagation rates in conditions of static and dynamic loadings. At last, micro-mechanisms of hydrogen effects on fatigue and geometric and mathematic models were promoted.In the test the annealed sheets of 2.0mm in thickness were employed, and specimens cut from the sheets were parted in four groups by different hydrogen contents, i.e. the natural hydrogen, 100 μg/g, 150μg/g and 270μg/g ones. Varied section specimens were used to test the curves of fatigue lives in different hydrogen and CT specimens were used to test the curves of fatigue crack propagation rates in different hydrogen. Metallographical and fracture morphological analysis were completed after all the tests.Researches reveal that for Ti-2Al-2.5Zr alloy, hydrogen charging results in great drop in fatigue crack initiation lives, in other words the natural hydrogen materials have the best fatigue lives. At the higher fatigue loads, little effects on fatigue lives are found within the hydrogen range of 100~270μ g/g, while at the lower loads, the fatigue lives decrease with the increase of hydrogen contents. Hydrogen has smaller effects on fatigue life of Ti-4Al-2V alloy, and little effects on fatigue lives are found within the hydrogen range of 100-280μ g/g, but specimens with natural hydrogen one exit the lowest fatigue life, which is owing to the lower yield strength. For the lower loads, it is same in the case of Ti-2Al-2.5Zr alloy.Hydrogen contributes the marked influences on the fatigue crack propagation rates of the two titanium alloys. For Ti-2Al-2.5Zr alloy, the higher in hydrogen amounts the lower in stress intensity factor at the shift of rapid crack growth, which is because of hydrides dropping the fracture toughness. For TJ-4A1-2V alloy, materialbearing 280 u g/g hydrogen exits the lowest da/dN in stage of near-threshold crack propagation, which is because of interstitial hydrogen atoms promoting the local plastic deformation, hence making the role of plasticity-induced crack closure bigger. Hydrogen hardly has effects on stable crack propagation for both the alloys.Frequency varying in the range of 3~15Hz has some influences on fatigue accumulating deformation and fatigue lives of hydrogen charged Ti-2Al-2.5Zr, but has little influences for natural hydrogen one. In the case of 116 u g/g hydrogen, rise in frequency makes the drop in cyclic deformation and rise in fatigue life, which is in relation to the mechanism of hydrogen atoms promoting plastic deformation.Load ratio gives the apparent influences on the fatigue deformation and fatigue lives of Ti-2Al-2.5Zr alloy. Higher load ratio makes plastic deformation rate down in the fatigue process, and makes the negative effects of hydrogen bigger.Comparing to the sustained loading cracking (SLC) characteristics of Ti-2Al-2.5Zr alloy bearing 110 u g/g hydrogen, it is found that the effect intensity of H on the crack propagation is distinct in for the fatigue loadings. For the SLC, the well-known mechanism of hydrogen diffusing accumulating in the crack tip and hydrides precipitating process happens, while for fatigue loading, the cyclic accumulating damage has covered the hydrogen effects, therefore the role of hydrogen is far weaker in fatigue loading than in SLC.Generally, hydrogen contributes marked influences on fatigue properties for both of the titanium alloys, but the detailed micro-mechanisms of hydrogen differ from each other in the two materials. In Ti-...
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