Explanation of an apparent abnormality in fatigue crack growth rate curves in titanium alloys |
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| Affiliation: | 1. Institute of Physics of Materials ASCR, Žižkova 22, CZ-61662 Brno, Czech Republic;2. CEITEC IPM, Žižkova 22, CZ-61662 Brno, Czech Republic;1. Charles University, Faculty of Mathematics and Physics, Department of Physics of Materials, Ke Karlovu 5, 12116 Prague, Czechia;2. Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, 12116 Prague, Czechia;3. Institute of Thermomechanics, Czech Academy of Sciences, Dolejškova 5, 18200 Prague, Czechia;4. Institute of Plasma Physics, Czech Academy of Sciences, Za Slovankou 1782/3, 18200 Prague, Czechia;1. Department of Materials Science and Metallurgical Engineering, Indian Institute of Technology Hyderabad, Kandi, Sangareddy, 502285, Telangana, India;2. Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee, 247667, Uttarakhand, India;3. Titanium Alloy Group, Defence Metallurgical Research Laboratory (DMRL), Kanchanbagh, Hyderabad, 500058, Telangana, India;1. Department of Materials Science and Metallurgical Engineering, Indian Institute of Technology Hyderabad, Kandi, Sangareddy 502285, Telangana, India;2. Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India;3. Titanium Alloy Group, Defence Metallurgical Research Laboratory (DMRL), Kanchanbagh 500058, Telangana, India |
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| Abstract: | A surprising phenomenon is investigated where titanium alloys exhibit no threshold fatigue crack growth value if Kmax in the Kmax-constant testing procedure exceeds a certain value. The crack growth rate increases with decreasing ΔK up to final fracture. The phenomenon was found repeatedly for Ti–6Al–2Sn–4Zr–6Mo above Kmax=21 MPa√m (equal to 72% of KIC), and its causes were investigated. The same crack growth rates as in the Kmax-constant test were reproduced by two independent experimental procedures, the so-called “jump” test and sustained K cracking experiments along with a calculation. It is demonstrated that the observed phenomenon is not a special crack growth feature or a new phenomenon, but simply caused by time-dependent crack growth, which is known to exist in titanium alloys or steels. Fractographic work revealed that intergranular crack growth along α and transformed β grain boundaries increases with decreasing ΔK and increasing Kmax value, accompanied by creep deformation in the transformed β grains. The conditions for time-dependent cracking are believed to be a sufficiently high stress and strain field in the crack tip region, along with hydrogen-assisted cracking. |
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