Explanation of an apparent abnormality in fatigue crack growth rate curves in titanium alloys

Abstract

A surprising phenomenon is investigated where titanium alloys exhibit no threshold fatigue crack growth value if K max in the K max-constant testing procedure exceeds a certain value. The crack growth rate increases with decreasing Δ K up to final fracture. The phenomenon was found repeatedly for Ti–6Al–2Sn–4Zr–6Mo above K max=21 MPa√m (equal to 72% of K IC), and its causes were investigated. The same crack growth rates as in the K max-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 Δ K and increasing K max 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.