EXPLANATION OF AN APPARENT ABNORMALITY IN FATIGUE CRACK GROWTH RATE CURVES IN TITANIUM ALLOYS

摘要

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 AK 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 experimen- tal 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 phenom- enon, 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 βgrain boundaries increases with decreasing ΔK 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.