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

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 Delta K up to final fracture. The phenomenon was found repeatedly for Ti-6Al-2Sn-4Zr-6Mo above K-max = 21 MPa root m (equal to 72% of K-IC), and its causes were investigated. The same crack growth rates as in the K-max-constant lest 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 alpha and transformed beta grain boundaries increases with decreasing Delta K and increasing K-max value, accompanied by creep deformation in the transformed beta 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. (C) 1999 Published by Elsevier Science Lid on behalf of Acta Metallurgica Inc. All rights reserved.