Abnormal da/dN-Delta K curves: New failure mode with Ti-alloys

Fatigue crack propagation (FCP) -rates and -threshold values have been determined on the titanium alloys IMI 834, IMI 685 (turbine disk materials) and Ti-6A1-6V-2Sn (plate material). K-max-constant tests were executed in laboratory air at room temperature and run with 50 Hz on C(T) specimens. It was found that FCP-rates in K-max-constant tests followed the well known FCP behavior up to a certain limiting value K-max, denoted as degrees K-max. Below degrees K-max, the FCP-rates da/dN decrease with decreasing Delta K down to the threshold value Delta K-T (Delta K for 10(-7) mm/cycle). For K-max-constant tests with K-max > degrees K-max, the FCP-rates initially decreased with decreasing Delta K, but reached 10(-7) mm/cycle at smaller Delta K. For K-max greater than or equal to over capK(max) > degrees K-max, FCP-rates of 10(-7) mm/cycle were never reached as Delta K decreased to and below Delta K-T. Instead, as Delta K approaches or gets smaller than Delta K-T, the FCP-rates stay either constant or increase again. The limit value OK,,, for this abnormal FCP-behavior had been determined for IMI 834 to be 22 to 28 MPa root m, for IMI 685 to be 46 MPa root m and for Ti-6Al-6V-2Sn to be 26 MPa root m. The important result from a practical standpoint is the large difference in degrees K-max for comparable Ti alloys, i. e., IMI 834 and IMI 685.