Abnormal da/dN – ΔK curves: New failure mode with Ti – alloys

Abstract

Abstract Fatigue crack propagation (FCP) ‐rates and ‐threshold values have been determined on the titanium alloys IMI 834, IMI 685 (turbine disk materials) and Ti‐6Al‐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 °K max . Below °K max , the FCP‐rates da/dN decrease with decreasing ΔK down to the threshold value ΔK T (ΔK for 10 −7 mm/cycle). For K max ‐constant tests with K max > °K max , the FCP‐rates initially decreased with decreasing ΔK, but reached 10 −7 mm/cycle at smaller ΔK. For K max ≧ ∧K max > °K max , FCP‐rates of 10 −7 mm/cycle were never reached as ΔK decreased to and below ΔK T . Instead, as ΔK approaches or gets smaller than ΔK T , the FCP‐rates stay either constant or increase again. The limit value °K max for this abnormal FCP‐behavior had been determined for IMI 834 to be 22 to 28 MPa√m, for IMI 685 to be 46 MPa√m and for Ti‐6A1–6V‐2Sn to be 26 MPa√m. The important result from a practical stand‐point is the large difference in °K max for comparable Ti alloys, i.e., IMI 834 and IMI 685.