Time-Dependent Crack Growth Thresholds of Ni-Base Superalloys
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INTRODUCTION
IN Ni-base superalloys, time-dependent crack growth can occur through several degradation mechanisms, including oxidation,[1–9] hot corrosion cracking,[10] stress corrosion cracking,[11] and creep cavitation[12–17] at elevated temperatures. In addition, time-dependent crack growth can appear as hold-time effects during dwell fatigue[18–21] where time-dependent and cycle-dependent crack growth occurs concurrently. In most cases, timedependent crack growth, also referred to as sustained load crack growth, environmentally enhanced crack growth, or creep crack growth, can be characterized on the basis of the stress intensity factor, K, when the crack-tip damage, creep, or plastic zone is small and constrained within the elastic stress field. For K-controlled crack growth, Ni-base alloys typically exhibit a large-crack growth threshold, Kth, below which time-dependent crack growth does not occur. The magnitude of the Kth is usually higher than the large-crack fatigue crack threshold, DKth.[2,14–17,19] Neither the origin nor the magnitude of the time-dependent crack growth threshold, Kth, is well understood for oxidation, stress corrosion, or creep crack growth. On the other hand, the crack growth rates during dwell fatigue at elevated temperature in air are significantly higher than KWAI S. CHAN, Institute Scientist, is with the Southwest Research Institute, San Antonio, TX 78238. Contact e-mail: kchan @swri.org Manuscript submitted October 10, 2013. Article published online April 5, 2014 3454—VOLUME 45A, JULY 2014
the crack growth rates under pure fatigue in air at the same temperature and stress intensity range, DK, levels. Ni-base superalloys in advanced turbo-propulsion systems operate at severe temperature and environments where competing cycle-dependent and time-dependent crack growth mechanisms may be operative concurrently.[21] Thus, there is a need for a better understanding of the interactions between cycle-dependent and time-dependent crack growth at the respective near-threshold regimes. Cycle-dependent fatigue crack growth and timedependent crack growth are generally treated as two independent processes whose crack growth increment, da, over a mission can be summed according to the expression given by[19–21] da da ðdaÞmission ¼ dN þ dt; ½1 dN cyclic dt where the first term on the right-hand-side of Eq. [1] treats cycle-dependent crack growth, while the second term treats time-dependent crack growth for an arbitrary loading history within a mission. For fatigue crack growth test data generated under a constant frequency with dwell, Eq. [1] can be expressed as[19–21] da da da ¼ þ½tdwell þ 1=f ; ½2 dN dwell dN cyclic dt where tdwell is the dwell time, and f is the frequency of the dwell fatigue cycles. To obtain the crack growth life, Eq. [2] is integrated over the fatigue cycle. The METALLURGICAL AND MATERIALS TRANSACTIONS A
fatigue crack growth rate, da/dN, can be represented in terms of the Paris power-law,[22] as given by da ¼ ADKn dN
for DK>DKth
½3
where DK is the s
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