Frequency interactions in high-temperature fatigue crack growth in superalloys
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INTRODUCTION
A L L O Y 718 is a nickel-based superalloy characterized by high creep resistance and good mechanical properties at elevated temperatures up to 923 K. tl.2,3] It is widely used in different industries, including aerospace applications where alloy 718 represents the main material in manufacturing hot section components of gas turbine engines. In these components, the load interaction effects due to sequential high/low frequency loadings on crack growth behavior, although important in design and failure prediction considerations, have received little attention. VanStone e t al. I41 have studied crack growth transients in alloy 718 where one type of cycling using a given waveform is followed by another waveform. In their investigation, fatigue loading with a 0.33 Hz cycle with a superimposed hold time of 300 seconds at Kmax was followed by a pure 0.33 Hz cycle at a constant gma x level higher than the one achieved in the prior cycle. It was observed that upon the change in waveshape, the crack achieved transient growth rates higher than the subsequent steady-state value under constant Kmax c o n d i t i o n s . This effect was observed at both 866 and 923 K and demonstrates the existence of a damage state ahead of the crack tip due to sustained loads which is more degraded than the one obtained under pure fatigue cycling. The influence of low frequency cycle loading on the subsequent high frequency crack growth behavior has also been investigated by Ghonem e t al. tSl for the case of alloy 718 at 923 ~ They have utilized constant load spectra, the basic structure of which is the repetition of a low frequency loading (0.05 Hz) with a superimposed 300 seconds hold time at maximum load level followed by a high frequency loading of 30 Hz. The time durations of both low and high frequency loading blocks were H. GHONEM, Professor, Department of Mechanical Engineering, and D. ZHENG, Research Associate, Mechanics of Solids Laboratory, are with the University of Rhode Island, Kingston, RI 02881. Manuscript submitted March 10, 1992.
METALLURGICAL TRANSACTIONS A
identical. It was observed that under all load conditions, the crack growth rate corresponding to the high frequency part of the cycle is higher than that corresponding to a pure, continuous cyclic loading with the same frequency. This accelerated crack growth behavior was interpreted as being a result of stress concentration positions produced along the unsmooth intergranular crack front at the end of the low frequency part of the loading spectrum. This conclusion was based on observations made on the fracture surface showing finger-like morphologies at different positions along the crack front within the low/high frequency transition regions. Once these regions attained a smooth front due to the transgranular fracture mode associated with high frequency loading, the crack growth rate became equal to that corresponding to continuous high frequency cycle at the same AK value. Furthermore, the validity of the linear summation rule was examined by calcu
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