Dwell Fatigue Design Criteria

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Book Chapter

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Many engineering components operate under steady state conditions for a period of time after the peak loads had been achieved. The time during which the loads are near constant is considered dwell time. Materials exhibit sensitivity to dwell times differently to the tension and compression loads and continue to challenge engineers designing such systems. New dwell sensitivity maps proposed in this paper showing the dwell sensitivity behavior of materials in low cycle fatigue (LCF) where life is determined by the development of plastic/inelastic strains and long crack growth (LCG) which is also known as high cycle fatigue life of components. While dwell sensitivity in LCF was significant in lower ranges of strains, higher strains and dwell time did not lower fatigue life as highly as it was under lower strains. LCF dwell sensitivity was predicted with the use of strength ratio, dynamic balance between the hardening and softening, and dwell time damage parameter. Each of these characteristic features predicted dwell sensitivity.

Dwell sensitivity in LCG has not been investigated as widely as in LCF. A new parameter proposed which measures the sensitivity in LCG called, normalized crack growth ratio (NCGR). It was a ratio of crack growth rates under continuous fatigue and dwell fatigue at the same mode I stress intensity factor range. The LCG dwell sensitivity was found from an order-of-magnitude to four orders-of-magnitude. Therefore, the dwell sensitivity was more significant in LCG. Dwell fatigue crack growth mechanism map was generated for Inconel 718 charting various zones in which a particular damage mode was observed. Remaining life assessment performed for a disk component with dwell fatigue crack growth models showed negligible life extension potential under the assumed conditions. There is a need for further collaboration and consensus development defining and/or refining the input parameters.



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