FATIGUE CRACK GROWTH IN INCONEL 718 SUPERALLOY FOIL AT ELEVATED TEMPERATURE

摘要

Brazed superalloy honeycomb panels with thin high-strength face sheets are under development for the thermal protection system of future reusable launch vehicles. These vehicles use the honeycomb panels as an active and load-bearing part of their primary structure. The panels will be subject to repetitive dynamic stresses, caused by mechanical, thermal and acoustic loading. One of the most important properties with respect to long-term dynamic loading is fatigue crack growth resistance. However, crack propagation in superalloy foils, used in lightweight panels, is still relatively unexplored. This paper reports on the fatigue crack growth behavior in 250 μm thick Inconel 718 foil at RT, 400℃, and 550℃. The principal goals of the paper are to describe the technique used for cyclic crack growth rate testing and to report cyclic fatigue crack growth rates at elevated temperature. The relationship between da/dN and ΔK obeys the Paris law da/dN = C ΔK~m but the values measured for the exponent, m, are higher than 5. This is significantly higher than the exponents typically found for fatigue crack growth in thicker Inconel 718 specimens. A possible explanation for the strong increase of the da/dN with ΔK is the low fracture toughness of Inconel 718 foil. The measured value for the foil (35 MPa m~(1/2)) is less than half of the reported plane strain fracture toughness.