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Isothermal and thermomechanical fatigue tests conducted in ultrahigh vacuum on Type 304 stainless steel and A286 alloy have shown significant effects of frequency and combined temperature-strain cycling on fatigue life. Results of isothermal tests at elevated temperatures in the creep range indicate reduced life with decreasing frequency. Combined temperature-strain cycling further reduces fatigue life with respect to isothermal lives measured at the maximum temperature of the applied thermal cycle. Life reductions observed with in-phase thermal cycling (tensile deformation at high temperature, compressive deformation at low temperature) are attributed to grain boundary ratcheting effects resulting from unreversed tensile grain boundary sliding. Results of out-of-phase tests (compressive deformation at high temperature, tensile deformation at low temperature) were confused in Type 304 stainless steel by the occurrence of geometric instabilities in the hourglass specimens. In the low ductility A286 alloy, where geometric instabilities were not observed, out-of-phase life reductions are attributed to grain boundary cavitation resulting from unreversed compressive grain boundary displacements (ratcheting) which could not be accomodated by intragranular deformation of the high-strength low-ductility matrix.
thermal fatigue, fatigue (materials), iron alloys, tests, stainless steels
Assistant project engineer, Materials Engineering and Research Laboratory, Pratt and Whitney Aircraft, East Hartford, Conn.