Thermal Fatigue of a Nickel-Base Alloy

The results of conventional (Coffin-type) thermal-fatigue tests of Hastelloy N are reported. The plastic-strains induced by thermal stresses ranged from a hundred micro-units to more than ten-thousand microunits and correlated well with fatigue life. The slope of the plastic-strain fatigue relationship differs from the conventional slope of minus one half being about −0.9. The thermal-fatigue data are in good agreement with the isothermal (1300 and 1500 deg F) strain fatigue data available on this alloy. The same plastic-strain criterion for failure describes the results of tests where plastic flow is produced by yielding and a combination of yielding and stress relaxation. The plastic-strain energy per cycle versus life suggests that a constant plastic work to failure exists for this alloy. The implications of such a criterion are discussed.