Influence of loading frequency on high cycle fatigue properties of b.c.c. and h.c.p. metals

The influence of the loading frequency on the high cycle fatigue properties of two b.c.c. metals, commercially pure (c.p.) niobium and c.p. tantalum in annealed and cold worked condition, and of two annealed h.c.p. titanium and of Ti–6Al–7Nb alloy were investigated. Endurance data in the regime of 105 to 2×108 cycles to failure obtained with rotating bending and ultrasonic fatigue testing equipment (loading frequencies 100 Hz and 20 kHz, respectively) coincide within the ranges of scatter for niobium and Ti–6Al–7Nb alloy. The mean endurance limits at 2×108 cycles of these metals are ≈60% of the respective yield stress of the as produced material. The high loading frequency leads to prolonged lifetimes and increased mean endurance limits for tantalum and (less pronounced) for titanium. Fatigue crack initiation in tantalum changes from a preferentially ductile and transgranular mode at 100 Hz to a more brittle, crystallographic and intergranular mode at 20 kHz. The mean endurance limit of tantalum is above the yield stress of the as produced material, and high initial rates of plastic deformation therefore result. Cold working of tantalum and niobium improves their static strength properties, but is of only minor importance for the high cycle fatigue behaviour.