Volume 21, Issue 2 (March 1993)
Fatigue and Corrosion Fatigue of Beryllium-Copper Spring Materials
Fine gage, 0.006-in. (0.15-mm) thick, beryllium-copper (Be-Cu) spring materials with tensile strength in the range of 70 to 145 ksi (482 to 999 MPa) were subjected to cyclic loading in air and salt water environments. Plain and notched (center hole) “hour glass” specimens were subjected to sinusoidal loading with R = (minimum maximum) stress = 0.1 at cyclic frequencies of 50 Hz in air and 1 Hz in salt water. Fatigue life was typically from 104 to 106 cycles with crack initiation as the dominant fatigue process.
The excellence fatigue performance of Be-Cu alloys in salt water is well-known, however, current findings demonstrate 10 to 37% reduction in fatigue strength of unnotched specimens in this environment for a life of 3 × 105 cycles. This strength degradation is attributed to the use of a lower cyclic frequency for present than for previous tests, i.e., 1 versus about 20 Hz. There was no effect of salt water on crack initiation in notched specimens.
The ratios of the fatigue strengths, namely (cold-rolled annealed) and (aged/annealed), for plain and notched specimens tested in air, decreased from 2 to about 1.4 as fatigue life increased from 104 to 106 cycles. This effect is attributed to cyclic hardening of the annealed material. The fatigue stress concentration factor, Kt = (plain notched) fatigue strength, increased by about 30% as fatigue cycles increased from 104 to 106. The ranking of Kf values of the various material conditions from highest to lowest was: cold-rolled, aged, and annealed.