Fatigue and corrosion fatigue of beryllium-copper spring materials

Abstract

Fine gage, 0.006-in. d(0.15-mm) thick, beryllium-copper (Be-Cu) spring materials with tensile strength in the range of 70 to 145 ksi 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 10[sup 4] to 10[sup 6] 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 x 10[sup 5] 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 10[sup 4] to 10[sup 6] cycles. This effect is attributed tomore » cyclic hardening of the annealed material. The fatigue stress concentration factor, K[sub f] = (plain/notched) fatigue strength, increased by about 30% as fatigue cycles increased from 10[sup 4] to 10[sup 6]. The ranking of K[sub f] values of the various material conditions from highest to lowest was: cold-rolled, aged, and annealed.« less