Application Properties for the Selection of Copper Alloys

5.1.7.1 Spring Bending Limit

The spring bending limit is defined as the boundary condition under which a standar-dized spring sample retains a deformation of 0.05 mm after initial bend stressing and subsequent force removal. The measurement is performed according to the standard EN 12384. The spring bending limit is strongly dependent on the direction of stressing with regard to the strip rolling orientation (Fig. 5.38). Higher values are obtained if bending is perpendicular to the rolling direction as compared to parallel. This has to be considered when designing contact springs.

Fig. 5.38: Direction dependence of the spring bending limit of selected copper materials (Wieland)

5.1.7.2 Fatigue Strength

The fatigue strength is a measure of maximum alternating bending force, symmetrical to the zero position, which a sample – for example a relay spring – can be exposed to for an “unlimited” number of cycles without breaking. (Rule of thumb: Fatigue strength = 1/3 of Tensile strength). The measurement is conducted using so-called Woehler– diagrams . With increasing bending force 7 the number of alternating cycles before breaking decreases. Above 10 cycles the influence of further cycling numbers becomes insignificant and therefore 7 the force value reaching 2x10 cycles can be used to define the fatigue strength. The multi-component alloys CuZn23Al3.5Co and CuSn1CrNiTi show high values of fatigue strength while CuFe2P and CuZn30 exhibit low ones (Figs. 5.39 and 5.40).

Fig. 5.39: Woehler curves for selected copper based materials. Strip samples: 0.3 mm thick, cold worked; Testfrequency; 1,500 / min (Wieland)

Fig. 5.40: Ranges of fatigue strength for selected copper materials (Wieland)