The torsional fatigue properties of titanium-alloy spring wire

The absence of any torsional fatigue-test data on currently used titanium-alloy spring wire along with inconsistent fatigue-test data derived from earlier test programs prompted this investigation. The torsional fatigue testing was performed on straight lengths of 13V?11Cr?3Al titanium-alloy spring wire in diameters of 0.148 in. (0.376 cm), 0.225 in. (0.572 cm) and 0.374 in. (0.950 cm) considered as representative. A second material selected as a possible alternate for 13V?11Cr?3Al spring was 3Al?8V?6Cr?4Mo?4Zr which was tested in diameters of 0.225 in. (0.572 cm) and 0.376 in. (0.955 cm). Testing as straight-wire lengths eliminated the manufacturing variables introduced by fabricating a helical-spring geometry. Two methods were considered for improving fatigue life, namely abrasive cleaning and shot peening. At a torsional stress of 100 ksi (689.5 MPa), the test data indicate that an improvement in fatigue life of at least one order of magnitude may be realized by shot peening 0.225-in. (0.572-cm)-diam 13V?11Cr?3Al wire to an intensity of 0.015A. A similar improvement of fatigue life may be gained by shot peening 0.148-in. (0.376-cm)-diam wire to a similar intensity. A substantial fatigue-life improvement was obtained by an obrasive-cleaning operation on all wire diameters tested. The fatigue life of the 3Al?8V?6Cr?4Mo?4Zr titanium-alloy wire in the cold-worked and aged condition was lower at all stress levels and diameters tested than for similar diameters of 13V?11Cr?3Al titanium-alloy wire.