10. Cylindrical rods of identical diameters (12 mm) were fabricated from seven different alloys and subjected to repeated rotating-bending stress cycling. The S-N curves of these samples are shown in the following figure (Fig. 8.21, Callister 10th ed.). a) Identify the materials with well-defined fatigue limits. b) List at least two ways to improve fatigue life. 700 600 Ti-5A1-2.5Sn titanium alloy 4340 steel 500 o 400 Maximum stress, S (MPa) 1045 steel 300 Ductile cast iron 200 70Cu-30Zn brass 2014-T6 Al alloy 100 EQ21A-T6 Mg alloy 0 104 105 108 109 106 107 Cycles to failure, N