A stepped shaft, as shown in Sketch e, was machined from high-carbon steel (AISI 1080, quenched and tem- pered at 800C). The loading is one of completely re- versed torsion. During a typical 30 s of operation under overload conditions the nominal stress in the 25-mm di- ameter section was calculated to be as shown. Estimate the life of the shaft when it is operating continually un- der these conditions. stress, MPa 50 mm- 200 100 -100 -200 r = 2.5 mm 30 s Sketch e, for Problem 7.37 25 mn +