We are currently designing a rather large stationary axle, that has a large moment on the ends of it. The axle is stationary but will see large forces, repeatedly applied, fluctuating from small nominal (almost 0) to a large value. Because of this loading the design is ok in normal loading but fatigue added to a stress concentration at one of the many step downs, fatigue is our limiting factor. We are trying to keep weight and size down for this axle. Currently we are considering using 1144 Fatigue proof or, maybe using a 4000 steel that is heat treated, we are looking for the best cost option betweeen some sort of heat treating and buying the stronger material so that we have 120ksi Yield, and 145ksi tensile, (or similar) anyone have experience making a similar decision? There is a lot of machining that has to happen to this axle which starts around 3.5" diameter down to about 1.5" at the ends, and it is almost 4 ft long, so machine-ability is important.
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