Design of a Fatigue-Resistant Shaft
Your company has produced several thousand shafts that have a fatigue strength of 20,000 psi. The shafts are subjected to high bending loads during rotation. Your sales engineers report that the first few shafts placed into service failed in a short period of time by fatigue. Design a process by which the remaining shafts can be salvaged by improving their fatigue properties.
Fatigue failures typically begin at the surface of a rotating part; thus, increasing the
strength at the surface improves the fatigue life of the shaft. A variety of methods might be used to accomplish this.
If the shaft is made from steel, we could carburize the surface of the part (Chapter 5). In carburizing, carbon is diffused into the surface of the shaft. After an appropriate heat treatment, the higher carbon content at the surface increases the strength of the surface and, perhaps more importantly, introduces compressive residual stresses at the surface.
We might consider cold working the shaft; cold working increases the yield strength of the metal and, if done properly, introduces compressive residual stresses. The cold work also reduces the diameter of the shaft and, because of the dimensional change, the shaft may not be able to perform its function.
Another alternative is to shot peen the shaft. Shot peening introduces local compressive residual stresses at the surface without changing the dimensions of the part. This process, which is also inexpensive, might be sufficient to salvage the remaining shafts.