From a practical standpoint, fatigue failures present a danger to you, the manufacturer, at three points in a components life. These are the design stage, the manufacturing process, and the service environment. In this part, we discuss aspects of the manufacturing process to monitor in order to prevent fatigue.
The Manufacturing Process
Manufacturing processes are a rich, though unintended, source of stress concentrations from which fatigue cracks can initiate. The list is almost endless, and includes rough machined surfaces from dull tooling or excessive feeds and speeds, burrs from cutting or drilling operations, and insufficient chamfers or corner radiuses. Mechanical fasteners – bolts, screws, studs, and rivets- are highly prone to fatigue failure. Prominent among these are thread laps, folds or seems, that are formed when the threads are cut into the fastener. Threads formed by rolling are much less susceptible to laps and consequent fatigue failure. Whether threads are cut or rolled, however, insufficient tightening torque during the assembly stage of the manufacturing process is probably the number one source of fatigue failure in fasteners.
Welds, even when technically faultless, provide geometric stress concentrations. Defective welds and welding procedures may result in porosity and high hardness heat affected zones from which fatigue can initiate. Similarly, braze and solder joints, by their very nature, typically produce a geometric configuration that can potentially invite fatigue initiation. Fatigue susceptibility resulting from these joining processes can be significantly mitigated by careful consideration in the design stage, but design quality cannot compensate for weld and joining defects such as undercut, porosity and slag inclusions.
Care in manufacturing and a good quality control program will avert many of these potential sources of fatigue initiation. However, despite the best quality control program, the manufacturer is often at the mercy of their raw material supplier. These suppliers may open the door to fatigue failure through castings which contain excessive porosity or microstructural defects, mill products which are work hardened, forgings with undetected laps or seams, or gross non-metallic inclusions in any of these products. Appropriate specifications on outsourced stock and components are vital in guaranteeing their quality, but as with so many aspects of production, they are a compromise. Loose specs solicit low cost bids, but a potentially high percentage of defective product, while tight specs limit the number of vendors capable of meeting them and drive costs higher, cutting into profits.
In Failure Analysis of Fatigue – Part 5 we will discuss fatigue prevention in service, the longest period of exposure in a component’s life.