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.
The designer or design engineer is the first line of defense against fatigue failure. He or she can’t prevent failures originating in the manufacturing process or service environment, but the designer lays the foundation of prevention.
In an ideal world, each design would be subjected to extensive stress calculations and fatigue testing.In the real world this is rarely cost effective for non-critical components. Instead, accepted and “proven” parameters are applied. These typically include safety margins which are more than adequate. Typically, but not always. And even a common “off the shelf” fastener can take complex products out of service if it fails.
A working understanding of material strengths and properties by the designer is optimal. Unfortunately, that is a relatively rare combination of expertise. And although material strength and property data is widely available, the effective application of this information is sometimes outside the experience of the designer.
The job of the designer becomes even more challenging when the many potential variables inherent in the manufacturing process are considered. Leaving aside the production of the raw material at the mill – the bar, plate and sheet – manufacture of the designer’s envisioned component may include a host of processes that he or would ideally be familiar with through which the seeds of fatigue failure could potentially be introduced.
Computer Aided Design (CAD), Finite Element Analysis (FEA) and a variety of other computer driven design and predictive technologies can greatly enhance the fatigue resistance of a component at the design stage. But they cannot prevent fatigue failures. That’s because the next two threats of fatigue failure are beyond the designer’s control.
In Failure Analysis of Fatigue – Part 4 we will discuss fatigue prevention at the manufacturing stage of a component’s life.