Why is Fatigue Testing of Fasteners Necessary?

Fatigue testing is an expensive and time consuming process, but when the specifications call for it, this type of testing is necessary. Why? Testing is required because fatigue can cause fasteners in a wide variety of circumstances to fail, resulting in damage and the risk of serious harm.

What is Fastener Fatigue?

This type of fastener failure normally occurs on externally threaded products as a result of cyclic load – or an ever changing load that can come from any angle at varying strengths. Windmills are a good example, since they are mounted high where air currents change rapidly and almost constantly. Fasteners on these applications are in danger of failure due to fatigue.

Fatigue happens on fasteners when that cyclic load causes cracks to develop on the surface of the metal, which will eventually deepen and cause the fastener to split. What makes fatigue unique is that generally the loads causing the failure are much less than the yield strength of the fastener. That means designers must look beyond those numbers when a cyclic load is present.

Another application where fastener fatigue may be a problem is within a truck body or engine. If that vehicle will be subjected to bumpy roads while carrying heavy or shifting loads the result can be fatigue. Street signs are another example, as the high poles are holding up against winds, rain and other elements of the weather that are ever changing and unpredictable.

Why is Fatigue Testing Done?

In applications where this type of load presents a danger the fatigue testing may already have been done. As is the case though, new designs and alternative processes are being introduced at a quick rate. Those designs require fatigue testing to be again (or for the first time) given that the unique design and loads are new or altered.

NASM 1312-11 is a common procedure standard for fatigue testing. This standard outlines the setup required for the testing and the steps to align the load. It does not specify the cycling rate for the test or how many tests are needed to ensure reliable results. NASM 1312-11 is the most common testing procedure however, and the results are widely accepted.