Fatigue Testing and Analysis
Nearly every material known to man is subject to fatigue failure. This is true of polymers, plastics, composites, and metals. The general rule is as follows: if a person can bend something back and forth enough times, he or she can eventually break it.
Metals tend to fatigue and fail with far fewer cycles than their plastic or polymer counterparts. This is because cyclic deformation of metals disrupts the microscopic arrangement of their crystalline grains, causing them to yield with progressively lower amounts of stress. Each deformation can eventually lead to failure.
Fatigue failure can also happen with cyclic loading as repeated stresses concentrate in microscopic cracks that are inevitably found in every material except for monocrystalline superalloys. This causes the stressed material at the roots of the cracks to yield in a cyclic fashion. Each yield or deformation in material at the root of a given crack results in a weakening of the material thereby, which causes the crack to become progressively worse to the point of failure. Bolts, studs, and threaded connectors are especially prone to this failure mode owing to the fact that their threads concentrate stress. This is why bolts or studs used in machinery should be torqued to proper specifications in order to preload them and reduce cyclic stress amplitude.
Unfortunately, fatigue failure is an inevitable reality. There is simply no known way to design anything that won’t eventually fail due to fatigue, regardless of the amount of cyclic loading. However, the best design practice will produce products and structures that sufficiently resist fatigue to provide long and reliable service life. Best design practice usually involves testing, and if fatigue failures are affecting your products, NTS provides fatigue testing services that can help.
Understanding the Fatigue Test Process
In a way, fatigue testing is like bending a paper clip back and forth and counting the number of times you’re able to bend it before it breaks. Thus, a fatigue test involves subjecting a test sample to repeated loading and unloading to evaluate how it will perform over time in its intended usage environment.
In order to contrive a viable fatigue testing and analysis program, the following parameters need to be established:
- Determine the points where cyclic stress should be applied as well as the type of stress that should be applied at those points.
- Determine the number of cycles that the test sample is expected to experience over its intended design life.
- Determine the amount of stress that the test sample is expected to experience for each cycle during its intended life.
- Determine the applicable safety factor.
Once these parameters are established, the fatigue test process is relatively straightforward. The Dynamics testing experts at NTS are able to develop a customized fatigue testing and analysis program to assess how your manufacturing materials will endure the cyclic stress of their intended application.
Please contact us today for more information about our materials fatigue testing services.