Within the field of physical testing, there is a fairly wide range of specific instruments devoted to particular measurements of the strength, hardness or other physical characteristics of materials and components. At the most theoretical level, physical testing involves the application of a stress, and the measurement of the resultant strain on the sample.

In fatigue testing, the applied stress is some form of dynamic force, often a loading force applied with a sinusoidal variation at a particular frequency. Ultimately, fatigue testing measures when the sample fails under the application of this dynamic load. The characteristics of failure may differ for different materials, with brittle samples failing through fracturing, while ductile or elastic materials may yield and deform beyond some limit. Fatigue testing provides insight into the physical process that leads to failure.

At its simplest, fatigue testing can measure the fatigue life of the sample. This measurement is a count of the number of times the stress has to be applied in order for the sample to fail. This failure point may be reached at a different number of cycles depending on the maximum applied force and the frequency of the cycle. This allows a fuller profile of the physical behavior of similar samples. One common representation of fatigue data is an “S-N” curve that plots the maximum stress against the number of cycles to failure, usually on a logarithmic scale.

More complex fatigue-testing systems provide still more information and options. Although the application of sinusoidal stress is common, the loading time-profiles can be altered to provide sharper impact-like stresses, or a more random mixture of stresses at different frequencies, which may better model the actual stresses a component might experience in the real world.

In addition to altering the dynamic qualities of the stress, fatigue-testing systems can also provide different stress modes, typically tension, compression and torsion. Before failure occurs, samples generally undergo physical changes and deformations that indicate the incipient failure. Fatigue testers may also be able to quantify this over the course of the test, providing more insight into the behavior and lifespan of materials and components. Some systems also include temperature and environmental control. Fatigue testing can be applied not only to basic materials, like alloys, polymers and elastomers, but also to finished components, ranging from chairs to aerospace components.

The focus on consumer products led to weakness in the fatigue-testing market during the global recession, but the outlook is beginning to be more favorable. The total market for fatigue testing was roughly $150 million in 2009. The competitive situation is dominated by the four major players in the general physical-testing market: Instron, a division of Illinois Tool Works; MTS Systems, which is particularly strong in the automotive market; Shimadzu; and Zwick.

Fatigue Testing at a Glance:

Leading Suppliers

• Instron (ITW)

• MTS Systems

• Shimadzu

Largest Markets

• Materials Science

• Automotive

• Polymers

Instrument Cost

• $20,000–$400,000