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Fatigue – A clear advantage of Fiber Optics Sensing over Strain Gauges

Fatigue – A clear advantage of Fiber Optics Sensing over Strain Gauges

Posted on by Pierrick Vulliez

One widespread application where FOS technology exhibits superior performance over foil strain gauges is in the fatigue testing of components, sub-assemblies, and full-scale structures. Demand for full-scale fatigue testing continues to increase, specifically in the aerospace industry where there is significant interest in extending the service life of aging aircraft well past the intended design life.

In order to extract as much life out of these aging aircraft as possible, new fatigue tests are required to appropriately plan future maintenance schedules.
Strain gauges are not ideal for this purpose as they often fail prematurely when subjected to fatigue or cyclic loading. As strain gauges are sufficiently fatigued, a drift of the zero signal occurs due to permanent damage in the gauge, known as a “zero-shift”. Depending on the stress amplitude and number of cycles that the strain gauge is subjected to, the “zero-shift” can range from ten to several hundred microstrain before the sensor stops working entirely. Regardless of the application, the fatigue performance of FBG optical fiber is vastly superior. For example, the nominal fatigue life (where zero-drift remains below 100 μϵ) of a typical commercial strain gauge is 1500 – 2500 μϵ at 10^6-10^7 cycles [9]. In comparison, optical fiber is essentially insensitive to fatigue. FBG fiber commonly used with Sensuron equipment has proven capable of withstanding over 20,000 μϵ at several million cycles [10]. Thus, FOS strain sensors are ideal for fatigue testing as the fatigue limit for fiber is well above the strain amplitudes witnessed during the testing of common structural materials. 
Summary
Reduced installation effort, increased sensor density, and excellent fatigue life are only a few of the unique advantages of FOS technology. Additional benefits include insensitivity to EMI, minimal measurement drift, corrosion resistance, and minimal lead cabling. Although FOS technology provides a variety of distinct advantages, it is not always optimal for all structural testing applications. For example, hard to reach areas or locations with restrictive space constraints are often best suited for traditional strain gauges. Additionally, traditional strain gauge rosettes are recommended to measure principal strains in areas with limited space. FOS technology has demonstrated its relevance as a critical structural testing tool. When used in conjunction with strain gauges, it provides the ability to thoroughly characterize the behavior of a structure or a component.

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