So, what is so compelling about distributed fiber optic sensing?!

Sandra M. Klute, Ph.D.
Director of Research, Contract Services
Lightwave Division

We recently presented our distributed fiber optic sensing technology at the October 2012 meeting of SAMPE TECH (The Society for the Advancement of Material and Process Engineering). The topic of the presentation, “Fiber Optic Distributed Strain Sensing used to Investigate The Strain Fields in a Wind Turbine Blade and in a Test Coupon with Open Holes,” provided an opportunity to discuss our distributed strain and temperature sensing systems and to highlight a couple of recent interesting applications.  We were thrilled when the talk inspired a team of scouts from Boeing and the Ed Wells partnership to select us to present our technology via live broadcast from the Boeing Education Network (BEN) studio in Seattle to the entire Boeing organization. 

So, what is so compelling about distributed fiber optic sensing?   Imagine that tens of meters of optical fiber about the size of a human hair can be quickly bonded to the surface of virtually any component under test, and that this enables the measurement of real time strain over thousands of points simultaneously.  Also imagine that the fiber can be easily embedded within the layers of a composite structure so that the structure can report its own internal strain field. 

5mm or better spatial resolution, > 200 sensing points per meter of fiber

With spatial resolutions of 5mm or less, it is possible to know in real time the strain field throughout a structure and to monitor its progression over time.  Because of the quality and density of the data, this measurement technique is a powerful and cost effective tool for verifying FEA (Finite Element Analysis) models and for providing data beyond what is possible with more traditional electrical foil gages.  In the testing of a 43 meter wind turbine blade shown below, for example, the data from the optical fiber (pink) agrees well with the foil strain gages (blue) but also points out ply drops, areas of stress concentration, and the onset of inelastic buckling.

Full-scale test of 43-meter wind turbine blade with fiber sensor along length of the blade (left) and corresponding strain profile (pink) showing excellent agreement with foil gages (blue, right)

(A. Guemes, A. Fernandez-Lopez, and B. Soller, Structural Health Monitoring, 9 (3), 2010, pp. 233-245)

We’ve recognized for a while that optical fiber does not fatigue at higher strains in the way that foil gages do.  A recent test emphasized the capability of the fiber in dynamic conditions in which many cycles are repeated at relatively large strains.  While the foil gages demonstrated extreme drift, the fiber optic sensor remained accurate over tens of thousands of cycles. 

Fatigue tests demonstrate drift in electrical foil gage vs stability in the fiber optic sensor over tens of thousands of cycles.

In a fatigue test of a 9-meter wind turbine blade, we demonstrated that we could embed fiber optics into the carbon spar cap of both halves of a blade containing intentionally engineered defects.  The distributed strain measurements painted a detailed picture of the defects in the blade from the time the blade was manufactured throughout the fatigue testing of the blade to failure 1.97 million cycles later.  In fact as early as 700k cycles, the strain field around the eventual failure location at 5meters showed rapid evolution and strain values many times greater than that of adjacent locations.  Monitoring the strain field of a structure over its lifetime with fiber optics can inform us as to the ongoing safety of the structure and equip us with information by which to intelligently maintain the structure.

Distributed strain measured at four different cycle counts during fatigue testing. The data are strain in the sensing fiber embedded under the defects within the carbon fiber spar cap. The growth of the defect located at 5 meters broke the sensing fiber between data in (c) 736,000 cycles and (d) 1,184,000 cycles. The blade ultimately failed at this location at 1,968,000 cycles.

More details can be found in our paper “Embedded and Surface Mounted Fiber Optic Sensors Detect Manufacturing Defects and Accumulated Damage as a Wind Turbine Blade is Cycled to Failure,” from SAMPE Baltimore 2012.

It’s hard to imagine another technology that could so significantly change the way we approach ground testing, flight testing, and SHM (Structural Health Monitoring).  And it’s this versatility and value to the aerospace community over a broad range of current and future applications that motivated Boeing to highlight distributed fiber optic sensing during its BEN webcast.

Leave a Reply