Multidimensional strain field measurements using fiber optic grating sensors

Udd, Eric; Schulz, Whitten L.; Seim, John M.; Haugse, Eric D.; Trego, Angela; Johnson, P.; Bennett, Thomas E.; Nelson, D. V.; Makino, Alberto

doi:10.1117/12.388113

Cited by 49 publications

(44 citation statements)

References 6 publications

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“…Other advantages of fiber optic sensors include their high sensitivity, wide bandwidth, EMI resistance, low power requirements, and environmental ruggedness. Fiber optic sensors can also be configured to monitor crack growth and corrosion in civil and aerospace structures [43][44][45]. Omni-directional fiber optic sensors can be used to measure large strains (up to 150% strain), small displacements (10 µm range), and crack growth in any material.…”

Section: Fiber Optic Sensorsmentioning

confidence: 99%

Development and Validation of Bonded Composite Doubler Repairs for Commercial Aircraft

Roach¹,

Rackow²

2018

Aircraft Sustainment and Repair

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A typical aircraft can experience over 2,000 fatigue cycles (cabin pressurizations) and even greater flight hours in a single year. An unavoidable by-product of aircraft use is that crack, impact, and corrosion flaws develop throughout the aircraft's skin and substructure elements. Economic barriers to the purchase of new aircraft have placed even greater demands on efficient and safe repair methods. The use of bonded composite doublers offers the airframe manufacturers and aircraft maintenance facilities a cost effective method to safely extend the lives of their aircraft. Instead of riveting multiple steel or aluminum plates to facilitate an aircraft repair, it is now possible to bond a single Boron-Epoxy composite doubler to the damaged structure. The FAA's Airworthiness Assurance Center at Sandia National Labs (AANC), Boeing, and Federal Express completed a pilot program to validate and introduce composite doubler repair technology to the U.S. commercial aircraft industry. This project focused on repair of DC-10 fuselage structure and its primary goal was to demonstrate routine use of this repair technology using niche applications that streamline the design-to-installation process. As composite doubler repairs gradually appear in the commercial aircraft arena, successful flight operation data is being accumulated. These commercial aircraft repairs are not only demonstrating the engineering and economic advantages of composite doubler technology but they are also establishing the ability of commercial maintenance depots to safely adopt this repair technique.This report presents the array of engineering activities that were completed in order to make this technology available for widespread commercial aircraft use. Focused laboratory testing was conducted to compliment the field data and to address specific issues regarding damage tolerance and flaw growth in composite doubler repairs. Fatigue and strength tests were performed on a simulated wing repair using a substandard design and a flawed installation. In addition, the new Sol-Gel surface preparation technique was evaluated. Fatigue coupon tests produced Sol-Gel results that could be compared with a large performance database from conventional, riveted repairs. It was demonstrated that not only can composite doublers perform well in severe offdesign conditions (low doubler stiffness and presence of defects in doubler installation) but that the Sol-Gel surface preparation technique is easier and quicker to carry out while still producing optimum bonding properties. Nondestructive inspection (NDI) methods were developed so that the potential for disbond and delamination growth could be monitored and crack growth mitigation could be quantified. The NDI methods were validated using full-scale test articles and the FedEx aircraft installations. It was demonstrated that specialized NDI techniques can detect flaws in composite doubler installations before they reach critical size. Probability of Detection studies were integrated into the FedEx training i...

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Section: Fiber Optic Sensorsmentioning

confidence: 99%

Development and Validation of Bonded Composite Doubler Repairs for Commercial Aircraft

Roach¹,

Rackow²

2018

Aircraft Sustainment and Repair

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“…A complementary analysis was conducted by comparing the DIC strain fields with the FBG spectra. It is well known that the shape of the reflected FBG spectrum depends on the strain gradients along the FBGs [2,17,20] as well as in the transverse direction [2,16,21,22,23]. If those are severe, a critical distortion of the spectrum may occur, leading to strain measurement inaccuracies.…”

Section: Tensile Test On Elementary Specimensmentioning

confidence: 99%

“…The related wavelength spectrum will show two major peaks, more or less distinctively. Using FBGs written on highly birefringent optical fibers and a polarization maintaining source setup, Udd et al [22] were able to measure multiaxial strain inside a stressed composite sample. In fact, they showed that the distance between the deformed spectrum peaks is proportional to the difference of strain in each related direction.…”

Section: Tensile Test On Elementary Specimensmentioning

confidence: 99%

Response of fiber Bragg gratings bonded on a glass/epoxy laminate subjected to static loadings

Mulle

Moussawi

Lubineau

et al. 2015

Composite Structures

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Response of fiber Bragg gratings bonded on a glass/epoxy laminate subjected to static loadings, Composite Structures (2015), doi: http://dx.doi.org/10. 1016/j.compstruct.2015.04.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. AbstractFiber Bragg gratings (FBG) may be used to monitor strain over the surface of a structure as an alternative technology to conventional strain gauges. However, FBG bonding techniques have still not been established to yield satisfactory surface measurements. Here, two adhesives were investigated, one with low viscosity and the other with high viscosity for bonding FBGs on glass/epoxy sandwich skins. First, instrumented elementary specimens were tested under tension. FBG strain results were analyzed together with digital image correlation (DIC) measurements. The influence of the bonding layer on the measured strain and on the integrity of the sensor was investigated by considering different regions of interest. Next, an instrumented structural sandwich beam was tested under four-point bending. FBG rosettes were compared to conventional strain gauge rosettes. The high viscosity adhesive demonstrated behaviors that affected FBG accuracy. Brittleness of the bonding layer and poor interface adhesion were observed using DIC and X-ray tomography. By contrast, the low viscosity adhesive demonstrated satisfactory results. The FBG strain measurements appeared to be consistent with those of DIC. The accuracy is also adequate as the FBGs and the conventional strain gauges had similar results in three directions, under tension and under compression.

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“…Figure 1a illustrates a multi axis grating written onto polarization preserving fiber, which is subject to uniform transverse loading. In this case the two spectral reflection peaks corresponding to the effective fiber gratings along each birefringent (polarization) axis will move apart or together uniformly providing a means to measure transverse strain 1,2,3,4 . In the case where load along the transverse axis is not uniform, as shown in Figure 1b, the peak associated with the nonuniform transverse load will split 1 .…”

Section: Multi Axis Fiber Grating Strain Sensorsmentioning

confidence: 99%

Monitoring of advanced composite weave structures using multi-axis fiber grating strain sensors

Davol¹,

Udd²,

Kreger³

et al. 2003

Sixth Pacific Northwest Fiber Optic Sensor Workshop

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The feasibility of using multi-axis fiber grating strain sensors to monitor transverse strain and transverse strain gradients to complex, woven composite structures has been evaluated. This paper overviews the multi-axis fiber optic grating strain sensors and how they can be applied to measuring multidimensional strain fields interior to composite parts with complex composite weave structures. Experimental results are given for the case of a bi-axially woven composite coupon as well as for an E-glass/epoxy composite sample . Multi Axis Fiber Grating Strain SensorsIn order to measure transverse strain and transverse strain gradients in textile composite materials multiaxis fiber grating strain sensors were used. A multi-axis fiber grating strain sensor is formed by writing a fiber grating onto birefringent polarization maintaining optical fiber. For this type of fiber grating strain sensor, a single fiber grating results in two distinct spectral peaks. These peaks correspond to each of the polarization axes of the polarization preserving fiber, which differ slightly in index of refraction. When the fiber is loaded transversely the relative index of refraction of the polarization axes of the fiber change and the net result is that the difference in wavelength between the spectral peaks changes as well. When the fiber is strained axially, the fiber elongates or compresses changing the fiber grating spectral period and the output spectrum goes to longer or shorter wavelengths respectively. Figure 1a illustrates a multi axis grating written onto polarization preserving fiber, which is subject to uniform transverse loading. In this case the two spectral reflection peaks corresponding to the effective fiber gratings along each birefringent (polarization) axis will move apart or together uniformly providing a means to measure transverse strain 1,2,3,4 . In the case where load along the transverse axis is not uniform, as shown in Figure 1b, the peak associated with the nonuniform transverse load will split 1 . The transverse strain gradient can be measured quantitatively by the spectral separation between the peaks. The response of the fiber is approximately 1/3 of that of axial strain along the length of the fiber. As a specific example at 1300 nm a spectral shift of 0.01 nm along the fiber axis corresponds to 10 microstrain. A peak to peak separation of 0.01 nm due to nonuniform transverse strain corresponds to approximately 30 microstrain for 125 micron diameter bow tie polarization preserving fiber.

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Multidimensional strain field measurements using fiber optic grating sensors

Cited by 49 publications

References 6 publications

Development and Validation of Bonded Composite Doubler Repairs for Commercial Aircraft

Development and Validation of Bonded Composite Doubler Repairs for Commercial Aircraft

Response of fiber Bragg gratings bonded on a glass/epoxy laminate subjected to static loadings

Monitoring of advanced composite weave structures using multi-axis fiber grating strain sensors

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