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Sensors & Transducers, Vol. 154, Issue 7, July 2013, pp. 143-148

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© 2013 by IFSAhttp://www.sensorsportal.com

Tensile and Fatigue Properties of Fiber-Bragg-Grating (FBG) Sensors

Gerrit FRIELING, Frank WALTHER

TU Dortmund University, Department of Materials Test Engineering (WPT) Leonhard-Euler-Str. 5, D-44227 Dortmund, Germany

Tel.: +49 231 755-8028, Fax: +49 231 755-8029 E-mail: [email protected]

Received: 15 April 2013 /Accepted: 19 July 2013 /Published: 31 July 2013 Abstract: As an innovative measurement technique, the so-called Fiber Bragg Grating (FBG) sensors are used to measure local and global strains in a growing number of application scenarios. FBGs facilitate a reliable method to sense strain over large distances and in explosive atmospheres. Currently, there is only little knowledge available concerning mechanical properties of FGBs, e.g. under quasi-static, cyclic and thermal loads. To address this issue, this work quantifies typical loads on FGB sensors in operating state and moreover aims to determine their mechanical response resulting from certain load cases. Copyright © 2013 IFSA. Keywords: Fiber-Bragg-grating sensors, FBG, Fatigue, Tensile test, High temperature properties, Fiber optic. 1. Introduction

As a measurement technique, FBGs are either connected to the component using gluing techniques or they are directly embedded in matrix material of a fiber-plastic composite [1-6]. Components with embedded sensor elements are often called “smart structures” [7].

FBGs are assembled similarly to fiber glasses. Their configuration is depicted in Fig. 1. A fiber core (5-9 µm) is surrounded by a less optically dense medium (~150 µm). Both mediums consist of pure glass (SiO2). Light is fully reflected at this interface up to a certain angle between the light beam and the direction of the fiber according to Snell´s law. An outer coating prevents failure of the FBGs, e.g. through corrosive media or careless handling.

The light conducting core of FBG sensors consisting of fiber glass has certain defined local modifications of the refraction index. These modifications are arranged periodically. The

arrangement of alternating areas of refraction indices works as an optical grating for entering light beams. Depending on the wavelength and the exact geometric arrangement of the diffraction grating, the beams are either reflected or transmitted [8]. Light with a specific (Bragg) wavelength is reflected from the array, whereas other wavelengths are transmitted.

The Bragg wavelength is related to the grating period which is inscribed into the fiber:

2B en d , (1)

where λB is the Bragg wavelength, ne is the effective refraction index and d is the grating period.

Thus, distances and especially changes in distances are related to local strain of the surrounding medium at this point. Reflected wavelengths can be directly measured by a measuring instrument, a so-called interrogator. This device provides the light signal send through the fiber and also analyses the wavelength of the reflected signal.

Article number 1262

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Fig. 1. Fiber Bragg Grating (FBG) sensor. Both, reflected and transmitted wavelengths

spectra can be used to quantify the strain level of the optical grating [9]. The strain εtot consists of mechanical and thermal strain:

tot mech therm

(2) So that both sources of strain can be quantified by

this measurement technique [5, 10]. Within a single fiber multiple gratings can be

realized with different spacings of the periodic array. This results in different Bragg wavelengths to be measured facilitating a simultaneous measurement of multiple points. Typical applications include the use in aerospace structures, ships and wind turbines [10, 11].

The exposure to high energy light sources and, thus, the grating inscription is either done by using a phase mask or a Talbot interferometer. In this device a periodic interference pattern is created at the fiber position [8]. The pulsed laser radiation induces a permanent and periodical modification of the refractive index of the fiber core. This is either achieved by the activation of dopants or the inscription of morphologic defects in the fiber core at high laser energy densities.

Depending on the laser energy densities used for inscription, weak and strong gratings are distinguished. For these gratings the reflectivity at the Bragg peak is lower than 30 % and higher than 90 %, respectively [12]. Already coated fibers are used for manufacturing strong gratings. To inscribe the gratings, the coating has to be stripped off first. Then the grating is inscribed and another coating is applied to the fiber. Gratings with lower reflectivity are

produced in a so-called draw tower process. In this process the fiber is drawn and the grating is inscribed before finally the coating is homogenously applied. Therefore a constant elasticity and durability of the coating in all areas of the fiber is ensured. This process is known for positive influence on the fatigue strength of the fiber [13].

As mentioned before, there are two methods for creating a grating in the fiber. The first is the phase mask method. A phase mask is an optical transmission grating. The mask is made of glass, into which periodical line shaped dents have been etched. If the phase mask is exposed to a beam of appropriate diameter, a line shaped interference pattern appears below the mask. By using this interference pattern, a periodical variation of the refractive fiber index is achieved.

A Talbot interferometer is used for the second method. In this interferometer a phase mask is used as beam splitter. The divided beam runs to two rotary mirrors which are disposed in such a way, that inference is achieved in the area of the fiber. A line shaped interference pattern is achieved, which leads to a periodical variation of the refractive index of the fiber [8].

Three different FBG designs (Fig. 2) are analyzed in this work. They have the following features:

• Design 1 – glass fiber (SIO2) with polyimide coating (highly reflecting grating);

• Design 2 – OptiMet-OMF (HBM) draw tower fiber with Ormocer (Organic Modified Ceramic) coating [14] (low reflecting grating);

• Design 3 – OptiMet-PKF (HBM), i.e. OptiMet-OMF draw tower fiber with an additional coating for increased robustness [15] (low reflecting grating).

Fig. 2. Schematic diagram of all fiber designs under investigation.


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2. Experimental Setup

The FBG specimens are exposed to various quasi-static, cyclic and thermal loads to analyze their mechanical properties in detail. Quasi-static loads are applied through a universal testing system 10 kN (Shimadzu AGS-X, load cell 5 kN) using a video-based extensometer (Shimadzu TRViewX) for contactless optical strain measurements and the fatigue properties are analyzed using a servohydraulic testing system 10 kN (Instron 8872), Fig. 3a.

A special clamping device has been developed by the authors to eliminate any fiber damage at the clamping device and to achieve reproducible results (Fig. 3b). The clamped fiber extends through the clamping device and is connected to the interrogator (HBM DI101) that emits and detects the laser light for the strain measurement with a sampling frequency of 100 Hz. The detection of the reflected light spectrum is carried out by a photoelectric unit in the interrogator. Using the information about the emitted wavelength together with the reflected light peak allows an interpretation of the strain state of the grating. Strain state measurements of the fibers without grating were performed by a video extensometer system.

Fig. 3a. Electromechanical and servo-hydraulic testing systems.

Fig. 3b. Clamping device.

To analyze any effect of the inscription process in the fiber grating by laser light, each of the three fiber designs with and without grating were analyzed in detail.

3. Results

3.1. Quasi-static Properties

Fig. 4 shows results of tensile tests of all

investigated designs (Fig. 2) for a testing speed of 1 mm/min. A video-based extensometer is used for strain measurements of specimens without grating, whereas FBG measurement technique is used whenever available. Since all specimens are tested until fracture, the applied strain is much larger than the maximum strain scope defined by the manufacturer of FBG sensors. Two different failure criteria were used: For designs 1 and 2 failure is considered as a complete fiber failure and destruction of the optical grating. For design 3 the jacket breaks first and the fiber with optical grating stays intact. As the traction with a component is established by the jacket, the fiber becomes inoperative by jacket failure (= failure criterion for design 3).

It becomes obvious by comparing the results of fibers with and without grating, that the maximum force and maximum strain (= strain to fracture) are reduced by grating inscription for all fiber designs. Design 2 fiber with and without grating is characterized by only small differences in both values. On the other hand, design 1 fiber with grating shows significantly lower values and much lower ductility compared to pure fiber, e.g. strain to fracture is reduced by 75 %. For design 3, fiber strain to fracture is reduced by 20 % due to grating insertion. The deformation behavior of fiber design 3 differs considerably as it shows non-linear behavior and a reduced strain to fracture of 3.1 % compared to 5 % for designs 1 and 2.

The non-linear force-strain relationship can be explained by the influence of the outer polymer coating (jacket) on the overall response of the FBG sensor.

Failure of fibers can be attributed to various reasons. In case of fiber designs 1 and 2 failure occurred for entire fiber at once, no failure origin could be determined directly but it can be interpreted to be at the grating as the weakest link. The weakening by grating inscription seems to depend on the outer coating, as it can be seen by comparing the fracture strains of designs 1 and 2. Design 2 fiber with OMF coating showed a roughly 4-times higher fracture strain. Further investigations should be carried out in order to confirm this observation and its interpretation.


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Fig. 4. Quasi-static properties of fibers and FBG (fiber+grating) sensors in tensile tests.

Regarding fiber design 3, primary fracture occurred at the outer coating, whereas fracture for the core part is not observed at this point of strain. Since designs 1 and 2 show a linear force-strain behavior up to fracture dominated by the glass properties, design 3 is non-linear due to the strong influence by the outer polymer coating (jacket). Weakening through inscription of grating should not be expected in this case. Higher strain values could be applied to neither of design 3 fibers as the load transmission is interrupted after failure of outer coating.

Summing up the results, fracture seems to be controlled by failure of glass, as inscription of grating leads to reduction in fracture strain.

3.2. Fatigue Properties

Fatigue properties of FBG sensors [16] have been studied using stepwise load increase tests [17, 18], at load ratio R = 0.1 and test frequency f = 2 Hz. The load ratio R is defined by the quotient of minimum and maximum displacement smin / smax. The measurement of amplitude sa was provided through a LVDT sensor (LVDT: linear variable differential transformer) of the servohydraulic testing system. Testing was carried out controlling the displacement sa, Start = 0.02 mm of the clamping device and increasing it by Δsa = 0.02 mm - 0.04 mm each ΔN = 104 cycles.

Fig. 5 shows the illustration of measured total strain εt for all FBG sensors as a function of cycles N. Applying stepwise load increase tests, FBG sensor design 1 reaches a max. cyclic strain of 1.1 %, close to quasi-static strain at fracture. With the max. cyclic

strain of 2.4 %, FBG sensor design 2 reaches approx. the half of the quasi-static value 5.1 %. Compared to 2.6 % in tensile test, fiber design 3 reaches a significantly lower max. cyclic strain of 0.8 %. In this case failure is controlled by fracture of the outer polymer coating (jacket).

All mentioned max. strain values are well outside usual working parameters given by the manufacturer [14, 15]. 3.3. High Temperature Properties

Application range of FBG sensors is commonly limited to T = 200 °C [14, 15]. As sample applications might include a forming step involving elevated temperatures, the behavior of FBG sensors at higher temperatures of 450 °C are investigated. These high temperatures are tested for a short exposure time of 10 min.

Fiber designs 1 and 2 were heated up to 450 °C in a furnace and the spectrum was analyzed by means of an interrogator. Design 3 was not analyzed as the outer coating polymer material is not suitable for these elevated temperatures.

All spectra are displayed in Fig. 6 for T = 20 °C and 450 °C. Design 1 (Polyimide) shows a distinctive peak in its wavelength spectrum at both 20 °C and 450 °C. The shift in the Bragg wavelength value is due to temperature-induced strain. Thus, this fiber design seems to be capable to withstand the elevated temperatures. In contrast to this, it was not possible to receive any signal from FBG design 2 at 450 °C due to a decay of the optical grating.


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Fig. 5. Cyclic properties of FBG (fiber+grating) sensors in stepwise load increase tests.

Fig. 6. High temperature properties of FBG (fiber+grating) sensor designs 1 and 2.


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4. Summary

Three different FBG sensor designs have been studied in tensile and fatigue tests as well as temperature stability tests. These tests visualize the differences in mechanical and thermal properties for the Polyimide, OMF and PKF design. PKF fibers consist of OMF fibers with an additional polymer jacket for increasing the robustness for service applications.

Influences by grating inscription were quantified under tensile loading. Applicable maximum forces of fibers with gratings were 4-5 times lower for Polyimide design and just 15 % lower for OMF design or PKF design compared to those without grating. Under cyclic loading Polyimide fibers endure less strain capability than OMF fibers. But in contrast to OMF fibers, Polyimide fibers can withstand elevated temperatures up to 450 °C.

PKF fibers show a distinctively different behavior under quasi-static and cyclic loading due to polymer jacket. All investigated sensor designs exhibit major differences under cyclic loads compared to quasi-static ones. Acknowledgements

We gratefully acknowledge the excellent collaboration with Hottinger Baldwin Messtechnik (HBM) GmbH (Darmstadt, Germany). References [1]. O. Hill, G. Meltz, Fiber Bragg Grating Technology -

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[2]. M. Trutzel, Dehnungsermittlung mit faser-optischen Bragg-Gitter-Sensoren, Dissertation, TU Berlin, 2001.

[3]. T. Kreger, A. K. Sang, D. K. Gifford, M. E. Froggatt, High Resolution Distributed Strain or Temperature Measurements in Single- and Multi-Mode Fiber Using Swept-Wavelength Interferometry, in Proceedings of the 18th Int. Conf. on Optical Fiber Sensors, Cancun, Mexico, 23-27 October 2006.

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[6]. VDI/VDE, Experimental Stress Analysis - Optical Strain Sensor Based on Fiber Bragg Grating - Fundamentals, Characteristics and Sensor Testing, VDI/VDE 2660, Part 1.

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