Embedded Fiber Bragg Grating Sensors in Polymer Structures Fabricated by Layered Manufacturing

Journal of Manufacturing ProcessesVol. 5/No. 12003

78

Embedded Fiber Bragg Grating Sensorsin Polymer Structures Fabricatedby Layered Manufacturing

Xiaochun Li, Assistant Professor, Dept. of Mechanical Engineering, University of Wisconsin-Madison,Madison, Wisconsin, USA. E-mail: [email protected] Prinz, Rodney H. Adams Professor in Dept. of Mechanical Engineering and Dept. of Materials Scienceand Engineering, Stanford University, Stanford, California, USA. E-mail: [email protected]

AbstractLayered manufacturing allows full access to any point of

interest during the production of a 3-D part, making feasiblethe embedding of sensors and components inside the func-tional part. Shape deposition manufacturing was used to inte-grate fiber Bragg grating (FBG) sensors during the productionof polymer components. Sensors can be placed close to pointsof interest prior to enclosure. For strain measurements, thesensors embedded in polyurethane yielded high sensitivity,accuracy, and linearity. The sensitivity of the embedded FBGswas in good agreement with that of bare FBGs. The embed-ded sensors were used to measure the strain field in layeredmaterials. Pressure sensors based on embedded strain gaugesand FBGs were also developed and tested, showing goodaccuracy and linearity.

Keywords: Embedded Sensors, Polymer Components, Lay-ered Manufacturing, Fiber Bragg Grating

IntroductionLayered manufacturing or solid freeform fabrica-

tion (Prinz 1997; Au and Wright 1993; Barkan andIasiti 1993; Ashley 1995; Marcus, Harrison, Crocker1996; Conley and Marcus 1997; Ashley 1997;Beaman et al. 1997) has emerged as a popular manu-facturing direction to accelerate product creation.Layered manufacturing can build parts that have tra-ditionally been impossible to build because of theircomplex shapes or variety in materials. Layeredmanufacturing can also build functional “smart” partswith sensors, integrated circuits, complete functionalassemblies, and actuators placed within the structureand fully embedded. In particular, sensors embeddedwithin the structural materials add intelligence tostructures and enable real-time monitoring at somecritical locations not accessible to ordinary sensors,which must be attached to the surface. Moreover,embedded sensors are also protected from damagecaused by extraneous environmental effects. These

sensors can be used to gain data for validating or im-proving designs during the prototype stage or to ob-tain information on the performance and structuralintegrity of functional components in service.

Shape deposition manufacturing (SDM), a novellayered manufacturing technology, was developedjointly at Carnegie Mellon University and StanfordUniversity. SDM builds fully dense parts by incre-mental deposition and CNC shaping of material lay-ers. SDM parts are made by a combination of directshaping of part material and replication of supportmaterial features into part material (Merz et al. 1994;Merz 1994). A schematic of the process is shown inFigure 1. First, a computer-aided design model of apart is sliced into layers. The layers are in the z-direc-tion and devised by a custom planning software. Theplanning software decomposes the part into manufac-turable “compacts”—segments of a single material thathave no undercut surfaces (Ramaswami 1997). Then,the material deposition paths and cutting tool pathsare generated to make the needed sequence of com-pacts. The deposition for a layer is in near-net shape.This near-net-shaped layer is then milled to final di-mensions by CNC machining. For overhanging fea-tures, support material is used. After CNC machiningof the support material to net shape, the next layer of

Figure 1Shape Deposition Manufacturing (courtesy of John Kietzman)



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the part material is deposited, and the process contin-ues. SDM can be used to fabricate complex shapes inpolymers (Kietzman et al. 1997), ceramics (Kietzman1999), and metals (Fessler et al. 1997).

Sensor Embedding in Layered Manufacturing

Taking advantage of layered manufacturing, manyresearchers have investigated the feasibility of em-bedding sensors or functional components to obtain“smart” functional products with complex geometry.In one earlier study, Nau (1991) embedded straingauges into tensile and bending specimens, whichwere fabricated via the stereolithography (SLA) pro-cess, and compared their outputs to these of externalstrain gauges during testing. Fusion deposition ofceramics (FDC), an FDM process, offers unique op-portunities for manufacturing electronic ceramics,specifically piezoelectric ceramics and PZT ceramic/polymer composites (for actuators and sensors) freeof the constraints of the design complexity (Danforthand Safari 1966). Many novel and complex compos-ite structures, including volume fraction gradients(VFG), staggered rods, radial and curved compos-ites, and actuator designs such as tubes, spirals, andtelescoping structures, were made using the flexibil-ity provided by the FDC technique (Safari andDanforth 1998; Safari et al. 1999; Bandyopadhyay etal. 1998). Denham et al. (1996, 1997) and Calvert,Denham, and Anderson (1999) studied the fabrica-tion of polymers and composites containing embed-ded sensors, which were manufactured in-situ by amodified FDM process. Luo et al. (1999) embeddedsensors into the core and cavity of a rapid resin moldfor direct measurement of pressure and temperaturefor the purpose of improving the injection moldingprocess. Using the information collected by embed-ded sensors, they developed a control system to en-sure the quality of products.

Fiber Optic Sensors Embedding

Embedding sensors in structural composites hasbeen a topic of research in the last two decades. Inthis arena, fiber optic sensors have emerged as thedominant technology (Udd 1995) in so-called fiberoptic smart structures. They allow critical parametersof materials and structures to be sensed while offer-ing light weight, immunity to electromagnetic inter-ference, nonobtrusive embeddability, resistance to

hostile environments, and extremely high bandwidthcapability. A network of embedded fiber optic sen-sors can allow a structure to monitor its own integrityor health during manufacturing and service. More-over, these sensors could replace many of the func-tions traditionally performed by human visualinspection and could provide real-time feedback inthe event of structure failure.

Lawrence (1997) applied embedded fiber opticstrain sensors for process monitoring of composites.Foedinger et al. (1998) studied structural health moni-toring of filament-wound composite pressure vesselswith embedded optical fiber sensors. Kim, Breslauer,and Springer (1992) studied the effect of embeddedsensors on the strength of composite laminates.

More recently, fiber Bragg grating (FBG) sensorshave become popular for process monitoring, espe-cially for temperature and strain measurements (Jinet al. 1998; Murukeshan et al. 2000; Murukeshan,Chan, Ong 2001). FBG sensors have been embeddedin nickel and stainless steel structures (Li 2001) byshape deposition manufacturing. The embedded FBGsensors were characterized for temperature and strainmeasurements. The embedded FBG sensors in nickeland stainless steel provided high sensitivity, goodaccuracy, and high temperature capacity for tempera-ture measurements. For strain measurements, the sen-sors embedded in metals yielded high sensitivity,accuracy, and linearity. The sensitivity of the embed-ded FBGs was in good agreement with that of bareFBGs. Moreover, a decoupling technique for embed-ded FBG sensors was developed to separate tempera-ture and strain effects (Li 2001).

Principle of Fiber Bragg Grating Sensors

Hill and coworkers (Hill et al. 1978; Kawasaki etal. 1978) first observed fiber photosensitivity in ger-manium-doped silica fiber in 1978. Meanwhile, anentire class of in-fiber components, called the fiberBragg grating (FBG), has been introduced. Basically,FBG consists of a periodic modulation of the index ofrefraction along the fiber core, as shown in Figure 2.Ultraviolet (UV) laser light can be used to write theperiodic modulation directly into photosensitive fi-bers. An FBG functions like a filter when a broad-band light is transmitted into the fiber core, reflectinglight at a single wavelength, called the Bragg wave-length. Thus, a single wavelength is filtered in thetransmitted light spectrum.


80

UV-written in-fiber grating technology has devel-oped very rapidly in recent years, and with gratingsnow commercially available, FBG is important intelecommunications applications and has great po-tential in the optical fiber sensor field. FBG as a sen-sor is compact, simple, and can be demodulated in awavelength-coding manner (Udd 1995; Othonos andKalli 1999). FBGs have been considered suitable formeasuring static and dynamic fields, such as tempera-ture, strain, and pressure (Kersey et al. 1997).

When an FBG is expanded or compressed, its grat-ing spectral response is changed. The temperaturedependence of the Bragg wavelength is related to thetemperature dependence of the index of refraction,n0, and the Bragg grating period, d0, through the fol-lowing equation:

�0 = 2n

0d

0(1)

If the equation is differentiated, the Bragg wavelengthshift, ��, is given by the following:

�� = 2n0�d + 2d

0�n (2)

The strain dependence of the Bragg wavelengtharises from the physical elongation of the sensor andthe change in refractive index due to photoelastic ef-fects. The wavelength shift due to axial strain can beobtained by the following equation:

�� = (1 – pe)�

0 � (3)

where � is the applied strain and pe is the photoelasticcoefficient term given by the following:

20

12 11 122e

np p p p�

(4)

where the pi,j coefficients are the Pockel’s coefficientof the strain-optic tensor and � is Poisson’s ratio. Sometypical values are listed as: � = 0.2, p11 = 0.113, p12 =0.252, and n0 = 1.46. Thus, the value of ��/µ� is ap-proximately 0.001051 nm/µ� at 1300 nm and0.001254 nm/µ� at 1550 nm under axial strain.

Sensor Embeddingin Polymer Materials

The feasibility of embedding fiber optic sensors inpolymers during shape deposition manufacturing wasinvestigated. The sensors can be interconnected intoa real-time monitoring network to detect structuralweaknesses, thereby becoming a basis in the devel-opment of smart polymer materials.

Experiments were performed in which optical fi-bers were embedded in materials such as polyurethane.The embedding in polyurethane was straightforwardbecause the melting point of the material is muchlower than that of silica fibers. A mold was first ma-chined to a designed shape in a specialized wax thatserved as support material. The wax was a mixture of25% Kindt-Collins Master File-a-wax and 75%Protowax. Optical fibers were then placed in a desig-nated path in the mold before a casting was used tofill the mold. After curing, the polyurethane part withthe embedded fiber optical sensors was completed.

Characterization of Embedded SensorsBare fiber Bragg grating sensors are available with

different Bragg wavelengths. FBGs near 1550 nmwere selected in this study because of their many ap-plications in the telecommunications industry.

Before studying the performance of embeddedFBGs, the characterization of the bare FBGs was in-vestigated. Figure 3 shows an experimental setup forcharacterizing a bare FBG in response to axial strain,which is applied by hanging weights on one end ofthe FBG. Because the mechanical properties of silicaoptical fibers are well known (E = 70 GPa, � = 0.2),the weights can be converted to axial strains. A broad-band ELED (edge light-emitting diode) light sourcewith a central wavelength at either 1300 nm or 1550nm was connected to a 3 dB 2×2 fiber coupler. Halfof the light was guided to the Bragg grating sensor,

Figure 2Schematic Representation of a Fiber Bragg Grating


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while the other half was guided to index-matchingliquid. The FBG sensor acted as a strain transducerand reflected a spectral peak back toward the lightsource, but allowed most of the optical power to passthrough. Through the fiber coupler, half of the re-flected peak was captured with an optical spectrumanalyzer with a resolution of ±0.001 nm. The Braggwavelength shifts were used to determine linearitywith axial strains.

The measured sensitivity for an FBG with a Braggwavelength of 1534.568 nm (from O/E Land, Inc.,Canada) was in good agreement with a value of 1.192× 10–3 nm/µ� for an FBG of 1550 nm (Reid 1998).

Metal embedded FBGs can provide accurate mea-surements of strain (Li 2001). Polyurethane embed-ded FBGs are also of interest, and samples were made

measuring 100 mm × 12.7 mm × 6 mm. The FBGswere embedded in a plane that was 1.45 mm fromthe top surface. The experimental setup is shown inFigure 4. The theoretical strain in the plane with theembedded FBG is half of the value measured by thestrain gauge on the top surface. To obtain the correla-tion between wavelength shifts and the theoreticalstrains, a best-fit line was used. Figure 5 presents atypical test result for an FBG with a Bragg wave-length of 1551.114 nm. The sensitivity of 1.215 ×10–3 nm/µ� matches that of the bare FBG well. Thetest results suggest that the bonding between poly-urethane and the optical fibers is strong and that FBGsembedded in polyurethane are capable of measuringstrains accurately.

The sensors embedded in metal and polyurethaneyielded high sensitivity, accuracy, and linearity forstrain measurements. The sensitivity of the embeddedFBGs was in good agreement with that of bare FBGs.

Applications of EmbeddedSensors in Polymers

This section investigates some potential applica-tions of embedded fiber Bragg grating sensors.

Strain Analysis with Two Embedded Sensors

In additive layered processes, flexure and simpleextension can coexist in a layered beam. With oneembedded sensor it is difficult, if not possible, to dis-tinguish the strains introduced by pure flexure fromthose introduced by simple extension. Thus, a two-

Figure 3Characterization of Strain Response of Bare FBGs

Figure 4Setup for Characterization of Embedded FBG in Polyurethane


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sensor technique is proposed and tested to verify theconcept. Figure 6 presents a configuration of twoembedded FBG sensors for strain analysis of a beam.Y1, y2, and t are the distance from the bottom to FBG1,FBG2, and the neutral axis, respectively.

If the beam is under unknown bending and stretch-ing, a theoretical strain at any point in the beam canbe expressed as follows:

�xx

= �bxx

(flexure) + �txx

(extension) (5)

Then the strain in the FBG1 is given by the following:

11xx txx

y

R

��

(6)

and in FBG2 by the following:

22xx txx

y

R

��

(7)

where R is the curvature of the beam under bending.From the equations of �1xx and �2xx, the following isobtained:

1 2

1 2xx xx

y yR

� �

(8)

Sensors FBG1 and FBG2 can be used to measure wave-length shifts, using the following equation:

0 1 ep

��

�

(9)

Therefore, �1xx and �2xx can be determined. The cur-vature R can then be calculated through the follow-ing equation:

1 2

1 2

01 021 1e e

y yR

p p

��

� �

(10)

With known R, the axial strain can be determined bothin flexure and simple extension at any point in thebeam. To verify the concept, two FBG sensors wereembedded in a polyurethane structure.

Two FBGs, one with an original Bragg wavelengthof 1551.123 nm and the other with an original Braggwavelength of 1555.457 nm, were embedded in a 100mm × 12.7 mm × 6 mm polyurethane beam. Y1 and y2

were set at 4.625 mm and 1.625 mm, respectively,and four-point bending to applied loads was used. Anoptical spectrum analyzer (OSA) was used to mea-sure the Bragg wavelengths. To verify the calculatedR from the Bragg wavelength shifts in the two FBGs,a mechanical dial with a resolution of 12.7 µm wasused to measure deflection at three different points.Figure 7 presents the principle of determining Rthrough deflection and L, which is the distance be-tween the two outside points. R can be given by thefollowing:

R

L

=2

22

+�

�

deflection

deflection2(11)

Figure 5Strain Response for FBG Embedded in Polyurethane

Figure 6Configuration for Two-Sensor Technique


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Figure 8 shows the results of the Bragg wavelengthshifts from the two FBGs against deflection, whichis called maximum deflection in the figure. The wave-length shifts from both FBGs is approximately linearwith �deflection.

Figure 9 shows a comparison of R’s measured bythe two-sensor technique to R’s measured by the me-chanical dial technique. The measured R’s are within5%, indicating the effectiveness of the two-sensortechnique.

Embedded Sensors for Pressure Measurements

With embedded sensors, it is possible to develop alow-cost fabrication methodology for the productionof differential pressure sensors that function just aswell as currently available structures with built-insemiconductor membranes. The basic concept is atubular vessel with a membrane embedded. The mem-brane has an FBG embedded close to the neutral axisto minimize the influence of bending. The selectionof suitable materials for diaphragms with embeddedstrain gauges is crucial for the sensor’s performance.

A model based on a thin diaphragm with an em-bedded FBG is considered. The thin diaphragm witha diameter of 2a and a thickness of t is deflected whena pressure difference, �P, is applied across its sur-face, as shown in Figure 10. The deflection causesstresses to appear in the disk. The correspondingstrains can be measured by the embedded FBG.

The maximum deflection, �max, is linearly relatedto the pressure difference, subject to the constraintthat it remains elastic (Ashby 1999), as follows:

4 2

max 3

3 1

16

a P

t E

� ��

(12)

where � is Poisson’s ratio and E is Young’s modulusfor the diaphragm. The maximum strain, �max, at thecircumference is also linear by relation to the pres-sure difference, as follows:

2

max

3 1

8

P a

E t

� ��

(13)

The pressure difference, �P, is then given by the fol-lowing:

max

28

3 1

E aP

t�

��

(14)

Figure 7Principle of Determining R Through Mechanical Dial

Figure 8Wavelength Shifts Against ��deflection

Figure 9Comparison of R’s Measured by Two-Sensor Technique with

R’s Measured by Mechanical Dial Technique


84

The strain causes the Bragg wavelength to shift inthe fiber Bragg grating sensor, which is embedded inthe thin diaphragm. The wavelength shift is deter-mined by the following:

�� = �0(1 – p

e) �

max(15)

By measuring the wavelength shift in the FBG, �Pcan then be calculated by the following:

2

0

8

3 1 1 e

E aP

t p

��

� �

(16)

As the equation shows, the sensitivity of the pres-sure sensor is determined by the resolution for wave-length shift measurements by a demodulation system,the material properties (E, �), and the geometry (a/t)of the diaphragm. A resolution of 1.0 µ� in strainmeasurements can be normally achieved via commer-cially available strain indicators. Moreover, a resolu-tion of 0.001 nm in wavelength shift can be normallyachieved via demodulation systems such as tunablelasers, tunable filters, or optical spectrum analyzers.

Pressure sensors were manufactured in polyure-thane by the shape deposition manufacturing process.The diaphragm was machined to 1.00 mm thicknessand the sensors were embedded in the middle of thediaphragm and in a layer 0.5 mm from the bottomsurface of the diaphragm. Two pressure sensors witha/t ratios of 15 and 25 were tested. In the pressuresensor with the a/t ratio of 15, a strain gauge wasembedded. In the other one, both a strain gauge andan FBG with an original Bragg wavelength of1533.478 nm were embedded. Because an optical fi-

ber has a diameter of 125 µm and a thin-film straingauge is approximately 50 µm thick, the sensing pointwas actually slightly above the neutral axis.

Figure 11 presents the results from the testing ofthe pressure sensor with the a/t ratio of 15. Strain isplotted against pressure. In the lower pressure region,a linear relationship between pressure and strain isobtained. Approximately a pressure of 105.91 Pa in-duces a strain of 1.0 µ�, as shown in Figure 11a.However, the ratio of pressure to induced strain be-comes larger in a higher pressure range, as shown inFigure 11b. The pressure sensor failed when the pres-sure was raised to 3.5 × 105 Pa (3.5 bar) and the in-duced strain was 2229 µ�.

Testing results of the embedded strain gauge forthe pressure sensor with the a/t ratio of 25 are shownin Figure 12. In the lower pressure region, a linearrelationship between pressure and strain is obtained.Approximately a pressure of 37.676 Pa induces astrain of 1.0 µ�, as shown in Figure 12a. Again, theratio of pressure to induced strain seems to be largerin the higher pressure range, as shown in Figure 12b.The pressure sensor functioned well at a pressure of1.6 × 105 Pa (1.6 bar) when the induced strain reached3445 µ�. This suggests that pressure sensors with ahigher a/t ratio can endure larger strain.

Embedded strain gauges function well for the pres-sure sensors. Embedded FBGs were also tested forpressure measurements. Wavelength shifts were mea-sured at different pressures, and the wavelength shiftwas plotted against the strain measured by the em-bedded strain gauge, as shown in Figure 13. Wave-length shifts follow the strain, thus pressure, linearly,at least for higher strains.

Figure 10Simple Model for a FBG-Based Pressure Sensor


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ConclusionLayered manufacturing can be used to integrate

fiber optic sensors during the production of polymercomponents. Sensors can be placed close to points ofinterest prior to enclosure. For strain measurements,the sensors embedded in polyurethane yielded highsensitivity, accuracy, and linearity. The sensitivity ofthe embedded FBGs was in good agreement with thatof bare FBGs. The embedded sensors were used tomeasure the strain field in layered materials. Pres-sure sensors based on embedded strain gauges andFBGs were also developed and tested, showing goodaccuracy and linearity.

Acknowledgments

The authors are grateful for the financial supportfrom National Science Foundation and the Office ofNaval Research.

Figure 11Pressure Sensor with a/t Ratio of 15

Figure 12Pressure Sensor with an a/t Ratio of 25

Figure 13Wavelength Shifts vs. Stain Reading From Strain Gauge

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Authors’ BiographiesDr. Xiaochun Li is an assistant professor of mechanical engineering at

the University of Wisconsin-Madison. Dr. Li’s primary areas of researchinterests include micro/nanomanufacturing, solid freeform manufacturing,laser material processing, sensors and actuators, MEMS, smart materialsand structures, and manufacturing processes monitoring and control. Heearned his PhD in mechanical engineering at Stanford University in 2001.He received a 2002 NSF CAREER award. He is also a member of theSociety of Manufacturing Engineers (SME), the Laser Institute of Ameri-ca (LIA), and the American Society of Mechanical Engineers (ASME).

Dr. Fritz Prinz is the Rodney H. Adams Professor in the departments ofmechanical engineering and materials science and engineering at StanfordUniversity. He currently serves as co-director for the Alliance for Innova-tive Manufacturing at Stanford University. Dr. Prinz was elected as a for-eign member of the Austrian Academy of Science in 1996. In 1997, he wasappointed as a board member of the National Research Council Commit-tee: Design and Manufacturing. His current research focuses on the designand manufacturing of microscale devices. He is interested in materials se-lection, scaling theory, geometric modeling, and abstraction. Recently, hebecame involved in a project on the synthesis of biological cell structures.

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Embedded Fiber Bragg Grating Sensors in Polymer Structures Fabricated by Layered Manufacturing