sensors Article

Localized Temperature Variations in Laser-Irradiated Composites with Embedded Fiber Bragg Grating Sensors R. Brian Jenkins 1, *, Peter Joyce 2 and Deborah Mechtel 1 1 2

*

Department of Electrical and Computer Engineering, US Naval Academy, 105 Maryland Ave, Annapolis, MD 21402, USA; [email protected] Department of Mechanical Engineering, US Naval Academy, 590 Holloway Rd., Annapolis, MD 21402, USA; [email protected] Correspondence: [email protected]; Tel.: +1-410-293-6159

Academic Editors: Christophe Caucheteur and Tuan Guo Received: 15 December 2016; Accepted: 25 January 2017; Published: 27 January 2017

Abstract: Fiber Bragg grating (FBG) temperature sensors are embedded in composites to detect localized temperature gradients resulting from high energy infrared laser radiation. The goal is to detect the presence of radiation on a composite structure as rapidly as possible and to identify its location, much the same way human skin senses heat. A secondary goal is to determine how a network of sensors can be optimized to detect thermal damage in laser-irradiated composite materials or structures. Initial tests are conducted on polymer matrix composites reinforced with either carbon or glass fiber with a single optical fiber embedded into each specimen. As many as three sensors in each optical fiber measure the temporal and spatial thermal response of the composite to high energy radiation incident on the surface. Additional tests use a 2 × 2 × 3 array of 12 sensors embedded in a carbon fiber/epoxy composite to simultaneously measure temperature variations at locations on the composite surface and through the thickness. Results indicate that FBGs can be used to rapidly detect temperature gradients in a composite and their location, even for a direct strike of laser radiation on a sensor, when high temperatures can cause a non-uniform thermal response and FBG decay. Keywords: fiber Bragg gratings; temperature sensors; polymer matrix composites; high energy radiation; strain sensors; structural health monitoring; smart structures

1. Introduction Optical fiber sensors provide a number of advantages for sensing either temperature or strain in polymer matrix composites. They are relatively nonintrusive, lightweight, and flexible. They respond rapidly, they can be used in harsh environments, and they are highly sensitive to both temperature and strain [1–3]. Fiber Bragg grating (FBG) sensors, in particular, can be used to accurately detect localized perturbations in strain and temperature with a high degree of spatial resolution due to their compact size [4,5]. This makes them especially attractive for structural health monitoring in polymer matrix composites to detect flames, radiation, or damage [6–8]. When used to measure high temperatures, FBGs may require appropriate annealing techniques or specialized fibers to prevent their decay or erasure [9–12]. For applications that simply require rapid detection of heat and its location on a composite surface, the high temperature decay of an FBG may be a secondary concern. For example, if an optical fiber sensor is used to detect flames [13] or to identify a directed energy assault [14] on an aircraft, the primary goal may be to simply recognize a spike in temperature as quickly as possible, similar to the way the human body responds to a burning sensation.

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The emphasis of this study is to determine the speed at which standard embedded FBG temperature sensors detect the temporal and spatial thermal response of a composite material or structure when high energy radiation is incident on the surface. CO2 lasers emitting at 10.6 µm have been used in previous research to rapidly heat FBGs in bare fibers [15–17]. In this research, high energy infrared (IR) radiation at 1064 nm was used to heat a polymer matrix composite with embedded FBG temperature sensors. The thermal response of various composite test specimens was observed in this manner. Initially, two simple composite laminates were fabricated using E-glass fiber and carbon fiber with epoxy resin. A single optical fiber containing one to three sensors was embedded between two plies of each type of composite. The compact size and close spacing of the FBGs ensured high-spatial resolution temperature measurements. Additional tests were then performed on a more complex architecture, with 12 FBGs embedded in a 2 × 2 × 3 array in a two-ply carbon fiber/epoxy composite [18]. Though precise measurement of temperature is not essential in this application, reliable estimates of the temperature are still important to optimally design an FBG sensor network that can detect high-energy radiation, so an oven was used initially to calibrate the low temperature sensitivity of bare sensors and embedded sensors. Subsequent tests with a heat gun demonstrated the influence of conduction and convection on the thermal response in all of the composite structures [19]. In the tests described here, in which energy from a high power laser was absorbed by the composite, the primary mechanism of heat transfer was radiation from the laser strike, but conduction and convective cooling were also evident. This study, using IR laser strikes at 1064 nm, demonstrated the feasibility of using networks of FBG temperature sensors to rapidly detect and locate radiation incident on the surface of a composite. The spatial response of the composite depended on the proximity of the sensor to the laser strike, whereas the temporal response was limited by the composite, not by the sensor. Development of an optimal sensor network and a detection algorithm that provides rapid and reliable detection of a thermal response will require additional research. Laser radiation directly incident on an embedded sensor caused a non-uniform temperature and strain profile [3,20], wavelength hopping due to peak wavelength interrogation at lower scan rates [21], and erasure (or partial erasure) of the grating at higher laser power levels. Because the sensors also measure any mechanical strain in the composite, interrogation of the full spectrum [5,22] may ultimately be required to fully identify and characterize the thermal and strain responses. Moreover, advanced signal processing may be needed to quickly isolate a laser-induced thermal response of an irradiated composite from mechanical strain. 2. Background Physically, a fiber Bragg grating is a sinusoidal variation in the core index of refraction in an optical fiber along its longitudinal axis. The index of refraction is typically modulated using interferometric techniques to exploit the photosensitivity of optical fiber to ultraviolet radiation [23]. The most common technique used to manufacture an FBG uses phase masks to precisely control the physical characteristics of the grating. For a grating period of Λ in a fiber with an effective index of refraction ne , light propagating down the fiber at the Bragg wavelength λ B = 2ne Λ will be reflected by the grating. Light at all wavelengths other than λ B will continue to propagate beyond the grating with high transmission and low loss. Hence, an FBG can be used to selectively filter one (or more) wavelengths of the light propagating down the optical fiber. Temperature sensing is possible because the Bragg wavelength shifts when the temperature changes. After taking the derivative of λ B with respect to temperature, the wavelength shift ∆λ B that corresponds to temperature variation ∆T is: ∆λ B =

 2

 dne dΛ Λ + 2ne ∆T. dT dT

(1)

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Equation (1) is often normalized, such that: ∆λ B = λB



 1 dne + α ∆T = 6.67 × 10−6 ◦ C−1 , ne dT

(2)

where the coefficient of thermal expansion of the optical fiber, α = 1/Λ(dΛ/dT ) is relatively small, contributing ~5% to the shift ∆λ B [4]. Based on (2), the sensitivity to temperature ∆λ B /∆T is near 10.3 pm/◦ C, at λ B = 1550 nm. FBGs are also sensitive to strain, since any change in length due to tension (or compression) will lengthen (or shorten) the grating period. The normalized shift in Bragg wavelength due to applied mechanical strain ε is given by: n   o ∆λ B = 1 − n2e /2 [ P12 − ν( P11 + P12 )] ε = (1 − pe )ε, λB

(3)

where Pij are Pockel’s coefficients and ν is Poisson’s ratio [4]. The quantity 1 − pe ≈ 0.78 × 10−6 µε−1 , so the sensitivity to strain at λ B = 1550 nm is 1.21 pm/µε. Strain may influence a temperature measurement if an FBG is intended for use purely as a temperature sensor. If an FBG is embedded in a structure, (1) and (2) must be modified to account for strain (thermal or mechanical) in the composite. To consider both temperature and strain, (2) and (3) are combined, and the total normalized shift in λ B is given by: ∆λ B = λB



 1 dne + α ∆T + (1 − pe )(ε + ε t ), ne dT

(4)

where ∆λ B , due to thermal strain in the composite ε t = αc ∆T depends on the coefficient of thermal expansion of the composite αc . Before performing experiments using FBGs to detect localized temperature variations in a composite caused by laser irradiation, Equations (2) and (4) were experimentally characterized for the sensors and composites used in this project [19]. Type I FBG [9] temperature sensors from ITF Technologies were used in all tests, with nominal reflectivity values near 70% at λ B . The sensor fibers were coated with polyimide, except for a 3–5 cm region around the FBGs where no recoating was applied. The vendor-specified maximum temperature was 300 ◦ C. An oven was used to calibrate the sensitivity of both bare and embedded sensors to temperatures as high as 250 ◦ C. In the preliminary characterization [19], the wavelength shift versus temperature in bare fiber was in good agreement with manufacturer specifications [24]. The resulting sensitivity ∆λ B /∆T was 11.9% ± 5% pm/◦ C in the measured temperature range, both for sensors in bare fiber and for sensors embedded in the carbon fiber/epoxy composite. Hence, the carbon fiber composite exhibited a near-zero value for αc such that ε t in Equation (4) was negligible; sensors embedded in the carbon fiber composite experienced minimal tensile or compressive strain as the temperature changed. In contrast, the sensitivity of sensors embedded in E-glass fiber/epoxy composite was 19.1 pm/◦ C. Based on the definition of ε t in Equation (4), αc ≈ 6 µε/◦ C in the E-glass fiber composite. The values for αc in each material are consistent with another work ([25], pp. 375, 377). Thus, for a given temperature shift, the wavelength shift ∆λ B in an E-glass/epoxy composite was expected to be >50% larger than that in a carbon fiber/epoxy composite. To measure shifts in λ B , an interrogation scheme similar to that shown in Figure 1 was employed. A tunable laser source was used in combination with a circulator to generate a reflected spectrum with energy at the Bragg wavelengths of all FBGs in the fiber. Shifts in the Bragg wavelengths were monitored at the reflected output port. Though only two sensors are illustrated in Figure 1, more were sometimes interrogated, depending on the size of the FBG array. The SmartScan interrogator (from Smart Fibres) used here could scan a 40-nm spectrum in four fiber optic channels at speeds up to 2.5 kHz with a spectral resolution around 0.1 nm, determining the peak wavelength of all the

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of all the sensors every 400 µ s. While the SmartScan interrogator scanned the spectrum at multi-kHz rates, the sensors time required to detect a temperature gradient is intrinsically limited by the response time of all the every 400 µ s. While the SmartScan interrogator scanned the spectrum at multi-kHz sensors every 400 µs. While the SmartScan interrogator scanned the spectrum atsensor multi-kHz rates,time the of the FBG sensor itself (i.e., when not embedded). Precise measurements of the response rates, the time required to detect a temperature gradient is intrinsically limited by the response time time required to detect a temperature gradient is intrinsically limited by the response time of the FBG were taken, though as a when roughnot estimate, the heating response time appeared to be ~50 ms, with of thenot FBG sensor itself (i.e., embedded). Precise measurements of the sensor response timea sensor itselfslower (i.e., when notresponse embedded). Precise measurements of the sensor response time were not somewhat cooling [19], similar to results reported elsewhere [15–17]. were not taken, though as a rough estimate, the heating response time appeared to be ~50 ms, with a taken, though as a rough estimate, the heating response time appeared to be ~50 ms, with a somewhat somewhat slower cooling response [19], similar to results reported elsewhere [15–17]. slower cooling response [19], similar to results reported elsewhere [15–17].

Figure 1. Typical sensor interrogation scheme for two inline fiber Bragg gratings (FBGs) with Bragg B1sensor wavelengths and B2 . Figure Figure 1. 1. Typical Typical sensor interrogation interrogation scheme scheme for for two two inline inline fiber fiber Bragg Bragg gratings gratings (FBGs) (FBGs) with with Bragg Bragg wavelengths λ and λ . wavelengths B1 and B2 . B1

B2

In this research, the goal was to use FBG temperature sensors to rapidly detect and locate a temperature gradient the in agoal laser-irradiated polymer matrix sensors composite. However, as Equation (4) In goal was to to use use FBG FBG temperature sensorstotorapidly rapidlydetect detect andlocate locate In this research, was temperature and a B gradient shows, can shift ineither because of external strain or a change in temperature An  T .  atemperature temperature gradient inaalaser-irradiated laser-irradiated polymer matrix composite. However, as Equation (4) polymer matrix composite. However, as shows, λ B can shiftshift either because of could external strain a change temperature ∆T. Anisembedded embedded sensor architecture that be used εstrain toorisolate from strain shown in shows, can either because of external a inchange in temperature T . An  ortemperature B sensor architecture that could be used to isolate temperature from strain is shown in Figureand 2, where n  m Figure 2, where an array of FBGs is embedded on each ply of the composite laser embedded sensor architecture that could be used to isolate temperature from strain is shown in an n × m array of FBGs is embedded on region each ply ofthe the surface. composite laser strain irradiation is incident on irradiation is incident on localized If and external uniform or  is m a array Figure 2, where an n  of FBGs is on embedded on each ply of the composite and laser a localized region on the surface. If external strain ε is uniform or predictable across the composite  due predictable is across the composite structure, it may to If isolate predictable irradiation incident on a localized region on be thepossible surface. external strain shifts uniform or  is in structure, it may be possible to isolate predictable shifts in λ B due to strain from localized shifts Bdue to  T totemperature strain from localized shifts duestructure, to by a temperature variation detected by onlyshifts a fewinsensors. B due across the composite it amay possible to isolate predictable apredictable variation ∆T detected only fewbe sensors. For the tests performed here, the optical fibers were embedded on the surface or between the For the tests performed the optical fibersvariation were embedded on thebysurface between T detected to strain from localized shiftshere, due to a temperature only a or few sensors.the plies of the composite. The initial experiments focused only on the material response to the laser plies For of the The initial experiments focused on the material response to the laser thecomposite. tests performed here, the optical fibers wereonly embedded on the surface or between the irradiation and did not include external strain, so in Equation (4). While precise measurement   0 irradiation and did not include external strain, so ε = 0 in Equation (4). While precise measurement plies of the composite. The initial experiments focused only on the material response to the laser of temperature temperature is not not the ultimate goal, estimates estimates of temperature willWhile be helpful helpful to diagnose diagnose and and of the ultimate goal, be to irradiation and is did not include external strain, so of in Equation will (4). precise measurement  the the 0 temperature characterize response, especially at high temperatures. Ultimately, rapid detection of characterize the system response, especially at high temperatures. Ultimately, rapid detection ofa of temperature is not the ultimate goal, estimates of the temperature will be helpful to diagnose and localized temperature shift may require an interrogator with sufficient memory to capture the full acharacterize localized temperature may require an interrogator with sufficient memory fulla the system shift response, especially at high temperatures. Ultimately, rapid detection of spectrum and high speed signal processing [5,21,22] in order to isolate the thermal and mechanical spectrum and high speed signal processing [5,21,22] in order to isolate the thermal and mechanical localized temperature shift may require an interrogator with sufficient memory to capture the full responses and due to to temperature temperature shifts andstrain. strain. responses due shifts and spectrum high speed signal processing [5,21,22] in order to isolate the thermal and mechanical responses due to temperature shifts and strain.

m Figure arraysofofsensors sensorson oneach eachlayer. layer. Figure2. 2.Multiple Multipleplies pliesof ofcomposite compositewith withembedded embedded nn× m arrays Figure 2. Multiple plies of composite with embedded n  m arrays of sensors on each layer.

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3. 3. Localized Localized Embedded Embedded Sensor Sensor Response Response To Toobserve observe the theresponse responseof ofembedded embedded temperature temperature sensors sensors to to laser laser irradiation, irradiation, aa class class IV IV laser laser was was used used to to generate generate rapid rapid heating heating at at specific specific locations locations on on the the surface surface of of each each composite composite specimen. The Thelaser laseremitted emittedCW CW(continuous (continuouswave) wave)light lightat at1064 1064nm nmwith withaa beam beam diameter diameterof of55 mm mm and and variable variable 2 . Figure 2. Figure output to ~100 ~100W Wwith witha amaximum maximumirradiance irradiance ~500 W/cm 3 shows the view rear output power up to of of ~500 W/cm 3 shows the rear view thesetup test setup an E-glass composite the article. test article. An exhaust pulled air of theoftest with with an E-glass fiber fiber composite as theastest An exhaust vent vent pulled air over over the specimen to remove smoke from chamber.NN wasapplied appliedas as necessary necessary to extinguish the specimen to remove smoke from thethe chamber. 2 gas extinguish 2 gaswas flames flames that that erupted. erupted. For initial tests [19], the laser power was set to 25% of its maximum power, corresponding corresponding to to ~25 ~25 W of total total integrated integrated power power across the the Gaussian Gaussian beam beam profile. profile. To To measure measure the the composite composite response response over over time, time, the the interrogator interrogator scanned scanned the the spectrum spectrum at at 500 500 Hz, Hz, acquiring acquiring the the peak peak wavelengthofofeach each sensor every 2 ms. tests described considered a 70 s time wavelength sensor every 2 ms. The The initialinitial tests described below below considered a 70 s time window, window, beginning at t =laser 0 s. was The laser was ons at t =off 10 at s and offs at = 30 test. s in each test. beginning at t = 0 s. The turned onturned at t = 10 and t = 30 inteach

Figure 3. Test setup with the high power laser to measure the localized response of the embedded Figure 3. Test setup with the high power laser to measure the localized response of the embedded sensors to high temperature. sensors to high temperature.

Figure Figure 44 shows shows aa picture picture of of the the two two composite composite specimens specimens after the initial tests were completed. Both Both specimens specimens were were manufactured manufactured with with room room temperature temperature curing curing epoxy epoxy resin resin using using either either plain plain weave E-glass fiber (GF) fabric or carbon fiber (CF) fabric. The legend in Figure 4 identifies the locations weave E-glass fiber (GF) fabric or carbon fiber (CF) fabric. The legend in Figure 4 identifies the of four laser at positions around1–4 each 1550 nm (nominal Bragg wavelength) sensor that locations of strikes four laser strikes at1–4 positions around each 1550 nm (nominal Bragg wavelength) was embedded each composite. strikes on the 1550-nm sensor in each specimen occurred at sensor that wasinembedded in eachDirect composite. Direct strikes on the 1550-nm sensor in each specimen position the GF4.composite, additional weresensors locatedwere 6 cmlocated and 12 6cm the12 right occurred4.atOn position On the GFtwo composite, twosensors additional cmtoand cm of to position nominal Bragg wavelengths of 1545 nm and 1540nm nm,and respectively. the right4,ofhaving position 4, having nominal Bragg wavelengths of 1545 1540 nm, respectively. Each Each FBG FBG sensor sensor is is 11 cm cm in in length. length. On On each each specimen, positions 1 and 2 are located 1 cm above, and and 1.2 1.2 cm cm to to the the right right and and to to the the left left of of the the 1550-nm 1550-nm sensor, sensor, respectively. respectively. Position Position 33 is is located located 11 cm cm below below and and 1.2 1.2 cm cm to to the the left left of of the the sensor. sensor. The The damage damage that that occurred occurred at at each each location location is is visible visible unless unless obscured obscuredby bychar charfrom fromlater latertests, tests, e.g., e.g., position position 22 is is obscured obscured on on the the CF CF composite composite (char (char from from previous previous experiments experiments is is also also seen seennear nearthe the1545-nm 1545-nmand and1540-nm 1540-nmsensors sensorsin inthe theGF GFcomposite.) composite.) Several Several general general observations observations can can be be made made based based on on the the thermal thermal response response of of the the composites composites during the laser strike at at position 1. during the thelaser laserstrikes strikesatatthe thelocations locationsshown shownininFigure Figure4.4.First, First,consider consider the laser strike position The responses measured by the 1550-nm sensors in each composite (1 cm and and 1.2 cm 1. The responses measured by the 1550-nm sensors in each composite (1 below cm below 1.2 to cmthe to left the of the are shown in Figure 5. The nitrogen gas was usedused intermittently to extinguish flames that left ofstrike) the strike) are shown in Figure 5. The nitrogen gas was intermittently to extinguish flames erupted from from the composite surface as theasepoxy resin resin in theincomposite laminate burned off. As seen that erupted the composite surface the epoxy the composite laminate burned off. As in both each composite responded immediately whenwhen the laser turned on aton t =at10t =s.10 Since the seen in plots, both plots, each composite responded immediately the laser turned s. Since the laser strike was above the sensor and the exhaust air flow (and N2) was upwards across the surface of each specimen, the shift in the Bragg wavelength and the temporal response depended primarily

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laser strike was above the sensor and the exhaust air flow (and N2 ) was upwards across the surface of each specimen, shift in theand Bragg wavelength and theoftemporal responseFor depended on on the thermal the absorptivity thermal conductivity the composite. example,primarily the higher the thermal absorptivity conductivity of the composite. Forsince example, the higher thermal thermal conductivity ofand the thermal CF composite is evident in Figure 5a the Bragg wavelength conductivity theupwards CF composite is evident in Figure Bragg wavelength continued to shift continued to of shift after the laser strike ended5aatsince t = 30the s; the measured temperature (detected upwards after the laser strike ended at t = 30 s; the measured temperature (detected by the sensor by the sensor at position 4, below the strike at position 1) continued to increase as laser energy at positionby4, the below the strike position 1) continued to increase as the laser energy the absorbed composite at at position 1 continued to conduct past FBG afterabsorbed the laser by strike composite at position 1 continued to conduct past the FBG after the laser strike ended. ended.

Figure 4. 4. Polymer Polymer matrix matrix composite composite (PMC) specimens tested for their high high temperature temperature response. Figure 4. composite (PMC) (PMC) specimens specimens tested tested for for their temperature response. response. Sensor positions in each PMC are indicated by their nominal Bragg wavelength. indicated by their their nominal nominal Bragg Bragg wavelength. wavelength. Air Sensor positions in each PMC are indicated Air flow flow to to remove remove smoke across each composite. CF: carbon fiber GF: E-glass fiber. upwards across across each each composite. composite. CF: CF: carbon carbon fiber fiber GF: GF: E-glass E-glass fiber. fiber. smoke was was upwards upwards

BBBininthe Furthermore, the magnitude magnitude of of ∆λ theCF CFcomposite compositeininFigure Figure was larger than shift Furthermore, the 5a5a was larger than thethe shift in thethe GFGF composite in Figure 5b since the thermal absorption of theofcarbon fiber was than that of in composite in Figure 5b since the thermal absorption the carbon fibergreater was greater than the E-glass fiber. Since the carbon opaque the E-glass translucent, more energy was absorbed that of the E-glass fiber. Since theiscarbon is and opaque and theisE-glass is translucent, more energy was by the CF composite the extent of the strike. is consistent absorbed by the CFduring composite during thelaser extent of This the laser strike. with This measurements is consistent from with a thermopile that was placed at the of each specimen. Noeach energy was measured bywas the measurements from a thermopile thatrear wasface placed at the rear face of specimen. No energy thermopilebyonthe the rear side of composite the test,during and though theand laser strikethe didlaser not measured thermopile onthe theCF rear side of theduring CF composite the test, though burn adid hole either specimen during the tests, energy was measured the rear facethe of strike notthrough burn a hole through either specimen during the tests, energy wasexiting measured exiting the GF after the laser was on. turned The wavelength shift of 175shift pm of in 175 Figure rear facecomposite of the GFsoon composite soon after theturned laser was on. The wavelength pm 5a in indicates the CFthat composite by approximately 15 ◦ C, based15on°C, a sensitivity 11.9 pm/◦of C. Figure 5athat indicates the CF heated composite heated by approximately based on aofsensitivity The temperature occurred nearoccurred t = 38 s, ~8 s after ended. contrast, theInGF composite 11.9 peak pm/°C. The peak temperature near t = 38the s, strike ~8 s after the In strike ended. contrast, the temperature increased only by a couple degrees, as indicated in Figure 5b by in a smaller pm, GF compositeonly temperature increased by a couple degrees, as indicated Figure shift 5b byofa ~40 smaller usingofa ~40 sensitivity of 19.1 pm/◦ C. As observed inAs Figure 5b, the increase in the shift pm, using a sensitivity of 19.1 pm/°C. observed inlargest Figure temperature 5b, the largest temperature E-glass occurred near toccurred = 21 s, when from the surface. increase in the E-glass nearflames t = 21 s,erupted when flames erupted from the surface.

B during Figure 5. The temporal response aa strike at 1, by the Figure 5. 5. The Thetemporal temporalresponse responseofof of strike at position position 1, measured measured the 1550-nm 1550-nm B during Figure ∆λ B during a strike at position 1, measured by theby 1550-nm sensor sensor in (a) or sensor in CF (a) the the CF CF composite composite or (b) (b) the the GF GF composite. composite. in (a) the composite or (b) the GF composite.

The thermal response also depended on when (or whether) N22 gas was applied to the surface of the composite. This is observed in Figure 5b during the strike at position 1 when puffs of N22 were briefly applied to the surface of the GF composite near t = 25 s and t = 27 s. This was also evident

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The thermal response also depended on when (or whether) N2 gas was applied to the surface of the composite. This is observed in Figure 5b during the strike at position 1 when puffs of N2 were briefly applied to the surface of the GF composite near t = 25 s and t = 27 s. This was also evident during strikes at positions 2 and 4 on the GF composite, as shown in Figure 6 (a and b, respectively), which show the response of the 1550-nm sensor. During each of these strikes, nitrogen was not applied until t = 33 s, ~3 s after the laser turned off. The responses in Figure 6a,b also highlight the distinction between temperature changes due to heating from absorption of radiation, conduction or combustion, and convective cooling (using the N17 2 ). Sensors 2017, 17, 251 7 of The large shifts in ∆λ B that occur near t = 21 s in each plot coincide with the presence of flames on the surface strikes duringat each strike, and the4 magnitude of ∆λ B in these plots indicates sensor in during positions 2 and on the GF composite, as shown in Figurethat 6 (athe and1550-nm b, respectively), the GF show composite eventuallyofachieved temperatures least 10 times observed in Figure 5b, which the response the 1550-nm sensor. at During each of hotter these than strikes, nitrogen was not when theuntil nitrogen was applied earlier, suppressing or extinguishing any combustion. As expected, applied t = 33gas s, ~3 s after the laser turned off. the temperature increase was 6a,b highest a direct laser strike,between as depicted by the erratic, fluctuating The responses in Figure alsoduring highlight the distinction temperature changes due to wavelength Figure 6b. of Figure 7 shows the flamesorerupting from the compositecooling at position 4 near heating frominabsorption radiation, conduction combustion, andGF convective (using the time t = 30 s. When the nitrogen was applied at t = 33 s, the combustion was extinguished and the N2). The large shifts in B that occur near t = 21 s in each plot coincide with the presence of flames temperature rapidly dropped. on the surface during each strike, and the magnitude of B in these plots indicates that the The sensor in the GF based composite was severely damaged by the direct strike at position 4, 1550-nm sensor GF composite achieved times hotter as indicated by in a the significant drop ineventually the optical power temperatures reflected backattoleast the 10 interrogator bythan the observed in Figure 5b, when the nitrogen gas was applied earlier, suppressing or extinguishing any 1550-nm sensor, ~65% before the strike and ~8% after the strike. Based on the square law analysis combustion. As expected, the temperature was during require a direct laserexposure strike, as and fiber characteristics in [12], such a partialincrease erasure of the highest grating would a 20-s at depicted by the erratic, fluctuating wavelength in Figure 6b. Figure 7 shows the flames erupting from ◦ ◦ a temperature of ~700 C (well above the manufacturer specified operating limit of 300 C for these the GF composite at position 4 near t = 30 iss.nonlinear; When theanitrogen was applied at t = to 33 higher s, the sensors). For the FBGs used here, the time sensitivity second order extrapolation combustion was extinguished and the temperature rapidly dropped. temperatures using data provided by the vendor [24] predicts wavelength shifts of at least 11 nm in GF the based composite wasof severely damaged by E-glass the direct strike at position 4, ◦ C in the bare The fibersensor at 700 ◦inC.the Using previous estimate αc ≈ 6 µε/ fiber/epoxy composite, as indicated by a significant drop in the optical power reflected back to the interrogator by the ◦ a total shift greater than 16 nm might be expected for temperatures near 700 C when measured by 1550-nm sensor, ~65% strike and ~8% after the strike. Based on the square law analysis and a sensor embedded in before the GFthe composite. fiber While characteristics in this [12], magnitude such a partial erasure of the grating would require a 20-s exposure a a shift of is somewhat consistent with the results in Figure 6b, atthe temperature of ~700 °C (well above the manufacturer specified operating limit of 300 °C for these temperature response during a direct laser strike was difficult to determine precisely because of sensors). Forreflectivity the FBGs used the sensitivity nonlinear; a second order extrapolation to higher the reduced in thehere, spectral peak at λ B .isThe peak wavelength measured by the interrogator temperatures using data provided by the vendor [24] predicts wavelength shifts of at least 11 nm in was inconsistent (note the discontinuities in the plot); sometimes the peak was near the upper bare fiber at 700 the previousatestimate µε/°C in the fiber/epoxy composite, instrument limit°C. ofUsing the interrogator 1568 nmof(ashift nearly 18E-glass nm) and sometimes the peak c  6 of between 5 nm and1610nm nm. The SmartScan interrogator also occasionally dropped out completely ashifted total shift greater than might be expected for temperatures near 700 °C when measured by a withoutembedded acquiring in a peak wavelength. sensor the GF composite.This phenomenon will be discussed in much greater detail later.

Figure 1550-nm sensor sensor in in the the GF GF composite compositeduring duringlaser laserstrikes strikesatat(a) (a)position position2 Figure 6. 6. The The variation variation in in the the 1550-nm 2and and(b) (b)position position4.4.

Figure 6. The variation in the 1550-nm sensor in the GF composite during laser strikes at (a) position 8 of 18 2 and (b) position 4.

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reduced reflectivity in the spectral peak at B . The peak wavelength measured by the interrogator was inconsistent (note the discontinuities in the plot); sometimes the peak was near the upper instrument limit of the interrogator at 1568 nm (a shift of nearly 18 nm) and sometimes the peak Figure ≈ interrogator 30 strike on the sensor out shifted between 5 nm and from 10 nm. The SmartScan also laser occasionally dropped 7. Flames erupting the GF composite at tt ≈ 30 ss during during aa direct direct at position 22 nozzle be just flame. completely without acquiring a peak wavelength. phenomenon will be discussed in much 4. The N nozzle can can be seen seen just below below the theThis flame. greater detail later. While shift of this magnitude is somewhat consistent with the results in 6b, are the For composite specimen Figure results strike the For the theaCF CF composite specimen (see (see Figure 4), 4), the the results of of aa direct direct strike on on Figure the sensor sensor are temperature a direct laser 8a strike wasthe difficult determine precisely because of the illustrated Figure The in shows sensortoresponse over time, Figure 8b illustrated in inresponse Figure 8. 8.during The plot plot in Figure Figure 8a shows the sensor response over time, and and Figure 8b shows the laser strike. In In contrast to the tests on shows the the reflection reflectionspectrum spectrumof ofthe thesensor sensorbefore beforeand andafter after the laser strike. contrast to the tests the GF composite depicted in Figures 6 and67,and the 7, nitrogen was applied to the CF composite during on the GF composite depicted in Figures the nitrogen was applied to the CF composite the entire test to suppress combustion, and the thermal response was much more controlled. The during the entire test to suppress combustion, and the thermal response was much more controlled. sensor in the waswas very hot,hot, given that thethe wavelength shift The sensor in CF the composite CF composite very given that wavelength shiftwas wasgreater greaterthan than10 10nm. nm. Furthermore, the response was very fast; the Bragg wavelength shifted by more than 10 nm in 1.5 Furthermore, the response was very fast; the Bragg wavelength shifted by more than 10 nm in 1.5 s,s, ◦ C within with shift of of ~200 ~200 °C with an an estimated estimated temperature temperature shift within the the first first 200 200 ms. ms. Surprisingly, Surprisingly, the the FBG FBG was was not not damaged damaged during during this this test. test. In In fact, fact, as as the the epoxy epoxy burned burned off off during during the the test test the the spectral spectral response response of of the the sensor was apparently restored (i.e., similar to its response in bare fiber before it was embedded in sensor was apparently restored (i.e., similar to its response in bare fiber before it was embedded in the the composite). This is illustrated plot Figure8b. 8b.Embedding Embeddingthe thesensors sensorsin in the the composite composite CF CF composite). This is illustrated byby thethe plot inin Figure distorted the spectrum (illustrated by sidelobes in the black curve, not present in the original distorted the spectrum (illustrated by sidelobes in the black curve, not present in the original reflection reflection spectrum for the sensor), but as the epoxy burned off, the reflection spectrum was restored spectrum for the sensor), but as the epoxy burned off, the reflection spectrum was restored to its initial to its initial apodized state (no in the blue [26], with slightly higher reflected apodized state (no sidelobes insidelobes the blue curve) [26], curve) with slightly higher reflected power and apower small and a small increase in the Bragg wavelength, likely due to the release of a small residual compressive increase in the Bragg wavelength, likely due to the release of a small residual compressive strain strain present after the sensor was embedded. present after the sensor was embedded.

BB and Figure 8. The and (b) (b) the the reflection reflection spectrum (before and after) for a direct strike at The (a) (a) shift shift ∆λ position 4 in the CF composite. position 4 in the CF composite.

Advanced Sensor Network Response 4. Advanced

The results in Figures 6b and 8 indicate that a large temperature shift could be quickly detected single sensor sensor embedded embedded in in aa composite. composite. To To more more precisely precisely determine during a direct laser strike on a single to to thethe speed of detection, a more complex sensor sensor network similar to Figureto2 how sensor sensorlocation locationrelates relates speed of detection, a more complex network similar was constructed to obtain multiple measurements in the planeinand the through thicknessthe of Figure 2 was constructed to obtainsensor multiple sensor measurements thethrough plane and a CF composite. 9 shows the 9structure. The configuration uses a 2 × 2 uses × 3 array sensors thickness of a CFFigure composite. Figure shows the structure. The configuration a 2 × 2of×12 3 array of 12 sensors in six optical fibers, with four sensors embedded on each surface, as well as between two plies of CF composite. As illustrated, two pairs of fibers are embedded on the front and back surfaces of the two-ply CF composite structure (see front surface view). A third pair of fibers is embedded between the two plies (see side view). The two fibers embedded at each layer are 2 cm apart, and the two sensors in each fiber are 1 cm in length, separated by 3 cm on centers. Three wavelength division

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in six optical fibers, with four sensors embedded on each surface, as well as between two plies of CF composite. As illustrated, two pairs of fibers are embedded on the front and back surfaces of the two-ply CF composite structure (see front surface view). A third pair of fibers is embedded between the two plies (see side view). The two fibers embedded at each layer are 2 cm apart, and the two sensors in each fiber are 1 cm in length, separated by 3 cm on centers. Three wavelength division multiplexers (WDM) route signals to/from the two fibers on each surface to communicate with three channels of the interrogator. The CF composite was fabricated by vacuum bag molding using West System® 105/206 Epoxy Resin and 3 K Plain Weave Carbon Fiber fabric (5.7 oz./sq.yd, 0.012” thick, Sensors 2017, 17, 251 9 of 17 12.5 × 12.5).

Figure Figure 9.9. Test Testsetup setupused usedto tomeasure measurethe theresponse responseofofaa2 2××22×× 3 array array of of sensors sensors embedded embedded on on the the surface surface and and between between two two plies plies of of CF CF composite. composite. The The nominal nominal Bragg Bragg wavelengths wavelengths of of the the FBGs FBGs are are as as shown. shown. The The actual actual composite composite specimen specimen is is shown shown in in the the upper upperright. right.

Todetermine determinehow howwell wellan anarray arrayof of sensors sensors can can isolate isolate aa localized localized temperature temperature gradient, gradient, the the high high To power laser laser energy energy was was applied applied to to the the front front surface surface of of the the specimen specimen in in Figure Figure 99 using using the the same same test test power setup as in Figure 3. As before, the laser was on for 20 s during each strike. The scan rate of the peak setup as in Figure 3. As before, the laser was on for 20 s during each strike. The scan rate of the peak interrogator was was 2.5 2.5 kHz kHz during during these these tests, tests, so so the the peak peak wavelength wavelength of of each each sensor sensor was was determined determined interrogator every 400 400 µs. µ s. Numerous Numerous tests tests were were run, run, and and the the laser laser was was used used to to assault assault the the composite composite at at many many every different locations. Figure 10 shows a representative sampling of positions and test conditions for six different locations. Figure 10 shows a representative sampling of positions and test conditions for laser strikes. TheThe strikes at at positions 1–3 were sensor at at locations locations six laser strikes. strikes positions 1–3 wereallall1 1cm cmaway awayfrom from the the nearest nearest sensor above, in between, and below the 1550-nm and 1532-nm sensors. The tests at positions 4–6 were direct above, in between, and below the 1550-nm and 1532-nm sensors. The tests at positions 4–6 were direct strikes on the 1555-nm, 1550-nm and 1532-nm sensors, respectively. As before, the beam diameter of strikes on the 1555-nm, 1550-nm and 1532-nm sensors, respectively. As before, the beam diameter of 2 the laser laser was was55mm mmand andthe themaximum maximumirradiance irradianceatat100% 100%power powerwas was~500 ~500W/cm W/cm2 .. The The power power level level the was set to 25% of maximum at positions 1, 4 and 5, 50% of maximum at positions 2 and 6, and 100% was set to 25% of maximum at positions 1, 4 and 5, 50% of maximum at positions 2 and 6, and 100% at at position N2 gas applied at each position during theextent full extent each to strike to prevent position 3. N3.2 gas waswas applied at each position during the full of eachofstrike prevent flames flames from erupting. from erupting.

strikes on the 1555-nm, 1550-nm and 1532-nm sensors, respectively. As before, the beam diameter of the laser was 5 mm and the maximum irradiance at 100% power was ~500 W/cm2. The power level was set to 25% of maximum at positions 1, 4 and 5, 50% of maximum at positions 2 and 6, and 100% at position 3. N2 gas was applied at each position during the full extent of each strike to prevent Sensors 17, erupting. 251 10 of 18 flames2017, from

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of 11.9 nm/°C for sensors embedded in the CF composite, a shift of B  0.5 nm on the vertical axis in Figure 11 corresponds to a temperature shift of just over 40 °C. In each test, the laser turned on at 20 s and off at 40 s. The responses of the sensors embedded on the front surface of the composite are shown in blue. The responses of the sensors embedded in the middle between the two plies of the composite are shown in red, and the sensors embedded on the back respond as shown in green. Figure 11 illustrates both the effects of radial distance and laser power on the thermal response of the FBG sensors. The closer the sensor is to the laser strike and the greater the laser power the more Figure 10. and conditions for laser strikes CF structure from Figure 9. Figurewavelength 10. Locations Locations shifts. and test testIn conditions for six six laser strikes on on CF composite composite from Figure 9. the the Bragg Figure 11a the 1550-nm sensors exhibitstructure a greater shift than 1532-nm sensors because they are closer to the laser strike. In Figure 11c the laser power is increased 4.1. ×3 3Embedded EmbeddedSensor SensorArray ArraytotoIndirect IndirectLaser LaserIrradiation Irradiation 4.1. Response Response of of 22 × ×22× from 25% to 100% and the Bragg wavelength shift increases irrespective of the sensor location relative peak interrogator responses of the and the 1532-nm to theThe laser strike, in this case the 1532 are1550-nm closer tosensors the laser and exhibit a sensors greater The peak interrogator responses ofsensors the three three 1550-nm sensors andstrike the three three 1532-nm sensors during the laser strikes at positions 1–3 are shown in Figure 11a–c, respectively. Based on a sensitivity response. sensors nm and nm were embedded ~3–4 cm away from strikes during theThe laser strikesatat1555 positions 1–31537 are shown in Figure 11a–c, respectively. Basedthe onlaser a sensitivity of positions 11.9 nm/◦1–3 C for sensors the CF composite,shift, a shift ofshown ∆λ B =here, 0.5 nm onrelatively the vertical axis at (see Figureembedded 10). Theirin Bragg wavelength not was small, in Figure 11 corresponds to a temperature of CF justcomposite, over 40 ◦ C.confirming In each test, laser turned on due to relatively poor thermal conductivityshift of the thethe localized thermal at 20 s and off at 40 s. The responses of the sensors embedded on the front surface of the composite are response to a laser strike and the ability to identify its location. Figure 11b shows the Bragg shown in blue. of at the sensors embedded in the to middle between the1550-nm two plies of the wavelength shiftThe for aresponses laser strike 50% power and equidistant the 1532-nm and sensors. composite are shown in red, and the sensors embedded on the back respond as shown in green. The responses of the FBGs on the front surface and in the middle are very similar.

Bvs.vs.time Figure (a–c) The The shift shift ∆λ time FBGs with nominal Bragg wavelengths of 1550 nm Figure 11. 11. (a–c) forfor thethe FBGs with nominal Bragg wavelengths of 1550 nm and B and 1532 nm during laser strikes at positions 1–3, respectively, in Figure 10. The laser (and N 2 gas) 1532 nm during laser strikes at positions 1–3, respectively, in Figure 10. The laser (and N2 gas) turned turned 20off s and offs.atThe 40 s.specimen The specimen was oriented air flow to in leftstrikes in strikes on at 20ons at and at 40 was oriented with with air flow fromfrom rightright to left 1–3. 1–3. location of strike, the strike, the relative power the corresponding shown in The The location of the the relative laserlaser power levellevel andand the corresponding FBGFBG are are shown in the the inset for each of graphs. inset for each pairpair of graphs.

Since rapid detection of a laser strike is paramount, the speed of response of the FBG sensors may be of greater concern than either the fidelity of the temperature measurements or even the location information. The results in Figure 11 clearly show speed of response is dependent both on the power of the laser strike as well as the proximity of the sensor to the laser strike. Only 2 s are required to detect a 10 °C temperature shift in the 1532-nm sensor at 100% power (in Figure 11c), whereas for the same proximity and 25% power it takes nearly 8 s after the laser strikes the target to effect a 10 °C increase in the 1550-nm sensor (Figure 11a). Figure 11 (a and c) also clearly shows a steeper shift in the Bragg wavelength the closer the sensor is to the laser strike.

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Figure 11 illustrates both the effects of radial distance and laser power on the thermal response of the FBG sensors. The closer the sensor is to the laser strike and the greater the laser power the more the Bragg wavelength shifts. In Figure 11a the 1550-nm sensors exhibit a greater shift than the 1532-nm sensors because they are closer to the laser strike. In Figure 11c the laser power is increased from 25% to 100% and the Bragg wavelength shift increases irrespective of the sensor location relative to the laser strike, in this case the 1532 sensors are closer to the laser strike and exhibit a greater response. The sensors at 1555 nm and 1537 nm were embedded ~3–4 cm away from the laser strikes at positions 1–3 (see Figure 10). Their Bragg wavelength shift, not shown here, was relatively small, due to relatively poor thermal conductivity of the CF composite, confirming the localized thermal response to a laser strike and the ability to identify its location. Figure 11b shows the Bragg wavelength shift for a laser strike at 50% power and equidistant to the 1532-nm and 1550-nm sensors. The responses of the FBGs on the front surface and in the middle are very similar. Since rapid detection of a laser strike is paramount, the speed of response of the FBG sensors may be of greater concern than either the fidelity of the temperature measurements or even the location information. The results in Figure 11 clearly show speed of response is dependent both on the power of the laser strike as well as the proximity of the sensor to the laser strike. Only 2 s are required to detect a 10 ◦ C temperature shift in the 1532-nm sensor at 100% power (in Figure 11c), whereas for the same proximity and 25% power it takes nearly 8 s after the laser strikes the target to effect a 10 ◦ C increase in the 1550-nm sensor (Figure 11a). Figure 11 (a and c) also clearly shows a steeper shift in the Bragg wavelength the closer the sensor is to the laser strike. The previous results in Figure 6 in the GF composite show the presence of flames (combustion) and air flow (convection) could also impact the speed (and magnitude) of the response. In more than one instance Figure 11 shows the Bragg wavelength shift or thermal response of the sensor on the back surface (green) is greater or more erratic than that of both the front and the middle (blue and red). Flames were sometimes observed erupting from the back surface of the composite during a strike. The flames appeared like a “blow-torch” projected away from the back of the composite, in contrast to the appearance of the flame that was directed upward by natural convection in Figure 7. Whereas combustion was suppressed on the front surface using the N2 gas nothing was done to extinguish the flames on the back surface of the composite. Heat transfer by conduction can be observed in Figure 11a–c. When the laser was turned off so was the N2 gas. Because the sensors in each case are a small distance from the laser strike, the heat of the laser strike is observed to diffuse to the sensor location; this is most evident in the slight temperature increase exhibited by the 1532-nm sensors at 40 s in Figure 11c. The temperature increase lasted for only ~5 s after the laser (and N2 ) was turned off. For these sensors the final wavelength was also less than the initial wavelength, likely due to relaxation of residual strain in the composite. Thru-thickness conduction is also evident in the 1532-nm sensors as the composite returns to thermal equilibrium; observe the temperature rise on the front surface once the N2 gas is turned off (Figure 11c). Likewise, in video taken during the tests with N2 gas, no flames were observed on the front surface. Figure 12 shows three single frames of the video of the front surface captured around t = 30 s during the three laser strikes at positions 1–3, respectively. Few flames were observed on the front surface of the composite because they were extinguished by the nitrogen. However, the increasing irradiance associated with each strike is evident, as indicated by progressively larger “hot-spots” at positions 2 and 3. The tows of the carbon fiber fabric in the composite, ~2 mm in width, can be discerned in each frame. The optical fiber 1 cm above the strike at positions 2 and 3 can also be seen. Bubbles of epoxy were also occasionally observed sizzling and burning off at the edge of a circular region that slowly expanded in diameter during the strike.

surface of the composite because they were extinguished by the nitrogen. However, the increasing irradiance associated with each strike is evident, as indicated by progressively larger “hot-spots” at positions 2 and 3. The tows of the carbon fiber fabric in the composite, ~2 mm in width, can be discerned in each frame. The optical fiber 1 cm above the strike at positions 2 and 3 can also be seen. Bubbles of17, epoxy Sensors 2017, 251 were also occasionally observed sizzling and burning off at the edge of a circular 12 of 18 region that slowly expanded in diameter during the strike.

Figure 12. Video frames of the front of the CF composite, captured halfway through the strikes at positions 1–3, respectively. respectively. N N22 gas flowed from right to left in each video video frame. frame.

4.2. 4.2. Response Response of of 22 × ×22××3 3Embedded EmbeddedSensor SensorArray ArraytotoDirect DirectLaser LaserStrikes Strikes Sensors 2017, 17, 251

12 of 17 More rapid and and larger larger shifts shifts in in wavelength wavelength and and temperature temperature were expected during More rapid were expected during direct direct laser laser strikes on the sensors located at positions 4–6 on the specimen in Figure 10. Figure 13 shows results strikes The on the sensors located atofpositions on the Figure 10. Figure resultsno of strike. Bragg wavelength the front4–6 sensor (inspecimen blue) wasinindeterminate near13t =shows 33 s (when of direct strikes at position 4 (on the 1555-nm sensors) and position 6 (on the 1532-nm sensors). Figure direct strikes at position 4 (on the 1555-nm sensors) and position 6 (on the 1532-nm sensors). Figure 13a peak could be acquired by the interrogator), similar to the response in Figure 6b for a direct strike on 13a shows the wavelength of front, the front, middle 1555-nm sensors, plotted as blue, red shiftshift ofalso the and and backback 1555-nm sensors, plotted as blue, red and ashows sensorthe in wavelength the GF composite, at 25%middle laser power. ◦ and green curves, respectively, during a direct strike at position 4 at 25% laser power. A ~100 °C shift greenThough curves,the respectively, a direct at position 4 at 25% laser(in power. A ~100 shiftmore was responsesduring in Figure 13a ofstrike the front and middle sensors blue and red) C were was measured in ~0.1 s infront the front sensor. The temperature shift was somewhat slower inmiddle the middle measured in ~0.1 s in the sensor. The temperature shift was somewhat slower in the and erratic during the strike than the response of the back sensor (in green), their Bragg wavelengths also and back sensors. The Bragg wavelength of the back sensor (in green) was relatively stable during back sensors. of the backtosensor green) was of relatively stable duringFBG the clearly shifted The by a Bragg larger wavelength amount. Extrapolation higher(in temperatures the vendor supplied the strike, similar to the response of the sensor embedded in the CF composite shown earlier in Figure strike, similar to the response of the sensor embedded in the CF composite shown earlier in Figure 8a. response [24] suggests that the temperatures in the composite were between ~600 °C on the back 8a. In contrast, peak Bragg wavelengths of the front middle sensors were more erratic, since In contrast, the the peak Bragg wavelengths of the front andand middle sensors were more erratic, since the Bwas  9 directly B specimen.  12 nm).At surface (for nm) and ~700 °C on the front surface (forofthe These are well the laser energy concentrated front surface highvalues temperatures, laser energy was directly concentrated onon thethe front surface of the specimen. At high temperatures, above thewavelengths specified maximum temperature of 300 °C for the sensors used here.values Partial erasure of the the of observed to the peak peak wavelengths of these these sensors sensors were were observed to hop hop rapidly rapidly between between values during during aa direct direct grating is expected [12], and the front and middle sensors were damaged during the strike. Their strike. The Bragg wavelength of the front sensor (in blue) was indeterminate near t = 33 s (when no reflectivity was reduced to only 10% and 20%, respectively, whereas the reflectivity of the back sensor peak could be acquired by the interrogator), similar to the response in Figure 6b for a direct strike on was approximately the same before and after the strike. a sensor in the GF composite, also at 25% laser power.

Figure 13. 13. (a) (a)Wavelength Wavelengthshift shiftduring duringa direct a direct strike at position in the front (blue), middle Figure strike at position 4 in4the front (blue), middle (red)(red) and and back (green) 1555-nm sensors (as shown in Figure 9); (b) Temperature on the back surface of the the back (green) 1555-nm sensors (as shown in Figure 9); (b) Temperature on the back surface of composite during a direct strike at position 6, estimated using the wavelength shift of the back 1532-nm composite during a direct strike at position 6, estimated using the wavelength shift of the back 1532-nm sensor (green dots) or measured with an IR camera calibrated for higher temperatures (black). sensor (green dots) or measured with an IR camera calibrated for higher temperatures (black).

To merely detect laser radiation on the composite surface, the precise temperature is not needed. However, reliable estimates of the temperature are helpful to optimize the design of the sensor network. Since the measured sensitivity of 11.9 nm/°C was only accurate at lower temperatures [19], and the epoxy resin begins to degrade near 250 °C (burning off completely near 400 °C), an

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Though the responses in Figure 13a of the front and middle sensors (in blue and red) were more erratic during the strike than the response of the back sensor (in green), their Bragg wavelengths also clearly shifted by a larger amount. Extrapolation to higher temperatures of the vendor supplied FBG response [24] suggests that the temperatures in the composite were between ~600 ◦ C on the back surface (for ∆λ B ≈ 9 nm) and ~700 ◦ C on the front surface (for ∆λ B ≈ 12 nm). These values are well above the specified maximum temperature of 300 ◦ C for the sensors used here. Partial erasure of the grating is expected [12], and the front and middle sensors were damaged during the strike. Their reflectivity was reduced to only 10% and 20%, respectively, whereas the reflectivity of the back sensor was approximately the same before and after the strike. To merely detect laser radiation on the composite surface, the precise temperature is not needed. However, reliable estimates of the temperature are helpful to optimize the design of the sensor network. Since the measured sensitivity of 11.9 nm/◦ C was only accurate at lower temperatures [19], and the epoxy resin begins to degrade near 250 ◦ C (burning off completely near 400 ◦ C), an independent measurement of temperature was desired. A FLIR® IR thermal imaging camera (FLIR Systems, Wilsonville, OR, USA), calibrated to higher temperatures, was placed behind the specimen to measure the thermal response on the back surface of the composite. Figure 13b shows the temperature measured by the IR camera and by the back FBG at position 6 during a direct strike on the 1532-nm sensors at 50% power. The black curve shows the temperature measured on the back surface by the IR camera. The green curve shows the estimated temperature response of the back FBG sensor, again based on an extrapolation of vendor specifications [24]. The IR camera measurement shows that the FBG sensor temperature response is roughly accurate during and after the laser strike, providing useful information about the sensor network. Both measurements in Figure 13b demonstrate that the temperature on the back surface approached 1000 ◦ C during the strike and returned to room temperature after the strike. Similar thermal imaging tests with the IR camera during a laser strike on a six-ply CF composite [14] showed that the temperature on the back surface remained relatively steady in time (at values between 500 ◦ C and 700 ◦ C), depending on the incident laser power (between 30% and 70% of maximum). The shapes of the responses in the thermal imaging measurements in [14] were nearly identical to the shape of the green curve in Figure 13a. The FBG peak wavelength response in Figure 13b at higher irradiance, however, is considerably more erratic than the response measured by the IR camera. To understand the cause of fluctuations in the Bragg wavelength λ B of an FBG during a direct strike, the spectra of several sensors were observed during direct strikes at different power levels. An example of the spectrum (reflectivity vs. wavelength) is shown in Figure 14 (a and b) for the front and back 1550-nm sensors, respectively, captured ~6 s after the beginning of a direct strike at position 5 (refer to Figure 10 for location). The laser power was 25% of maximum, so the behavior of the peak wavelengths of the front and back sensors (nominally near 1550 nm) in Figure 14 should mimic the response observed in the front and back sensors (nominally at 1555 nm) in Figure 13a. Based on the spectral responses shown in Figures 13 and 14, precise acquisition of a peak wavelength in the FBG response during a direct strike on a sensor can be hindered by two effects: First, highly localized temperature spikes during a direct strike can result in a non-uniform temperature profile across the length of the sensor that can cause erratic fluctuations in the reflection spectrum and in the peak wavelength if the temperature is high enough. The resulting spectrum is similar to that caused when a non-uniform strain field is applied across the length of an FBG sensor [3,20,21,27,28]. Secondly, for direct laser strikes at higher power levels, the gratings were often erased soon after the strike began, especially for sensors embedded closer to the front surface of the composite.

observed during direct strikes at different power levels. An example of the spectrum (reflectivity vs. wavelength) is shown in Figure 14 (a and b) for the front and back 1550-nm sensors, respectively, captured ~6 s after the beginning of a direct strike at position 5 (refer to Figure 10 for location). The laser power was 25% of maximum, so the behavior of the peak wavelengths of the front and back sensors (nominally near 1550 nm) in Figure 14 should mimic the response observed in the front 14 and Sensors 2017, 17, 251 of 18 back sensors (nominally at 1555 nm) in Figure 13a.

BλB1550 Figure 14.14. Spectral response of the (a) (a) front andand (b)(b) back sensors (with nm, nominally) forfor Figure Spectral response of the front back sensors (with = 1550 nm, nominally) a strike position 5, initially, and s after strike begins. a strike at at position 5, initially, and ~6~6 s after thethe strike begins.

Based on the in Figures 13 and 14, one precise acquisition of a peak Consider thespectral spectralresponses distortionshown in Figure 14, which shows instance of a multi-peaked wavelength in the the front FBG response direct strike a sensor be hindered effects: of spectrum in and back during sensorsa(nominally at on 1550 nm) at can a time ~6 s after by thetwo beginning First, highly localized temperature spikes during a direct strike can result in a non-uniform the laser strike. The distortion in this multi-peaked spectrum varied dynamically during a direct temperature profile thethe sensor thatpeak can cause erratic fluctuations in hopped the reflection strike. For the frontacross sensorthe inlength Figure of 14a, spectral with the highest reflectivity rapidly spectrum and in the peak wavelength if the temperature is high enough. The resulting spectrum is of between neighboring peaks [21], changing in wavelength by several nm, consistent with the behavior similar to that caused when a non-uniform strain field is applied across the length of an FBG the blue curve in Figure 13a. For the sensor on the back surface of the composite, shown in Figure 14b, sensor Secondly, were for direct laser strikes at higher power gratings were often shifts[3,20,21,27,28]. in the peak wavelength much smaller, similar to the greenlevels, curvethe in Figure 13a. erased Given soon after the strike began, especially for sensors embedded closer to the front surface that the beam profile for the high energy laser was Gaussian with intensity of I (the z) =
2 2 composite. I0 exp −2z /w , during a direct strike the peak irradiance I0 occurred at position z = 0 (the center 0

of the grating), and the energy was highly localized with a laser spot size 2w0 = 5 mm for an FBG gauge length of 10 mm. A precise explanation for the resultant spectral response in Figure 14 can be developed using a modified transfer matrix approach as described in [27], used by Prabhugoud and Peters to characterize non-uniform quadratic strain fields on strain sensors. Based on the results in [27], a transfer matrix analysis using a modified form of the grating period which is here defined consistent with Equation (4), 

Λ ( z ) = Λ0 1 +



  1 dne + α + (1 − pe )αc ∆T (z) + (1 − pe )ε , ne dT

(5)

would demonstrate that the spectrum in Figure 14b and the incident Gaussian intensity are consistent h i ∼ with the presence of a non-uniform quadratic temperature profile ∆T (z) = ∆T0 1 − (z/z0 )2 across

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the grating length, where ε = 0 in this experiment. The actual values of ∆T0 and z0 would depend on the energy absorbed by the composite, its thermal conductivity, and its coefficient of thermal expansion αc . Based on Equation (5), a non-uniform temperature profile causes non-uniform thermally-induced strain αc ∆T (z) for any material in which αc is not negligible. The spectrum in Figure 14b shows that a highly localized direct laser strike results in a highly localized, dynamically varying thermal response by the sensor. In the carbon fiber/epoxy composite described by Figure 14, for which αc is small, the dominant contribution is due to changes in the index of refraction ne with temperature. In the E-glass fiber/epoxy composite, described earlier in Figure 6, the non-uniform thermal response also includes thermally induced strain since αc is markedly larger. Finally, although the initial spectrum of the front and back sensors were very similar (a single peak near 1550 nm), the front sensor also experienced a much sharper drop in its reflectivity during the strike, as can be seen by comparing the graphs in Figure 14a,b. This is likely due to higher temperatures on the front surface than on the back surface of the carbon fiber/epoxy composite. After the strike at position 5, the peak wavelengths of both the front and back sensors returned to values close to their nominal Bragg wavelength, i.e., a single reflection peak near 1550 nm. However, the peak reflectivity of the front sensor was less than 40% after the strike, indicating partial erasure of the front sensor due to high temperature grating decay. This suggests the second reason it is more difficult to determine a peak wavelength for an FBG sensor experiencing a direct strike. For direct strikes with a power level of 50% or greater on a two-ply composite, the gratings usually erased completely, whether on the front surface, in between the plies, or on the back surface. When this occurred, the peak wavelength measurement was indeterminate. The spectral response looked similar to that in Figure 14a, but within seconds the small peaks in the spectrum shifted to higher wavelengths (beyond the instrument measurement limit of 1568 nm), and the distorted spectrum gradually dissipated into the noise as the grating decayed completely, with no recovery of a spectral peak at the nominal Bragg wavelength. Interestingly, although the measured temperatures were sometimes quite high at laser power levels of 25%, there was often no significant damage to the sensor after the strike, permitting its reuse. Further tests would be needed to determine the accuracy of the analysis in [12] (a work describing grating decay at high temperatures) when localized thermal energy is suddenly applied to a grating under laser irradiation. These tests, however, clearly demonstrate the ability to rapidly detect temperature spikes in a composite structure using embedded FBG sensors. 5. Conclusions In this study, structural health monitoring is performed using embedded FBG temperature sensors to rapidly detect laser radiation and its location on the surface of a polymer matrix composite. Rapid detection might be necessary to detect flames or to counter a directed energy attack. During a direct laser strike on a sensor, large thermal gradients of ~100 ◦ C within ~0.1 s were detected. In sensors located 1 cm to 3 cm away from a strike, temperature shifts were also observed quickly, and they were much smaller in magnitude, producing results that indicate the proximity of the laser strike and the laser power. Arrays of FBG sensors were also used to characterize the thermal conductivity and absorptivity of carbon fiber and E-glass fiber/epoxy composites based on their response to the localized temperature gradient caused by the laser radiation. The material-dependent response is important, for example, laser radiation was detected more rapidly by sensors embedded in a carbon fiber/epoxy composite than in an E-glass fiber/epoxy composite [19], so that fewer sensors would be needed per unit area in materials with higher thermal conductivity. The influence of convection on the temperature of the composite surface was observed only as it related to the use of exhaust and N2 gas (intermittently applied to suppress flames). The importance of convection as it relates to high speed airflow across a structure (such as over an aircraft wing) must be explored further.

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The depth at which sensors were embedded in a composite did influence the temperature response. It was observed that FBG sensors on the front surface rapidly detected high temperatures, but could be damaged or erased as the irradiance increased. Sensors embedded beneath the front surface also detected laser radiation, with estimated temperatures approaching 600 ◦ C during a direct strike, but with a somewhat reduced speed of detection and with considerably less damage to the sensor. Further research is needed to determine the optimal depth at which sensors should be embedded to rapidly detect higher laser power levels. Standard FBG temperature sensors were used in this project instead of specialized high temperature FBG sensors. While high temperatures did cause damage to sensors under certain conditions, standard type I sensors were acceptable to use here since the goal was solely to detect high energy radiation as quickly as possible, before the polymer matrix composite itself was destroyed. Furthermore, even when sensors were damaged or destroyed during direct strikes at higher power levels, the initial temperature gradient could still be obtained, and neighboring sensors in the array also detected the strike. There are conditions when high temperature FBGs may be beneficial, for example, if groups of sensors were destroyed too fast for detection to occur or if sensor functionality needed to be maintained after a strike. Results here indicate that embedding the sensor more deeply in the composite would likely alleviate these concerns. High temperature sensors would also be needed if reliable high temperature measurements were needed for instrumentation or diagnostic purposes. For example, high temperature sensors might be required to precisely characterize the quadratic temperature profile that was observed across the grating length during a direct laser strike. The non-uniform temperature profile detected at high temperatures resulted from the small spot size of the laser and the intrinsically poor thermal conductivity of the polymer matrix composite structure. At even higher temperatures, the dynamically varying spectral response caused wavelength hopping that prevented a peak wavelength interrogator from acquiring reliable values for the Bragg wavelength. A transfer-matrix analysis is likely the best approach to understand the details of such a response. Ongoing experiments are examining the influence of strain on the detection of a localized temperature gradient [18]. Although no external mechanical strain ε was applied in the tests described here, external sources of strain that could prevent accurate identification of a localized temperature gradient ∆T must be considered. An array of sensors embedded within one or more layers of the composite may provide sufficient monitoring of the Bragg wavelength to rapidly identify both effects. Ultimately, the sensor array must be designed to ensure that the strain and thermal responses can be isolated, considering both the energy levels capable of causing damage as well as the spot size of the laser. Signal processing algorithms that can isolate these responses due to strain and temperature are being developed. While a full spectral interrogation at a higher scan rate may be useful to provide the necessary data acquisition capability, alternative fiber optic sensing technologies are also being considered. These could include specialized FBGs that perform well at high temperatures [9] as described above, distributed optical fiber sensors based on Rayleigh scattering, or optical frequency domain reflectometry [29,30]. The insensitivity of optical fiber temperature sensors to the incoming wavelength of IR radiation is an important advantage over other techniques for sensing laser radiation on a polymer matrix composite. Acknowledgments: We would like to thank the Office of Naval Research and the High Energy Laser-Joint Technology Office for their support, as well as R. Joseph Watkins, Kyle Elam, Jorge Garcia, Cody Brownell and Kyle Milden for their assistance in the experiments. Author Contributions: Brian Jenkins is the primary author. He performed all experiments with assistance from the other authors. All three authors contributed to the conception and design of the experiments and the analysis of the results. Peter Joyce also advised students who fabricated the composite specimens with embedded FBG sensors.

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Conflicts of Interest: The authors declare no conflict of interest. The sponsors had no role in the design of the study; in the collection, analysis, or interpretation of data; in the writing of the manuscript or in the decision to publish the results.

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