The design and manufacture of future space transportation and delivery systems will be strongly driven by safety, cost, maintenance and reliability considerations. Advanced composite structural components are likely to be a key element in realizing these system objectives. Composites have the additional potential of enabling the embedment of sensors for system health monitoring, which supports requirements for low cost safety, maintenance and repair diagnosis

Messinger, Ross H.

Schoenwald, Jeffrey S.

English

Jeffrey S. Schoenwald

Ross H. Messinger

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UHF Modulation and Fourier Transform Differential Time Domain Techniques for Measuring Strain Via Fiber Optics

The design and manufacture of future space transportation and delivery systems will be strongly driven by safety, cost, maintenance and reliability considerations. Advanced composite structural components are likely to be a key element in realizing these system objectives. Composites have the additional potential of enabling the embedment of sensors for system health monitoring, which supports requirements for low cost safety, maintenance and repair diagnosis.</p

Schoenwald, Jeffrey

Messinger, Ross

Digital Repository @ Iowa State University (ISU)

UHF MODULATION AND FOURIER TRANSFORM DIFFERENTIAL TIME DOMAIN TECHNIQUES FOR MEASURING STRAIN VIA FIBER OPTICS Jeffrey S. Schoenwald Science Center Rockwell International Corporation P.O. Box 1085 Thousand Oaks, CA 91358 Ross H. Messinger Space Systems Division Rockwell International Corporation P.O. Box 7009 Downey, CA 90241-7009 INTRODUCTION The design and manufacture of future space transportation and delivery systems will be strongly driven by safety, oost, maintenance and reliability considerations. Advanced composite structural components are likely to be a key element in realizing these system objectives. Composites have the additional potential of enabling the embedment of sensors for system health monitoring, which supports requirements for low cost safety, maintenance and repair diagnosis. Composite structures will incorporate bonded joints, and a suitable sensor will be needed to monitor the integrity of such joints for dis bonds and delaminations. Fiber optic sensors have been recognized as very attractive for measurement of strain, vibration and detection of other structural phenomena (e.g., cracks, fatigue, delamination)[I]. If real-time distributed inspection of critical composite joints in space transportation structures can be achieved using networked fiber optic sensors, a significant improvement can be made in assuring the reliability and safety of manned and unmanned space systems while also streamlining servicing and maintenance. We describe a fiber optic sensor system intended for spatially resolving strain levels of 10-6 between closely spaced regions within composite sampies. The phase-amplitude response of swept ultra-high frequency (UHF) laser amplitude modulated light transmitted through oomposite embedded optical fibers is being used to measure strain in test structures subject to tensile loading. This is accomplished by embedding multiple optical fibers of different penetration lengths. The phase-magnitude characteristics of the reflected signals from the three fibers are determined by a UHF network analyzer [2]. A Fast Fourier Transform (FFf) performed on the network analyzer yields a transmission-mode time domain image of the metallized fiber ends, which appear as distinct reflections. Using a signal subtraction process, small strains are detectable and differences between strain in small spatial regions around a defect should be resolvable. The system and results to date will be described. Review 0/ Progress in Quantitative Nondestructive Evaluation, Vol. 11 Edited by D.O. Thompson and D.E. Cbimenti, Plenum Press, New York, 1992 1103 -----~ ..--------..-..-FEM Location I ,/ I , " , , -Teflon disk Optic Fiber 3 reqd Teflon disk AIR ~App1ied Load Figure 1. Dual bonded joint showing placement of simulated dis bonds. BONDED JOINT SPECIMEN DESIGN AND FABRICA TION A bonded joint was designed to test the feasibility of observing strain tields - and strain differences due to joint dis bonds - by means of tiber optic sensors incorporated in the composite material in the region of interest. Three specimens were fabricated - one with no dis bond in the joint interface, and two with dis bonds simulated by Teflon inserts of different size. The joint material is graphite/epoxy composite throughout. Figure I is a schematic of the specimens, showing where the optical tibers and Teflon inserts were placed relative to the bond region. The tibers were placed in the mid-plane of the central tongue of the joint and penetrate to different lengths across the joint. The Teflon inserts were 1.0 and 2.0 inches in diameter and were placed in two sampies to form semicrrcular disbonds, as shown in the figure. The specimens were to be placed under tensile load (see Figure I), resulting in an expected lateral (transverse) contraction, parallel to the optical tibers, which would experience the same strain as the surrounding medium. The different degree of penetration of each tiber into the joint region would result in a different degree of net strain in each fiber. A 21 x 18 node mesh covering a 2.0 x 1.9 in.2 portion of the bonded region (as indicated in Figure 1) was modeled by fmite element methods (FEM) to predict the two-dimensional stress field in the substrate as a function of disbond area in the adjacent adhesive bond placed under axial tension. This region is indicated in Figure 2. Transverse strain results of the analysis are shown in two-dimensional plots in Figures 3a-c. The strain experienced by the optical tibers, located at axiallocation 16, is thus indicated by values between curves at axiallocations 15 and 17. In Figure 3a, adherend compressive strain increases monotonically from the outer edge (axiallocation 21) as the adherend gradually accepts the distributed shear load. Transverse strain levels vary from about 125 Il-inches/inch to 300 Il-inches/inch for an applied load of 1104 18 17 16 15 14 1.7 13 t-----2.0-----------I~ 1.0----l~ r-o.5 1/ f--L:V 1/ ~ V .If ~f><; 12 11 10 9 8 7 6 5 4 3 2 1 / V ~M odel presen13tion I Re I / 1 2 3 4 5 6 7 89 1011 12131415161718192C21 Lateral Location Axial Location Figure 2. FEM nodal mesh of bonded region. 1000 pounds per inch. Predicted strain in a11 three optic fibers will be about -160 micro strain. With a 0.5 inch disbond results, shown in Figure 3b, indicate that the transverse strain field remains unchanged only at the greatest distance from the dis bond (laterallocation 1). Compared to the no-disbond case, strain generally decreases near the disbond due to the lower total applied load. Within the disbond itself, however, compressive strain rapidly increases, caused by the elongation of the unloaded area in the axial direction. Along the optic fibers, strain levels remain unchanged between laterallocations 1 and 10, then decreases from about -170 microstrain at laterallocation 10 to -100 microstrain at lateral location 18. Results of the 1.0 inch dis bond analysis, shown in Figure 3c, further accentuate the effects seen in the 0.5 inch dis bond analysis. While the transverse strain field remains unchanged at the greatest distance from the dis bond (laterallocation I), strain decreases rapidly near the disbond due to the lower total applied load, and in fact becomes positive "below" the disbond due to the eccentricity between applied loads to the left of the dis bond and reaction along the 10wer edge. Within the unloaded disbond area, a high compressive strain is again caused by the elongation of the unloaded area, which inc1udes a displacement of the disbond area in the transverse direction. Along the optic fibers, strain levels again remain relatively unchanged along the entire width of the model - major shifts are predicted only near the edge of the substrate or below the dis bond. Thus, optimum location of the optic fibers may be at either of these two locations, rather than at the center of the bond, to provide greater induced strain changes. FIBER OPTIC STRAIN SENSOR SYSTEM The fiber optic system designed to measure strain in the double lap joint is shown in Figure 4. A network analyzer provided a swept rf signal from 300 kHz to 3 GHz [3]. This signal modulated the amplitude of a high speed diode laser [4], which is conveniently supplied with a 50 n rf input port and was coupled to a 50 micron core multi mode fiber at the optical output port. This fiber was spliced to one fiber of a 3 x 3 beam splitting coupler. The three output fibers were spliced to three fibers embedded in the center tongue 1105 Slrain (a) micro-inlin (b) Straio (c) micro-inl io $lraio micro-inlio nodi bond Or-------------~======~--x= l000 Ib/in -50 l---------=========--Axial Location ·e · 1 ~. 3 .• . 5 (]. 7 / ..... 9 ~E::~~~::!:::tl!""'lf"~=~=" " ~=""" .... 11 .)C. 13 .. - 15 17 ..... 19 -250 "~" ""''''''''''' ''''''''~'''''a''''~_~~~ ~ ~_~~ I 0-0 - 0 - .- .- .-e-e-.-.- e-u - o -300 ~"i"=~=~~!=~=~~-'1-~-"-'-v-~",=~~; -Ir 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 L...-_-' Lateral Locatioo 0.5-ioch radius disbood Or---------------------------------50 ....................................................................................................................... . 345 10 11 Lateral Locatioo 1.0-inch radius disbood ~ f~~~~:,;~;;';:I:~~:: --------100 ....------------------:b . ... IiI-" 0 ···················································· ................................ :.;:d-~ ~~~: -"-)(- )C -100 ... IC_- =='::::O __ =--.-'---'--.- a_._ !!: ---S -200 t::~~--k:::::: .. !-::::!--:~ "" Axial Locatioo ·e · 1 ~. 3 . .. 5 ~. 7 ... 9 -Ir 11 . )C. l3 .. - 15 17 ... 19 -Ir 21 Axial Locatioo ... 1 ~- 3 .. - 5 (]. 7 ..... 9 -Ir 11 .)c . 13 .. - 15 17 ... 19 -Ir 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 L...-_-' Lateral Locatioo Figure 3. FEM predicted strain in bonded region with (a) no disbond, (b) 0.5 in radius disbond, and (c) 1.0 in radius disbond_ of the double lap joint, as described earlier, and shown in Figure 4. Each embedded fiber is a 50 micron core/125 micron cladding step index mode fiber coated with polyimide to a total outer diameter of 145 microns. The lengths of the 3 fibers from the 3 x 3 coupler to the specimen are arbitrary, and can be chosen to simplify signal processing, described later. 1106 rfout network analyzer rf in computer data acquisition & analysis Specirnen in tension L..-_ _ __ LOAD & EXTENSION SIGNALS ------I~ Figure 4. Configuration of optical fibers in specimen and data acquisition system. The embedded fibers were end coated with gold to produce a substantial reflection. The reflected light returned through the coupler and is split three ways. Only the fibers carrying the light from the laser to the specimen and back to the detector are of interest - the third return fiber is ignored, and is not shown in the figure. The high speed detector has a 50 ohm ac coupled output port which was connected to the input port of the network analyzer. A transmission measurement provides a magnitude and phase record of the sum of the three optical signals, which are of different path length, and which therefore produces an interference pattern in the frequency domain measured from 300 kHzt03 GHz. SIGNAL PROCESSING The different lengths of the optical fibers (both in their penetration into the specimen and in absolute length) produce a complex amplitude pattern versus frequency, shown in Figure 5. We have omitted the corresponding phase plots. Axially loading the double lap joint specimen will produce a complex shifting of the phases of the reflected waves, and the resulting interference spectrum. Interpretation of strain effects from the frequency spectrum is difficult. Instead, we are able to perform a network analyzer-based Fast Fourier Transform (FFT) and convert the frequency domain information to a transmission time domain representation. In effect, reflections from the fiber ends are observable as optical sources at a given distance along the optical path. Figure 6a is a plot of the FFT obtained from the frequency spectrum of Figure 5. The relative delays between reflection peaks inc1ude arbitrary lengths of fiber between the coupler and the embedded fibers, not just the embedment distances. Figure 6b is an oscilloscope trace obtained with a high resolution optical time domain reflectometer, which confrrms the accurate representation of the time domain calculation obtained by FFT. To observe strain effects in the specirnen using the fiber sensor, a subtraction procedure was used. As load was applied to the specimen, time-averaged waveforms of new time dornain data (TDD) were subtracted from a reference waveform of TDD of the specirnen in the zero load state. These differences represent the degree to which the strained fiber endpoints have shifted relative to the unloaded state. Electronic markers placed at time locations corresponding to the peak locations at zero load and indicating the difference signal (in arbitrary units) will increase monotonically as load increases and the reflection peaks shift in time relative to the zero load reference peaks. The expected time shift is so small that the difference signals are in no danger of saturating, Le., the strains are not large enough to shift a strained reflection peak completely away from its reference. 1107 CHI TRN/M 11n MAG Avg 16 200 . 300 000 MHz I l... 864.98 mU - 3. 412 66 . r r - 'I =- r. , __ .J •. _._ +____ _ _ _ . . __ 1-_____ -1.. _ STOP 3 000. 000 000 MHz CHI START Figure 5. RF power versus frequency response of optical signal reflected from specimen. Analog voltages provided by the load cell and an axial extensometer placed on the lap joint region of the specimen were monitored by programmable digital multi-meters. Under computer control and upon keyboard command, the load and longitudinal strain could be computed from the multi-meter readings, and the amplitude markers were read from the network analyzer. Load was set manually, which explains the need for a semi-automated data acquisition procedure. EXPERIMENT AND DISCUSSION Tensile loads up to 16,000 lbs were applied to the specimen containing a 1.0 in. radius Teflon insert. Figure 7 is a representative set of curves for this specimen. The first difference curve indicates a nearly negligible response, whereas curves 2 and 3 indicate a trend of increasing signal with load, but with variations in the trends. These variations were somewhat repeatable with repetition of the measurements; since considerable time averaging was performed, noise is discounted as a probable reason for such variations. We suspect the variations are due to interfacial disbonding between the fibers and the composite matrix. Hysteresis lag during load reduction tends to support this interpretation. Further tensile testing and ultimately destructive sectioning and photornicrograph analysis may explain the observed behavior of the sensor. Interfacial adhesion between optical fiber coatings and both the fiber and composite matrix are of critical concem. An informal analysis of the predicted absolute changes expected in the embedded fibers sheds some light on the results. Assume a fiber embedded to the extent of 3 in. (7.5 cm) is strained 1000 microstrains. The net absolute change in length is then 75 rnicrons. This corresponds to a time delay change in the propagation of light along the fiber (speed of light = 2 x lOe8 mls) of 0.375 picoseconds! This is indeed small, and the important aspect is that it is intended to detect changes on the order of 100-300 rnicro-in./in. of joint width between the three fiber sensors that are indicative of incipient or apparent joint failure, as implied by the large shifts in strain fields predicted by the FEM. SUMMARY We have described a fiber optic sensor designed with the goal of detecting disbonds in double bonded joints of graphite/epoxy composite. The ability to observe such disbonds has implications for health monitoring systems applications in a variety of structures, inc1uding space launch vehic1es. 1108 (a) CH2 CH2 1n MAG 8.995 ~U/ REF 0 U --- ---- _. - - - hI!:::---4rl-I-.-AR , ER --l7--L_~-!_ ~~..!.:ml (b) a, 60. 06 ~U n .. S TOP 67 . 007 n. Figure 6. (a) FIT offrequency response ofFigure 5, and (b) optical time domain reflectivity measurement of reflections from three embedded fibers. CD " " 0-E <t 1.200.6 ,----------------------. \.000·6 8.000·7 - Amplitude t 6.000·7 - Amplitudo 2 - AmplitudoJ 4.000·7 2.000·7 2000 4000 6000 8000 10000 12000 Figure 7. Differential signal response of FIT waveforms versus applied load. 1109 The sensor is based on the signal processing of the complex amplitude and phase response of the summed reflections of radio frequency amplitude modulated light in three optical tibers embedded in a composite structure to different penetration distances. An FFT reduces the complex frequency information to a time domain representation of reflection pulses, which can be digitized and differenced from reference time domain waveforms. Observable trends occur which indicate that tibers of greater penetration depth show greater strain response, but questions remain as to the bond integrity between the tibers and the composite matrix, and possible sources of noise which affect the predicted monotonic behavior of the processed signals. Further analysis and experiments are needed to ACKNOWLEDGEMENTS This work is supported by the Rockwell International Independent Research and Development Program. The authors thank L. Bivins, M. Wang and G. Halweg for technical assistance. REFERENCES 1. B. Culshaw and J. Dakin (1989). Optical Fiber Sensors: Systems and Applications, Vol. 2, Artech House, Inc., Norwood, MA. 2. 1. S. Schoenwald and R. H. Messinger, "A Fiber Optic rfResonant Cavity Sensor for Strain Sensing - FORRCS," in Rev. Prog. Nondestructive Evaluation, lOB, D. O. Thompson and D. E. Chimenti, Eds., (Plenum Press, NY, 1991), pp. 1239-1246. 3. HP-8752A vector network analyzer. 4. Ortei, Inc., Alhambra, CA: laser transmitter TSL-300; detector/receiver RSL-50. 1110 

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UHF Modulation and Fourier Transform Differential Time Domain Techniques for Measuring Strain Via Fiber Optics

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