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Feasibility of component state awareness of high strain rate events using fiber Bragg grating sensors
International Journal of Impact Engineering 100 (2017) 166174
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International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng
Feasibility of component state awareness of high strain rate events using fiber D6X XBragg grating sensors TagedPD7X XJames AyersDa8X X , D9X XTusit WeerasooriyaD10XaX , D1X XAnindya GhoshalD12Xa,X *, D13X XCollin PecoraD14XaX , D15X XAllan GunnarssonD16XaX , D17X XBrett SanbornD18XbX , D19X XPeter TurneyD20XcX TagedPa US Army Research Laboratory, Aberdeen Proving Ground, MD, D21X X United States b Oak Ridge Institute for Science and Education, Oak Ridge, TN, D23X X United States c Aberdeen Test Center, Aberdeen Proving Ground, MD, D25X X United States
TAGEDPA R T I C L E
I N F O
Article History: Received 8 May 2014 Revised 20 October 2016 Accepted 29 October 2016 Available online 1 November 2016 TagedPKeywords: Material state D27X X awareness Impact monitoring High strain events Fiber optic sensors
TAGEDPA B S T R A C T
Strategically located Fiber Bragg Grating (FBG) Sensors have been proposed as an in situ method to increase the signal to noise ratio (SNR) for metallic and composite components. This paper presents a systematic study that investigates the viability of FBG Sensors under high strain rate loading by initially measuring 1Dstrains in a compression Hopkinson bar experiment, followed by 2D full-field strain-tensor in impact and blast experiments on plates. Specifically, high strain rates from commercialized FBG Sensors are compared to traditional resistive and semi-conductor based strain gages under various levels of 1D high strain rate loading. In the projectile-plate impact experiments, full-field back-surface strain measured using FBG Sensor arrays are compared with that measured from 3D surface Digital Image Correlation (3D-sDIC) strain measuring technique. Finally, strains in welded steel plates subjected to high explosive discharge are monitored with mounted FBG Sensors on the back surface. From this study, potential improvements in the SNR of FBG Sensors are recommended, and the survivability of these sensors under more complex, dynamic loading is evaluated. Published by Elsevier Ltd.
1. Introduction TagedPStructural health monitoring of high strain rate events, such as blast and impact by embedded or surface mounted sensors, is an existing challenge. Most recently, particular emphasis has been placed on sensor survivability during the event and the requirement to increase the signal to noise ratio (SNR) due to the highly complex response. The ability to measure the response of a structure under extreme dynamic environments has great interest to military applications and civil and aerospace structures, such as re-entrant space vehicles. In terms of military applications, structural impacts from terminal ballistics achieve peak strain rates on the order of 105 s¡1 to 106 s¡1, and in general, strain rates above 102 s¡1 are herein classified as high strain rates [1]. In order to achieve such strain rates in a controlled environment, experimental setups including flier plates, expanded rings, and ballistic impact have been developed. The state of the art for high strain rate measurements is succinctly reviewed in [2], with particular attention given to Split-Hopkinson or Kolsky Bars and dynamic failure experiments. The most common setup for high strain rate material characterization is the Hopkinson bar, *
Corresponding author. E-mail addresses: [email protected], [email protected] (A. Ghoshal). http://dx.doi.org/10.1016/j.ijimpeng.2016.10.012 0734-743X/Published by Elsevier Ltd.
TagedPwhich can generate controlled one dimensional (1D) stress waves with known type (tension, compression or shear), strengths, magnitude, and strain-rates [3]. TagedPThe ability to measure high strain phenomena has been investigated by optical techniques [46], piezoresistive and modified strain gages [7]. Fiber optic sensors provide the potential for greater signal to noise ratio, and several publications provide evidence of using fiber optics in a high strain rate loading conditions [810]. Specifically, development of a passively demodulated optical fiber sensor system for in-line fiber etalon is shown to agree with resistive strain gages to within 2% accuracy, despite the dimension of strain gage being six times longer [8]. TagedPFBG Sensors are based on the photosensitivity of germanium (Ge) when illuminated with UV light [11, 12]. Since the Ge dopant is usually confined to the core of the fiber, the photosensitivity effect is observed only in the core. FBG Sensors have periodic variations of the refractive index in the Ge-doped core of the fiber. The periodic change in the refractive index is created by focusing an interference pattern of UV radiation onto the core. The difference of refraction indexes between the inner core and the cladding causes the light to propagate inside the inner core. The glass fiber is normally coated with acrylate, polyimide or other coating material. The periodicity of the refractive index is represented by the grating pitch, L, as seen in Fig. 1. The pitch of the grating is typically on the order of »0.5 mm,
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Fig. 1. Schematic of FBG sensor with reflected and transmitted spectra. Fig. 2. Schematic of typical scanning filter FBG demodulator.
TagedPwhile the amplitude of the refractive index variation is typically 10¡3 [13, 14]. TagedPLight travelling in an optical fiber will produce a back-reflected peak, centered at the Bragg wavelength, lB, when the Bragg condition is satisfied. The Bragg condition is simply the requirement that satisfies both energy and momentum conservation. Conservation of energy requires that the frequency of the incident radiation and the reflected radiation are equal. Conservation of momentum requires that the incident wave vector, ki, plus the grating wave vector, K, equal the wave vector of the scattered radiation, kf. Together the two conditions simplify to the firstorder Bragg Condition:
λB D 2neff L
ð1Þ
where the Bragg grating wavelength, λB, is the free-space center wavelength of the input light that will be back-reflected from the Bragg grating; neff is the effective refractive index of the fiber core at the free space center wavelength; and L is the periodicity of the grating [1315]. Therefore, for a given pitch (L) and average refractive index (neff), the wavelength λB will be reflected from the Bragg Grating (Fig. 1). TagedPUsing EqD D28X X .29X X (1), the shift in the Bragg grating center wavelength, DλB, due to an incremental change of length (DL), and ignoring thermal effects, is given by: @h @L DλB D 2 L eff C heff DL ð2Þ @L @L TagedPAssuming that: (a) the strain field is uniform across the Bragg grating length, (b) strains are low (less than 1%), (c) the FBG Sensor is surface mounted, and (d) perfect adhesion with no shear lag enabling perfect strain transfer between structural substrate and FBG sensor, a simplified form of EqD D30X X .31X X (2) is:
DλB D λB ð1¡pe Þez
ð3Þ
where: ez is the longitudinal strain on the FBG, and pe is the effective photo-elastic constant of the core material which is defined as: pe D
h2eff 2
½p12 ¡ðp11 C p12 Þ
ð4Þ
TagedPIn EqD D32X X .3X X 4, p11 and p12 are components of the strain-optic tensor, and is the Poisson's ratio. TagedPFBG sensing interrogators measure the Bragg Wavelength shift, DλB, induced by strain and temperature. The thermal effects and compensation techniques are well known and documented in the available literature [1016]. The interrogator chosen depends on such considerations as number of sensors, type of multiplexing, desired frequency response, and measurement resolution and accuracy. Existing techniques for interrogation include tunable filters, scanning lasers, linear optical filters, and spectrum analyzers [1117]. One popular filter-based technique, especially for high frequency applications, uses a tunable passband filter such as a scanning Fabry-Perot (FP) filter (Fig. 2). TagedPThe FP filter is characterized by a high finesse, Lorentzian transfer function, and filter tuning is controlled by the physical mirror separation using a piezoelectric (PZT) element. As seen in Fig. 2, the light reflected from the FBG is passed through a Fabry-Perot filter which
TagedP asses over one narrow band wavelength component, depending by p the spacing between the mirrors. As the filter is tuned - via voltage control of the PZ element - the FP passband scans over the return signals from the FBG, and the wavelengths can be determined and recorded from the voltage applied to the PZ element. TagedPThe advantages of FBG Sensors include: (1) the sensed information is encoded directly into the wavelength, which is an absolute, self-referencing parameter, (2) the output does not depend on the total light levels, losses in the connecting components (fibers, couplers, etc.), or source power, (3) the optical response allows many FBG sensors to be serially multiplexed via Wavelength Division Multiplexing (WDM) or Time Division Multiplexing (TDM), and Optical Frequency Domain Reflectometry (OFDR) on a single strand of optical fiber, thereby enabling distributed sensing, (4) FBGs are immune to electromagnetic interference (EMI), (5) FBGs are lightweight and small, allowing suitable for placement into, or onto, a structure, and (6) the availability of commercial, off-the-shelf (COTS) measuring systems reduces the complexity for the end user [13,14]. TagedPThe main limitation of FBG sensors is their sensitivity to both temperature and strain. The resulting problem is that temperature variations along the fiber path can lead to erroneous temperatureinduced strain measurements. It is recognized that the inherent characteristics of FBG Sensors (e.g., fragility, strain-temperature cross sensitivity, and anisotropic loading) requires consideration for encapsulation -or packaging - before being handled and inserted into, or onto, test items for monitoring. Numerous encapsulation techniques for temperature compensation have been developed and successfully demonstrated. The challenges include faithfully transmitting the strains or temperatures from the test item to the FBG sensor, discriminating between thermal and mechanical strain, and eliminating peak splitting/birefringence effects due to anisotropic and lateral forces on the FBG. On the advantage side, this sensitivity property allows FBG Sensors to be used as both temperature and strain sensors. TagedPFBG sensors have been deployed in structural health monitoring applications, such as bridges and composite airframe structures [18]. More recently, carbon fiber plates under impact loading was monitored using an FBG sensor and resistive strain gauge as a reference [19, 20]. At a sampling rate of 100 kilosamples per second, the experimental results illustrated that FBG sensors could measure strain with higher signal-to-noise ratio in wider frequency range compared with conventional strain gauges [18]. For this reason, it is of interest to systematically investigate the use of high strain rate capability of FBGs for more complex loading conditions. The spectrally-encoded, linear optical response of Fiber Bragg Gratings (FBG) to external strain and temperature has led to the rapid development of FBGbased sensing and interrogation systems for Structure Health Monitoring. TagedPThe following paper describes four independent experiments that utilize the FBG Sensors under various loading conditions and attached to different materials. A 1D test setup utilizing the well known compression Hopkinson bar is followed by a discussion of the time history and frequency results. Next, projectile-impact
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Fig. 3. Schematic of Hopkinson bar setup used to compare FBGs with typical strain gages.
TagedPexperiments are performed on metallic and polycarbonate (PC) using a compressed air gun to compare 2D strain histories of FBG Sensors with DIC and resistive strain gages. Finally, blast loading experiments are performed on welded steel plates, where time-history and frequency results from FBG Sensors are analyzed. The steel plates are Rolled Homogeneous Armor (RHA). 2. Hopkinson bar monitoring (1D) TagedPPrior to complex loading experiments, where multi-axial strain fields are present, an experiment was designed to generate a 1D high rate stress wave. This was done using the input-bar of an aluminum compression split-Hopkinson bar setup was impacted by another bar (named the striker bar) which was propelled by a gas gun. After impact, a compression stress wave travels down the bar and is reflected at the free end as a tensile stress wave. The reflected pulse is almost identical in shape and amplitude to the incident pulse, with subsequent pulses gradually attenuated as they travel back and forth between ends of the bar at the longitudinal bar wave speed [21]. 2.1. 1D strain D34X X monitoring setup
TagedP ages was adjusted until the output matched that of the resistive g gages. Resistive gages are thermally more stable and consequently have a more stable gage factor compared to the semiconductor gages. However, semi-conductor gages are used in very low signal experiments due to higher SNR than resistive gages. TagedPThe sensitivity (defined as the ability to measure relatively small strains) as well as the time and frequency response of FBG strain measurements, are compared to that obtained from traditional and semi-conductor strain gages. The Micronoptics OS 1105 FBG sensor was located 1626 mm from the impact end of the Hopkinson bar. The data acquisition speed of the si920 optical sensing interrogator is 500 kHz for a single channel. The sensitivity of the strain gages used in the Hopkinson Bar experiments shows less than C/- 0.2% change in the gage factor for temperature range of C/- 14 deg C above/below ambient (24 degree Celsius) [23], so these gages should be quite stable for our laboratory-based experiments. The Micronoptics FBG sensor length is 10 mm with a strain limit of 5000 me. The strain sensitivity is » 1.2 pm/me with the operating temperature range is - 40 to 120 degree Celsius. TagedPThe FBG and strain sensors are attached using widely accepted/ known standard attachment process. The surface preparation includes removal of oxides and smoothened using sandpaper. The
TagedPThe compression Hopkinson bar used in this study was 3658 mm in length and 31.75 mm in diameter. The striker bar used to generate the stress pulse in the incident bar was 458 mm in length and 19.05 mm in diameter. All bars were made of 7075-T6 aluminum. A schematic of the setup is shown in Fig. 3. TagedPResistive and semiconductor strain gages, both with nominal resistance of 1000 V, were located 1219 mm and 1981 mm from the impact end of the bar, respectively. The circuit for both the resistive and semiconductor strain gages consisted of a pair of gages attached to the surface of the bar symmetrically across the bar diameter. The output of the gages was conditioned using a Wheatstone bridge circuit, a differential amplifier (LDS PB 70), and was read by the oscilloscope. The strain in the bar is calculated by: ɛD
2Vgage GF:Vext
ð5Þ
where, Vgage is the output voltage from the strain gage, GF is the gage factor, and Vext is the excitation voltage supplied to the strain gages. Due to the sensitivity of the semiconductor gages to variations in temperature and humidity, the gage factor of the semiconductor
Fig. 4. Time history strain response per sensor from compression Hopkinson bar test.
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TagedPsurface is cleaned using isopropyl alcohol. The FBG sensors are then attached using Loctite epoxy adhesive. The strain gages are mounted using standard M-Bond 200 cyanoacrylate adhesive.
2.2. 1D strain monitoring results TagedPThe time history and frequency content of the 1D-tests are provided in Figs. 46. The phase difference of the pulse arrival and subsequent reflections in Fig. 4 is attributed to the separations in sensor spatial locations. TagedPThe peak amplitude of the FBG on the reflected pulse (at approximately 3.2 ms) is 6.4% larger than the incident (first) pulse, which is an artifact of the FBG ringing at specific frequencies between the pulse arrivals and not physically representative of the stress wave, as the semi-conductor strain gage exhibits a nominal variation of less than 1%. TagedPIn order to assess the discrepant signal variation between the FBG and strain gages, Fig. 5 depicts the standard deviation of the time history response. For the given Micronoptics 4-channel si920 sensing interrogator instrumentation, each channel has a predefined maximum strain range: 60 (one channel), 600 (two channels) and 6000 (one channel) micro-strain. This particular FBG hardware instrumentation was specifically customized for high strain rate measurement from high velocity impacts and blast loads. From Fig. 5, the maximum strain range of 6000 micro-strain produces a standard deviation of 22.6, the highest within the FBG sensor measurements by an average factor of 12.9. For the maximum range levels of 60 and 600 micro-strains, the average FBG sensor output standard deviation is 1.6. In comparison, the resistive and semi-conductor strain gage yields an average standard deviation of 22.5 and 0.86, respectively. TagedPThese results indicate that the FBG sensor is comparable to the semi-conductor precision when the appropriate maximum strain range is selected, whereas the resistive strain gage admits a relatively large noise content, as expected due to EMI noise. TagedPThe frequency content of the stress wave per sensor is depicted in Fig. 6. The fixed sampling rate of the Micronoptics si920 instrumentation for an individual channel is 500 kHz, and the maximum frequency content per sensor does not exceed at 12 kHz, and therefore removes the possibility of aliasing. The initial peak resonances between the resistive and semi-conductor strain gages follow the resonant trends by the FBG measurements, with peaks at 0.7, 2.1, and 2.8 kHz. However, the measured FBG resonant amplitude is approximately 31% higher than the resistive and semi-conductor
Fig. 6. Frequency content per sensor from the compression Hopkinson bar test.
sTagedP train gage amplitude, and may highlight the relative sensitivity of the FBG to specific low frequency bandwidths. 3. Plate impact experiments (2D) TagedPPrevious studies have measured strain measurements from impact loading using resistive strain gages coupled with piezoelectric sensors [7]. The damage site is inspected after impact using nondestructive techniques, such as ultrasonic scans, X-ray diffraction, and eddy current techniques. However response monitoring sensors of large scale structures under explosive environments are exposed to electromagnetic interference (EMI), which presents serious drawbacks for electronic sensors. FBG Sensors are immune to EMI and allow for multi-functional, multiplexibility not previously enabled for built-in sensors [22]. The following section provides a description of metallic and polymeric test specimens under projectile impact loading. 3.1. Metallic plate impact setup (2D) TagedPA series of air gun ballistic shots on a steel plate are performed (Fig. 7), whose dimensions are nominally 10.2 cm (4D0035X X ) £ D36X X 91.4 cm (3600 )£1.27 cm (0.500 ). The steel plates are Rolled Homogeneous Armor (RHA). They are produced to the military standard MIL-DTL12,560. TagedPTwo FBG sensors are perpendicularly collocated on the steel specimen near the point of impact to measure the high strain rate response (Fig. 7a). The sensor configuration allowed for the axial and transverse strain measurements. Resistive strain gages, on both sides of the impact zone, are mounted to the steel specimen as a baseline measurement. The sequential impacts utilized a 7.62 cm (300 ) length slug at 0.62 MPa (90 psi) breech air pressure. The ballistic slugs consist of hard steel, whose cylindrical shape has a 2.54 cm (100 ) base diameter (Fig. 7b). TagedPFigs. 8D37X X and 9 shows the ballistic test setup at the Aberdeen Test Center (ATC) Shock Laboratory. The high speed video camera captures the visual motion at a 30 kiloframes per second, while the laser vibrometer measures velocities and corresponding displacements at 100 kHz sampling rate. The strain gage response is measured with a sampling rate of 1 MHz. 3.2. Metallic plate monitoring results
Fig. 5. Standard deviation per sensor for 4 maximum strain range levels (Level 1 DD1X X60; Levels 2, 3D2X XDD3X X600; Level 4D4X XDD5X X6000).
TagedPFig. 10 compares the first 60 ms of the axial FBG Sensors and resistive strain gage measurements, and Fig. 9 shows the first 100 ms of strain readings from the transverse FBG sensors from one
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Fig. 7. (a) Steel test specimen with mounted FBG Sensors and resistive strain gages, and (b) Test slugs used in impact tests on nominal steel plate.
TagedPof the impact tests using the 7.62 cm (300 ) slug. From Fig. 8, it is observed that the absolute maximum axial micro-strain measured by the FBG is 2204 micro-strain compared to the resistive gage maximum micro-strain of 1769, with a 19.7% difference. The FBG Sensor measurements exhibited significant noise during this impact experiment, with a relative standard deviation of 28.5 micro-strain compared to the 0.72 micro-strain of the resistive strain gage. TagedPThe maximum transverse micro-strain is 748.1 micro-strain with a relative standard deviation of 2.0 micro-strain (Fig. 11). As can be seen in the inset of Fig. 11, after the initial overshoot produced by the slug impacting the plate, the transverse strain measurements exponentially attenuate until reaching steady state at about 100 ms. TagedPThe frequency spectrum of the transverse stress wave from FBG sensor is depicted in Fig. 12. The fixed sampling rate of the si920 instrumentation for both channels in operation is 225 kHz. The maximum frequency content in both transverse and axial FBG sensors does not exceed 30 kHz, and therefore removes the possibility of aliasing. The axially mounted FBG Sensor shows the low frequency resonances upto 5 kHz, whereas the transverse FBG Sensor strain exhibits peak resonant frequencies at 6.3, 15.9, and 28.4 kHz, respectively. The discrepancy between the axial and transverse peak resonances illustrates the need for multiple component sensors in understanding the vibratory response during complex impact loading.
TagedP he plate is instrumented with two FBG Sensors perpendicular to T each other and the back-surface of the plate was speckled to measure the full-field strain using DIC during impact. After testing, the images from the cameras are post-processed using commercial DIC software from Correlated Solutions Inc. to obtain the three dimensional displacement data of the back-surface of the PC panel Fig. 13. TagedPThe size of each panel is 305 mm by 305 mm (12 in by 12 in). The DIC system used in the impact experiments consisted of two Photron APX-RS high-speed digital cameras. The DIC allows for transient fullfield measurements of deformation and strain in the plate to be made during impact. During the experiment, the PC panel is clamped between an aluminum mounting frame and an aluminum support. The support increased distribution of the clamping force along the perimeter of the panel. The frame and the support are 25.4 mm thick, providing a 254 mm by 254 mm area of the target exposed to
3.3. Polycarbonate (PC) plate impact TagedPIn this impact experiment, a blunt-projectile propelled from a gas gun is impacted on an instrumented PC plate of 5.8 mm thickness.
Fig. 8. Ballistic test air gun barrel with steel plate specimen.
Fig. 9. Ballistic test setup with laser vibrometry and data acquisition.
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Fig. 10. Axially mounted FBG and resistive strain gage measurements for 7.62 cm (300 ) slug Test.
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Fig. 13. Axial and transverse FBG sensors on speckled composite plate.
Fig. 11. Transverse mounted FBG strain measurement for 7.62 cm (300 ) slug test. Fig. 14. Air gun test setup of composite plate with DIC and FBG DAQ.
TagedPa pair of Photron high-speed cameras. The cameras recorded the panel back surface motion during the impact event at a resolution of 256D38 X £ X D39 X 2X 56 pixels and a frame rate of 30,000 fps. The panel is impacted by a steel projectile with a hemi-spherical tip of radius 6.35 mm at a velocity of 57 m/s (see Fig. 14 for the experimental set-
TagedP p). The FBGs are located 63.5 mm from the impact point, and offset u by 19.4 mm from each other.
3.4. PC plate impact results (2D and 3D)
Fig. 12. 300 Slug Test: Frequency content for axial and transverse located FBGs.
TagedPThe transient deformation data generated by the DIC is used to determine how the PC behaved during the blunt impact. The DIC data is presented both graphically and numerically. The full field 2-D and 3-D contour plots for the impact velocity experiment is shown in Fig. 15(a) and D40X(b), X respectively. Both contour plots show horizontal strain (exx) at the time of maximum out-of-plane displacement (z-direction) 1.1 ms after impact. The 2D full-field is overlaid with the corresponding deformed image whereas the 3-D contour is displayed on an x-y-z coordinate axes plot. The zscale is not proportional to the x-y axes and thus the exaggerated profile in the z-direction. The graphical representations shown are typical of the data acquired for all of the experiments performed. TagedPFrom Figs. 15 and 16, the resulting maximum exx, and eyy, strain is 1.49 and 1.35% strain at the impact point, respectively. This value is relatively high and typically beyond the capacity of a traditional FBGS system. TagedPThe time history from the impact center region of the plate is shown in Fig. 17. It is of interest to note the similarity of the strain in
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Fig. 15. Strain contours, exx, on composite plate at maximum deflection measured by DIC system: (a) 2D planform view, and (b) enhanced 3D surface map at 1.1 ms after impact.
Fig. 16. Strain contours, eyy, on composite plate at maximum deflection measured by DIC system: (a) 2D planform view, and (b) enhanced 3D surface map at 1.1 ms after impact.
Fig. 18. Time history strain response comparison between the FBG and DIC on composite plate: (a) exx, and (b) eyy.
4.1. 2D Blast Plate Setup
Fig. 17. Maximum strain response from DIC at impact point for exx and eyy.
TagedPboth phase and amplitude for both exx, and eyy, which is not observed in regions away from the impact region. TagedPComparisons between the FBG strain data and the measured DIC strain data is shown in Fig. 18(a-b). Qualitatively, it is clear that the eyy remains in-phase and approximately similar amplitudes, whereas the measured exx is out of phase by more than 45 degrees and peak strain by more than 30%. The amplitude discrepancy may be partly due to the limitations of the hardware for the selected channels, where the maximum range is nominally limited by a D41X X §D42 X 6X 00 microstrain range, as evident by peak amplitude clipping in Fig. 18(a).
T he test specimens consist of two rectangular 6.35 mm (1=4 ”) agedPT rolled homogeneous armor (RHA) steel plates welded together to form a square 72.4 mm (28.500 )D43X £ X D4X 7 X 2.4 mm (28.500 ) target. The target was then welded to a 12.7 mm (1/200 ) wide frame (Fig. 20). The targets were subjected to various blast loads until weld failure was obtained. Various non-destructive techniques were used to track
4. Plate blast monitoring (2D) TagedPThe measured experimental strain fields are used to calibrate and improve 3D finite element simulation methodologies that incorporate state of the art blast modeling techniques and material models that represent the base-metal, weld-metal and the critical heat affected zone (HAZ) (Fig. 19).
Fig. 19. Stress gradients from detailed dynamic FEM under 40% maximum load.
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Fig. 20. Mounted and marked FBG sensors placed radially from the center. Fig. 22. Time history for multiple rounds of discharge on the RHA steel plate.
Fig. 23. Frequency content for multiple rounds of discharge on the RHA steel plate.
Fig. 21. Testing schematic with welded plate specimen.
TagedPboth surface strains and weld crack growth. Dynamic plate deformations using a honeycomb core and incident overpressure was measured during each iteration. TagedPThe schematic of the test setup is provided in Fig. 21. The discharge explosive is similar to C4 material, which is a relatively stable, solid explosive with a consistency similar to modeling clay The plate is subjected to several rounds of C4, incrementally increasing from 400 g up to 1200 g. For this test a constant 800 g of C4 was detonated thrice at a stand-off distance of 1.2 m. The FBG Sensors are located at 5 cm from the center of the plate. 4.2. Blast plate results (2D) TagedPThe time history and frequency spectrum from the sensors in proximity are presented in Figs. 22D45X X and 23. Several observations are made from Fig. 22. Successful transient measurements from 200 to 3000 micro-strain were recorded. The initial blast wave impacted the plate at 6.5 ms, and the subsequent vibration effects were experienced at 16.5 ms. TagedPThe primary resonant frequency occurred at 6.1 kHz, which is consistent with previous measurements by high speed accelerometers (Fig. 23). Significant amount of inherent noise was apparent throughout the measurement interval. The noise is largely attributed
tTagedP o the outdoor temperature fluctuation, the length (500 m) of the optical wire to ensure sufficient stand-off from the detonation, and the receptor interface to the DAQ system. Additionally anti-aliasing filters are not built into the given Micronoptics si920 hardware, although primary resonant frequencies are less than 20 times the Nyquist frequency. Despite the unwanted noise levels, the feasibility of the FBG Sensor system for blast loading is demonstrated.
5. Conclusions TagedPThe independent experiments demonstrated the concept of an optical fiber Bragg grating (FBG)-based real-time instrumentation system for quantitative evaluation of the full-field dynamic response of ballistic impacted specimens. Several conclusions may be drawn from the results for future improvements of experimental measurements using FBG Sensors. Specifically, a need for multi-physic measurements such as high impact accelerometers and strain gages collocated at the FBG Sensor site. This would allow for improved baseline, and an accurate test setup should include the following, when using FBG Sensors on high strain rate measurements: 1) increase time increment of the pre-trigger to eliminate environmental or trigger-induced noise, 2) implement an optical or soft backing such as clay rather than honeycomb structure in measuring deformation under blast loading conditions to ensure no prestress on FBGs, and 3) rectify and monitor drifting of the hardware by cleansing of the FBG receptor and in-line calibration.
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TagedPFinally, the successful validation of the peak strain value of the instantaneous response during impact and the transient response after impact demonstrates the potential usage of FBG Sensors in future impact and blast loading environments. Particularly, future work exists in the development of composites, where FBG Sensors can be embedded into the design of the structure, especially for health monitoring. The most recent advances in FBG interrogation systems may provide improved insights in micro-vibration mechanics. Acknowledgments TagedPThe authors gratefully acknowledge the test facilities at the Aberdeen Testing Center and Mr. Vincent Urbanski for his expertise in deploying the impacting projectile. The comparable numerical blast simulations were performed by Dr. Rahul Gupta from the US Army Research Laboratory. References TagedP [1] Hamouda AMS, Hashmi MSJ. Testing of composite materials at high rates of strain: advances and challenges. J Mat Process Tech 1998;77(1-3):327–36. TagedP [2] Ramesh KT. High strain rate and impact experiments. In: Sharpe WN, editor. Springer handbook of experimental solid mechanics. Springer; 2008. Chapter 33. TagedP [3] Lo Y, Sirkis JS, Chang CC. Passive signal processing of in-line fiber etalon sensors for high strain-rate loading. J Lightwave Tech 1997;15(8):1578–86. TagedP [4] Gogulya MF, Dolgoborodov AY, Brazhnikov MA. Investigation of shock and detonation waves by optical pyrometry. Int J Imp Eng 1999;23(1):283–93. TagedP [5] Longana ML, Dulieu-Barton JM, Syngellakis S. Application of optical measurement techniques to high strain rate deformations in composite materials. In: Proc. 7th Asian conference on composite materials; 2010. p. 1–4. TagedP [6] Rosakis AJ. Application of coherent gradient sensing (cgs) to the investigation of dynamic fracture problems. Opt Lasers Eng 1993;19(1-3):3–41.
TagedP [7] Gupta YM. Use of piezoresistance gauges to quantify the stress state in a shocked solid. In: Ramesh KT, editor. Experimental techniques in the dynamics of deformable solids. New York: ASME; 1993. p. 89–101. TagedP [8] Nellen P, Frank A, Kenel A. High strain and high strain gradients measured with fiber bragg gratings in structural engineering applications. In: Proc. 15th optical fibers sensors conference; 2002. p. 111–4. A. Dynamic strain measureTagedP [9] Cusano A, Cutolo A, Nasser J, Giordano M, Calabro ments by fibre Bragg grating sensor. Sens Actuators A 2004;110(1-3):276–81. TagedP[10] Majumder M, Gangopadhyay TK, Chakraborty AK, Dasgupta K, Bhattacharya DK. Fibre bragg gratings in structural health monitoring—present status and applications. Sens Actuators A 2008;147(1):150–64. TagedP[11] Hill KO, Fujii Y, Johnson DC, Kawasaki BS. Photosensitivity in optical fiber waveguides: application to reflection fiber fabrication. Appl Phys Lett 1978;32(10). TagedP[12] Meltz G, et al. Formation of Bragg gratings in optical fibers by a transverse holographic method. Opt Lett 1989;14(15). TagedP[13] Othonos A, Kalli K. Fiber bragg gratings: fundamentals and applications in telecommunications and sensing. Artech House Optoelectronics; 1999 Chaps. 2-3. TagedP[14] Baldwin C. Optical fiber strain gages. In: Sharpe WN, editor. Springer handbook of experimental solid mechanics. Springer; 2009. Chapter 14. TagedP[15] Kersey AD, Davis MA, Berkhoff T, Bellemore D, Koo K, Jones RT. Progress towards the development of practical fiber bragg grating instrumentation systems. SPIE, 2839; 1996. p. 40–63. TagedP[16] Digonnet Michel. Short course on fiber optic sensors: principles & applications. In: Conference on lasers & electro-optics; 2005. TagedP[17] Kersey AD. Interrogation and multiplexing techniques for fiber bragg grating stain-sensors. SPIE, 2071; 1993. p. 30–48. TagedP[18] Hu J, Dong H, Zhang G. Study on a new kind of surface sticking strain sensor with sensitivity enhanced based on FBG. SPIE, 6595; 2007. p. 65953K. TagedP[19] Tsuda H, Lee JR. Strain and damage monitoring of CFRP in impact loading using a fiber Bragg grating sensor system. Comp Sci Tech 2007;67(7-8):1353–61. € r T, Coric D. Internal strain measurements in TagedP[20] Frieden J, Cugnoni J, Botsis J, Gmu CFRP plates subjected to impact load using FBG sensors. In: Proc. of the 17th int. conf. on comp. mats. ICCM-17; 2009. p. 1–9. TagedP[21] Chen W, Song B. Split hopkinson (Kolsky) bar: design, testing, and applications. Springer; 2010 Chaps. 1-2. TagedP[22] Othonos A. Fiber Bragg grating. Rev Sci Instrum 1997;68(12):4309–41. TagedP[23] MICRO-MEASUREMENTS: “Strain gage thermal output and gage factor variation with temperature,” Tech Note TN-504-1, Document Number: 11054, http:// www.vishaypg.com/docs/11054/tn504.pdf, Revision 14- Aug-2014
Contents lists available at ScienceDirect
International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng
Feasibility of component state awareness of high strain rate events using fiber D6X XBragg grating sensors TagedPD7X XJames AyersDa8X X , D9X XTusit WeerasooriyaD10XaX , D1X XAnindya GhoshalD12Xa,X *, D13X XCollin PecoraD14XaX , D15X XAllan GunnarssonD16XaX , D17X XBrett SanbornD18XbX , D19X XPeter TurneyD20XcX TagedPa US Army Research Laboratory, Aberdeen Proving Ground, MD, D21X X United States b Oak Ridge Institute for Science and Education, Oak Ridge, TN, D23X X United States c Aberdeen Test Center, Aberdeen Proving Ground, MD, D25X X United States
TAGEDPA R T I C L E
I N F O
Article History: Received 8 May 2014 Revised 20 October 2016 Accepted 29 October 2016 Available online 1 November 2016 TagedPKeywords: Material state D27X X awareness Impact monitoring High strain events Fiber optic sensors
TAGEDPA B S T R A C T
Strategically located Fiber Bragg Grating (FBG) Sensors have been proposed as an in situ method to increase the signal to noise ratio (SNR) for metallic and composite components. This paper presents a systematic study that investigates the viability of FBG Sensors under high strain rate loading by initially measuring 1Dstrains in a compression Hopkinson bar experiment, followed by 2D full-field strain-tensor in impact and blast experiments on plates. Specifically, high strain rates from commercialized FBG Sensors are compared to traditional resistive and semi-conductor based strain gages under various levels of 1D high strain rate loading. In the projectile-plate impact experiments, full-field back-surface strain measured using FBG Sensor arrays are compared with that measured from 3D surface Digital Image Correlation (3D-sDIC) strain measuring technique. Finally, strains in welded steel plates subjected to high explosive discharge are monitored with mounted FBG Sensors on the back surface. From this study, potential improvements in the SNR of FBG Sensors are recommended, and the survivability of these sensors under more complex, dynamic loading is evaluated. Published by Elsevier Ltd.
1. Introduction TagedPStructural health monitoring of high strain rate events, such as blast and impact by embedded or surface mounted sensors, is an existing challenge. Most recently, particular emphasis has been placed on sensor survivability during the event and the requirement to increase the signal to noise ratio (SNR) due to the highly complex response. The ability to measure the response of a structure under extreme dynamic environments has great interest to military applications and civil and aerospace structures, such as re-entrant space vehicles. In terms of military applications, structural impacts from terminal ballistics achieve peak strain rates on the order of 105 s¡1 to 106 s¡1, and in general, strain rates above 102 s¡1 are herein classified as high strain rates [1]. In order to achieve such strain rates in a controlled environment, experimental setups including flier plates, expanded rings, and ballistic impact have been developed. The state of the art for high strain rate measurements is succinctly reviewed in [2], with particular attention given to Split-Hopkinson or Kolsky Bars and dynamic failure experiments. The most common setup for high strain rate material characterization is the Hopkinson bar, *
Corresponding author. E-mail addresses: [email protected], [email protected] (A. Ghoshal). http://dx.doi.org/10.1016/j.ijimpeng.2016.10.012 0734-743X/Published by Elsevier Ltd.
TagedPwhich can generate controlled one dimensional (1D) stress waves with known type (tension, compression or shear), strengths, magnitude, and strain-rates [3]. TagedPThe ability to measure high strain phenomena has been investigated by optical techniques [46], piezoresistive and modified strain gages [7]. Fiber optic sensors provide the potential for greater signal to noise ratio, and several publications provide evidence of using fiber optics in a high strain rate loading conditions [810]. Specifically, development of a passively demodulated optical fiber sensor system for in-line fiber etalon is shown to agree with resistive strain gages to within 2% accuracy, despite the dimension of strain gage being six times longer [8]. TagedPFBG Sensors are based on the photosensitivity of germanium (Ge) when illuminated with UV light [11, 12]. Since the Ge dopant is usually confined to the core of the fiber, the photosensitivity effect is observed only in the core. FBG Sensors have periodic variations of the refractive index in the Ge-doped core of the fiber. The periodic change in the refractive index is created by focusing an interference pattern of UV radiation onto the core. The difference of refraction indexes between the inner core and the cladding causes the light to propagate inside the inner core. The glass fiber is normally coated with acrylate, polyimide or other coating material. The periodicity of the refractive index is represented by the grating pitch, L, as seen in Fig. 1. The pitch of the grating is typically on the order of »0.5 mm,
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Fig. 1. Schematic of FBG sensor with reflected and transmitted spectra. Fig. 2. Schematic of typical scanning filter FBG demodulator.
TagedPwhile the amplitude of the refractive index variation is typically 10¡3 [13, 14]. TagedPLight travelling in an optical fiber will produce a back-reflected peak, centered at the Bragg wavelength, lB, when the Bragg condition is satisfied. The Bragg condition is simply the requirement that satisfies both energy and momentum conservation. Conservation of energy requires that the frequency of the incident radiation and the reflected radiation are equal. Conservation of momentum requires that the incident wave vector, ki, plus the grating wave vector, K, equal the wave vector of the scattered radiation, kf. Together the two conditions simplify to the firstorder Bragg Condition:
λB D 2neff L
ð1Þ
where the Bragg grating wavelength, λB, is the free-space center wavelength of the input light that will be back-reflected from the Bragg grating; neff is the effective refractive index of the fiber core at the free space center wavelength; and L is the periodicity of the grating [1315]. Therefore, for a given pitch (L) and average refractive index (neff), the wavelength λB will be reflected from the Bragg Grating (Fig. 1). TagedPUsing EqD D28X X .29X X (1), the shift in the Bragg grating center wavelength, DλB, due to an incremental change of length (DL), and ignoring thermal effects, is given by: @h @L DλB D 2 L eff C heff DL ð2Þ @L @L TagedPAssuming that: (a) the strain field is uniform across the Bragg grating length, (b) strains are low (less than 1%), (c) the FBG Sensor is surface mounted, and (d) perfect adhesion with no shear lag enabling perfect strain transfer between structural substrate and FBG sensor, a simplified form of EqD D30X X .31X X (2) is:
DλB D λB ð1¡pe Þez
ð3Þ
where: ez is the longitudinal strain on the FBG, and pe is the effective photo-elastic constant of the core material which is defined as: pe D
h2eff 2
½p12 ¡ðp11 C p12 Þ
ð4Þ
TagedPIn EqD D32X X .3X X 4, p11 and p12 are components of the strain-optic tensor, and is the Poisson's ratio. TagedPFBG sensing interrogators measure the Bragg Wavelength shift, DλB, induced by strain and temperature. The thermal effects and compensation techniques are well known and documented in the available literature [1016]. The interrogator chosen depends on such considerations as number of sensors, type of multiplexing, desired frequency response, and measurement resolution and accuracy. Existing techniques for interrogation include tunable filters, scanning lasers, linear optical filters, and spectrum analyzers [1117]. One popular filter-based technique, especially for high frequency applications, uses a tunable passband filter such as a scanning Fabry-Perot (FP) filter (Fig. 2). TagedPThe FP filter is characterized by a high finesse, Lorentzian transfer function, and filter tuning is controlled by the physical mirror separation using a piezoelectric (PZT) element. As seen in Fig. 2, the light reflected from the FBG is passed through a Fabry-Perot filter which
TagedP asses over one narrow band wavelength component, depending by p the spacing between the mirrors. As the filter is tuned - via voltage control of the PZ element - the FP passband scans over the return signals from the FBG, and the wavelengths can be determined and recorded from the voltage applied to the PZ element. TagedPThe advantages of FBG Sensors include: (1) the sensed information is encoded directly into the wavelength, which is an absolute, self-referencing parameter, (2) the output does not depend on the total light levels, losses in the connecting components (fibers, couplers, etc.), or source power, (3) the optical response allows many FBG sensors to be serially multiplexed via Wavelength Division Multiplexing (WDM) or Time Division Multiplexing (TDM), and Optical Frequency Domain Reflectometry (OFDR) on a single strand of optical fiber, thereby enabling distributed sensing, (4) FBGs are immune to electromagnetic interference (EMI), (5) FBGs are lightweight and small, allowing suitable for placement into, or onto, a structure, and (6) the availability of commercial, off-the-shelf (COTS) measuring systems reduces the complexity for the end user [13,14]. TagedPThe main limitation of FBG sensors is their sensitivity to both temperature and strain. The resulting problem is that temperature variations along the fiber path can lead to erroneous temperatureinduced strain measurements. It is recognized that the inherent characteristics of FBG Sensors (e.g., fragility, strain-temperature cross sensitivity, and anisotropic loading) requires consideration for encapsulation -or packaging - before being handled and inserted into, or onto, test items for monitoring. Numerous encapsulation techniques for temperature compensation have been developed and successfully demonstrated. The challenges include faithfully transmitting the strains or temperatures from the test item to the FBG sensor, discriminating between thermal and mechanical strain, and eliminating peak splitting/birefringence effects due to anisotropic and lateral forces on the FBG. On the advantage side, this sensitivity property allows FBG Sensors to be used as both temperature and strain sensors. TagedPFBG sensors have been deployed in structural health monitoring applications, such as bridges and composite airframe structures [18]. More recently, carbon fiber plates under impact loading was monitored using an FBG sensor and resistive strain gauge as a reference [19, 20]. At a sampling rate of 100 kilosamples per second, the experimental results illustrated that FBG sensors could measure strain with higher signal-to-noise ratio in wider frequency range compared with conventional strain gauges [18]. For this reason, it is of interest to systematically investigate the use of high strain rate capability of FBGs for more complex loading conditions. The spectrally-encoded, linear optical response of Fiber Bragg Gratings (FBG) to external strain and temperature has led to the rapid development of FBGbased sensing and interrogation systems for Structure Health Monitoring. TagedPThe following paper describes four independent experiments that utilize the FBG Sensors under various loading conditions and attached to different materials. A 1D test setup utilizing the well known compression Hopkinson bar is followed by a discussion of the time history and frequency results. Next, projectile-impact
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Fig. 3. Schematic of Hopkinson bar setup used to compare FBGs with typical strain gages.
TagedPexperiments are performed on metallic and polycarbonate (PC) using a compressed air gun to compare 2D strain histories of FBG Sensors with DIC and resistive strain gages. Finally, blast loading experiments are performed on welded steel plates, where time-history and frequency results from FBG Sensors are analyzed. The steel plates are Rolled Homogeneous Armor (RHA). 2. Hopkinson bar monitoring (1D) TagedPPrior to complex loading experiments, where multi-axial strain fields are present, an experiment was designed to generate a 1D high rate stress wave. This was done using the input-bar of an aluminum compression split-Hopkinson bar setup was impacted by another bar (named the striker bar) which was propelled by a gas gun. After impact, a compression stress wave travels down the bar and is reflected at the free end as a tensile stress wave. The reflected pulse is almost identical in shape and amplitude to the incident pulse, with subsequent pulses gradually attenuated as they travel back and forth between ends of the bar at the longitudinal bar wave speed [21]. 2.1. 1D strain D34X X monitoring setup
TagedP ages was adjusted until the output matched that of the resistive g gages. Resistive gages are thermally more stable and consequently have a more stable gage factor compared to the semiconductor gages. However, semi-conductor gages are used in very low signal experiments due to higher SNR than resistive gages. TagedPThe sensitivity (defined as the ability to measure relatively small strains) as well as the time and frequency response of FBG strain measurements, are compared to that obtained from traditional and semi-conductor strain gages. The Micronoptics OS 1105 FBG sensor was located 1626 mm from the impact end of the Hopkinson bar. The data acquisition speed of the si920 optical sensing interrogator is 500 kHz for a single channel. The sensitivity of the strain gages used in the Hopkinson Bar experiments shows less than C/- 0.2% change in the gage factor for temperature range of C/- 14 deg C above/below ambient (24 degree Celsius) [23], so these gages should be quite stable for our laboratory-based experiments. The Micronoptics FBG sensor length is 10 mm with a strain limit of 5000 me. The strain sensitivity is » 1.2 pm/me with the operating temperature range is - 40 to 120 degree Celsius. TagedPThe FBG and strain sensors are attached using widely accepted/ known standard attachment process. The surface preparation includes removal of oxides and smoothened using sandpaper. The
TagedPThe compression Hopkinson bar used in this study was 3658 mm in length and 31.75 mm in diameter. The striker bar used to generate the stress pulse in the incident bar was 458 mm in length and 19.05 mm in diameter. All bars were made of 7075-T6 aluminum. A schematic of the setup is shown in Fig. 3. TagedPResistive and semiconductor strain gages, both with nominal resistance of 1000 V, were located 1219 mm and 1981 mm from the impact end of the bar, respectively. The circuit for both the resistive and semiconductor strain gages consisted of a pair of gages attached to the surface of the bar symmetrically across the bar diameter. The output of the gages was conditioned using a Wheatstone bridge circuit, a differential amplifier (LDS PB 70), and was read by the oscilloscope. The strain in the bar is calculated by: ɛD
2Vgage GF:Vext
ð5Þ
where, Vgage is the output voltage from the strain gage, GF is the gage factor, and Vext is the excitation voltage supplied to the strain gages. Due to the sensitivity of the semiconductor gages to variations in temperature and humidity, the gage factor of the semiconductor
Fig. 4. Time history strain response per sensor from compression Hopkinson bar test.
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TagedPsurface is cleaned using isopropyl alcohol. The FBG sensors are then attached using Loctite epoxy adhesive. The strain gages are mounted using standard M-Bond 200 cyanoacrylate adhesive.
2.2. 1D strain monitoring results TagedPThe time history and frequency content of the 1D-tests are provided in Figs. 46. The phase difference of the pulse arrival and subsequent reflections in Fig. 4 is attributed to the separations in sensor spatial locations. TagedPThe peak amplitude of the FBG on the reflected pulse (at approximately 3.2 ms) is 6.4% larger than the incident (first) pulse, which is an artifact of the FBG ringing at specific frequencies between the pulse arrivals and not physically representative of the stress wave, as the semi-conductor strain gage exhibits a nominal variation of less than 1%. TagedPIn order to assess the discrepant signal variation between the FBG and strain gages, Fig. 5 depicts the standard deviation of the time history response. For the given Micronoptics 4-channel si920 sensing interrogator instrumentation, each channel has a predefined maximum strain range: 60 (one channel), 600 (two channels) and 6000 (one channel) micro-strain. This particular FBG hardware instrumentation was specifically customized for high strain rate measurement from high velocity impacts and blast loads. From Fig. 5, the maximum strain range of 6000 micro-strain produces a standard deviation of 22.6, the highest within the FBG sensor measurements by an average factor of 12.9. For the maximum range levels of 60 and 600 micro-strains, the average FBG sensor output standard deviation is 1.6. In comparison, the resistive and semi-conductor strain gage yields an average standard deviation of 22.5 and 0.86, respectively. TagedPThese results indicate that the FBG sensor is comparable to the semi-conductor precision when the appropriate maximum strain range is selected, whereas the resistive strain gage admits a relatively large noise content, as expected due to EMI noise. TagedPThe frequency content of the stress wave per sensor is depicted in Fig. 6. The fixed sampling rate of the Micronoptics si920 instrumentation for an individual channel is 500 kHz, and the maximum frequency content per sensor does not exceed at 12 kHz, and therefore removes the possibility of aliasing. The initial peak resonances between the resistive and semi-conductor strain gages follow the resonant trends by the FBG measurements, with peaks at 0.7, 2.1, and 2.8 kHz. However, the measured FBG resonant amplitude is approximately 31% higher than the resistive and semi-conductor
Fig. 6. Frequency content per sensor from the compression Hopkinson bar test.
sTagedP train gage amplitude, and may highlight the relative sensitivity of the FBG to specific low frequency bandwidths. 3. Plate impact experiments (2D) TagedPPrevious studies have measured strain measurements from impact loading using resistive strain gages coupled with piezoelectric sensors [7]. The damage site is inspected after impact using nondestructive techniques, such as ultrasonic scans, X-ray diffraction, and eddy current techniques. However response monitoring sensors of large scale structures under explosive environments are exposed to electromagnetic interference (EMI), which presents serious drawbacks for electronic sensors. FBG Sensors are immune to EMI and allow for multi-functional, multiplexibility not previously enabled for built-in sensors [22]. The following section provides a description of metallic and polymeric test specimens under projectile impact loading. 3.1. Metallic plate impact setup (2D) TagedPA series of air gun ballistic shots on a steel plate are performed (Fig. 7), whose dimensions are nominally 10.2 cm (4D0035X X ) £ D36X X 91.4 cm (3600 )£1.27 cm (0.500 ). The steel plates are Rolled Homogeneous Armor (RHA). They are produced to the military standard MIL-DTL12,560. TagedPTwo FBG sensors are perpendicularly collocated on the steel specimen near the point of impact to measure the high strain rate response (Fig. 7a). The sensor configuration allowed for the axial and transverse strain measurements. Resistive strain gages, on both sides of the impact zone, are mounted to the steel specimen as a baseline measurement. The sequential impacts utilized a 7.62 cm (300 ) length slug at 0.62 MPa (90 psi) breech air pressure. The ballistic slugs consist of hard steel, whose cylindrical shape has a 2.54 cm (100 ) base diameter (Fig. 7b). TagedPFigs. 8D37X X and 9 shows the ballistic test setup at the Aberdeen Test Center (ATC) Shock Laboratory. The high speed video camera captures the visual motion at a 30 kiloframes per second, while the laser vibrometer measures velocities and corresponding displacements at 100 kHz sampling rate. The strain gage response is measured with a sampling rate of 1 MHz. 3.2. Metallic plate monitoring results
Fig. 5. Standard deviation per sensor for 4 maximum strain range levels (Level 1 DD1X X60; Levels 2, 3D2X XDD3X X600; Level 4D4X XDD5X X6000).
TagedPFig. 10 compares the first 60 ms of the axial FBG Sensors and resistive strain gage measurements, and Fig. 9 shows the first 100 ms of strain readings from the transverse FBG sensors from one
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Fig. 7. (a) Steel test specimen with mounted FBG Sensors and resistive strain gages, and (b) Test slugs used in impact tests on nominal steel plate.
TagedPof the impact tests using the 7.62 cm (300 ) slug. From Fig. 8, it is observed that the absolute maximum axial micro-strain measured by the FBG is 2204 micro-strain compared to the resistive gage maximum micro-strain of 1769, with a 19.7% difference. The FBG Sensor measurements exhibited significant noise during this impact experiment, with a relative standard deviation of 28.5 micro-strain compared to the 0.72 micro-strain of the resistive strain gage. TagedPThe maximum transverse micro-strain is 748.1 micro-strain with a relative standard deviation of 2.0 micro-strain (Fig. 11). As can be seen in the inset of Fig. 11, after the initial overshoot produced by the slug impacting the plate, the transverse strain measurements exponentially attenuate until reaching steady state at about 100 ms. TagedPThe frequency spectrum of the transverse stress wave from FBG sensor is depicted in Fig. 12. The fixed sampling rate of the si920 instrumentation for both channels in operation is 225 kHz. The maximum frequency content in both transverse and axial FBG sensors does not exceed 30 kHz, and therefore removes the possibility of aliasing. The axially mounted FBG Sensor shows the low frequency resonances upto 5 kHz, whereas the transverse FBG Sensor strain exhibits peak resonant frequencies at 6.3, 15.9, and 28.4 kHz, respectively. The discrepancy between the axial and transverse peak resonances illustrates the need for multiple component sensors in understanding the vibratory response during complex impact loading.
TagedP he plate is instrumented with two FBG Sensors perpendicular to T each other and the back-surface of the plate was speckled to measure the full-field strain using DIC during impact. After testing, the images from the cameras are post-processed using commercial DIC software from Correlated Solutions Inc. to obtain the three dimensional displacement data of the back-surface of the PC panel Fig. 13. TagedPThe size of each panel is 305 mm by 305 mm (12 in by 12 in). The DIC system used in the impact experiments consisted of two Photron APX-RS high-speed digital cameras. The DIC allows for transient fullfield measurements of deformation and strain in the plate to be made during impact. During the experiment, the PC panel is clamped between an aluminum mounting frame and an aluminum support. The support increased distribution of the clamping force along the perimeter of the panel. The frame and the support are 25.4 mm thick, providing a 254 mm by 254 mm area of the target exposed to
3.3. Polycarbonate (PC) plate impact TagedPIn this impact experiment, a blunt-projectile propelled from a gas gun is impacted on an instrumented PC plate of 5.8 mm thickness.
Fig. 8. Ballistic test air gun barrel with steel plate specimen.
Fig. 9. Ballistic test setup with laser vibrometry and data acquisition.
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Fig. 10. Axially mounted FBG and resistive strain gage measurements for 7.62 cm (300 ) slug Test.
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Fig. 13. Axial and transverse FBG sensors on speckled composite plate.
Fig. 11. Transverse mounted FBG strain measurement for 7.62 cm (300 ) slug test. Fig. 14. Air gun test setup of composite plate with DIC and FBG DAQ.
TagedPa pair of Photron high-speed cameras. The cameras recorded the panel back surface motion during the impact event at a resolution of 256D38 X £ X D39 X 2X 56 pixels and a frame rate of 30,000 fps. The panel is impacted by a steel projectile with a hemi-spherical tip of radius 6.35 mm at a velocity of 57 m/s (see Fig. 14 for the experimental set-
TagedP p). The FBGs are located 63.5 mm from the impact point, and offset u by 19.4 mm from each other.
3.4. PC plate impact results (2D and 3D)
Fig. 12. 300 Slug Test: Frequency content for axial and transverse located FBGs.
TagedPThe transient deformation data generated by the DIC is used to determine how the PC behaved during the blunt impact. The DIC data is presented both graphically and numerically. The full field 2-D and 3-D contour plots for the impact velocity experiment is shown in Fig. 15(a) and D40X(b), X respectively. Both contour plots show horizontal strain (exx) at the time of maximum out-of-plane displacement (z-direction) 1.1 ms after impact. The 2D full-field is overlaid with the corresponding deformed image whereas the 3-D contour is displayed on an x-y-z coordinate axes plot. The zscale is not proportional to the x-y axes and thus the exaggerated profile in the z-direction. The graphical representations shown are typical of the data acquired for all of the experiments performed. TagedPFrom Figs. 15 and 16, the resulting maximum exx, and eyy, strain is 1.49 and 1.35% strain at the impact point, respectively. This value is relatively high and typically beyond the capacity of a traditional FBGS system. TagedPThe time history from the impact center region of the plate is shown in Fig. 17. It is of interest to note the similarity of the strain in
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Fig. 15. Strain contours, exx, on composite plate at maximum deflection measured by DIC system: (a) 2D planform view, and (b) enhanced 3D surface map at 1.1 ms after impact.
Fig. 16. Strain contours, eyy, on composite plate at maximum deflection measured by DIC system: (a) 2D planform view, and (b) enhanced 3D surface map at 1.1 ms after impact.
Fig. 18. Time history strain response comparison between the FBG and DIC on composite plate: (a) exx, and (b) eyy.
4.1. 2D Blast Plate Setup
Fig. 17. Maximum strain response from DIC at impact point for exx and eyy.
TagedPboth phase and amplitude for both exx, and eyy, which is not observed in regions away from the impact region. TagedPComparisons between the FBG strain data and the measured DIC strain data is shown in Fig. 18(a-b). Qualitatively, it is clear that the eyy remains in-phase and approximately similar amplitudes, whereas the measured exx is out of phase by more than 45 degrees and peak strain by more than 30%. The amplitude discrepancy may be partly due to the limitations of the hardware for the selected channels, where the maximum range is nominally limited by a D41X X §D42 X 6X 00 microstrain range, as evident by peak amplitude clipping in Fig. 18(a).
T he test specimens consist of two rectangular 6.35 mm (1=4 ”) agedPT rolled homogeneous armor (RHA) steel plates welded together to form a square 72.4 mm (28.500 )D43X £ X D4X 7 X 2.4 mm (28.500 ) target. The target was then welded to a 12.7 mm (1/200 ) wide frame (Fig. 20). The targets were subjected to various blast loads until weld failure was obtained. Various non-destructive techniques were used to track
4. Plate blast monitoring (2D) TagedPThe measured experimental strain fields are used to calibrate and improve 3D finite element simulation methodologies that incorporate state of the art blast modeling techniques and material models that represent the base-metal, weld-metal and the critical heat affected zone (HAZ) (Fig. 19).
Fig. 19. Stress gradients from detailed dynamic FEM under 40% maximum load.
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Fig. 20. Mounted and marked FBG sensors placed radially from the center. Fig. 22. Time history for multiple rounds of discharge on the RHA steel plate.
Fig. 23. Frequency content for multiple rounds of discharge on the RHA steel plate.
Fig. 21. Testing schematic with welded plate specimen.
TagedPboth surface strains and weld crack growth. Dynamic plate deformations using a honeycomb core and incident overpressure was measured during each iteration. TagedPThe schematic of the test setup is provided in Fig. 21. The discharge explosive is similar to C4 material, which is a relatively stable, solid explosive with a consistency similar to modeling clay The plate is subjected to several rounds of C4, incrementally increasing from 400 g up to 1200 g. For this test a constant 800 g of C4 was detonated thrice at a stand-off distance of 1.2 m. The FBG Sensors are located at 5 cm from the center of the plate. 4.2. Blast plate results (2D) TagedPThe time history and frequency spectrum from the sensors in proximity are presented in Figs. 22D45X X and 23. Several observations are made from Fig. 22. Successful transient measurements from 200 to 3000 micro-strain were recorded. The initial blast wave impacted the plate at 6.5 ms, and the subsequent vibration effects were experienced at 16.5 ms. TagedPThe primary resonant frequency occurred at 6.1 kHz, which is consistent with previous measurements by high speed accelerometers (Fig. 23). Significant amount of inherent noise was apparent throughout the measurement interval. The noise is largely attributed
tTagedP o the outdoor temperature fluctuation, the length (500 m) of the optical wire to ensure sufficient stand-off from the detonation, and the receptor interface to the DAQ system. Additionally anti-aliasing filters are not built into the given Micronoptics si920 hardware, although primary resonant frequencies are less than 20 times the Nyquist frequency. Despite the unwanted noise levels, the feasibility of the FBG Sensor system for blast loading is demonstrated.
5. Conclusions TagedPThe independent experiments demonstrated the concept of an optical fiber Bragg grating (FBG)-based real-time instrumentation system for quantitative evaluation of the full-field dynamic response of ballistic impacted specimens. Several conclusions may be drawn from the results for future improvements of experimental measurements using FBG Sensors. Specifically, a need for multi-physic measurements such as high impact accelerometers and strain gages collocated at the FBG Sensor site. This would allow for improved baseline, and an accurate test setup should include the following, when using FBG Sensors on high strain rate measurements: 1) increase time increment of the pre-trigger to eliminate environmental or trigger-induced noise, 2) implement an optical or soft backing such as clay rather than honeycomb structure in measuring deformation under blast loading conditions to ensure no prestress on FBGs, and 3) rectify and monitor drifting of the hardware by cleansing of the FBG receptor and in-line calibration.
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TagedPFinally, the successful validation of the peak strain value of the instantaneous response during impact and the transient response after impact demonstrates the potential usage of FBG Sensors in future impact and blast loading environments. Particularly, future work exists in the development of composites, where FBG Sensors can be embedded into the design of the structure, especially for health monitoring. The most recent advances in FBG interrogation systems may provide improved insights in micro-vibration mechanics. Acknowledgments TagedPThe authors gratefully acknowledge the test facilities at the Aberdeen Testing Center and Mr. Vincent Urbanski for his expertise in deploying the impacting projectile. The comparable numerical blast simulations were performed by Dr. Rahul Gupta from the US Army Research Laboratory. References TagedP [1] Hamouda AMS, Hashmi MSJ. Testing of composite materials at high rates of strain: advances and challenges. J Mat Process Tech 1998;77(1-3):327–36. TagedP [2] Ramesh KT. High strain rate and impact experiments. In: Sharpe WN, editor. Springer handbook of experimental solid mechanics. Springer; 2008. Chapter 33. TagedP [3] Lo Y, Sirkis JS, Chang CC. Passive signal processing of in-line fiber etalon sensors for high strain-rate loading. J Lightwave Tech 1997;15(8):1578–86. TagedP [4] Gogulya MF, Dolgoborodov AY, Brazhnikov MA. Investigation of shock and detonation waves by optical pyrometry. Int J Imp Eng 1999;23(1):283–93. TagedP [5] Longana ML, Dulieu-Barton JM, Syngellakis S. Application of optical measurement techniques to high strain rate deformations in composite materials. In: Proc. 7th Asian conference on composite materials; 2010. p. 1–4. TagedP [6] Rosakis AJ. Application of coherent gradient sensing (cgs) to the investigation of dynamic fracture problems. Opt Lasers Eng 1993;19(1-3):3–41.
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