We demonstrate the measurement of and applications for reflected spectral signatures obtained from FBG sen- sors in dynamic environments. Three uses of the spectral distortion measurements for monitoring of airframe structures are presented: the measurement of the dynamic response of a laminated plate to an impact event; the measurement of damage induced spectral distortion in a thin plate during vibration loading; and the measurement of the change in dynamic response of an adhesively bonded joint with the progression of fatigue damage.
In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor embedded at the adhesive layer of a single composite lap joint subjected to harmonic excitation after fatigue loading. After a fully-reversed cyclic fatigue loading is applied to the composite lap joint, the full spectral response of the sensor is interrogated in reflection at 100 kHz during two states: with and without an added harmonic excitation. The dynamic response of the FBG sensor indicates strong nonlinearities as damage progresses. The short-time Fourier transform (STFT) is computed for the extracted peak wavelength information to reveal time-dependent frequencies and amplitudes of the dynamic FBG sensor response. Pulse-phase thermography indicates a progression in defect size at the adhesive layer that strongly suggests non-uniform loading of the FBG sensor.
We present a fiber Bragg grating (FBG) interrogation method using a micro-controller board and optical filter that
achieves high strain sensitivity and high dynamic range. This interrogation method allows high sensitivity detection of
ultrasonic waves superimposed on low-frequency (on the order of 100Hz) vibrations of arbitrary magnitude. One
possible application is in-situ structural health monitoring of windmill blades exposed to strong winds by using FBG sensors for detection of ultrasonic waves. Interrogator operation is based on the edge filtering method using a broadband source, fiber Fabry-Perot filter and a micro-controller board which acts as a control feedback loop that locks the filter wavelength to the mid-reflection point on the FBG spectrum. Wavelength locking method allows high sensitivity for edge filtering of high-frequency waves, while the feedback signal is the measurement of low-frequency vibration with high dynamic range. The concept of the interrogator operation and different implementations are described and discussed with experimental results.
We used high-speed full-spectrum interrogation of a Fiber Bragg Grating (FBG) sensor to measure dynamic strain in
different sensor packages in real-time. In this effort we performed solenoid impact tests on a variety of sensor mounting
structures made with FR4, steel, and carbon fiber composite materials. Full spectrum FBG interrogation at 40 kHz
repetition rate was the key that allowed us to measure and compare dynamic strain in the structures, with measurement
resolution on the sub-millisecond scale. With this interrogation method we were able to measure the full character of the
dynamic strain including the strain non-uniformity and distribution manifested in peak-splitting and spectrum
broadening. Results showed that the FR4 board with soft epoxy responded with a maximum dynamic strain on the order
of 3000 micro-strain. Adding hard materials such as steel and graphite fiber composite reduced the strain about 7 times.
However, the FR4 board mounted in a free-floating configuration using hard epoxy reduced the maximum strain to a
value below the noise threshold of the full spectrum interrogation configuration. Here we proposed using edge detection
method of FBG interrogation due to its increased strain sensitivity which enabled us to further analyze the critical results
obtained by full spectrum interrogation. We also proposed using edge detection to measure sensor strain in real time for
the purpose of filtering out the strain noise from useful signal. We will use the results and data obtained with both
methods to analyze and enhance the performance of our electric field sensors in environments of high static and dynamic
strain.
In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor subjected to a non-uniform static
strain state and simultaneously exposed to vibration loading. The full spectral response of the sensor is interrogated
in reflection at 100 kHz during two loading cases: with and without an added vibration load spectrum. The static
tensile loading is increased between each test, in order to increase the magnitude of the non-uniform strain field
applied to the FBG sensor. During steady-state vibration, the behavior of the spectral shape of the FBG reflection
varies depending on the extent of non-uniform strain. With high-speed full spectral interrogation, it is potentially
possible to separate this vibration-induced spectral change from spectral distortions due to non-uniform strain. Such
spectral distortion contains valuable information on the static damage state of the surrounding host material.
This paper presents a high repetition rate fiber Bragg grating (FBG) interrogation system that is able to capture the entire
reflection spectrum at a rate of up to 300 kHz. The system uses a high speed MEMS based tunable optical filter that is
driven with a sinusoidal voltage. The time varying FBG reflection spectrum in transmitted through the tunable filter.
The time varying signal is then mapped into time varying reflection spectra. This interrogation system is used during
two dynamic strain tests, in which the reflection spectra are measured at a repetition rate of 100 kHz. The first test is the
impact of a woven carbon composite and the second test is on an electromagnetic railgun.
In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor subjected to a non-uniform static
strain state and simultaneously exposed to vibration loading. The full spectral response of the sensor is interrogated in
reflection at 100 kHz during two loading cases: with and without an added vibration load spectrum. The static tensile
loading is increased between each test, in order to increase the magnitude of the non-uniform strain field applied to the
FBG sensor. The spectral distortion due to non-uniform strain is observed to change once the sensor is exposed to a non-transient
150 Hz vibration spectrum. With high-speed full spectral interrogation, it is potentially possible to separate this
vibration-induced spectral change from spectral distortions due to non-uniform strain. Such spectral distortion contains
valuable information on the static damage state of the surrounding host material.
Various densities of optical fibers are embedded into a total of eighty woven, graphite fiber-epoxy composite laminates,
for which the response to low velocity impacts are evaluated. The goal of this work is to determine the role of hostsensor
interaction on the lifetime of the host material system. The woven composites are subjected to multiple impacts
at 14.5 J until perforation of the specimen. We obtain the energy dissipated by the laminate and the maximum contact
force between the impactor laminate for each strike. From these experimental data we calculate the statistical distribution
of the total energy dissipated at failure as a function of embedded optical fiber density. The total dissipated energy, a
measure of the specimen lifetime, decreased with increasing embedded optical fiber density, however remained constant
after a threshold density was reached. The total maximum contact force per specimen, a measure of the specimen stiffness, continued to decrease with the number of embedded optical fibers.
This paper presents a means for the high repetition rate interrogation of fiber Bragg gratings (FBG's). The new system
highlights a method that allows a tradeoff between the full spectrum capture rate and the wavelength range and/or the
spectral resolution of the technique. Rapid capture of the entire reflection spectrum at high interrogation rates shows
important features that are missed when using methods that merely track changes in the peak location of the spectrum.
The essential feature of the new system is that it incorporates a MEMs tunable filter driven by a variable frequency openloop
sinusoidal source. The paper demonstrates the new system on a laminated composite system under impact loading.
In this study we evaluate the measurements of fiber Bragg sensor spectra from a sensor embedded in a composite
laminate subjected to multiple low velocity impacts. The full-spectral response of the sensor is interrogated in reflection
at 100 kHz during the impact events. The measurement of the time dependent spectra features are compared with
previous results obtained at a 534 Hz interrogation rate. With the increased interrogation rate, we can observe a smooth
transition in the full-spectra response of the sensor between strikes and the presence of peak-splitting due to transverse
compression from the beginning of the laminate lifetime. Finally, at the 100 kHz acquisition rate, it is possible to
determine the maximum wavelength and accurately determine the duration of the impact event for all of the strikes.
This paper presents the full-spectral measurement of fiber Bragg grating sensor responses during impact testing of
composite laminates. The sensors are embedded in carbon fiber/epoxy laminates which are subjected to multiple low
velocity impacts until perforation of the laminate occurs. Applying a recently developed high-speed interrogator, the
Bragg grating sensor interrogation is demonstrated at 534 Hz over a 14.9 nm bandwidth. The measurement of the
transient response of the grating sensors during impact reveals unique spectral signatures that could not be detected
through peak-wavelength monitoring or post-impact full-spectral scanning of the sensors, including local relaxation of
the laminate.
KEYWORDS: Fiber Bragg gratings, Sensors, Composites, Analog electronics, Optical filters, Field programmable gate arrays, Digital electronics, Data storage, Data conversion, Data communications
This paper presents a new means for collecting fiber Bragg grating (FBG) data
during drop tower measurements used to assess damage to composite structures. The
high repetition-rate collection process reveals transient features that cannot be
resolved in quasi-static measurements. The experiments made at a repetition rate of
about 500 Hz show that the detected FBG spectrum broadens for a short period of
time and relaxes quickly to a narrower static state. Furthermore, this relaxation time
increases dramatically as the strike count increases. The information gained by such
measurements will enhance the ability to characterize and distinguish failure modes
and predict remaining lifetime in composite laminate structures.
This paper presents experimental measurements of the response of woven composite laminates to multiple low-velocity
impacts. Damage initiation and progression occur at multiple physical and temporal scales in heterogeneous materials,
including fiber breakage, matrix cracking, delamination and matrix relaxation. The sensor/interrogators were therefore
chosen specifically to provide insight into the order and progression of different failure modes. Measurements of the
contact force between the impactor and composite are measured throughout impact. Additionally, the dissipated energy
per impact event is also calculated from the impactor velocity. Surface mounted and embedded fiber Bragg grating
sensors are used for the measurement of the laminate response. Peak wavelength measurements are performed during
impact at 1 kHz, while full-spectral scanning is performed at 5 Hz during relaxation period of the laminate immediately
after impact and quasi-statically to measure post-impact residual strain states within the laminate. The results highlight
the depth of information embedded within the FBG full-spectral data sensors, as well as the added insight to be gained
from combined global-local measurements.
In this study, the development and optimization of embedded fiber Bragg grating (FBG) sensor networks within
composite materials was investigated. Various densities of optical fibers were embedded within composite laminates,
and low-velocity impact damage responses were evaluated to determine the effects on the mechanical behavior of the
laminates. The woven composites were subjected to multiple strikes at 2 m/s until perforation occurred, and the
impactor position and acceleration were monitored throughout each event. From these measurements, we obtained
dissipated energies and contact forces for specimens with and without embedded optical fibers. Embedded fibers were
interrogated with light to determine the degree to which light could pass through them for each density and arrangement.
Cross sectional optical micrographs of the specimens were used to determine the local effects of the embedded fibers on
neighboring fibers and the surrounding matrix material, both before and after impact events. Currently FBG sensors are
being calibrated and prepared for embedment in specifically chosen configurations within the composite. They will be
serially multiplexed together to create a single fiber sensing network capable of monitoring damage over a large area.
Real time strain information will be gathered as future embedded laminates are subjected to impact events, and the
resulting data will be used to better monitor and predict damage in the composite system.
In this study, measurements from low-impact velocity experiments and surface mounted optical fiber Bragg grating (FBG) sensors were used to obtain detailed information pertaining to damage progression in two-dimensional laminate woven composites. The woven composites were subjected to multiple strikes at 2m/s until perforation occurred, and the impactor position and acceleration were monitored throughout each event. From these measurements, we obtained dissipated energies and contact forces. The FBG sensors were surface mounted at different critical locations near penetration-induced damaged regions. These FBG sensors were used to obtain initial residual strains and axial and transverse strains that correspond to matrix cracking and delamination. The transmission and the reflection spectra were continuously monitored throughout the loading cycles. They were used, in combination with the peak contact forces, to delineate repeatable sensor responses corresponding to material failure. From the FBG spectra, fiber and matrix damage were separated by an analysis based on the behavior of individual Bragg peaks as a function of evolving and repeated impact loads. This provided an independent feedback on the integrity of the Bragg gratings. Thus, potential sources of error such as sensor debonding were eliminated from the strain data throughout the measurements. A comparison by number of impact strikes and dissipated energies corresponding to material perforation indicates that these measurements can provide accurate failure strains.
In this study, measurements from low-impact velocity experiments and embedded and surface mounted optical fiber Bragg grating (FBG) sensors were used to obtain detailed information pertaining to damage progression in two-dimensional laminate woven composites. The woven composites were subjected to multiple strikes at 2m/s until perforation occurred, and the impactor position and acceleration were monitored throughout each event. From these measurements, we obtained dissipated energies and contact forces. The FBG sensors were embedded and surface mounted at different critical locations near penetration-induced damaged regions. These FBG sensors were used to obtain initial residual strains and axial and transverse strains that correspond to matrix cracking and delamination. The transmission and the reflection spectra were continuously monitored throughout the loading cycles. They were used, in combination with the peak contact forces, to delineate repeatable sensor responses corresponding to material failure. From the FBG spectra, fiber and matrix damage were separated by an analysis based on signal intensity, the presence of cladding modes, and the behavior of individual Bragg peaks as a function of evolving and repeated impact loads. This provided an independent feedback on the integrity of the Bragg gratings. A comparison by number of strikes and dissipated energies corresponding to material perforation indicates that embedding these sensors did not affect the integrity of the woven systems and that these measurements can provide accurate failure strains.
This article presents the use of Bragg reflection and cladding mode measurements to independently measure axial strain and the integrity of a Bragg grating sensor. While the Bragg reflection is known to be sensitive to applied strain, the cladding modes are shown to be sensitive to expected damage within the sensor such as microcracking and debonding from the host structure. This phenomenon allows the intelligent self-testing of the Bragg grating sensor without additional instrumentation and permits the separate identification of sensor failure from the failure of the host structure.
The growth of cladding modes during degradation of a Bragg grating is experimentally demonstrated in controlled tension tests with different fiber-host interface conditions.
The long-term goal of this project is the development of embedded, optimally distributed, multi-scale sensing methodologies that can be integrated into material systems for failure identification in structural systems. The coupling of sensor data fusion with a three-dimensional predictive framework will provide insight and understanding of events that are difficult, if not impossible, in
any experimental study, such as subsurface damage and crack nucleation in structural systems. The current work presents an
experimental study of the survivability and degradation behavior of an optical fiber Bragg grating sensor, surface mounted on a woven fiber composite material system during multiple low velocity impacts. The results reveal that as sensor degradation occurs, additional coupling phenomena other than Bragg reflection are observed in the grating sensor. From these additional modes, information on the sensor/host bond and fiber degradation is obtained.
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