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 an integrated formulation for the calculation of the spectral response of a fiber Bragg grating sensor embedded in a host material system, as a function of the loading applied to the host structure. In particular, the calculation of the transverse strain sensitivity of a fiber Bragg grating sensor through the calculation of the change in effective index (or indices) of refraction of the fiber cross-section due to the applied load is presented in detail. For the calculation of the fiber propagation constants, a two-step finite element formulation is used moeling the optical, geometric and material properties of the cross-section. Once the propagation constants and principle optical axes are known along the fiber, a modified transfer matrix method is applied to calculate the spectral response of the FBG. It is shown that the inclusion of the change in index of refraction throughout the cross-section yields close agreement with previous methods. However the current method provides the potential to evaluate the effects of high strain gradients across the optical fiber core for some loading applications.
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.
Optical fiber Bragg gratings are unique among strain sensors due to their potential to measure strain distributions over gage lengths of a few centimeters with a spatial resolution of a few nanometers. The application of these sensors requires modeling of the grating output spectrum due to an applied axial strain profile. The most computationally efficient method for this calculation is the transfer-matrix model (T-matrix) derived originally for chirped gratings. This approach models a grating with varying properties as a series of smaller grating segments with constant parameters. Huang and colleagues first applied the T-matrix approach to model the inverse problem of a grating subjected to non-uniform strain by varying the period of each segment. The current work shows that, in the presence of strain gradients, this approach does not converge to the numerical solution of the grating coupled mode equations in the limit of a large number of segments. A modified T-matrix representation is then derived for the sensor problem and is shown to approach the coupled mode solutions for a large number of segments. Finally, the application of the modified T-matrix model to Bragg grating sensors is outlined, including inversion of the grating spectrum via a genetic
algorithm.
The goal of a structural health monitoring system is to detect, locate, and identify damages in a structure during its lifetime. The concept of structural health monitoring is particularly important for fiber reinforced composites due to the complexity of the possible failure mechanisms. The goal of this work is to simulate the response of optical fiber Bragg grating sensors to multi-component loading for their implementation in structural health monitoring algorithms for composites. A simulation method is presented to determine the effects
of axial, bending and shear loading on an embedded optical fiber Bragg grating sensor. The effect of fiber bending on the Bragg grating sensor is experimentally verified by embedding the sensor in a solid cone, clamped at the base and subjected to a point load at the apex. Next, a numerically efficient method to calculate the
response of sensors embedded in a unidirectional composite is developed using both finite element analysis and optimal shear-lag theory and taking into account the above effects. The limitations of the optimal shear-lag theory are derived through comparison with the
finite element results. The application of this method is demonstrated through a numerical example, simulating the response of sensors embedded in one fiber layer to a transverse crack.
Fiber Bragg gratings have demonstrated great potential as sensing elements for the structural health monitoring (SHM) of composites. However, the complexity of stress and strain fields due to one or multiple damages makes the health-monitoring problem extremely difficult and sensitive to errors. Current approaches consider only the Bragg grating averaged axial strain as input to the health-monitoring algorithm. This paper presents a strategy to improve SHM information. In previous experiments, Bragg grating sensors embedded to measure the strain field near a crack demonstrated significant loss in their reflected spectrum at high strain. This loss was not a constant but a function of wavelength and is shown to be due to bending. The effect of shear loading is shown to be negligible. In order to demonstrate the technique an axially loaded unidirectional composite with a transverse crack is considered. The axial stress distribution is calculated using a modified shear lag theory suitable for multiple fibers and cracks. The bending stress at the crack tip is calculated using linear elastic fracture mechanics and a FEM model. From the calculated stress field a grating response simulation is performed taking into account the local bending of the fiber.
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