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This paper presents a genetic algorithm for the interrogation of optical fibre Bragg grating strain sensors. The method encodes the axial strain distribution along the Bragg grating, here represented through the local period distribution, into a gene. To facilitate rapid calculation of the grating reflected intensity spectrum, the transfer-matrix approach is applied. The genetic algorithm inversion method presented requires only intensity information from the sensor and reconstructs non-linear and discontinuous distributions well, including regions with significant gradients. The development of this algorithm will permit the use of Bragg grating sensors for structural damage identification, allowing them to be located in regions where strong strain non-uniformities occur.

The state of the local fiber–matrix interface highly influences the propagation of cracks in fiber-reinforced composites and thus the stress distribution in any bridging fiber. This paper demonstrates that by embedding a long optical fiber Bragg grating into a reinforcing fiber and using an established model of the grating response to non-uniform stress distributions, one can determine key parameters of a crack bridging model. The grating extending into the epoxy on each side of the crack is subject to a strain function as a result of all micro-mechanical phenomena acting along the fiber. Furthermore, this technique does not require that one knows a priori the exact location of the crack. Two types of central crack specimens with an artificial crack were fabricated and tested, one with a strong interface and one with a weaker interface resulting in frictional sliding. The results demonstrate that this technique is efficient for the measurement of the bridging forces through validation by previous measurements using short Bragg gratings and the deduction of interface parameters. Analysis also shows that the sensitivity of the Bragg grating sensor to the bridging force is sufficient, even for the more realistic case of an initially zero-width crack e.g. grown by fatigue.

The main objective of this work is to investigate the bridging tractions in a model composite using optical fiber Bragg grating (FBG) sensors written into selected reinforcing fibers. Simultaneously, the crack opening displacement (COD) is measured using a speckle interferometry technique. The measurements are useful in the verification of the relation between the COD and bridging tractions established with the use of the weight function method. Center crack specimens made of epoxy and reinforced with one layer of optical fibers are prepared and tested under remote tension parallel to the fibers. Bragg gratings of 0.17 to 0.38 mm in length are introduced in selected fibers for direct, non invasive, local measurements of axial strains in these fibers. A controlled central crack, bridged by intact fibers, is introduced by a laser technique such that the FBGs are located between the crack faces. The results on the forces obtained from the FBGs and the COD-weight function method show good agreement. The experimental results also compare very well with 3-dimensional numerical simulations of the actual specimen geometry and loading configuration.

This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e., the strong nonuniformity of the strain field), the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth is not valid. Compact tension specimens including a controlled notch shape are fabricated, and optical fiber Bragg gratings with different gage lengths are embedded near the notch tip. The form of the spectra in transmission varies between gages that are at different distances from the notch tip under given loading conditions. This variation is shown to be due to the difference in the distribution of strain along the gage length. By using the strain field measured using electronic speckle pattern interferometry on the specimen surface and a discretized model of the grating, the spectra in transmission are then calculated analytically. For a known strain distribution, it is then shown that one can determine the magnitude of the applied force on the specimen. Thus, by considering the nonuniformity of the strain field, the optical fiber Bragg gage functions well as an embedded strain gage near the stress concentration.

Resin-based laboratory dental composites for prosthetic restorations have been developed in the past years as a cost-effective alternative to conventional porcelain-fused-to-metal or full ceramic restorations. The fracture toughness at different stress states (K(Ic), K(IIc), and mixed-modes K(I), K(II) ) was assessed for three laboratory dental composite resins used for prosthetic restorations that were aged up to 12 months in a food simulating fluid (10% ethanol) at 37 degrees C. The materials were mainly di- methacrylate based resins reinforced with submicron glass filler particles. The Brazilian disk test was used on precracked chevron-notched specimens, and different stress states were obtained by angulating the precracked chevron notch relative to the diametral compressive loading direction. The stress intensity factors were calculated using Atkinson et al.'s relation. For all three materials, mode I fracture toughness values ranged between 0.48-0.64 MPa. m(0.5) and mode II values ranged between 0.93-1.2 MPa. m(0.5). Overall, aging time and storage media had little effect on toughness. Considering the inherently low toughness of these restorative materials, their use should be limited to low stress masticatory areas.

‘Edge permeability’ between habitat patches in a mosaic landscape of mixed intense agriculture and semi-natural areas was investigated with directional trap devices along field borders and in a 300-m long transect through crap fields, pasture, wetland and a dry meadow. Almost all identified arthropod species performaned population exchanges over the field borders. Species abundances clearly depend on habitat quality rather than area or distance to related habitat islands. Most insect species collected in sticky traps over crop fields were never or rarely encountered in the vegetation layer. Surface-dwelling flightless species showed gradual transitions over the field borders. ‘Hard edge’-species tended to be specialists for undisturbed perennial habitats, while ‘soft-edge’-species with a diffuse distribution were mainly associated with annual crops. It is concluded that in cultivated areas a mosaic landscape of small-sized crop fields and semi-natural habitats maximizes arthropod diversity and decreases the probability for overall extinction even of rare species.

Damage identification is an important component for accurate lifetime predictions of any structure. In the case of a composite structure, however, damage can occur at several material scales: it can vary from micro damage, like fiber debonding or micro-cracking, to global damage such as buckling or delamination. These different material scales make damage identification difficult with a single type of sensing device. A single embedded optical fiber, causing little perturbation to the surrounding host structure, can multiplex hundreds of sensors, and furthermore, sensors measuring at different length scales. For example, short Bragg gratings can measure strain at given locations; long Bragg gratings can measure strain gradients; interferometric techniques can measure integrated strain along a given fiber length. The use of multi-scale measurements has been shown by the authors to improve the precision of damage identification. Still the treatment and fusion of these data is a non-trivial problem. This work presents a back propagation Neural Network algorithm used to fuse simulated multi-scale sensor data in order to identify damage. An analytical model of an isotropic plate subjected to a known load and specific forms of damage is used to train the network. The input data are: localized strain, localized strain gradient, and integrated strain measurement along a regularly spaced sensor network. This method is tested against a randomly generated set of damages. The combined use of multi-scale measurements and Neural Network analysis shows a great potential in damage identification for composite structures.

Optical fiber Bragg gratings are unique among embedded strain sensors due to their potential to measure strain distributions with a spatial resolution of a few nanometers over gage lengths of a few centimeters. This article presents a genetic algorithm for the interrogation of optical fiber Bragg grating strain sensors. The method calculates the period distribution along the Bragg grating which can then be directly related to the axial strain distribution. The period distribution is determined from the output intensity spectrum of the grating via a T-matrix approach. The genetic algorithm inversion method presented requires only intensity information and reconstructs non-linear and discontinuous distributions well, including regions with significant gradients. The method is demonstrated through example reconstructions of Bragg grating sensor simulated data. The development of this algorithm will permit the use of Bragg grating sensors for damage identification in regions close to localized damages where strong strain non-linearities occur.Copyright © 2003 by ASME

Multi-scale measurements, i.e. measurements of strain, strain gradient and integrated strain data, throughout a structural volume have demonstrated a great potential for improved damage identification. However, the large number of data and their different forms make fusion of the data difficult. To overcome this problem, a neural network data fusion approach is proposed. A simulation of damage identification in an isotropic cracked plate is presented. The crack position, angle and crack length are used as test parameters to be determined. A back-propagation neural network is trained to reproduce the crack angle and length as a function of all sensor responses. The improvement gained by using both multi-scale sensing and neural network data fusion for this specific case is significant. Testing of the sensitivity of the method to measurement errors or missing data demonstrated the robustness of the neural network to errors.Copyright © 2003 by ASME

Fiber reinforced composites offer increased resistance to fracture as compared to isotropic materials. In addition, they have demonstrated great potential to support embedded sensor systems. However, to develop a truly reliable, embedded sensor for composites, the failure modes of such materials, including the influence of the embedded fiber sensor, must be known. Crack bridging by intact fibers is considered to be one of the most efficient mechanisms to slow down transverse crack propagation in a fiber reinforced composite. This paper presents non-invasive, direct measurements of bridging fiber stresses in a model epoxy/glass composite, using long gage length optical fiber Bragg gratings. Several central crack specimens, containing artificially bridged cracks, were fabricated and tested. The Bragg grating gage length of 12 mm permitted measurement of the force distribution in the reinforcing fiber extending from the crack surface to the far field region. A T-matrix simulation was used to model the grating response. Results from specimens involving both a strong and mixed interface are presented. The measured strain distribution in the bridging fibers compared well with previous analytical models. Discussion of the application of these results to structurally embedded sensors for damage detection is also presented.

This paper presents an efficient technique to uniquely identify damage through the creation of a multi-scale map of material deformations and behavior within a representative volume element (RVE) of the host structure. Optimally distributed, embedded fiber optic sensors provide strain, strain gradient, and integrated strain fields throughout the RVE. As a demonstration, an isotropic, homogeneous RVE is modeled instrumented with an evenly spaced grid of sensing elements. The multi-scale damage identification technique and an equivalent single-scale method are evaluated on the basis of damage detection and identification. A large number of induced, stochastic damage cases are analyzed, generated by introducing a crack defined by three random variables: center location, length, and orientation angle. The multi-scale sensing capability is shown to provide a higher quality strain map of the RVE from the distributed sensors, resulting in significantly improved damage identification.Copyright © 2002 by ASME

This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e. the strong non-uniformity of the strain field) the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth may not be valid. Compact tension specimens including a controlled notch shape are fabricated with embedded optical fiber Bragg gratings at identical locations but with different gauge lengths. The spectra in transmission varies between such specimens for given loading conditions. This variation is shown to be due to the difference in gauge length. By using the strain field measured on the specimen surface with electronic speckle pattern interferometry and a discretized model of the grating, the spectra in transmission are then verified analytically. Thus, by considering the non-uniformity of the strain field, the optical fiber Bragg gauge functions well as an embedded strain gauge near the stress concentration. Due to the distributed nature of the measurements within a specified gauge length, the optical fiber Bragg gauge has a large potential to measure debonding in fiber reinforced composites.