Michel LeBlanc

Bragg intragrating structural sensing

When a fiber-optic intracore Bragg grating is subject to an appreciable strain gradient, its reflective spectrum will not only be shifted but also be distorted because of the chirp of the grating. We employed the J-matrix formalism to calculate the influence of different strain gradients on the reflective spectra of Bragg gratings and have undertaken experiments to test these calculations. The results of these experiments have confirmed that intracore Bragg gratings can be used to evaluate strain gradients and can be thought of as quasi-distributed strain sensors. This adds a new dimension to structural sensing, permitting measurements in any situation where strain gradients exist. It also provides a warning of any sensor/host debonding.

Distributed strain measurement based on a fiber Bragg grating and its reflection spectrum analysis

A method of extracting the strain profile along a fiber Bragg grating from the intensity reflection spectrum is described. The procedure is based on a filter synthesis theory that relates the aperiodicity of a grating with its reflection spectrum. To illustrate the approach, we measured the strain profile near a hole in a plate and obtained a strain resolution of 80 micro. The spatial resolution depends on the strain gradient; i.e., the higher the gradient, the better the resolution. A resolution of 0.8 mm was achieved for a 5-mm grating with a gradient of 250 micro/mm.

Fiber optic sensors for smart structures

Abstract An overview is presented of our research towards the development of structurally integrated fiber optic sensors for Smart Structures. This includes the development of the first full-scale fiber optic damage assessment test system in the form of a composite aircraft leading edge and the fabrication, characterization and evaluation of the first fiber optic strain rosette. This optical strain rosette was shown to be capable of mapping the strain tensor from within composite materials.

Impact damage assessment in composite materials with embedded fibre-optic sensors

Abstract A review of the application of embedded optical fibres as damage detection sensors based on fracture of the sensor is presented. Impact damage and the mechanisms of its formation are surveyed. The role of critical matrix cracking occurring under tension and shear is shown to be fundamental to the mechanics of embedded damage sensors. The current developments toward practical sensors are also presented. System issues for the practical use of this approach are discussed. The results of the development of a prototype Fibre Optic Damage Assessment System for an aircraft wing leading edge are presented. The literature on the effect of embedded sensors on the material properties is also surveyed.

Arbitrary strain transfer from a host to an embedded fiber-optic sensor

The problem of an arbitrarily varying axial strain fields transfer from material host to embedded fiber-optic sensor is studied. A derivation is proposed by which the axial in-fiber strain field is predicted given an arbitrary, axially varying, strain field at some distance in the material host. The spatial fields are considered in terms of spatial wavelength content and a spatial wavelength-dependent transfer function is outlined, to assist in the solution of the transfer problem. Results from the proposed approach are plotted against conventional finite-element results for the same physical problem.

Bragg grating fiber optic sensing for bridges and other structures

We have demonstrated that fiber optic intracore Bragg grating sensors are able to measure the strain relief experienced over an extended period of time by both steel and carbon composite tendons within the concrete deck support girders of a recently constructed two span highway bridge. This is the first bridge in the world to test the prospects of using carbon fiber composite tendons to replace steel tendons. This unique set of measurements was accomplished with an array of 15 Bragg grating fiber optic sensors that were embedded within the precast concrete girders during their construction. We have also demonstrated that these same sensors can measure the change in the internal strain within the girders associated with both static and dynamic loading of the bridge with a truck. We are now studying the ability of Bragg grating fiber optic sensors to measure strong strain gradients and thereby provide a warning of debonding of any Bragg grating sensor from its host structure...one of the most important failure modes for any fiber optic strain sensor.

Fiber optic intra-grating distributed strain sensor

Fiber-optic intracore Bragg gratings have been widely used as strain sensors. In most applications, the strain along a fiber grating is uniform and the strain is measured by the shift of the reflective wavelength of the grating. If the grating is under a gradient strain field, however, its reflective spectrum will be not only shifted but also distorted due to the chirp of grating periods. The shape of this spectrum depends on the strain distribution. Therefore the location and the shape of the reflective spectrum together can provide information on the strain distribution over a small region (sub-centimeter). We have calculated the reflective spectra of gratings by using the T-matrix formalism which allows us to simulate gratings with any distribution of the refractive-index modulation depth and the chirp of the period length. The calculation results show a strong dependence of the reflective spectrum of a grating on the distributions of its refractive-index modulation depth and period length. Experimental results have verified the T-matrix calculations. These results indicate that fiber-optic gratings can be used as quasi-distributed strain sensors to detect strain gradients that is very useful for monitoring and studying the presence and the evolutionary process of fiber sensor malfunction or material fringe/damage.

Micromechanical considerations for embedded single-ended sensors

Single-ended optical fiber sensors, for which the same fiber length serves as lead-in and lead- out paths of the optical signal, offer many practical advantages over dual-ended sensors (particularly in their installation and interconnection). However, and this is especially relevant to optical fiber strain gauges, high interfacial stresses can develop at the tip of the sensor when the host material is under load. This can result in debonding of the sensor and, consequently, the loss of the sensors function. The tip region can also be the site of initial crack formation in the host material. Using a shear-lag model and FEM analysis, we show how the use of coatings with appropriate properties can reduce or eliminate these problems. This analysis brings to attention important properties of the glass/coating and coating/resin interfaces that need to be determined from experiments. The conclusions from this analysis are discussed in the context of the broader issue of sensor/host interaction and sensor life-performance.

Study of interfacial interaction of an optical fibre embedded in a host material by in situ measurement of fibre end displacement- : Part 2. Experiments

This paper describes the experimental approach and the experimental results of a new technique for the characterization of host-to-sensor interaction based on the measurement of the embedded fibre end face displacement. The method consists of monitoring the reflected signal from the Fabry–Perot cavity formed at the end of an embedded fibre while the test sample is under increasing axial load. The curve of displacement versus applied strain in the host is analysed to describe the quality of the strain transfer and the condition of the interfacial region. Results on a test configuration consisting of an optical fibre surrounded by epoxy in an aluminium matrix are presented. In this configuration, the epoxy acts as the fibre coating and the aluminium is the host material. The results show that for this configuration the mode of failure of the interfacial region is by plastic deformation of the epoxy material. The results also make evident the visco-elastic and visco-plastic response of the epoxy.

Fiber optic Bragg intra-grating strain gradient sensing

By looking at the full reflection spectrum of a Bragg grating strain sensor, one cal tell when strain gradients are presented along the sensing length and estimate their magnitude. Furthermore, if certain conditions of monotonicity of the strain profile along the sensor are met, complex strain profile shapes can be interpreted from the gratings spectrum. We show how this technique can be applied to the study of the strain transfer at the tip of an embedded fiber. A fiber embedded in epoxy was loaded up to 2.5% strain, which induced fiber end-zone damage. The strain profile along the fiber, estimated from the reflection spectrum of the grating, gives details about the failure process which could not be obtained by any other known method.