Geert Luyckx

Strain Measurements of Composite Laminates with Embedded Fibre Bragg Gratings: Criticism and Opportunities for Research

Embedded optical fibre sensors are considered for structural health monitoring purposes in numerous applications. In fibre reinforced plastics, embedded fibre Bragg gratings are found to be one of the most popular and reliable solutions for strain monitoring. Despite of their growing popularity, users should keep in mind their shortcomings, many of which are associated with the embedding process. This review paper starts with an overview of some of the technical issues to be considered when embedding fibre optics in fibrous composite materials. Next, a monitoring scheme is introduced which shows the different steps necessary to relate the output of an embedded FBG to the strain of the structure in which it is embedded. Each step of the process has already been addressed separately in literature without considering the complete cycle, from embedding of the sensor to the internal strain measurement of the structure. This review paper summarizes the work reported in literature and tries to fit it into the big picture of internal strain measurements with embedded fibre Bragg gratings. The last part of the paper focuses on temperature compensation methods which should not be ignored in terms of in-situ measurement of strains with fibre Bragg gratings. Throughout the paper criticism is given where appropriate, which should be regarded as opportunities for future research.

Microstructured Optical Fiber Sensors Embedded in a Laminate Composite for Smart Material Applications

Fiber Bragg gratings written in highly birefringent microstructured optical fiber with a dedicated design are embedded in a composite fiber-reinforced polymer. The Bragg peak wavelength shifts are measured under controlled axial and transversal strain and during thermal cycling of the composite sample. We obtain a sensitivity to transversal strain that exceeds values reported earlier in literature by one order of magnitude. Our results evidence the relevance of using microstructured optical fibers for structural integrity monitoring of composite material structures.

Multi-axial strain transfer from laminated CFRP composites to embedded Bragg sensor: I. Parametric study

Embedded optical fibre sensors are considered in numerous applications for structural health monitoring purposes. However, since the optical fibre and the host material in which it is embedded, will have different material properties, strain in both materials will not be equal when load is applied. Therefore, the multi-axial strain transfer from the host material to the embedded sensor (optical fibre) has to be considered in detail. In the first part of this paper the strain transfer will be determined using finite element modelling of a circular isotropic glass fibre embedded first in an isotropic host and second in an anisotropic composite material. The strain transfer or relation depends on the mechanical properties of the host material and the sensor (Youngs modulus and Poissons ratio), on the lay-up of the composite material (uni-directional lay-up/cross-ply lay-up) and the position of the sensor in a certain layer. In the second part of the paper the developed strain transfer model will be evaluated for one specific lay-up and sensor type.

Multi-axial strain transfer from laminated CFRP composites to embedded Bragg sensor: II. Experimental validation

Embedded optical fibre sensors are considered in numerous applications for structural health monitoring purposes. Since the optical fibre and the host material in which it is embedded have different material properties, the strain in both materials will not be equal when external load is applied. Therefore, the strain transfer from the host material to the embedded sensor (optical fibre) was studied in more detail in the first part of the paper. This second part presents an experimental evaluation of the response of uni-axial fibre Bragg grating sensors embedded in small cross-ply composite laminates subjected to out-of-plane transverse loading. This loading case induces high birefringence effects in the core of the optical fibre. Using the numerically determined strain transfer coefficients (Luyckx et al 2010 Smart. Mater. Struct. 19 105017) together with multi-axial strain formulations, the authors were able to measure with reasonable accuracy the total strain field inside a carbon fibre reinforced plastic specimen.

Dynamic Strain Measurements on Automotive and Aeronautic Composite Components by Means of Embedded Fiber Bragg Grating Sensors.

The measurement of the internal deformations occurring in real-life composite components is a very challenging task, especially for those components that are rather difficult to access. Optical fiber sensors can overcome such a problem, since they can be embedded in the composite materials and serve as in situ sensors. In this article, embedded optical fiber Bragg grating (FBG) sensors are used to analyze the vibration characteristics of two real-life composite components. The first component is a carbon fiber-reinforced polymer automotive control arm; the second is a glass fiber-reinforced polymer aeronautic hinge arm. The modal parameters of both components were estimated by processing the FBG signals with two interrogation techniques: the maximum detection and fast phase correlation algorithms were employed for the demodulation of the FBG signals; the Peak-Picking and PolyMax techniques were instead used for the parameter estimation. To validate the FBG outcomes, reference measurements were performed by means of a laser Doppler vibrometer. The analysis of the results showed that the FBG sensing capabilities were enhanced when the recently-introduced fast phase correlation algorithm was combined with the state-of-the-art PolyMax estimator curve fitting method. In this case, the FBGs provided the most accurate results, i.e., it was possible to fully characterize the vibration behavior of both composite components. When using more traditional interrogation algorithms (maximum detection) and modal parameter estimation techniques (Peak-Picking), some of the modes were not successfully identified.

Disbond monitoring in adhesive joints using shear stress optical fiber sensors

We present dedicated shear stress optical fiber sensors for in situ disbond monitoring of adhesive bonds. The shear stress sensitivity of these sensors is about 60 pm MPa−1, which corresponds to a shear strain sensing resolution of 50 μϵ. By integrating a combination of three such sensors in the adhesive bond line of a single lap joint, we can assess the internal shear stress distribution when the joint is tensile loaded. Disbonding of this joint was initiated by cyclic tensile loading, and the sensor responses were monitored during this process. Our results show that this sensing system can detect disbonds as small as 100 μm.

Towards micro-structured optical fiber sensors for transverse strain sensing in smart composite materials

We developed a highly birefringent micro-structured optical fiber that, in combination with a fiber Bragg grating sensor, allows measuring transverse strains in reinforced composites. The first generation of this dedicated fiber sensor featured a hydrostatic pressure sensitivity of −15 pm/MPa and yielded a transverse strain sensitivity of −0.16 pm/µε when embedded in a carbon fiber reinforced polymer. The second generation of this sensor has now been fabricated and hydrostatic pressure experiments and FEM simulations show that this generation returns a sensitivity of more than twice that of the first generation. FEM simulations additionally show an increased sensitivity when this sensor is embedded in a reinforced composite, achieving an unprecedented transverse strain sensitivity of 0.29 pm/µε. We explain how the optimized micro-structure yields this record-high sensitivity. In addition we demonstrate the selectivity of the bare fiber sensor, which remains insensitive to temperature changes or axial strain. This sensor can therefore play an important role in the domain of structural health monitoring.

Highly Sensitive Waveguide Bragg Grating Temperature Sensor Using Hybrid Polymers

A waveguide Bragg grating temperature sensor implemented using a hybrid inorganic-organic material (Ormocer) with a 25-times higher temperature sensitivity than a typical silica fiber is presented. The sensor consists of second order gratings (1010-nm pitch) in 5 μm × 5 μm waveguides fabricated on a planar substrate using the replication-based methods. The gratings were imprinted in the under-cladding layer, and the waveguide cores were patterned on top by capillary filling of microchannels, which were defined in a transparent and flexible mold. The somewhat larger, slightly multimode waveguides facilitate pigtailing with an optical fiber but lead to three reflection peaks corresponding to the different excited waveguide modes. The peak at the longest wavelength (Bragg wavelength at 1539 nm, corresponding to the fundamental mode) was tracked during temperature testing, and a sensitivity of -249 pm °C-1 was found.

Microstructured optical fiber Bragg grating as an internal three-dimensional strain sensor for composite laminates

In this article, we study the possibility to use a pair of specifically designed microstructured optical fiber Bragg gratings (MOFBGs) as a multi-component strain sensor when embedded within composite materials. The dependence on the orientation of the transverse sensitivity of the MOFBGs is exploited to build a sensing device able to measure the strain field along the three principal mechanical directions of a laminate composite. We developed an analytical and numerical model of such a sensor and benchmarked it with experiments performed on laminated composite coupons equipped with this sensor. We report on a theoretical strain resolution of about 5 μ in the transverse directions of the composite material, which is a six-fold improvement over results reported in literature.

Strain monitoring of FRP elements using an embedded fibre optic sensor

This paper presents a strain monitoring approach for following up FRP elements (in this case a [90°] CFRP laminate) using an embedded fibre optic sensor. The sensor exists of two fibre Bragg gratings (FBGs) written in a polarization maintaining fibre (PMF). First, the strain response of the non-embedded sensor is determined which makes it possible to relate the different bragg peak shifts with the induced strain field in the core of the optical fibre. Secondly, a transfer coefficient matrix is presented and calculated using finite element simulations which relates the measured strain field of the sensor with the adjacent one existing in the structure as if no sensor would be present.