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.

https://biblio.ugent.be/publication/1152717/file/1265945.pdf

Strain measurements of composite laminates with embedded fibre Bragg gratings : criticism and opportunities for research

Small-diameter fiber Bragg grating (FBG) sensors were applied for the monitoring of delamination induced by low-velocity impact. The FBG sensors were embedded into carbon fiber reinforced plastic (CFRP) laminates [04/904/04]. Using a drop-weight impact tester, an impact loading was applied to the laminates at four impact energy levels. After the impact tests, the internal damages including delaminations were observed by ultrasonic C-scan, and the reflection spectra from the embedded FBG sensors were measured. The form of the spectrum changed sensitively depending on the delamination size. Furthermore, the spectra were calculated theoretically for confirmation of the measured spectra. Since the change in the measured spectrum was consistent with that in the calculated spectrum, the relationship between the delamination size and the form of the spectrum could be clarified. From the results, the present method using small-diameter FBG sensors was found to be effective for the monitoring of the delamination.

Delamination monitoring of laminated composites subjected to low-velocity impact using small-diameter FBG sensors

Chirped fiber Bragg grating (FBG) sensors were applied for the identification of crack locations in carbon fiber reinforced plastic (CFRP) laminates since the reflection spectrum from a chirped FBG was expressed as a function of the position along the grating. For the crack identification in the 90° ply of a cross-ply laminate, a chirped FBG sensor, whose grating length and spectrum width were 50 mm and about 5 nm respectively, was embedded into the 0° ply of the CFRP cross-ply laminate, and the reflection spectra were measured after tensile loadings were applied to the laminate. As a result, the spectra had dips corresponding to locations of transverse cracks. This change in the form of the spectrum was also confirmed by a theoretical calculation. Hence, the crack locations could be identified from the form of the spectrum. Furthermore, it was demonstrated that a small-diameter chirped FBG sensor could also be applied for the identification of the crack locations.

Application of chirped fiber Bragg grating sensors for identification of crack locations in composites

When FBG sensors are embedded in CFRP laminates, the reflection spectrum from the FBG sensors splits into two peaks because of the non-axisymmetric thermal residual stress. This deformation of the spectrum will lead to misreading in strain measurements or crack detection in the laminates. In the present research, three types of FBG sensors: uncoated normal, polyimide-coated normal, and polyimide-coated small-diameter FBG sensors, were embedded in CFRP cross-ply laminates, and reflection spectra from the sensors were measured during the fabrication process of the laminates. Through the comparison of results obtained for the three FBG sensors, it was found that the effect of thermal residual stress on the reflection spectrum could be decreased when the optical fiber was coated with polyimide and its diameter was small in the present laminate configuration and embedment position. Furthermore, these changes of the spectra during the curing process could be simulated by theoretical calculation considering the birefringence effect.

Effect of thermal residual stress on the reflection spectrum from fiber Bragg grating sensors embedded in CFRP laminates

This article reviews the principles and application of optical fiber Bragg gratings (FBGs) as strain and temperature sensors for structural health monitoring applications. FBG sensors have many advantages for strain sensing in such applications including their small size, the potential to multiplex hundreds of sensors with a single ingress/egress fiber, their immunity to electromagnetic interference, and their corrosion resistance. The spectral properties of FBGs and their response to applied strain and temperature fields are derived. Multiplexing and interrogation strategies for large sensor networks are presented. Special focus is placed on the response of FBGs to multiaxis strain fields and rapidly varying strain fields such as near the location of structural damage. Common approaches to invert the spectral response of an FBG subjected to a complex strain field are also reviewed. Finally, this article reviews common fabrication techniques for FBGs and their relative impacts on the strength of these sensors.


Keywords:

optical fiber sensor;
fiber Bragg grating;
long period grating;
wavelength division multiplexing;
multiaxis sensor

Philip Pattis

Papers

Experimental verification of response of embedded optical fiber Bragg grating sensors in non-homogeneous strain fields