Microstructured Optical Fibre Bragg Grating Sensors for Production and Health Monitoring of Carbon Fibre Reinforced Polymer Materials and Structures

Abstract

Structural health monitoring (SHM) has emerged as an exciting topic for multi-disciplinary research and development in the field of civil engineering. SHM refers to the broad concept of assessing the in-service performance of civil engineering structures such as bridges, dams, aircrafts and even wind turbines, using a variety of measurement techniques. Materials or structures equipped with various types of SHM sensors are known as 'smart' materials or 'smart' structures. Smart typically refers to the ability to respond to the environment or to the capacity for diagnosing internal deterioration before critical failure occurs. A 'smart wing', for example, will warn aircraft engineers that internal damage is about to occur or has occurred, and that maintenance might be required in order to avoid catastrophic wing failure.
High-end structures such as aircrafts and wind turbines are increasingly fabricated using high-performance materials such as fibre reinforced polymers (FRPs) or 'composites'. To ensure that these new materials are performing as planned, and that the safety and the integrity of structures made thereof is not compromised, the engineering community is investing considerable efforts in developing new systems that enable in-situ SHM. These are meant to be integrated in a non-intrusive manner within the materials, for example to measure the mechanical load experienced by the material and to obtain information on the internal mechanical stress concentrations.
This PhD thesis contributes to these efforts. We exploit the capabilities of a new type of optical fibre based sensor to obtain 'smart composite materials'. Such sensors feature a number of well-know advantages over conventional electrical sensors including insensitivity to electromagnetic interference, small dimensions, light weight, multiplexing capabilities and resistance to corrosion. Owing to their small diameter, the fibre sensors can be embedded in a non-intrusive manner inside composite materials to monitor their internal strain state for SHM purposes.
The innovation and advancement on the state-of-the-art that we bring stems from our particular choice for the optical fibre sensor: we choose to use sensors based on Bragg gratings in novel highly birefringent microstructured optical fibres (MOFs). A MOF is an optical bibre that holds a regular pattern of micron-sized air holes in its cross-section, which run along the entire fibre length. The microstructure of the MOF used in this PhD work is highly asymmetric and has been designed in such a way that its phase modal birefringence features sensitivity to transverse strain that is one order of magnitude larger than that reported for conventional highly birefringent fibre, whilst being quasi-insensitive to temperature changes. The region of the fibre core contain a GeO2-doped inclusion that allows for the fabrication of fibre Bragg gratings(FBGs) using conventional ultraviolet inscription techniques. The combination of this particular MOF with conventional FBGs (MOF-FBGs) enables multi-axial strain field mapping inside materials.
Our grand objective is to demonstrate the capabilities of our sensors for smart material applications, and to do so we integrate our sensors inside carbon fibre reinforced polymer (CFRP) materials. On the way to achieving this objective we tackle the following challenges.
First, we evaluate the mechanical strength of our MOFs in order to define the mechanical stress range to which MOFs can be submitted without being damaged. We use a systematic statistical approach and we conduct an extensive experimental campaign on various types of MOF s under two loading conditions (tensile and bending loads). This allows us to study the mechanical strength of the MOFs as a function of their geometrical features and to identify specific failure mechanisms of the MOFs. Furthermore and since we dedicate our sensor to smart composite applications, the mechanical strength of the MOFs is also studied as a function of the ageing conditions often encountered in composite material applications. We conclude that the failure mechanisms of MOFs can differ from those of conventional optical fibres, but that the resulting strength is still sommensurate with the requirements for composite SHM.
Second, we explore the concept of smart materials with MOFs embedded within carbon/epoxy laminate materials. We take advantage of the specificity of this MOF which responds in a different manner to transverse force depending on the direction of the load with respect to the microstructure. This allows us to propose a multi-strain sensor consisting of two parallel MOF-FBGs integrated within the material under different angular orientations, in order to assess the multi-axial strain field within the composite material. In a first step, we characterize the bare MOF-FBG sensor responses to different loads, and we compute the sensitivity matrix of the sensor. In a second step, we develop a model for the strain transfer between the core of the embedded MOF and the host material. This approach allows relating the amount of strain present within the laminate and that reaching the MOF core. The strain transfer of the different MOF sensor configurations is calculated by means of finite element analyses. Finally, we benchmark our finite element simulations of the different sensor configurations against experimental tests for longitudinal and transverse in-plane and out-of-plane strains. The experimental data are in good agreement with the theoretical models. We obtain relative errors on the strain in transverse directions that are smaller than the state-of-the-art.
Third, we combine our MOF-FBG sensors with flexible ultrasonic transducers and conventional optical fibre sensors to monitor the manufacturing of a large-scale composite part. This large-scale experiment demonstrates the compatibility of our sensors with industrial production constraints. Moreover, this multi-instrumentation approach allows gaining insight in the physical state of the composite material during the cure process. We demonstrate the possibility to use MOF-FBG sensors to identify the main phases and material state changes during the cure cycle of a CFRP and we prove that we are able to estimate the residual strain built up within the composite as a result of the manufacturing.
Finally, using the calibration of our embedded sensors and our strain transfer models, we monitor the manufacturing of composite laminate coupons, using in-situ MOF-FBG sensors during a vacuum bagging autoclave process. This allows us to estimate, for the first time to our knowledge and with such sensors, the level of through-the-thickness strain built up in the coupons during the cycle. This strain in the cross-ply laminates is almost 4 times larger than the in-plane strains.
Whilst additional work is still required to bring our technology up to full maturity, we can safely conclude that our microstructured optical fibre Bragg grating sensors carry great potential in the field of production and health monitoring of composite materials and structures.

Date of Award	Dec 2013
Original language	English
Supervisor	Francis Berghmans (Promotor), Hugo Thienpont (Promotor), Danny Van Hemelrijck (Jury), Rik Pintelon (Jury), Erik Stijns (Jury), M. Gomina (Promotor), S. Eve (Jury), Joris Degrieck (Jury), Francis Collombet (Jury), Geert Luyckx (Jury) & Alexis Beakou (Jury)