Fibre Bragg Grating as a Multi-Stage Structure Health Monitoring Sensor
There is a clear need to implement models and measurement systems through the entire life of the wind turbine blade. In this chapter will be presented some work conducted to implement optical fibres as a multi-stage sensor, capable to measure different structural properties, and link them with all the different life stages and support a better design of the wind turbine blades. The characteristics and functionality of fibre Bragg grating sensors are briefly introduced. Their application as multi-stage structure health monitoring sensors for polymer laminate composite is then described. At the manufacturing stage, where the sensors can measure several parameters of infusion and curing, sensor feedback can help control the process, avoid residual strain, and contribute to the product certification; and then in operation where cracks can be detected and monitored. Experimental mechanical testing involving crack growth and fibre Bragg sensing is described that highlights the response from the fibre optic which will correctly detect the presence and growth of damage. Models to implement these results in a damage detection system for a wind turbine blade can then be developed.
KeywordsDigital Image Correlation Fibre Reinforce Polymer Fibre Bragg Grating Sensor Wind Turbine Blade Digital Image Correlation Technique
4.1 Fibre Bragg Grating Sensors
Fibre Bragg grating (FBG) sensors are the most commonly used type of sensors in the fibre optic field. An FBG sensor can be embedded in the Fibre reinforced polymer material (main material of the wind turbine blade), without compromising its structural resistance. This is due to the FBG reduced size, with a diameter of 125 μm it is virtual non-intrusive to the material. Also, FBG sensors present other interesting features, such high resolution, multiplexing capability, immunity to electro-magnetic fields, chemical inertness and long term stability.
4.2 Manufacturing Stage: Residual Stress Induced by Resin Shrinkage and Curing Process Control
The FBG sensors are an excellent choice to monitor the curing process of wind turbine blades, where their capability of surveying the curing process is combined with their capability to monitor strain and other features over the structure lifetime. Because the FBG sensor small size that makes them virtually non-intrusive to the structure, they can be embedded in the composite layers from the first manufacturing step.
This embedded sensor will monitor several parameters of the curing process: temperature of the resin, which enables a retro-feedback of temperature to the process controller; residual stress that are a big issue in the fatigue performance of the composite; resin flow, by measuring the position of dry spots in the laminate; etc. Additionally, the FBG sensor can be used as part of the process certification, by giving information about the curing profile of the structure, the residual stress, the load history during manufacturing, transport and installation.
4.2.1 Embedded FBG Response to Strain and Temperature Variation
4.2.2 Residual Stress Measurement
By measuring the temperature in the resin using a thermocouple, it is possible to decouple the temperature-strain cross sensitivity of the FGB and calculates the epoxy shrinkage.
4.3 Operation Stage: Crack Growth Detection by Embedded FBG Sensors
It is possible to divide the strain distribution in two distinct contributions: crack tip singularity/material damage and fibre bridging. Near the crack tip, the stress field closely approaches the singular stress field of linear elastic fracture mechanics. This means that the stress tends to infinity and has a fast variation (high gradient). Also, with the progression of the crack the material/structure losses stiffness increasing the strain, here it is possible to observe higher values of strain ε11 at the crack faces, as showed in Fig. 4.4a. In the fibre bridging zone (L < x < 0), a positive strain ε22 was observed, due to the forces transferred by the fibre that are connecting the two crack faces, as shown in Fig. 4.4c. These forces are balanced by a compression stress that appears ahead of the crack tip (x > 0), which creates a negative strain ε22, shown in Fig. 4.4c as a blue area.
4.3.1 Crack/Delamination Detection by Embedded Fibre Bragg Gratings
As measured using DIC technique, during a crack/delamination event different fracture features will be present near the crack tip. Being able to identify and measure these specific phenomena with a FBG sensor is the key factor to correctly determine the presence of damage and its growth.
126.96.36.199 Embedded FBG Response: Strain
188.8.131.52 Embedded FBG Response: Transverse Stress
184.108.40.206 Embedded FBG Response: Non-Uniform Strain
4.3.2 Delamination Detection in Fibre Reinforced Polymer Specimen Using Embedded FBG Sensor: Material and Testing Procedure
To validate the crack detection technique double cantilever beam (DCB) specimens were tested in a fracture testing machine, developed by Sørensen (2010). The DCB specimens were loaded with different combination of moments, giving different type of fracture modes that simulates different crack/delamination cases. The DCB specimens were manufactured using two FRP material arms, made of unidirectional and triaxial glass fibre layers (SAERTEX UD and TRIAX), with a layup stacking of : [90/+45/−45/04/04/+45/−45/90], glued by a commercial epoxy structural adhesive (Epikote MGS BPR 135G/Epikote MGS BPH137G). A thin slip foil was placed in the edge of the structural adhesive, to act as a pre-crack and ease crack initiation.
220.127.116.11 Experimental Results
4.4 Application of the FBG Crack Detection Method
By using this method it becomes possible to extract two types of information from the sensor: one type dependent of the loading and geometry, εzz, which give information about the global strain/loading state of the structure; The other type,εzz (z) and σx,y, independent and only affected by the proximity of a crack.
4.5 Fibre Bragg Grating as a Multi-Stage Structure Health Monitoring Sensor: Published Work
Pereira G, Mikkelsen LP, McGugan M (forthcoming) Crack Detection in Fibre Reinforced Plastic Structures Using Embedded Fibre Bragg Grating Sensors: Theory, Model Development and Experimental Validation. Plos One
Pereira G, Mikkelsen LP, McGugan M (forthcoming) Fibre Bragg Grating Sensor Signal Post-Processing Algorithm: Crack Growth Monitoring in Fibre Reinforced Plastic Structures. Springer Proc Phys
Pereira G, Mikkelsen LP, McGugan M (2014) Damage tolerant design: failure and crack propagation in composites. In: Abstracts of the 10th EAWE PhD Seminar on Wind Energy in Europe. European Academy of Wind Energy, Orléans, 28–31 Oct 2014
Pereira G, Mikkelsen LP, McGugan M (2014). FEM model of Embedded Fibre Bragg Grating Sensor Response: Crack Growing Detection. In: Abstracts of the NAFEMS NORDIC – Simulation Verification and Validation (V&V): A Key Enabler for Virtual Product Development, NAFEMS, Copenhagen, 3–4 November 2014
Pereira G, Mikkelsen LP, McGugan M (2015) Crack Growth Monitoring by Embedded Optical Fibre Bragg Grating Sensors. In: Abstracts of the 3rd International Conference on Photonics, Optics and Laser Technology, Berlin, 13–15 March 2015
Pereira G, Mikkelsen LP, McGugan M (2015) Embedded Fibre Bragg Grating Sensor Response Model: Crack Growing Detection in Fibre Reinforced Plastic Materials. In: DAMAS 2015. 11th International Conference on Damage Assessment of Structures, Ghent, August 2015. Journal of Physics: Conference Series, Vol 628. Institute of Physics, Temple Back, p 012115. doi: 10.1088/1742-6596/628/1/012115
Pereira G, Mikkelsen LP, McGugan M (2015), Structural Health Monitoring Method for Wind Turbine Trailing Edge: Crack Growth Detection Using Fibre Bragg Grating Sensor Embedded in Composite Materials. Paper presented at the 20th International Conference on Composite Materials, ICCM20, Copenhagen, 19–24 July 2015
- Jülich F, Roths J (2010) Comparison of transverse load sensitivities of fibre Bragg gratings in different types of optical fibres. In: Berghmans F, Mignani AG, van Hoof CA (eds) Optical sensing and detection. SPIE, Brussels, May 2010. SPIE proceedings, vol 7726. SPIE, Washington. doi: 10.1117/12.854019
- Pereira G, Mikkelsen LP, McGugan M (2015) Embedded fibre Bragg grating sensor response model: crack growing detection in fibre reinforced plastic materials, 2015 In: DAMAS 2015. 11th International conference on damage assessment of structures, Ghent, August 2015. Journal of Physics: Conference Series, Vol 628. Institute of Physics, Temple Back, p 012115. doi: 10.1088/1742-6596/628/1/012115
- Sørensen L (2010) Cohesive laws for assessment of materials failure: Theory, experimental methods and application. Dissertation, Technical University of DenmarkGoogle Scholar
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