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Conference programme 

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Poster session

Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Gilmar Pereira DTU- Technical University of Denmark, Denmark
Co-authors:
Gilmar Pereira (1) F P Lars Mikkelsen (1) Malcolm McGugan (1)
(1) DTU- Technical University of Denmark, Roskilde, Denmark

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Poster
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Presenter's biography

Biographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited

Mr. Pereira is a PhD student at the Technical University of Denmark (DTU). His PhD project is part of a Marie-Curie action- European Offshore Wind Energy project- MARE-WINT. His current PhD research is in experimental and numerical analysis of wind blade sub-structures, with special focus in fracture and damage of composite materials. He holds a Master Degree in Mechanical Engineering, with specialization in structural engineering and machine design, from the Faculty of Engineering, University of Porto-Portugal. Previously, he has been involved in several research projects in the area of Composite Materials, Experimental Mechanics, Structural Health Monitoring and Manufacturing process.

Abstract

Condition monitoring of composite material and bondlines in wind turbine blades: failure and crack propagation

Introduction

The trend for wind energy structures designs shows an increase in the size of the components, the industry relies on advances in materials technology and design philosophy to deliver the most cost-effective light-weight structures. Historical design philosophy of composite materials is based on conservative analysis methods, with large safety factors, underestimating the material properties, and considering only the linear behavior of the material. As knowledge about material behavior increased it became possible to safely adopt more advanced design philosophies.

Approach

In a Damage Tolerant design, structural health monitoring and models describing crack propagation permit the structure to operate despite the presence of damage and fully exploits the capability of the material, leading to a more competitive structure.
One of the most common types of damage that can develop in the trailing edge of a blade is the delamination/debonding (in composite material/adhesive interface). This delamination is accompanied by the formation of a crack bridging zone, where intact fibers connect the crack faces behind the tip thus increasing the energy required for the crack to propagate (Damage tolerance mechanism).



Main body of abstract

The aim of the present project is to develop a damage tolerance approach for wind turbine blade sub-structures, focusing on the crack growth mechanisms and detection methods. To do this, a finite element model of the crack growth mechanisms in a double cantilever beam (DCB), representative of a delamination on real blade trailing edge under different fracture modes was developed. Experimental tests were conducted in order to fully characterize the material and support the model.
Afterwards, a crack monitoring technique was implemented using embedded Fiber Bragg Grating (FBG) sensors into the composite material/adhesive bonding, in order to determine the existence of a crack and its growth. Different features present in the crack mechanism that can induce a change in the FBG response were identified, making it possible to identify specific phenomenon that will only happen with the proximity of a crack, such as compression fields ahead the crack or non-uniform strain, and then identify the presence of such damage in the real structure. An algorithm was built using these assumptions and a Finite Element Model of the crack growth was developed in order to predict the sensor output response for this structure-failure-sensor case. Some DCB specimens were instrumented with one array of FBG sensors embedded into the host material, using a digital image correlation technic to determine the presence of specific phenomena caused by the crack, and to correlate with the FBG sensor.

Conclusion

These experiments validate the coupled structure/sensor model, so it becomes possible to study the application of this monitoring technique in other locations, predict the sensor output and track different damage features. The application of damage tolerant materials and structural monitoring can lead to safe operation of loaded components even when in damage condition.


Learning objectives
-Damage tolerant design vs Historical design philosophy;
-Damage tolerance mechanism that increase the fracture resistance;
-Crack detection by the use of Fibre Bragg Gratings;
-Structure/Sensor model and application of this monitoring technique in other locations;