Wind turbine rotor blades

Today the coin-tap test is a widely used technique on wind turbine rotor blades for inspection of thin GFRP laminates for disbonded and delaminated areas. However, since the sensitivity of this technique depends not only on the operator but also on the thickness of the inspected component, the coin-tap testing technique is most sensitive to defects positioned near the surface of the laminate. Therefore, there has been an increasing demand for alternative non-destmctive testing techniques which is less operator dependent and also more sensitive to delaminations and disbonded areas situated beyond thicker GFRP-laminates. 

Fig. 1. Simple illustration of the basic design of the wind turbine rotor blade scanner.

Based on a preliminary set of acceptance criteria s developed by LM Glasfiber a standard data-set has been developed for each of the above mentioned set-ups, in order to minimize the scanning time. During the performance demonstration at LM Glasfiber the effective scanning time for a complete 21m wind turbine rotor blade based on the preliminary acceptance criteria s, was found to be less than hour. 

After the performance demonstration a number of damaged rotor blades were scanned followed by a number of destructive verifications of the results achieved by ultrasonic scanning. Based on this examination it was concluded that the wind turbine rotor blade scanner is capable of detecting defects such as delaminations, inclusions, missing adhesion, lack of adhesive, porosities and variations in thickness. 

J. Rusborg, Non-Destructive inspection of wind turbine rotor blades, part I (In Danish) FORCE Institute 1995, ISBN 87-7784-049-6... 

ALTERNATIVE FATIGUE FORMULATIONS FOR VARIABLE AMPLITUDE LOADING OF FIBRE COMPOSITES FOR WIND TURBINE ROTOR BLADES... 

This paper looks at modem wind turbine rotor blades from the point of view of material fatigue. Characteristics of the rotor blades and loads are discussed and a simple commonly used lifetime prediction method is reviewed. Also, possible modifications to the various components of the fatigue calculations are discussed. 

Composite materials are extensively used in modem wind turbine rotor blades. These blades have exhibited a significant increase in length over the past few years (blade lengths exceed 30 m). In addition, wind turbines are increasingly being installed collectively in wind farms, to serve as large electricity plants rather than stand-alone power supply. The associated high investments call for accurate and reliable lifetime prediction methods for the whole turbine, but especially for the blades, which are difficult to repair and to reinstall. 

However, there are indications that the commonly used lifetime prediction methods can lead to inaccurate results for the composite blade s lifetime under the strongly varying loads to which they are subjected. The present paper presents the preliminaries to an extensive study into the fatigue behaviour of wind turbine rotor blade materials under variable amplitude loading. This paper will discuss some of the problems and drawbacks of the currently prescribed lifetime prediction methods, and describes possible ways of tackling these. 

First, the most important loads on wind turbine rotor blades are described and a brief review of fatigue research in composites is presented to place the following approach in a broader perspective. The approach described in this paper concerns modification of the existing formulations. Possible modifications are illustrated with predictions for an existing data set, which was generated by subjecting composite specimens to different variable amplitude load sequences. 

The wind turbine rotor blades, both for on-shore and off-shore wind farms, are exposed to various hostile conditions such as extreme temperatures, humidity, rain, hail impact, snow, ice, solar radiation, lightning and salinity. In order to withstand these external conditions without diminishing safety, a sound knowledge of the fatigue behaviour of the material and structural properties is needed. 

Mohamed, M.H., Wetzel, K.K., 2006. 3D woven carbon/glass hybrid spar cap for wind turbine rotor blade. Trans. ASME J. Solar Energy Eng. 128, 562-573. 

Zangenberg J, Brondsted P, Koefoed M. Mat. and Design. 2014, Vol 56, pp 635-641. Design of a fibrous composite preform for wind turbine rotor blades. 

Complete sub-stmctures, such as the root attachment construction inserted into the lay-up of a wind turbine rotor blade... 

Kensche, C.W. and Kalkul, T. (1990) Fatigue testing of Gl-ep in wind turbine rotor blades, Proc, EWEC 90 Conference Madrid. 

---