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Phased array ultrasonics inspection of wind turbine blade

A bespoke inspection programme developed by TWI has enabled replication of an inspection conducted by a wind turbine manufacturer and subsequent detection of flaws within the aluminium ring of the blade root. TWI was able to supply information on the circumferential length of the flaws, as well as their position and through-wall extent within the ring. The inspection programme has meant that blades can now be accepted or rejected according to the manufacturer’s acceptance criteria and plans are in place for future work requirements.

An Industrial Member company contacted TWI with a view to inspecting wind turbine components to detect suspected cracking. This followed an earlier component failure, where a number of phased array ultrasonic inspections had been conducted by the wind turbine manufacturer, reporting cracking within the blade root area. The exact location of flaws and specific details of the technique applied were not available, therefore the member company asked TWI to devise a phased array ultrasonic technique (PAUT) to replicate the manufacturer’s inspection of the aluminium blade root and determine the location of any suspect cracking.

Figure 1. Photograph of a sectioned aluminium ring showing the size of one of the 60 bolt holes equally distributed around its circumference.
Figure 1. Photograph of a sectioned aluminium ring showing the size of one of the 60 bolt holes equally distributed around its circumference.

The TWI team worked to:

  • Develop a suitable inspection technique for the blade root – an aluminium ring glued in the composite which allows the mounting of the blade onto the wind turbine rotor. This blade root contained 60 bolt holes, measuring about 1m in diameter and giving an external circumference of around 3m;
  • Report any cracks within the aluminium ring main body;
  • Validate the results provided by the inspections previously carried out by the manufacturer’s personnel.

TWI began by making a number of visits to wind farms to collect information about blade configuration, geometry and accessibility. The team was then able to develop a PAUT inspection programme and start a number of trials.

The programme succeeded first in the detection of cracks within the ring – confirming the results of the manufacturer’s inspections. TWI continued its work by carrying out an encoded inspection to achieve accurate measurement of crack length and through-wall extent.

In terms of acceptance criteria and applicability to any future work on this component, the inspections appear to lie outside the scope of current codes. With over 40 years’ experience in the Engineering Critical Assessment (ECA) approach or 'fitness-for-purpose analysis' widely accepted by a range of engineering industries, TWI considers it more appropriate to adopt ECA. This will allow NDT results to be interpreted against tolerable flaw sizes and enable classification of the root in terms of safe remaining life (e.g. Class 1: Unfit for purpose, Class 2: 1 year remaining life, Class 3: two years remaining life, etc.). The approach will make use of information from the inspection results on size (length and through-wall extent), position and orientation of flaws within the ring rather than just their circumferential extent.

For information please email contactus@twi.co.uk

Figure 2. Data plot of a crack found at the end of a bolt hole in the aluminium blade root
Figure 2. Data plot of a crack found at the end of a bolt hole in the aluminium blade root
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