NDT FOR WIND TURBINES

NDT techniques for wind turbines

Because mass production of wind turbines is fairly new, no manufacturing standards have been set yet. Efforts are now being made in this area on the part of both the government and manufacturers.

While wind turbines on duty are relied on to work 90 percent of the time, many structural flaws are still encountered, particularly with the blades. Cracks sometimes appear soon after manufacture. Mechanical failure, due to alignment and assembly errors, is common. Electrical sensors frequently fail because of power surges. Non-hydraulic brakes tend to be reliable, but hydraulic braking systems often cause problems

Manufacturing flaws on turbine blades

Manufacturing flaws can cause problems during normal operation. For example, blades can develop cracks at the edges, near the hub or at the tips . Fiberglass rotor blades are regarded as the most vulnerable components of a wind turbine.

Typical manufacturing flaws on the blades may be summarized as delaminations , adhesive flaws and resin-poor areas. Here are some specific flaws at particular locations:

•     Skin/adhesive: this is bad cohesion between the skin laminate and the epoxy or the epoxy is missing.
•     Adhesive/main spar: this is when there is no cohesion between the adhesive and the main spar.
•    De lamination in main spar laminate.

•    High damping in skin or main spar laminate, which could be caused by porosities or change of thickness of laminate.

Manufacturing flaws on the tower

A tubular tower is made of a lot of sheets of iron that are welded together. A flange is welded onto each end of the sections. The welding is checked thoroughly using ultrasonic NDT.

 NDT techniques at manufacture

Visual inspection

Relatively advanced NDT testing methods are used to examine rotor blades. The methods employed include penetrant testing and visual inspection with the use of miniature cameras or endoscopes.

At present, it appears that mechanical components aren’t tested at manufacture but the cause of their damage can be determined. For example, endoscopes are used for the visual inspection of planetary gear transmissions. However, damaged components are usually examined in a materials laboratory.

Ultrasonic NDT

An ultrasonic test can be carried out to investigate if any damage is present in the wind turbine blade. Ultrasonic inspection reveals these flaws quickly, reliably and effectively and is the most often used non-destructive composite inspection method in industry. The main advantage of ultrasound scanning is that it enables us to see

beneath the surface and check the laminate for dry glass fibre and delamination.

Tap test

The tap test can be used to verify some of the results from the ultrasonic test and it is also a good method to discover irregularities in the structure. The method is based on the fact that the sound emitted when knocking on the structure changes when the thickness or material type changes or when porosities are present. It could also be caused when there is a dis bond between the skin laminate and the main spar. There are three types of tap testing equipment; a manual tapping hammer, the ‘Woodpecker’ portable bond tester and the Computer Aided Tap Tester (CATT) system.

All the automated tap methods have the advantage that they can produce a print of the damaged area, which is both useful and a permanent record of the damage found. All the tapping methods work well for thin laminates, honeycomb structures and other sandwich panels but are not so effective on thicker parts.

 Infrared thermography

The adhesive joints are critical points in the blade structure. That is why they are inspected with particular care. Infrared (IR) scanners are used to examine the blade throughout its length, measuring exactly the same points each time. The scanner is able to see through the laminate and check the adhesive joint. It records temperature differences in the adhesive, possibly identifying flaws, and takes a series of pictures. If there are any doubts, a point can be highlighted

and later analyzed using electronic image processing. If flaws are found, they can almost always be repaired immediately.

Flaws and NDT in-service

As the force of the wind is so irregular, the driving mechanism of a wind turbine is subject to much greater dynamic loads. Virtually all components of a wind turbine are subject to damage, including everything from the rotor blades to the generator, transformer, nacelle, tower and foundation.Wind turbines do have regular maintenance schedules in order to minimize failure. They undergo inspection every three months, and every six months a major maintenance check-up is scheduled. This usually involves lubricating the moving parts and checking the oil level in the gearbox. It is also possible for a worker to test the electrical system on site and note any problems with the generator or hook-ups

Flaws on turbine blades

In-service flaws have been identified in the following report: Risø-R-1391(EN) ‘Identification of Damage Types in Wind Turbine Blades Tested to Failure’ Christian P. Debel, AFM. ISBN 87-550-3178-1; ISBN 87-550-3180-3  ISSN 0106-2840

Flaws on rotor bearings

Cyclic stresses fatigue the blade, axle and bearing material, and were a major cause of turbine failure for many years. When the turbine turns to face the wind, the rotating blades act like a gyroscope. As it pivots, gyroscopic precession tries to twist the turbine into a forward or backward somersault. For each blade on a wind generator’s turbine, precessive force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. This cyclic twisting can quickly fatigue and crack the blade roots, hub and axle of the turbine.

Spalling (breaking up into fragments) of rotor shaft bearings can result in cracked rings, and in some cases, revolution of the inner rings around the shaft causes cracks in the shaft that can result in a total loss of the turbine. illustrates what can happen when the rotor bearings fail (in this case the blades landed 1 km away after having crossed a road)

TESTING PROCEDURE

Acoustic emission

A substantial amount of work has gone on in AE since 1993. A fatigue test of a wind turbine blade which was conducted at the National Renewable Energy Laboratory shows that fatigue tests of large FRP wind turbine blades can be monitored by AE techniques and that the monitoring can produce useful information. AE testing procedures, developed during a laboratory blade testing programme, have been applied to an in-service wind turbine blade in 2003. In the Framework of the Non-Nuclear Energy Programme, JOULE III from 1998 to 2002, the partners successfully developed a methodology for the structural integrity assessment of wind turbine blades, either in-service, or during certification testing, based on Acoustic Emission monitoring during static or fatigue loading. Within the framework of JOULE III, specialized pattern recognition software for AE data analysis and wind turbine automatic structural integrity assessment has been developed

Full scale testing of wind turbine blade to failure

A 25 m wind turbine blade was tested to failure when subjected to a flap wise load. With the test setup, it was possible to test the blade to failure at three different locations. The objective of these tests is to learn about how a wind turbine blade fails when exposed to a large flap wise load and how failures propagate. The report also shows results from the ultrasonic scanning of the surface of the blade and it is seen to be very useful for the detection of flaws, especially in the layer between the skin laminate and the load carrying main spar. AE was successfully used as sensor for the detection of damages in the blade during the test.

Wireless detection of internal delamination cracks in CFRP

In this study, a wireless system using a tiny oscillation circuit for detecting delamination of carbon/epoxy composites is proposed. A tiny oscillation circuit is attached to the composite component. When delamination of the component occurs, electrical resistance changes, which causes a change in the oscillating frequency of the circuit. Since this system uses the composite structure itself as a sensor and the oscillating circuit is very small, it is applicable to rotating components. The wireless method is found to successfully detect embedded delamination, and to estimate the size of the delamination.

Structural health monitoring techniques for wind turbine blades

These experiments indicate the feasibility of using piezoceramic

patches for excitation and a Scanning Laser Doppler Vibrometer or piezoceramic patches to measure vibration to detect damage.

Further testing of different smaller damages and types of damage is needed to verify the sensitivity of the methods. The resonant comparison method can be used for operational damage detection while the operational deflection shape method produces non-symmetric contours that are an easily interpretable way to detect damage in a structure that is not moving.

Infrared thermography for condition monitoring of composite wind turbine blades

Infrared thermography has the potential for providing full-field non-contacting techniques for the inspection of wind turbine blades(10). For application to turbine blades, the sensitivity of the thermal imaging has been shown to be suitable for non-destructive examination during fatigue testing; furthermore, it is thought that for blades in situ, the wind loading conditions may be sufficient to create effects detectable by thermal imaging.