Unseen flaws in turbine rotor blades can lead to foreshortened product lifetimes, or sudden, dangerous failure. Morgan Troedsson, product manager, FORCE Technology, reveals how the very latest developments in ultrasonic inspection techniques can help guarantee manufacturers’ peace of mind.

Any manufacturer of rotor blades for wind turbines is concerned with efficient and safe quality assurance (QA). In what is a rather manual lay-up and assembly process, variability in quality is a key issue for the ultimate product performance.

Automated ultrasonic inspection systems have been successfully applied for a number of years. They follow a continuous evolution, from uncomplicated manual inspection systems, to more advanced track and crawling scanner concepts. A regular meander-scan pattern is currently the state-of-the-art. This provides high-resolution 3D data on inherent composite structures.

In recent years, the fatal issue of fibre waviness (wrinkles) in spar caps and girders was commonly recognised. Span-wise ultrasonic line scanning was established as the most suitable method to properly capture data on this problem. Multiple line-scanning now sets a new standard for high-performance ultrasonic blade inspection.
The industry faces a dramatic need for automated processing during blade manufacture. Robotised systems for fibre lay-up and for finishing are currently use; blade coating processes have already been automated for some time, due to the concern for workers’ health and safety.

FORCE Technology has recently exploited the concept with autonomous scanner systems with great success. They are mounted on self-propelled, fork-lift chassis that carry all the scanner mechanics, cranes, ultrasonic probes and required control units for safe data handling and physical manipulation.

Fibre waviness
Hidden fibre waviness, where wrinkles and waves appear in the unidirectional reinforcement layers of spars, girders and spar caps, is a common defect in old and new rotor blades. Dynamic operational loads may cause cracks to occur in spar laminates early in a blade’s lifetime. These hidden flaws are known to cause catastrophic failures, with whole sections of blades falling to the ground.
Today, several NDT (non-destructive testing) methods are available to indicate the existence of wrinkles. FORCE’s proven ultrasonic technique provides not only basic indications, but also proper mapping, contour sizing and positioning of wrinkles laterally and in depth. Ultrasonic observations are often denoted and classified accordingly.

Each and every OEM blade designer should establish a critical level for unidirectional fibre waviness. If this critical level is exceeded, the residual lifetime of that blade is significantly reduced.

Several examples have revealed a knock-down factor of three on the actual lifetime. It means that a 20-year blade lifespan is reduced to only seven years. These fatigue-load catastrophic failures will occur suddenly, without any specific warning or visible alert.

Vertically oriented blades
A means of NDT for vertically oriented rotor blades has been sought for years and its evolution has brought with it a couple of new scanner systems. FORCE Technology introduced a novel autonomous mobile blade scanner concept in 2013. A regular meander-scanner manipulator in vertical orientation is attached to a customised fork lift truck base structure equipped with a crane-handling device.

Perfect coupling of the ultrasonic contact probes to vertical surfaces proved to be a major challenge. The result of all this effort, however, was a unique and successful coupling solution that made high-performance ultrasonic blade inspection feasible, even when blades were only available in vertical orientation (the normal storage position). Continuous NDT on very long blades also became feasible, managed by a single operator.

The ultrasonic array
FORCE Technology’s revolutionary inspection concept is based on the mobile blade array scanner and has a unique, full-size, 24-channel probe array that can collect NDT data in a multiple line-scan mode. P-scan multichannel electronics and software may record, collect and analyse scanning data from the load-bearing spar caps (girders) and bonding to internal shear webs.

The greatest revolution comes from the inherent performance of the scanner. Simultaneous multichannel line-scan data handling allows for an unbeaten high inspection speed, reaching 5-7m a minute. This is obtained with a predetermined resolution for detecting hidden flaws. Anything invisible in the vital blade structure may be disclosed for proper QC of any OEM rotor blade.

Full spar cap inspection on 60-80m-long rotor blades can be achieved in an hour, with both sides covered. Another advantage of this revolutionary device is that it requires only one inspector to operate it.

Keeping the line scan principle in mind, the accuracy for monitoring and sizing of any UD fibre waviness (wrinkles) is excellent. This is not compromising the ability for full and accurate girder bond-line assessment. Data on the embedded spar cap (girder) structure will be included without extra work.

Future prospects
A current trend in the OEM blade industry is to invest in smart automation in the various steps of blade manufacture. These are:

  • fibre lay-up, for the basic FRP composite shell,spar and web components
  • infusion and moulding of the basic FRP composite blade shell and web components
  • adhesive bonding of the assembled integer rotor blade
  • finishing, with flash milling, surface grinding and protective painting.

The blade array scanner concept may quite easily be adapted to fit contemporary automated robot systems, to accommodate the full capacity for an integrated system approach. The array head with multiple probes may be conveniently attached to the grip of an autonomous system. When the OEM blade industry is ready for the natural evolution in automated technology, the required advanced ultrasonic NDT inspection technology is available.