Strain measurement for extremely large blades

Larger blades pose special needs for measurement technology

 

Current rotor blades are more than 60 meters long. The next generation will go far beyond. This changes not only the demands on the materials, logistics and installation.

In particular, development engineers are facing new challenges: Large blades enforce new and different material and web structures. Effects can be relevant which played no role in previous structures. The flutter tendency of rotor blade tips must be measured precisely to exclude relevant damage mechanisms. The more measurement points are realized, the more detailed is the validation.

In addition to vibration, material failure and failure of the adhesive seams due to high stress due the narrow design is to be avoided.

This also changes the requirements for the measurement equipment for validation of FE models. Already today continuous load tests up to 5,000 µstrains at 20 x 106 last changes are measured, to test the continuous load capability of the new blades.

But not only large strains challenge the developers, the large number of measuring points must be installed with minimal effort.

Conventional measurement technology fails at high strains over a longer period

At these extreme strains over many load cycles the conventional electrical measurement technology often fails: Electrical strain gauges are drifting or fail completely, so tests have to be interrupted or even restarted. With these requirements, the current generation of electric DMS comes to the limits of what is physically possible.

Up to 5,000 µstrains with fiber optic sensors

Fiber-optic sensor, are in principle, more resistant: expansions up to 10,000 µstrains can be precisely measured during fatigue tests. The Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) tested our fiber optic strain sensor until failure of the carrier GFK material.

Non-destructive tests up to 5,000 µstrains with 20 x 106 cycles with fiber optic strain sensors enable high-quality results about the tested composite material and the rotor blade design.

Distributed strain measurement at several radii

By integrating up to 18 sensors per fiber, it is possible to manage a slender test setup with very little wiring. Thereby an extensive distributed strain measurement becomes realistic for the first time.

Using chain sensors, with only one fos4Test nSens series measurement device and 4 fiber cables strains at up to 18 radii or 72 positions can be measured.

The 4 optical fibers with integrated strain- and/or temperature sensors are combined in a field distributor. A robust blade hub connection guides the fibers from the blade to the measurement device. This allows to disconnect the measurement technology outside of the blade from the sensors inside of the blade with a single connector.

Equip yourself for the future.