Blade Failure

Wind & Water General
1

Hello all. I’m glad to be a part of the nwtc forum now.

I’ve been trying to read up on the means of blade failure. Intuitively, it seems that the blade would fail from fatigue, whether that be ply delamination or any other failure mechanism. However, all the reports I’ve read on blade tests describe the blade being flap-wise loaded to its breaking point. The papers then look at the damage on the blade and identify several modes of damage.

I could see using this to validate a FE model, but it seems that a blade is never going to be subjected to an extreme flapwise load that would cause failure.
So, I guess my questions are:

  1. Am I right in assuming blade related failures are predominantly fatigue based?

  2. What are the significant gains from loading a blade to failure in a laboratory? Are all of the damage modes found from extreme loading relevant to fatigue?

  3. Is there non-proprietary information available regarding fatigue damage found in the field? or FE modeling of fatigue? or lab testing of expected fatigue loads?

I’m a wind-power rookie, so I apologize if these questions seem too basic. I’m trying to quickly orient myself so I can aim my thesis towards work that will benefit the community. Thanks!

2

It’s not basic at all. Your question goes to one of the fundamental changes of focus in a number of industries: how to translate knowledge of the bulk properties of a material into the detail behaviour of its fine structure, and how to do this with complex materials such as composites.

Your literature search will turn up data developed at Sandia: windpower.sandia.gov/topical.htm Your subject has been studied already, but I wouldn’t take it as a “done deal”.

And there is a large body of this information available in the aircraft structures world, too. For comparison, fiberglass airframes are not loaded much like wind turbine blades, but if you were to look for parallels in, say, composite helicopter rotor blades, you would probably find a wealth of information to research. Of the body of data in the public domain, probably an equal amount is proprietary, due to the strategic value of some of that research to governments and commercial enterprise.

From my reading of other’s experience, fatigue is a major cause of blade failure (or at least repair) on WT’s big and small. There is a lot of value in developing the field of blade design to prevent it.
Good luck!

3

Hi Alex,

I would like to add a few comments more from the previous excellent reply. When it come to the blade failure, you have to think about the material’s time-dependent behavior and very unique working conditions of turbine blades. Frequently, turbine blades are compared to helicopter rotor blades which are made of composite material as well; however, wind turbine blade is much larger that is comparable to a small bridge and its loading condition is fully-reversed (edge-wise bending at least). And also composite material is changing over time without any mechanical load unlike metallic materials. So, here are a few sloppy answers:

  1. Am I right in assuming blade related failures are predominantly fatigue based?
    Yes, if you include composite materials’ time-dependent behavior into the fatigue category as well. Some are trying to model this time-varying behavior as fatigue models like S-N curve. A few examples of blade failures are:
    Delamination (or Debonding) → Buckling → Fail
    Matrix Crack (-> Delamination) → Fiberbreakage → Fail
    Composites are initially ductile, but becomes more brittle as time goes by, and also easily affected by UV-rays, moisture (oxidation), corrosion, creep, etc.

  2. What are the significant gains from loading a blade to failure in a laboratory? Are all of the damage modes found from extreme loading relevant to fatigue?
    People design blades primarily based on the lab testing, fatigue tests. And impulse loading is also important to consider. Think about bird strike or lighting events.

  3. Is there non-proprietary information available regarding fatigue damage found in the field? or FE modeling of fatigue? or lab testing of expected fatigue loads?
    I believe there are large amounts of literature to answer for those questions. Especially for 3), people from Sandia, NWTC and other universities including Virginia Tech are active in the field of “Structural Health Monitoring of Wind Turbine Blades”.

Hope this helps.

4

Thanks Steven and Dongwon. Your replies are appreciated. I have a better idea of where to go from here, now.