Fatigue crack growth analysis considering spectral load

[usecheijk]

Hi
I want to perform a fatigue crack growth analysis for a thin walled fractured vertical mast under win loads. ¿How can I perform this kind of analysis if spectral win load (aerodynamic pressure load over mast) is provided?. I have experience with crack propagation analysis under quasi-static conditions (using Miner's rule and Paris Law). But in this case, dynamic condition can't be negligible.

Thanks,

Jack

 

[Sparweb]

Wind speed at the tower site is variable, and typically it is modeled with a Weibull distribution where wind speeds are binned and probabilities are plotted on a histogram. Once you select the correct parameters, you can determine the spectrum of wind speeds that will be applied to the tower. You can consult the local weather office for detailed historical data about the wind speed where the tower is sited. Below is the histogram of wind speeds where I live:

The response of the structure to gusts, on the other hand, will depend on its stiffness, and turbulence around the site. That subject is studied in the design of wind turbine towers so that's one place to start for you.

STF

 

[rb1957]

if "dynamic condition can't be negligible" means that you have to account for dynamic loading effects on the tower, then you need a time domain FEA. But time domain loading (sinusoidal waveforms) would be quite intensive analysis.

I'd've thought that there were codes that provided simple design or qualification guides to account for this. Also, I'd expect that a thin walled mast would be cable stayed ... so the cables are doing the bulk of the work ... Steel cables with an endurance limit (ie infinite life for a certain stress level). Steel cables with a preload ...

another day in paradise, or is paradise one day closer ?

 

[Jason8zhu]

Hi usecheijk,

To perform the fatigue crack growth analysis, is there any FAR or AC can be followed?

Thanks.

 

[usecheijk]

Thanks to everyone for you answers.

Jason8zhu, what FAC and AC means?

 

[Sparweb]

usecheijk,

What jason8zhu is referring to are the design standards for aircraft structures. I don't think that is a suitable path for you to follow, however. Those requirements and design guidance documents are very specific to aircraft, and will have only passing value to you designing wind turbine structure. That's not to say that at a material and structural level, the mechanisms of crack growth aren't identical. Now I'm going to state the obvious here, but you probably don't like your wind turbine rotor spinning at 2000 RPM, and the radius of your blade is probably much larger than 2 meters. So I don't think it would be realistic for you to take guidance for an aircraft propellor and apply it to a wind turbine blade.

You seemed to be looking for specific guidance about WT rotors, and aircraft design guidance simply won't hit the right target for you.

Have you visited the NREL designer discussion groups yet? I don't have the link handy but I can find it for you, if you want to try that. The tower load spectrum may be a part of their WTPERF analysis code. I remember that it does tower loads, but I don't remember at what level of detail. You can ask if you want to visit that forum yourself.

STF