# Mlife and Fatigue

Hello everyone,

Recently I use the Mlife to calcuate the wind turbine fitague lifetime.I have some doubt and hope you can help .
1> As we know,we calculate the fitague by S-N curve,we need the stress or strain,but I can get the results of Force and Moment from the FAST,Is the calculation theory of S-N curve available or can you recommand some papaer about the theory?

2>About the S-N curve,I want to know how I confirm the “m” values about the blade and tower ,can you recommand the values of “m” about 5MW wind turbine?
Hope your help!Thank you very much!
Best Regard!

Ruiling.Wang

Dear Ruiliang,

Wind turbine analysis typically follows a two-step calculation process, where tools such as FAST are used to predict the dynamic response of the complete wind turbine system, including the coupled aerodynamics, structural dynamics, and controller dynamics. The outputs from FAST are the loads (shear forces, bending moments, etc.) throughout the various components of the turbine, such as the blades, hub, drivetrain, and tower. These outputs are then used to feed FE-based models of individual components for calculations such as stress, strain, and buckling (or simple analytical models for simple cross sections and stress states, e.g. axial stress = M*y/I for pure bending). Because the loads output by FAST are the reaction loads accounting for both the applied (e.g., aerodynamic) loads and the inertia loads from structural dynamics, there is no need to capture the dynamics within the FE-based model.

That said, fatigue is often computed in terms of loads instead of in terms of stress e.g. when computing damage-equivalent loads (DELs).

In the absence of more specific data, the Whoeler material exponent (m) is often taken to be between 3-5 for steel and 8-12 for composites. In fact, many loads-analysis reports show the DELs for m = 3, 4, 5 for steel components and the DELs for m = 8, 10, 12 for composite components.

Best regards,

Hi all,

I am trying to use MLife in order to calculate the fatigue life of an offshore wind turbine’s components. First of all, I would like to say that when I started reading MLife’s documentation I had the same questions with Ruiliang, but Jason’s post gave me the answers.

My major issue though is that I am not able to use MLife since I cannot perform the certification test which is required at the beginning according to MLIfe_User manual. In specific, I cannot find the Matlab script mlife_certification_tests.m in the archive which I downloaded from NWTC. I specifically downloaded the latest version (alpha v1.00.00f-gjh) which is available online. However, I cannot find the specific scipt in any of the folders that the archive contains.

I would appreciate if any other user of MLife could tell me if he/she encountered the same issue and of course where the mlife_certification_tests.m is available.

Best,

Dimitrios Bilionis

Dear Dimitrios,

I believe the script is actually called mlife_regression_test.m, which is located in the CertTest directory.

Best regards,

Dear all,
I have a question here to consult, I am also trying to use MLife in order to calculate the fatigue life of an offshore wind turbine’s components. My target is the jacket structure of offshore wind turbine. I found that the results of different members but the same joint load is not the same, As the picture. So is it to be able to get a real joint fatigue, we must use MLife’s Calculated Channel function to add up the two loads, so as to reflect the real fatigue result of the joint.

Best regards,

Shao-Hua. Yang

Dear Shao-Hua Yang,

I’m not sure what you mean when you refer to “add up the two loads”. If multiple members intersect at a single point and properties such as the orientation, thickness, diameter, or material properties change, there are likely stress concentrations that need to be calculated to analyze the loads at the joint. SubDyn is really giving you the loads in the members where they connect to the joint (without consideration of stress concentrations).

Best regards,

Dear Jason,

In typical fatigue analysis for tubular elements we have to calculate the hot spot stresses at the interested location. In MLife theory file as the loads are considered rather than stresses
I have not found any description about hot spot stresses calculation. Furthermore, no comparing process is carried out in the theory file between S-N method and MLife method so that I am confused a bit.
Would you please clarfy that why the DEL and MLife theory would be correct in order to estimate fatigue damage and structure’s life before failure due to fatigue given that tubular elements such as Jackets?

Best regards,

Dear Arsalan,

A similar question has been discussed on the forum e.g. see: http://forums.nrel.gov/t/fatigue-calculation/1306/1. FAST will not calculate hot-spot stresses directly, but you could convert the cross-sectional loads to stresses yourself before post-processing with MLife.

Best regards,

Dear Jason,

I appreciate you as always.

Sincerely,