Dear @Miguel.Pereira,
You may be able to calculate the value of LUlt for simple cross sections composed of an isotropic material, but to truly calculate it for a complex airfoil cross section composed of composite material, you’d have to use a sectional analysis tool. See related topics regarding LUlt on this forum for further discussion e.g.: Mlife - User Defined Distribution - #15 by Jason.Jonkman.
Best regards,