Dear Subramani,
Your plots make sense to me, except for the damage-equivalent load (DEL) for RootFy, which I would expect to asymptotically approach a fixed value for large LUlt, but the plot seems to drop a bit at LUlt = 1000 kN.
I’m not sure I understand your equation LUltIxx/y = M. I think you are trying to use the flexure formula sigma = My/Ixx, but you’ve exchanged sigma for LUlt, where sigma is the stress. LUlt is a load, not a stress. Using an LUlt lower than the load derived from an extreme load case would imply failure.
Ideally, LUlt would be based on a strength (e.g., FEA) analysis of the cross section of the component in question, as discussed in our FAQ: nwtc.nrel.gov/FAQ#TypeLMF. For a simple beam and pure bending, for example, LUlt could be derived from the flexure formula above: LUlt = sigma_yield*Ixx/y_max, where sigma_yield is the yield strength of the material (perhaps reduced by a safety factor) and y_max locates the point in the cross section of maximum stress.
I hope that helps.
Best regards,