sentences:  Failure in ultimate loading of wind-turbine rotor blades exposed to wind and gravity loading is a failure mode that needs to be considered when the rotor blades are designed.The wind speed that causes bending of the rotor blades exhibits a natural variability, and so does the stress response in the rotor blades.Also, the resistance of the rotor blades, in terms of their tensile strengths, exhibits a natural variability.