It is not plastic Plastic is not known to be physically rigid, strong. I have seen children bend straws. Fiberglass is nothing like plastic. Carbon fiber is nothing like plastic. The force applied across a wind turbine blade that is spinning at over a 100 mph at it's tip, much less at it's center? That is great question. How much force is being applied? How does it vary across the length of the blade? How much torque is felt at the tip? In a 20 mph wind? How does that tip buffet, vibrate? Load and unload as the wind varies erratically? That is over a hour of google searching for that answer. But the simple logic would dictate stress fractures, fatigue cracks. Lets add freezing and heating?