“How does a plastic blade ‘wear out’”
_____________
Stress fractures from the slight bending due to torque placed on the blade....
______________
If you take a look at the first two paragraphs in the ‘Introduction’ section of the Researchgate paper below, you’ll have a good explanation of the basic issues. The problem is fatigue due to cyclical loads. While there are many specific cyclical loads on an IWT blade that need to be analyzed, here’s one that is fairly straightforward to understand even for the non-engineers....
One of the most common areas of fatigue problems is at the root of a blade. Here’s what happens as a blade rotates and all you have to do is examine two locations.... When a blade going in a clockwise direction is at say the 9 o’clock position, gravity is acting down and the moment that is being applied at the root of a blade is in a counter clockwise direction due to the weight of the blade. Now rotate the blade 180 degrees so that it’s at the 3 o’clock position. At this location, gravity is still of course acting down but this time the moment due to the blade weight that is being applied at the root is in a clockwise direction. So... what happens at the root of a blade is that the stresses that are due just to the gravity portion of the loading have to alternate. A typical IWT has a rotation speed of approximately 16 times a minute. This then means that the cyclical load just due to gravity on a blade occurs that frequently.
And this is just one (and the simplest) of the cyclical forces acting on the blades and is just looking at two locations. If you look at the 12 o’clock and the 6 o’clock positions for the blades, you have something very similar.... the stresses at the root due to the weight of the blade are very different because of the rotational location and this changes with every revolution.
https://www.osti.gov/biblio/12694
https://www.researchgate.net/publication/314755353_FATIGUE_ANALYSIS_OF_WIND_TURBINE_BLADES
Yeah, plus the rebound, wind pressure, uneven tightening on the attachment bolt, the foundation being a tiny bit off level causing the blades to stress the spindle unevenly, small anomalies or defects in the carbon weave, tiny air bubbles in the resin or slightly uneven thickness of the resin can all cause stress fractures. And those things do not even take into account a defective design or square/sharp edges.
I don’t know much about windmill blades but I know some of the engineering that goes into bicycle frames and preventing stress fractures are a huge issue in the design and build process. I am assuming the issues are similar, just on a different scale.